Edmund (1)

Applied Geochemistry. Vol. 2, pp. 251-274. 1987 (1883-2927/87 $3.(X) + .00 Printed in Great Britain Pergamon Journals Ltd.

Baseline geochemical conditions in the Chalk aquifer, Berkshire, U.K.: a basis for groundwater quality management

W. M. EDMUNDS, J. M. COOK, W. G. DARLING, D. G. KINNIBURGH

and D. L. MILES British Geological Survey, Wallingford, U.K.

A. H. BATH British Geological Survey, Keyworth, U.K.

M. MORGAN-JONES

Thames Water Authority, Reading, U.K.

and

J. N. ANDREWS University of Bath, U.K.

(Received 9 February 1987; accepted in revised form 31 March 1987)

Abstract--The Chalk aquifer is the most important British aquifer and is also important over much of northern Europe. Aquifer protection requires a sound knowledge of the baseline conditions and how these might vary, or have varied, with time. This detailed geochemical study of a representative area of Chalk in Berkshire, U.K., includes a consideration of several components: (1) the inputs from the atmosphere; (2) the interstitial water of the soil and the unsaturated zone; (3) the interstitial water in the confined and unconfined sections of the aquifer; and (4) the saturated, mainly fissure flow, along the hydraulic gradient which forms an important water supply of the Thames Valley region.

Atmospheric inputs form an important source of some elements, but the dominant chemical characteristics of the Chalk groundwater are acquired during percolation through the soil and the upper unsaturated zone. During saturated flow downgradient the chemistry is modified mainly by incongruent reactions of the carbonate matrix and by redox reactions, and only to a minor extent by exchange reactions and mixing with residual saline connate water. The incongruent reaction of carbonate results in a marked increase in the Mg/Ca ratio and the Sr and ~3C contents of the groundwater with increased residence time. Oxygen concentrations are reduced mainly by oxidation of Fe 2+, and the onset of reducing conditions allows dissolved Fe 2+ to increase and rapid denitrification to occur. The salinity profile through the confined Chalk confirms that residual connate water, up to one-fifth sea water concentration, still remains at depth, and this accounts for some salinity increase in the confined groundwater resulting from fissure water/pore water diffusional exchange.

Timescales for groundwater movement have been established using tritium, radiocarbon, and indirectly using inert gas ratios and stable isotope ratios. On balance, it is concluded that all abstracted water is of Holocene age, although inert gas temperatures indicate cooler climatic conditions for recharge for some of the confined groundwater.

The implications for development and aquifer protection are discussed, especially the prospect of natural in situ denitrification, problems of Fe solubility, and the recognition of groundwater of different maturities.

INTRODUCTION

THE CHALK is the most important aquifer in England. It has been extensively developed for public supply for over a century and has been investigated in detail hydrogeologically, particularly in connection with river augmentation schemes (OwEN and ROBINSON, 1978). However, the water quality variations within the aquifer are less well understood. The region is extensively developed both for agriculture and urban settlement, presenting various possibilities for pollution. Nitrate contamination of the Chalk in Berk- shire and elsewhere has been studied in some detail

kG 2 : 3 - X

(FosTER et al., 1982). An improved geochemical framework for understanding the existing water quality distribution in both the saturated and unsaturated zones is desirable as a basis for long-term water quality management. In particular it is desirable to establish:

(1) the baseline contributions for various elements from natural (geochemical) sources, so that long term changes and incipient pollution may be identified;

(2) the present extent of uncontaminated groundwater;

(3) the principal geochemical processes in the 251

252 W. M. Edmunds et al.

* ~ a 40 iO an

s

FIG. 1. Hydrogeological map of the western end of the London Basin (Berkshire). Sample locations and borehole details are summarised. Large circles denote public supply boreholes and small circles indicate

private boreholes.

aquifer and their role in the release and attenuation of solutes during infiltration and flow downgradient;

(4) the residence time and origin of groundwater in the system.

In this study, it has been possible to investigate a wide range of constituents in relation to groundwater residence in one representative area of the Chalk, at the western rim of the London Basin. Stable and radioisotope analysis of the groundwaters are used .to expand the model of groundwater residence time in the central London Basin (SMITH et al., 1976), and the carbon stable isotope data, together with major and some minor and trace element chemistry, are re- examined to elucidate the carbonate geochemistry. Del~ailed control information on the chalk water-rock system is provided by three cored boreholes in which interstitial water profiles and host rock geochemical variation have been studied. Previous work on interstitial waters from the Chalk in East Anglia (BArn and EDMUNDS, 1981) has shown that the most saline water in the system represents connate water (sensu stricto), little changed from Cretaceous sea water; lower salinities in the profile can be explained using a diffusion model mainly by mixing with overlying freshwater. Previous work in the Berkshire area (ED- MUNDS et al., 1973) identified slightly saline interstitial water which is also likely to be diluted connate

water. This water is of a different chemistry to that obtained during pumping. Preliminary analyses of interstitial waters in the confined Chalk of Berkshire (EDMONDS, 1976) indicate that diagenesis of the Chalk, as well as dilution of the connate water, has taken place during freshwater recharge, resulting in increasing Mg/C1, Sr/C1 and K/CI ratios as salinity decreases. Water - rock interaction is also implied by results obtained from the central London Basin (SMITH et al., 1976), where 613C in groundwater becomes considerably more positive during downgradient flow, approaching values characteristic of the Chalk matrix.

The main objective of this paper is to bring together this wide range of hydrogeochemical evidence to contribute to a better scientific understanding of the Chalk aquifer. This should form a basis for improved groundwater management of the Chalk and should demonstrate an approach which is applicable in other areas.

HYDROGEOLOGICAL SETTING

The study area lies at the western end of the easterly plunging London Basin syncline and has a mean annual rainfall of around 750 mm/a. The north-

Baseline geochemical conditions in the Chalk aquifer, Berkshire. U.K. 253

N S A S H B U R Y Winterbourne Faircross B A S I N G S T O K E

Borehole Borehole 1 3 4 5 7 8 9 1 0 1 1 1 2 1 3 1 4 1 7 1 9 3 1 3 5 3 8

i i i I I I i I

u i , i I

m I I lil i I I L t , l~ .~ i , , . . . . . . . . " . ~ l N i i i i I I [ ] [ ~ I - ~ ~ ~ Z ~ " i ' - ' ~ ' - - _ ' - 1 ~ m I I I I I I l n l l ~ ' ~ + i l l i i

/ T E R T I A R Y S T R A T A " " ' .,4 l i e .

r ; / / / / / / / ' / / ~ . + + . . o ~ . . ~ . , . . , I , ' , 't..e+,j,,..,,+ . . . . . . . . . . . i, , . , . , . , ~ - - 0 0 . . . . . . ~ m l ~ , , . . . . . . i i I i [ ] i i i i i [ I ~ • • ~ ~ ~ ] ~

I [ ~ ' . l . ' . ' . ' , ' , ~ - s t ~ i + • t t t s . . -~::'+"

FI~. 2. Hydrogeological section through the Berkshire syncline showing representative boreholes along the line of section (location shown in Fig. 1).

ern half of the area, as indicated by the extensive outcrop of Upper Chalk in Fig. 1, is dominated topographically by the rolling hills of the Berkshire Downs. Tertiary strata occupy the lower land along the axis of the syncline (Fig. 2) and the southern limb of the Chalk rises to form Salisbury Plain.

The Chalk (Upper Cretaceous) is the major aquifer and is overlain unconformably by the Reading Beds (Lower Eocene) and underlain by the minor Upper Greensand (Lower Cretaceous) aquifer, which in turn overlies the Gault (Lower Cretaceous), a blue- grey clay and marl which serves as the basal aquiclude for the water-bearing formations.

The Reading Beds consist of fluviatile sand, loam, clay, bands of concretionary limestone, and beds of flint pebbles and gravels, overlain by red mottled clays. The sands near the base of the sequence are generally in hydraulic continuity with the Chalk.

The Upper Chalk (Maastrichtian-Senonian) is a soft, white, fine-grained limestone containing regu- larly spaced bands of nodular and tabular flints. Fissure flow predominates and of the three sub- divisions (Figs 1 and 2), the Upper Chalk is the most important in terms of water supply. The "Chalk Rock" at the base of the Upper Chalk frequently forms a preferential flow zone if less than 100 m below ground surface. At outcrop, the Upper Chalk is often capped by clay-with-flints which locally reduces infiltration, but which results in increased surface run-off and recharge to the aquifer around the edge of the deposit. The Middle Chalk (Turo- nian) is very similar to the Upper Chalk, but contains smaller flints. The hard, nodular Melbourn Rock at the base of the Middle Chalk may also form a preferential flow zone. The Lower Chalk (Cenomanian) contains no flints and the increase in clay content from the top to the bottom is reflected in a colour change from pale buff grey to dark blue grey. The Chilton Stone, a hard band within the Lower Chalk, forms a preferential flow zone which manifests itself on the scarp slope of the Berkshire Downs in a line of

springs. The base of the Chalk is marked by a thin greenish-grey glauconitic marl.

In the unconfined aquifer, the majority of fissures lie in the upper 60 m of the Chalk and are well developed along the valley systems. There is a large lateral variation in transmissivity between 50-2500 m2/d (OwEN and Romr~sor% 1978); the high values are associated with the valleys, and the transmissivity decreases rapidly away from these areas. The water table reflects the land surface in subdued form and shows seasonal fluctuations of up to 20 m.

In the confined part of the aquifer, the frequency of fissures is less, although their vertical distribution is similar. Transmissivity values lie between 270--450 mZ/d (OwEN and Roazr~soN, 1978) and the transmissivity tends to decrease with increasing depth of burial but there is no pronounced lateral variation. Both fluctuations in pumping regimes and the barometric pressure affect the piezometric surface in the confined zone more than at outcrop. Over the last 30 years a limited long-term decline in the pressure surface of the confined aquifer has occurred south of Reading and east of Newbury.

Springs are rare on the dip slope of the Chalk, drainage occurring via a few major streams which gain their flow mainly by seepage from the aquifer. These streams are characterised by intermittent sections in their upper reaches. On the scarp slope, springs are more common and occur at the junction of the Lower Chalk and Upper Greensand.

SAMPLE LOCATIONS AND COLLECTION

For this study a wide range of hydrochemical samples was collected, representing inputs to the Chalk aquifer (rainfall, soil solutions and unsaturated zone solutions) as well as groundwaters within the saturated aquifer (interstitial waters and pumped groundwater).

254 W.M. Edmunds et al.

(1) Precipitation. Bulk precipitation was collected at a research site at Lambourn (201 m O.D.) located in a small exposed beech woodland (Fig. 1). The Lambourn data are based on four bulk precipitation collectors sampled at monthly intervals between 1980-82. Two of these collectors sampled throughfall and were located directly beneath beech trees. The other two collectors were in open clearings and sampled the rainfall directly. The Lambourn data are compared with longer term rainfall chemistry from an open site at Chilton (123 m O.D.) which represents the average of bulked monthly samples collected over a six-year period, 1975-80 (CAwsE, 1976, 1977, 1978 and pers. comm.).

(2) Soil and unsaturated zone water. The Lam- bourn and Chilton sites were used to investigate element cycling between the atmosphere, soil, and unsaturated zone (KINNmURGH et al., in prep.). The two sites were chosen to be representative of natural recharge conditions beneath grassland (Chilton) and woodland (Lambourn). They represent baseline conditions and neither is typical of the intensively man- aged farmland characteristic of much of the Berkshire Downs. In the latter areas, additional solute inputs would be expected from fertilisers and a general increase in organic matter breakdown resulting from increased agricultural activity.

Samples were collected from soils and the unsaturated zone to a depth of 8 m by excavation and dry percussion drilling, and the results given here are the average for each site for the unsaturated zone, being the "input" to the aquifer below the soil zone.

(3) Saturated zone interstitial water. Interstitial water profiles have been obtained from two boreholes, Winterbourne and Faircross (Fig. 1), which also provide stratigraphic, mineralogical and geochemical control. One of the boreholes, at Win- terbourne, is in unconfined Chalk, details of which have already been published (EDMONDS e ta l . , 1973). The Faircross borehole was drilled near the centre of the basin into Upper Chalk confined beneath 94 m of Tertiary strata, and fully penetrated 229 m of Upper, Middle and Lower Chalk, terminating in the Upper Greensand at 330 m total depth. Interstitial waters were extracted by centrifugation (EDMONDS and BATH, 1976). Alkalinity and pH were measured on site by microanalysis, and filtered and acidified samples were retained for chemical analysis.

(4) Saturated zone pumped samples. Groundwater samples were collected on three separate occasions (July-August 1979, July 1981 and March 1982) from 36 boreholes and two springs in the confined and unconfined aquifer, the sites of which are shown in Fig. 1. The map also shows the total depth of each borehole and the strata penetrated.

Most of the samples represent the fissure derived groundwater integrated over a vertical section of the aquifer. Therefore, they may be a mixture of waters from different flow systems having a range of residence times. This should be borne in mind when

considering some variations in the hydrochemistry in the subsequent discussion.

Water for inorganic analysis was fed under well- head pressure or by manual filtration through in-line 0.45 /zm membrane filters into acid washed poly- ethylene bottles; one bottle from each site was acidified by the addition of 1% v/v Aristar grade HCI, while a second bottle was left unacidified and used subsequently for the determination of F, CI, Br, I, NO 3 and HCO3. During the 1981 and 1982 fieldwork, separate 1 1 filtered samples were collected for trace element analysis. These were stored in bottles which had been subjected to a more rigorous cleaning pro- cedure, involving soaking in 50% v/v Aristar HCI, rinsing and soaking in deionised distilled water followed by drying in a laminar flow hood; they were transported to and from the field in sealed polythene bags to reduce further the risk of contamination. Samples were refrigerated until required for analysis.

Unfiltered samples were collected in 30 mi neo- prene sealed Macartney vials for 62H and 61sO. For U, 25 1 samples were collected and acidified to pH 2. 222Rn was determined on 1 1 samples which were collected in sealed glass bottles with inlet and outlet tubes so that degassing of 222Rn was avoided. 226Ra was determined on 0.45/.,m filtered 5 1 samples which were acidified to pH 2 on collection. For inert gas analysis, samples were collected in glass sample tubes, 1 cm 3 being isolated between glass stopcocks.

ANALYTICAL METHODS

Measurements of pH, EH, dissolved oxygen and, in 1979, HCO3 were made on-site using the techniques described by Cook and MILES (1980).

Most of the major element determinations made on samples collected in 1979, including Na, K and Mg, were carried out by flame atomic absorption spectrometry (AAS); SO4 by gravimetry; F by ion-selective electrode; CI, Si, HPO4, NO3 and NH4 by automated colorimetry; Ca was determined by subtraction of the Mg value from the total hardness measured by titrimetry, Ferrous iron was measured colorimetrically, St, Ba and Li by flame AAS, and total Fe by flameless AAS (CooK and MILES, 1980).

Samples collected in 1981 and 1982 were analysed for Na, K, Ca, Mg, SO4, Si, St, B, Li, total Fe, Mn and Ba, by plasma emission spectrometry. The trace metal determinations required evaporative pre-concentration of samples by a factor of twenty. Automated colorimetry was used to determine CI, NO3, IO3 and total I. Bromide was determined by neutron activation and subsequently by automated colorimetry.

Initially, 62H was determined on H 2 gas produced by reduction of the water samples over U at 800°C; subsequently, the reduction was carried out over Zn at 450°C (COLEMAN eI al., 1982). 6t80 was determined on CO 2 equilibrated with 5 ml aliquots of sample at 25°C. The carbon isotopes, 14C and 613C, were both determined on samples precipitated in the field using BaCI2/NaOH under controlled conditions.

Following the addition of 232U tracer (about 2 dpm) and 300 mg Fe 3+ carrier, U was recovered by co-precipitation on Fe(OH)3, and subsequently separated from Fe, Ca, Mg and other elements by solvent extraction and anion exchange procedures. The U was then electrodeposited on a stainless

Baseline geochemical conditions in the Chalk aquifer, Berkshire, U.K. 255

steel planchet and counted in a silicon surface-barrier alpha- spectrometer. 234U/23aU activity ratios were measured directly from the alpha-spectra and U-contents calculated from the 238U count using the 232U count to determine chemical yield.

For Rn, the groundwater was outgassed with a N 2 stream and Z22Rn was trapped on charcoal at -80~C. Subsequently, 222Rn was transferred from the charcoal trap to an evacuated scintillation flask. The alpha-activity of Z2°Rn was determined by placing the flask on the photocathode of a 3 in. photomultiplier and counting after the elapse of a period of 3 h to permit ingrowth of 222Rn daughters. Counting rates were corrected to the groundwater collection time, and absolute 222Rn contents were determined from flask cali- brations with standard 226Ra solutions.

Radium was measured in the acidified sample, following transfer to glass de-emanation bottles and removal of excess 222Rn by outgassing with nitrogen. Following storage for at least one month, to allow ingrowth of 222Rn, the 222Rn in equilibrium with 226Ra was determined as described above.

Samples for inert gas analysis were introduced to a vacuum system, and mixed with a known volume of tracer gas. The individual gases were separated and their isotopic ratios determined by mass spectrometry. The gas contents of the sample were then calculated from the standard relation for isotope dilution analysis.

The Ca, Mg and Sr contents of the carbonate fraction of Chalk from the Faircross borehole were estimated by 1 M acetic acid dissolution of samples from which interstitial water had been abstracted.

R E S U L T S

Atmospheric inputs

The chemical da ta for rainfall at Chi l ton and rainfall and throughfa l l at L a m b o u r n are given in Table 1.

The throughfa l l vo lume was a b o u t 11% less than the rainfall at L a m b o u r n , a measu re of the in te rcep t ion loss. The throughfa l l is two to th ree t imes more concen t r a t ed for many of the solutes. This c a n n o t be accounted for mere ly by evapora t ive concen t r a t ion on the foliage and the re fo re represen t s e i the r (1) an increased input due to washoff of dry deposi t ion on the vege ta t ion surfaces, or (2) leaching of solutes f rom the vegeta t ion . This dis t inct ion is an i m p o r t a n t one because the first process gives rise to a net increase in solute input to the ecosystem, whereas the second process only reflects a more rapid cycling of solutes a l ready present . A t this t ime it is not possible to dist inguish be tween these two processes and probably bo th are opera t ing . Never the less , t he re is a possibili ty of a relat ively high solute input f rom areas covered by vegeta t ion .

The re are cons iderab le , as yet unexp la ined , differences in the chemis t ry of rainfall be tween the two sites; for example , d i f ferences of more than a fac tor of two were found for K, Mn, and Na.

Interstitial water o f the soil and unsaturated zone

The Chi l ton and L a m b o u r n sites overl ie the Middle and U p p e r Chalk , respect ively, and the infi l trat ing rainfall and throughfa l l rapidly read jus t to the new env i ronmen t . Most of the m a j o r changes occur in the soil zone. The most i m p o r t a n t changes are the reduc- t ion in H ÷ concen t r a t ion by th ree or four orders of magni tude ( f rom pH 4-5 to abou t p H 8) reflecting the

Table 1. Average concentration of dissolved solutes in bulk precipitation and in unsaturated zone Chalk interstitial water at two sites within the study area. Results in mg/l

Precipitation Interstitial water Chilton Lambourn Chilton Lambourn

Rainfall Rainfall Through fall Open Open 1975-80 1980-82 1980-82 site site

688 mm/a 1000mm/a 890mm/a 0.6-3.00 m 1.3-7.8 m n = 24 n = 12-20 n = 12-20 n = 48 n = 48

Woodland site

3.0-7.8m n = 7 0

Ba - - - - - - 0.020 0.034 Br 0.026 . . . . Ca 1.92 3.49 6.79 60.2 105 CI 5.70 9.17 26.2 10.6 52.5 F 0.096 . . . . Fer 0.055 0.038 0.045 0.009 0.014 HCO3 - - - - - - 121 85 K 3.06 1.21 4.03 0.74 0.92 Li - - 0.0005 0.0012 <0.004 <0.001 Mg 0.75 0.61 1.90 0.83 0.92 Mn 0.010 0.044 0.18 0.003 0.007 NO3-N 1.53 - - - - 1.23 24.2 Na 2.64 6.18 14.5 4.12 61.9 S 4.8 9.2 23.0 37.2 147 Si - - 0.129 0.256 1.82 1.79 Sr - - 0.015 0.023 0.158 0.261 Zn 0.068 0.058 0.100 0.002 - - I <0.005 . . . .

0.049

179 302

0.004 89

1.16 <0.001

4.71 0.011 4.8

134 207

3.48 0.433

Fe T = total Fe. - not determined.


gain of significant bicarbonate alkalinity. An increase in dissolved Ca by about a factor of 30 rapidly leads to calcite saturation. Dissolved S increases by an order of magnitude, and there is a reduction in the concentration of some trace elements, notably Zn, by a similar amount. Four principal processes are operat- ing in the unsaturated zone: (1) the concentration of all solutes due to the evaporation of water; (2) the removal or release of solutes by precipitation or dissolution of solid phases; (3) the temporary removal of solutes by plant uptake and biomass accumulation; (4) the temporary release or removal of solutes by ion exchange reactions. Virtually all solutes are affected by processes 1 and 2 whilst processes 3 and 4 result in a " temporary" loss or gain of some dissolved solutes. Cation exchange reactions, for example, tend to buffer rapid changes in the concentration of dissolved solutes but, because the cation exchange capacity is more or less fixed, these reactions do not account for a permanent loss or gain of solutes. Weathering reactions, here principally the dissolution of calcium carbonate, do result in a permanent transfer of solutes from the solid phase to solution.

Chloride in the unsaturated zone groundwater is derived almost exclusively from atmospheric inputs (fertilizer inputs can be ruled out at the sites studied here) and so the extent of evaporation can be estimated from the increase in CI concentration going from rainfall to the interstitial water deep in the unsaturated zone. This is illustrated by interstitial

water profiles at the Chilton site, which show that below about 1.5 m, the CI concentration is almost constant with depth (~12.6 mg/l). This compares with an average rainfall concentration of 5.7 mg/1 and a rainfall of 688 mm/a giving a concentration factor of 2.2 and implying recharge of approximately 45% or 311 mm/a. This is in reasonable agreement with estimates using physical measurements (Penman equations) at similar sites. Clearly, those solutes with a concentration factor much greater than 2.2 (e.g. Ca and S) must be derived from mineral dissolution, or some other form of input such as dry deposition, and those with a factor considerably less (e.g. Mg, K, NO3-N, and many trace elements) must be reacting and/or being removed within the profile. The most likely sink for the major elements (mostly nutrients) is in the slow build up of solid organic matter in the soil. Although this build-up is in a sense only temporary, it can take decades to reach a true steady s ta te - -a time interval much greater than the few years that it takes for the water to pass through the top few metres of the unsaturated zone.

The situation at Lambourn is quite different. Large differences in Chalk interstitial water chemistry were found between '~open sites" and "woodland sites", and in none of the four profiles sampled was a clear steady state C1 profile found. It seems likely that some of the increased C1 found beneath the trees reflects the higher CI concentration in the throughfall and increased dry deposition of Ci on the tree surfaces, but must also reflect an increased evaporation

I N T E R S T I T I A L WATER C H E M I S T R Y ( r a g / I )

Ga

NIl K

/ :..

c, ;'~ '; ... Na

.'.~ . . . . . . . . . . . . . -~_L2 ,~___ "' " ~ .3"

..,~ ~ ~ ~:.

Sr "~.

F]~. 3. Interstitial water profiles (CI, Na, K, Ca, Mg, Sr) through the Upper, Middle and Lower Chalk at Faircross.


Table 2. Field analyses of pH and alkalinity of extracted interstitial fluids from various depths in the Faircross borehole. Calcite and dolomite saturation indices (Sic, SId) are derived from field analytical and temperature data. Ratios of the

activities of Mg and Ca are also shown

Depth interval Alkalinity KIA P KIA P

(m) pH HCO3 (mg/l) calcite Sic dolomite Sld aMg/aCa

90-120 7.99 298 0.79 × 10 -s +0.29 0.73 x 10 -~6 +0.15 1.07 120-140 8.07 382 1.76 x 10 -8 +0.61 5.5 x 10 -16 +0.60 1.18 140-160 8.24 336 1.71 x 10 -8 +0.66 4.6 × 10 -16 +0.68 1.35 160-180 8.28 365 1.88 x 10 -8 +0.68 6.5 × 10 -16 +0.73 1.53 180-200 8.26 400 2.12 x 10 -8 +0.73 6.7 × 10 -16 +0.75 1.29 200-220 8.37 354 2.30 x 10 -8 +0.82 6.1 × 10 -16 +0.77 1.06 220-240 8.39 361 3.05 x 10 -8 +0.91 6.8 x 10 -16 +0.91 1.05 240-260 8.33 386 2.42 x 10 -8 +0.82 8.3 x 10 -16 +0.82 1.04 260-280 8.50 385 4.30 x 10 -s +1.06 25 × 10 -16 +1.10 1.13 320-330 8.61 521 5.30 x 10 -s +1.16 29 × 10 -16 +1.14 0.99

of water by the trees because the average interstitial water: throughfall C1 ratio was 11:1. Chloride concentrations of up to 930 mg/1 were found in the interstitial water beneath beech trees. Recharge must have been very low for a considerable period at these sites (less than 100 mm/a). The NOa-N data at Lam- bourn show considerable variation; in particular, concentrations were high in the interstitial water at open sites, where NO 3 removal is minimal, and relatively low directly beneath trees, suggesting uptake of NO3-N by the trees, an effect opposite to that for CI.

Saturated zone vertical profiles

(a) Unconfined chalk. Results from a borehole in the unconfined Chalk at Winterbourne (EDMuNDs et al., 1973) have shown that there is a slight increase in the salinity of interstitial water with depth up to a maximum of 300 mg/1 Na. The chemistry of the interstitial water differs from that of water in fissures at similar depths. Interstitial water, therefore, represents the very large intergranular storage which is not readily abstracted by pumping but which retains a faint memory of its evolution over a lengthy period of geological time. The diffusional exchange between the intergranular and fissure water is important, nevertheless, in determining the evolution of groundwater composition over an extended period of time.

(b) Confined chalk. In contrast with Winterbourne, the results from the Faircross borehole are an example of a situation where large scale groundwater circulation has been far less important. Profiles for Li (EDMuNDS et al., 1986) and for CI, Na, K, Ca, Mg, and Sr (Fig. 3) illustrate the overall increase in pore water mineralization with depth. The overall increase in salinity is from 100 to 2000 mg/l; that is, up to one-fifth of sea water composition. The predominant features of the profiles are the smooth increases in CI and Na through the Upper Chalk, with a marked increase in both elements below the Chalk Rock

zone; this overall pattern is also shown by most of the other elements.

The pH of interstitial water increases consistently with depth (Table 2) from 8.00 to 8.50, while the total alkalinity remains relatively constant except for the uppermost interval where there is some disturbance due to fissure flow. All of the interstitial waters show significant supersaturation with respect to calcite (Table 2) and the reasons for this are not obvious. Three possibilities are: (1) that the Chalk particles are not homogeneous but, although small, may have a Mg-rich surface composition; (2) that the small grain size needs to be taken into account in the calculation of the saturation index; and (3) that SO4 substitution in the calcite structure (BuSENBERG and PLUMMER, 1985) may increase the solubility. At the present time none of these can be substantiated but (2) is unlikely to provide supersaturation of sufficient magnitude. Either (1) or (3) seem most probable and, although no SO4 data are available for the Chalk, it is noted that gypsum cement is often found in the Chalk matrix (MORGAN-JONES, 1977).

Hydrogeochemical changes downgradient

All data are given in Table 3 and the principal changes in hydrogeochemistry taking place along the hydraulic gradient are summarised in Figs 4-8, where the data are projected onto the approximately N-S line of section shown in Fig. 1. The position of the confined-unconfined boundary is shown on each diagram, and the redox boundary, which is discussed in more detail below, is also indicated.

Major ions

Chloride in the range 9-31 mg/l characterises the groundwaters in the unconfined zone where Na ranges from 5-17 mg/l (Fig. 4). In the confined aquifer the molar Na/CI ratio increases from <1.0 to ~2.0; maximum Na and CI values of approximately 220 and

258

Table 3. Hydrogeochemical analyses cross-section (Fig. 1).

W. M. Edmunds et al.

of groundwaters from the Chalk of Berkshire tabulated according to the line of Data are from three seasons' sampling, (i) 1979, (ii) 1981, (iii) 1982

Site No. Location pH

bI3C(HCO~) E,v O: Temp. 3H 14C 6ZH 6180

(mV) (rag/l) (°C) (TU) (pmc) (Y~oSMOW) (%.SMOW) (%PDB) Bulk

1 Ashbury Spring (iii) 7.25 2 LetcombeRegisSpring (iii) 6.80 3 MileEnd (i) 7.25 4 EastburyGrange (i) 7.22 5 HenleyFarm (i) 7.24 7 Brightwalton (i) 7.00

(ii) 6.88 (iii) 7.05

8 Welford (i) 6.95 (iii) 7.18

9 BrightwaltonHolt (i) 7.16 10 ChapelFarm (i) 7.18 I1 Peasemore (i) 6.92 12 WinterbourneNorth (i) 7.12 13 Chieveley (i) 7.15

(ii) 6.9 (iii) 7.15

14 ColdAsh (i) 7.07 15 BradfieldValley (iii) 7.2 16 ColthropBoardMills (iii) 7.25 17 Woolhampton (i) 7.21

(iii) 7.20 18 ShalfordFarm (i) 7.23

(iii) 7.22 19 OIdMiltHotel (i) 7.3

(iii) 7.35 20 AldermastonCourt (i) 7.27

(iii) 7.3 21 UftonNervet (i) 7.17

(ii) 7.0 (iii) 7.2

22 Theale (i) 6.92 (ii) 6.8

(iii) 6.8 23 Grazeley (i) 7.54

(iii) 7.4 24 Shinfield (i) 7.48 25 ArborfieldMill (i) 7.4 26 DairyResearchlnst. (i) 7.48

(iii) 7.50 27 GrazeleyCourtFarm (i) 7.64 28 MisselsBridge (i) 7.44

(iii) 7.59 29 Mortimer (iii) 7.49 30 LittleParkFarm (i) 7.50

(iii) 7.45 32 StanfordEnd (i) 7.58

(iii) 7.68 33 Wellington Park (i) 7.14 34 Wellhouse Farm House (i) 7.52 35 LawnFarm (i) 7.60 36 SheffieldManor (i) 7.34 37 Redlands (i) 7.00

(iii) 7.3 38 Greywell (i) 7.05

(iii) 7.05

10.1 10.0

355 8.9 10.4 12 -46 -7.9 - 12.4 385 9.8 11.0 5 -46 -7.1 -13.6 335 8.8 10.5 6 -46 -7.7 390 8.4 i1.5 7 -46 -7,6

9.3 11.7 411 10.3 10.5 41 62.1 -48 -7.4 -14.8 -13.9 370 9.0 11.2 4 -49 -7.0 -12.8 372 9,9 10.3 9 -50 -7.3 -7.1 352 10.0 11.0 23 360 10.5 11.5 29 -45 -7.9 380 10.5 11.0 2 -49 -6.2 -5.2 360 8.9 11.0 19 -13.5 415 8.2 11.1 1 -48 -6.9 -12.8 245 8.0 11.2 417 9.2 l l .0 6 56.3 -49 -7.2 -13.5 -12.3 415 7.0 11.5 33 -47 -6,6 -10,6 396 4.8 10.6 22 -48 -7.2 -6.8 166 0.5 12.8 20 7.0 -47 -7.0 -6.3 -9.6 60 0.2 11.6 I 10.9 -48 -7.0 -8.1 -8.3

161 40.1 11.5 50 0.3 12.5 3 -48 -7.8 -2.8

-99 40.1 12.1 5 0.2 12.7 1 5.8 -47 -7.2 -6.7 -6.2

100 0.2 11.2 -47 -7.2 -9.7 75 0.1 12.0 1 -49 -7.3 -3.3

120 0.1 11.1 6 2.0 -49 -7.4 -3.9 -2.1 4 0.2 12.8 5 2.6 -48 -6.9 -1.7 -2.5 0 0.1 12.1

30 0.2 10.9 21 -46 -7.0 -4.8 130 0.2 10.8 72 -46 -7.4 -12.2 165 0.2 11.5 185 0.1 10.3 42 55.4 -44 -6.9 -13.4 -11.7 80 0.4 12.8 2 1.1 -47 -7.8 -2.1

120 0.2 11.8 -49 -7.5 -6.9 60 ~0.I 12.8 0 -49 -7.8

12.5 40 0.2 12.8 0 1.9 -49 -7.6 -4.7 -4.4

119 0.2 12.4 50 0.2 13.5 0 -50 -7.8 -2.9 50 0.1 12.8 I -46 -7.2 -1.8

-59 40.1 12.6 41 40.1 12.5 6 11.7 -49 -7.2 -5.6 -8.6 90 40.I 13.5 1 22.9 -47 -6.7 -13.3 99 0.2 12.1 8 23.9 -45 -6.8 -15.9 -13.3 80 0.2 13.5 1 4.8 -47 -7.6 -6.4 -5.4

- 9 40.1 13.3 30 0.3 13.6 2 10.2 -48 -7.2 -5.6 -7.3

-50 0.2 0 7.3 -48 -7.6 -8.6 -8.6 40 0.1 13.5 0 -51 -7.9 -1.7

110 0.3 12.9 2 3.4 -46 -7.5 -3.1 -2.5 90 0.1 12.0 0 -43 -6.9 -2.1 -2.4

130 0.1 11.6 5.9 -45 -7.0 340 7.8 11.6 8 -44 -7,0 -12.7 445 8.5 11.2 33 68.7 -45 -6.8 -14.2 -13.2

200 mg/1, respectively, are found. Sulphate (Fig. 5) increases from 5-15 mg/l in the unconfined aquifer to a maximum of around 70 mg/l in the confined aquifer, following broadly the same pattern as chloride. The borehole at Theale is an exception to the general behaviour in these and other parameters, and its anomalous chemistry is considered to reflect a component of the recharge being derived from the overlying Eocene and alluvial deposits. Because it is anomalous, it has not been plotted on Figs 4-8.

Overall trends in the carbonate system can be followed through the variations in pH, total alkalinity (expressed as n c o 3 ) , Ca and Mg (Fig. 4). Total alkalinity is typically 220-340 mg/l in the unconfined aquifer and between 260--330 mg/l in the confined aquifer; comparable values for pH are 6.9-7.3, and generally 0.2--0.3 units higher in the deeper confined groundwaters. The principal and most striking lea-

ture in the aquifer is the antipathetic change in Ca and Mg. Calcium in the unconfined aquifer is 85- 120 mg/1 but drops to around 40 mg/l in the confined zone, whereas Mg increases from 0.9-3.5 mg/I to a maximum of 30 mg/1. These trends are discussed in detail below, but it should be noted here that the increase in Mg precedes the increase in Na.

Redox system

The redox potential E H remains buffered at +330 to +420 mV in the oxygenated groundwater (Fig. 5), where the platinum electrode probably behaves as a Pt-O electrode (WHrrHELO, 1974) which is sensitive to pH but not oxygen. Dissolved oxygen levels remain fairly constant in the unconfined aquifer (about 9.0 mg/l) and fall to zero coincident with the


Table 3. Continued

Na K Ca Mg Cl SO4 HCOs NO Si Sr F (m~).

Ba Li B Fe

Br "~ I 103 NH~ total Fe:" Mn .0zg/I)-

7.1 3.1 110 2.1 15 33 286 22 10.7 0.29 7.6 2.3 122 2.3 13 34 315 33 10.5 0.58 8.0 1.1 90 1.6 13 15 221 22 6.5 0.9 86 0.9 17 7.6 224 30 8.9 1.1 93 2.0 12 12 241 27 7.0 1.3 116 2.0 9 4.9 328 21 5.1 0.99 121 1.6 11 6.3 342 24 5.6 0.6 124 1.6 11 2.5 348 22

10.6 5.0 113 2.3 18 4.3 326 19 8.5 0.8 117 1.3 15 4.2 341 18

12 1.0 98 1.4 26 13 272 31 10 1.0 113 1.8 21 18 268 49 9.6 2.8 109 2.1 31 8.1 300 31 9.8 1.3 118 1.5 14 27 289 30

17 2.0 103 3.5 18 8.6 294 30 14.5 1.9 107 3.3 16 14 305 35

3.3 4 19 2.5 4 26

6.2 0.2 0.21 ~ 6 0 ~<5 ~<20 4 6.2 0.17 0.07 ~<60 50 6.7 0.18 0.15 <~100 ~<5 50 5 5.5 0.22 0.17 ~<60 ~<5 40 6.6 0.26 33 0.6 6 5.8 0.25 0.11 27 0.6 ~<6 41 5 4 7.3 0.23 0.13 ~<100 ~<5 ~<20 6 7.4 0.25 0.12 25 11.4 3 211 5 4.1 0.04 0.13 ~<60 ~<5 ~<20 4,7 0.14 0.15 ~60 <~5 ~<20 5.3 0.07 0.15 ~<100 <~5 ~<20 6 4.9 0.11 0.15 ~<I00 <~5 <~20 6.5 0.26 0.29 ~<100 <~5 30 7.7 0.53 28 2.3 43

7 ~0 .2 4 ~0 .2

<10 7 ~<10 2 ~<10 8 ~<10 2 ~ 1 0 8 ~<10 2 ~<10 7 ~<111 1

~0 .7 ~0 .03 ~<0.3 ~0 .03

<~10 10 ~<10 2 <~20 1 0.3 ~<I0 4 ~<10 ~I).5 ~<10 <~2 ~<I0 ~0 .5 ~<10 8 ~<10 ~0 .5 ~<10 <~2 ~<10 ~0 .5 ~<11) 9 -<.10 I

0.7 ~0 .03 14.9 1.5 109 3.0 17 11.7 316 32 12 3.2 118 3.9 23 33 287 29

8.4 1.5 105 2.6 16 12.9 295 22 8.2 0.33 0.13 30 3 13.4 2.4 86 8.3 I8 14.5 301 ~ 2 9.7 2.0 0.74 146 8 9.8 1.9 97 5.4 20 24 292 ~0 .1 10.0 1.0 0.49 <~100 11

10.4 2.0 100 5.4 18 21.7 300 ~ 2 10.9 0.99 0.40 98 7 13 3.4 87 I5 I6 13.5 323 ~0 .1 9.7 4.5 1.40 130 9 12.7 3.2 84 13.9 13 15.0 324 ~ 2 10.7 4.22 1.30 131 8 12 5.1 84 16 16 15 324 ~0 .1 7.8 1.8 0.63 70 6 11.6 5.0 79 17.3 15 15.5 328 ~ 2 8.2 3.59 0.48 69 10 36 6.9 63 24 33 15 349 ~0 .1 9.8 6.3 1.70 120 13 31.7 6.8 63 22.5 30 14.8 342 ~ 2 10.5 6.17 1.33 112 12 19.4 3.9 83 16.4 27 20 292 ~0 .1 9.3 4.4 1.10 I00 12 19.0 3.8 84 14.5 22 21.9 320 ~0 .5 11.4 4.36 119 9

6.6 0.46 0.31 24 2.6 42 58 7 6 <~20 ~<0.5 ~0 ,03 5.5 0.18 0.18 80 <~5 ~<20 ~10 11 ~<10 ~0 .5

11 35 3 ~20 1 0.1 39 42 8 53 212 8 30 5 10 260 21~ 7 19 32 4 138 3 70 18 10 180 140 5 45 28 16 146 4 30 12 60 310 290 35 56 67 12 ~1 205 6711 57

130 20 250 7~1 350 19 111 143 24 ~1 460 355 6.4

77 50 110 55 8 61 I15 8

21.5 3.8 85 15.1 25 19.9 324 ~ 2 10.6 4.3 1.05 115 24 4.3 169 8.9 41 197 293 0.4 5.0 1.5 0.35 <~100 9 21,9 4.2 147 8.4 39 137 300 8.0 6.9 1.53 58 7 21.9 3.5 150 8.2 41 124.6 318 7.1 5.8 1.4 0.32 47 9 91 9.3 44 30 78 56 299 ~0 .1 9.5 5.7 1.47 ~<100 22 76 9.3 47 29.4 74 55.6 302 ~ 2 10.7 6.21 1.45 88 21

113 9.8 53 26 117 45 304 ~0 .1 6.7 4.1 1.55 80 25 72 9.4 48 28 63 48 297 ~0 .1 5.3 2.9 1.25 ~<100 15

115 9.6 42 26 129 47 321 ~ 0 . I 7.3 4.4 1.90 80 24 115 9.6 4I 26 114 42 324 ~ 2 7.5 4.3 1.67 52 25 94 9.2 42 2I 101 41 271 ~ 0 . I 9.5 4.6 2.0 ~<100 24 92 8.2 55 19 96 34 309 ~0.1 10.2 4.4 1.80 90 19

115 9.7 54 23 102 36 285 ~ 2 13.5 5.00 1.7 73 20 80 5,1 52 10.3 72 34 268 ~ 2 9.8 2.05 1.25 56 14 53 9.7 42 29 30 45 316 ~ 0 . I 4.7 4.4 0.68 90 18 55 111.2 46 27.5 29 48.2 330 ~ 2 4.9 4.29 0.64 51 16

132 10.0 57 19 183 52 321 0.I 5.8 2.9 2.60 70 15 173 9.9 35 19.5 184 51 313 ~ 2 6.6 3.6 2.4 48 29

70 8.7 57 25 66 33 318 ~0 .1 5.9 3.8 1.18 ~<11~1 22 61 9.1 57 28 55 54 323 ~0 .1 4.8 3.6 1.20 711 25

10.8 61 109 22 ~1 130 98 6.8 120 10 9tl 43811 19011 282 107 1810 91 100 9 ~1 89 1230

290 531) 70 232 315 35 ~1 720 61 640 890 2811 3311 54O 23O

540 47 700 2711 526 540 54 ~1 1150 217 420 34 630 240 310 28 440 ~ 370 210 31 71 282 320 29 640 72 680 8 690 64(1 253 88 8 1 1060 582 NI0 740 110 7411 I140 67 141 230 27 50O 500 190 26

139 124

38 4 2.8

130 8 110 13

1.311 9 6.2

130 8 55 3

2 3

295 211 16

120 9 9

270 2O 470 25611 1380 34

219 9.4 36 14.4 204 68 319 ~0 .1 8.6 2.7 4.8 ~<100 22 1100 850 220 120 16 26 6.1 67 20 21 20 318 ~ 0 . I 9.8 3.8 1.92 ~<100 6 1711 19 260 650 16 15 3.8 76 16 17 20 291 ~ 0 . I 9.1 3.3 2.20 120 9 11~1 7 80 260 1411 3 16.3 4.3 70 19,9 15 20.7 312 ~<2 9.5 3.55 2.10 71 6.8 103 62 11 ~<1 190 292 3 9.9 1.9 113 2.1 18 19 295 31 4.8 0.3 0.16 ~<100 ~<5 411 5 ~<111 ~<2 ~<10 2

111.4 1.6 116 2.2 17 15.1 316 31 5.1 0.33 0.11 18 1.4 I4 55 5 5 ~<20 1 11.3

drop in En to +160 mV or less. Similar behaviour is been detected in most of the Chalk groundwaters: observed for NO3 which persists at 20-30 mg/l in the Li, NH4, K, Sr, Ba, B, F, Br, I, Mn, and Si. In unconfined aquifer and then decreases to below the addition, Cu, is above 1 /zg/1 in the majority of detection limit, coincident with the fall in E n and oxidising waters but is not discussed further in this dissolved O2. A well defined redox boundary there- paper. Chromium, Co, Ni, Zr, Cd, AI, Sc, Y, La fore exists in the Chalk aquifer, which is primarily and Pb were not found above their detection limits controlled by the complete reaction of O2, probably of 1, 0.8, 3, 0.5, 1, 5, 0.05, 0.05, 0.5 and 10 gtg/1, by inorganic reduction by Fe 2+ in the chalk sediment; respectively. this, in turn, allows rapid reduction of NO 3 and Potassium (Fig. 6) shows some variability in the, solution of Fe 2÷ (EoMuNOS et al., 1984). The total unconfined aquifer but is generally between 1-2 mg/l dissolved (~0.45 gtm fraction) Fe concentrations with anomalies up to 5 mg/1 possibly due to anthro- remain very low (~<0.01 mg/1) under aerobic condi- pogenic contamination. In the confined aquifer K tions but rise to 0.1-0.7 mg/l in the reducing section of increases to almost 10 mg/l. Strontium, Mn and Li all the aquifer, increase sharply at the redox boundary; Sr is 0.3 rag/l,

Mn is very low, between <0.03 and 2 p~g/l, and Li is Minor elements typically around 3 tzg/l in the unconfined zone; these

values increase to maxima of ~ 6 mg/l, -35/zg/1 and The following minor and trace elements have 25 /zg/l, respectively, in the confined aquifer. It is

260 W . M . Edmunds et aL

~,---- UNCONFINED----~J • .I AEROBIC

290

A

~ 1 5 0

IOO

50

o

"~t50

,oo] 5°]1t~

o o

~ 813 t 75 0

7.0 o

6"5 0

o

t .) 501

o;

!:t o

~b /o

CONFINED ) ~"UNC.-- J -,w~-- ANAEROBIC ~ - A E R O . ~

Rldox Redox Oou~dlry Boultdar y

A~

S

4b ~b 70

• l I• • I in

4b ~o 6o r •

o o o

i• ab

10 20 30 40

°° r , :

~ ~b ~ 4b

~b ~b ab ~b

•

4k"• B 1

• " I . o ~

Distance along line of sect ion (k in )

FIG. 4. Major ion (Na, CI, HCO3, Ca, Mg) and pH variation along flow gradients across the Berkshire syneline. Distance along line of section is measured from the base of the Chalk on the northern outcrop shown in Fig. 1. Open symbols denote unconfined groundwaters and solid symbols indicate

confined groundwaters.

~ UNCONFINED'-'I" I ~ CONFINED D I"-UNC.~ AEROBIC i~ J ~ ANAE ROE IC "~-.1~ ~"AI~O.~ "=' "=i'

o/ • D

[ o o ~8

• 1 --4b- -r,~o" -~0 ~b

! o lb /o ~o

C 0 lb 2b ab - - ~ -

0"6 A

}o,

U. O.

m

E g

c

n n n ~ rll ' l ~b ~o ~b 7o

| lb ab ab

~ A ~ '~

,b e•

• t74o f2eeo

4 , |

n

J ab

Distance along line of sect ion ( km) 70

FIG. 5. Variation in redox related parameters (En, 02, NO3, FeT and SO4) along flow. gradients across the Berkshire syncline. Distance along line of section is measured from the base of the Chalk on the northern outcrop shown in Fig. 1. Open symbols denote unconfined groundwaters and solid

symbols indicate confined groundwaters.

significant that the increase in Sr, Mn and Li precede the increases in Na and CI in the confined aquifer and seem to be more associated with the changes in Ca and Mg concentrations.

Fluoride concentrations are generally below 0.3 mg/1 in the unconfined aquifer, increasing to a maximum of 4.8 mg/1 at depth. The rapid increase in F across the area of the redox boundary is in contrast to C1 behaviour. Bromide and total I (Fig. 6) are present in the unconfined aquifer at levels below 0.06 and 0.008 mg/1, respectively, increasing to maxima of 1.1 and 0.067 mg/l, respectively, at depth. It is evident that Br concentrations change parallel with those of C1, while I and F show a similar variation to each

other, and to Ca, Mg and Sr (which is different from that for Cl); a similar relation was observed in the Lincolnshire Limestone (EDMur~DS, 1973).

Stable isotopes, tritium and radiocarbon

Stable isotope ratios (180/160 and 2H/1H) for the majority of groundwaters lie in the overall range - 6 . 6 to -7.9%0 for 61so, and - 4 3 to -51%o for 62H. These values are very similar to those reported for the centre of the London Basin by SMrrn et al. (1976). It can be seen from Fig. 7 that there is no clear variation either along the aquifer section or (with

Baseline geochemical conditions in the Chalk aquifer. Berkshire, U . K . 261

15

~ . U N C O N F I N E D ~ ~ C O N F I N E O - - ~ UNC-" i A E R O O I C . . ~ . - - ~ . ~ A N A E R O ~ I C ~:~ AERO.- a Nodox •odox

Boundary Boun4ary

~o 0. .I ! i ~o ° ° ~ ~o ,~ *

015

0.O5

U.

0 0 6 }o, 50

~b ~b ab ~b

15 ~,o ab ~b

tx

r~ eo

,£t "~o

~b ~b m

:h. , ~

. I ~b m

o uoO o o •- - • ii~/ o lo 20 ~o 40 50 60 ~0

I. 5" 0 i o j

o o o o ~b ,,b ~b 6o n)

~ .~o 1.o!

.e o5

z o

075

050"

e~,~ o 25 .

5 ~

^ ^

l;..0. _ ~ D o ° ° ° ° ~

D is tance a long line of sect ion (ks )

A

- ~

7O

. ° t o

-45 t

~ - 4 ~ ]

-521 0 -6"5

' * - - U N C O N F I N E D ---t,- I .n CONFINED n,. F U N C . - ~ " ' - " - " ~ - A E S O S I C - i ~ ~N AE ROSIC . ~ l ~ ~ A E SO..,

~o:,.. sod. SoundBry mou~ery

n

i •

. . . . . . - ~ - " , - - ~ - ~ . . . . i - , - . - ' ~ . - - . - * ' . z r - ~ -

lb ab ab 4o ~o eo

o

I . t •

~b ~ ~ 4b r,b 60

~ e ~ eo o ~o • •

° o o o d~ ~ • o ib 2b ab ~ ~b 6o

N , lb N 3b 4b ~ 60

• ~ • ~o o ~ . 1 , . - .

o o O • miD •

o o • m R lb 2b ~b *b ~ 6o

D i s t a n c e a l o n 0 l ine o f s e c t i o n ( k s )

ZERO TRITIUM ~- -

.I

70

FIG. 7. Variation of stable and radioisotopes (C, H, O) along flow gradients across the Berkshire syncline. Distance along line of section is measured from the base of the Chalk on the northern outcrop shown in Fig. 1. Open symbols denote unconfined groundwaters and solid symbols indicate

confined groundwaters.

reference to 3H and 1 4 C ) correlation with groundwater age. The scatter of values probably represents a variety of recharge sources and some variation in recharge composition with time. There must be components of older (Pleistocene) water in the deeper aquifer, particularly in areas of unfissured Chalk where low permeability intergranular storage domi- nates. However, the absence of a clear trend in stable isotopic composition suggests that the groundwater is relatively well mixed and does not have an unmixed pre-glacial component in the deep aquifer. In comparison, Pleistocene recharge has been identified in the East Midlands Sandstone aquifer and is characterised by 6180 below -9%0, which is around 2%0 lighter than recent groundwater (BArn et al . , 1979).

Thermonuclear tritium was measured in duplicate for most sites in 1979. Tritium values are below 1 TU in the confined aquifer indicating that recent recharge is but a minor component. Tritium in the unconfined Chalk aquifer is variable. Positive values indicate

FIG. 6. Minor element (Si, Sr, Ba, F, Br, I, K, Li, NH4, B, Mn) variation along flow gradients in the Berkshire syncline. Distance along line of section is measured from the base of the Chalk on the northern outcrop shown in Fig. 1. Open symbols

denote unconfined groundwaters and solid symbols indicate confined groundwaters.


"~--" UNCONF]NED---~- [ 9 AEFI081C

o

0

lO

,,~ 0.5

° o

15

2"O

~5 1.o.

0.5.

o

400

o

- m ~ 13

o ooo

1'o ~o

, , C O N F I N E D , ~ - - - ~ . - U N C : '~ - I ~1.---. ANAEROSIG -~-o- ~-AEIqO: ~

Rodox Redox Ilovndltry Bo4mdltry

i |

• t,.al i o

-,io- •- -oh m

lb ab ~b 4'o 5b sb

° g ! o •

" , e :

0 0 . o

lb 2b 3'o ~ ~ eo ro

l" Oo o • [ • o • ~o

lb 2b 3b ,~o so eo ro Di,,mnee 81on0 line of soetion (kin)

Fro. 8. Variation of radioelements (U, Ra, Rn), U isotope ratios and aHe along flow gradients across the Berkshire syncline. Distance along line of section is measured from the base of the Chalk on the northern outcrop shown in Fig. 1. Open symbols denote unconfined groundwaters and solid

symbols indicate confined groundwaters.

areas where recent recharge is predominant, but the presence of several borehole samples with zero or low tritium (~10 TU) must indicate a large proportion of older (pre-1950's) groundwater. The low tritium values found in at least three boreholes in the unconfined aquifer must imply residence times in excess of 30 a. Two values of ]4C in the unconfined aquifer are 56 and 59 pmc (per cent modern carbon), with b13C between -13 .9 and -12.3%o for bulk carbonate precipitates. Each sample reporting 14C also has detectable thermonuclear tritium and, therefore, the ~4C values represent appropriate "initial" ~4C values for recent, carbonate-saturated groundwater in the aquifer. In the confined aquifer all but two ~4C values are below 12 pmc, with a minimum of 1.1 pmc at Grazeley. Thus, detectable 14C is also found in all confined aquifer samples. The 613C values in the confined aquifer are considerably more positive than

those in the younger groundwaters of the unconfined aquifer, and fall in the general range - 9 . 6 to -2.1%0. The one exception ( -13 .3 and -15.9%o at Little Park Farm) signifies influx of younger, less chemically evolved water, for which L3C is a good tracer. There is a general inverse correlation between ]4C and ~ 13C, and the c~t3c values increase in the confined aquifer to values close to those of the carbonate matrix (+2 to +3%0). Quantitative age interpretation of these data is inconclusive due to the various possibilities for carbonate exchange, in addition to the mixing in the deep aquifer. This is similar to the situation already identified by WIGLEY (1976) and DOWNING et al. (1979) for the Chalk aquifer in the centre of the London Basin.

Radioelements, radiogenic 4He and inert gases

The 222Rn contents of groundwaters from the unconfined and confined parts of the aquifer are generally similar and have a modal value of about 60 pCi/! (Table 4). Some exceptionally high 222Rn contents (Fig. 8) however, occur in the confined aquifer, and at the Stanford End borehole, a value of 1067 pCi/l was recorded. Contents of 222Rn about the modal value could be derived by flow in fractures, with fracture widths of about 1 mm for a U-content of about 1 ppm in the Chalk. The higher 222Rn contents imply either flow through more U-rich zones or flow in a system with smaller fracture openings.

Higher U concentrations are likely to be associated with phosphatic zones, especially in the Upper Chalk (HAWKINS, 1940). The 226Ra contents are generally less than 1 pCi/1 (average 0.64 + 0.38 pCi/l) and show no significant variation over the aquifer.

The decrease in redox potential from about +400 mV in the unconfined aquifer to about + 150 mV in the confined aquifer is reflected by a general decrease in the U-content of the groundwater from >0.2/xg/l to ~<0.05/xg/1 in the unconfined and confined zones, respectively. The 234U/238U activity ratio is enhanced (1.5-2.0) even for samples from the unconfined aquifer, and increases further as the U-content decreases for samples from the confined zone.

Contents of 4He in samples from or near the unconfined zone are close to the value for air-saturated water (4.8 × 10 -8 cm 3 STP/cm 3 H20 at 10°C). East- wards along the profile there is significant excess radiogenic 4He (Fig. 8).

DISCUSSION

Geochemistry o f the Chalk

The Chalk is generally a very pure carbonate sediment, although it contains significant concentrations of Mg and Sr as impurities and these elements are involved in water-rock interactions. The geochemical


i HOST ROCK GEOCHEMISTRY INON-CARBOI~ ~ ( m g / g ) RESIDUE

i Ca Mg Sr (wt%) ~o 4oo 15 z5 o5 ~o o 5 io

!." ." ...~ ..

. °

• . ~ . :..'.

,•.. • . ' .

.! :';" • t • ~•

• -'-. .'~

-" . :

..

C a

.; "....,

Mg St ..

F1G. 9. Geochemical profiles (Ca, Mg, Sr) through the Upper, Middle and Lower Chalk at Faircross. Results refer

to the acetic acid soluble fraction.

logs for the three divisions of the Chalk show small variations (Fig. 9) which are considered to be related to effects of sedimentation and early diagenesis. The amount of acetic acid insoluble residue in the Upper Chalk is low, generally between 1-2% (Fig. 9). The Middle Chalk can be distinguished from the Upper Chalk by residues of 3--6%, except near its base where purer carbonate is found. The Lower Chalk is considerably less pure than either the Upper or Middle Chalk, with insoluble residues between 5 and 12%. Quartz, montmorillonite, white mica and apatite are present throughout the profile with minor palygorskite, feldspar, marcasite and cristobalite; kaolinite and mixed layer clays are restricted to the Lower Chalk (MORGAN-JoNES, 1977).

Significant variations in the carbonate chemistry with depth can be distinguished based upon the evidence from the Faircross borehole:

(1) The Mg content of the upper section of the Upper Chalk is around 1.4 mg/g, and is lower than that of the basal 40 m where Mg increases to near 2.0

mg/g. In the Middle and Lower Chalk the Mg content is more variable but higher, overall, than in the Upper Chalk.

(2) In contrast to Mg, Sr concentrations are slightly higher in the uppermost 70 m of the Upper Chalk (around 0.7 mg/g), compared with 0.5-0.6 mg/g in the basal section. In the Middle Chalk, Sr is more variable (between 0.5-0.9 mg/g). In the Lower Chalk nearly constant values of 1.0 mg/g are found.

(3) Three zones are found (at 195,220 and 260 m) where Mg exceeds 2.0 mg/g and Ca is correspondingly low. These zones probably correspond to hardgrounds, where syndepositional recrystallisation has occurred. Detailed studies of the geochemistry of the Chalk elsewhere (JARVIS, 1980) show that Fe and Mn, as well as Mg, are enriched during hardground cementation whereas Sr is usually depleted; however, this does not appear to be so in the Faircross profile.

The overall extent of alkaline earth substitution appears to be lower than in modern counterpart biogenic oozes which have carbonate compositions of 1.4-1.6 mg/g Sr and around 1.5 mg/g Mg (SAYLES and MANHEIM, 1975). There appear to be two expla- nations for this difference; either (1) that the coc- colith compositions have changed with time, or (2) that Sr is preferentially lost to solutionl GRAHAM e t a l .

(1982) found that the Sr/Ca ratio in foraminifera has increased by 10-15% since the end of the Cretaceous and explain this by suggesting a change in aragonite precipitation and/or a hydrothermal supply of Ca. The Mg contents are similar in modern and ancient samples. During present day diagenesis in deep marine sediments, some Sr is lost during calcite recrystallisation (SAYLES and MANHEIM, 1975). The

6.0

50

E

?~ 340- ÷ %

o

o o

o o o ~ oo

o o o 3 ~ •o

o . ,.,,,~, , ~ v - -

0 Unconfk~d grourldwlters • • 2.0" • Confined groundwater8

1.O 4:0 5:0

HCO 3- (rag e¢luiv / I )

FIG. 10. Calcium-sulphate plotted against H C O ; for confined and unconfined groundwaters, showing stoichiometry

of carbonate dissolution.

264 W. M. Edmunds et al.

Table 4. Dissolved radioelement and noble gas contents of groundwaters from the Berkshire Chalk

222Rn 226Ra U content No. Site (pCi/kg) (pCi/kg) (/~g/l) 234U/23sU H-10 s Ne.107 Ar-104 Kr.10 s Xe.10 s

3 Mile End 30 0.15 0.281 1.93 2.8 4.64 3.26 8.31 5 Henley Farm 63 1.15 0.279 1.45 8.6 2.85 4.45 9.74 3.26 7 Brightwalton 54 0.44 0.241 2.36 9.0 2.91 4.73 10.22 1.52 8 Welford 67 1.66 0.279 1.50 5.9 2.88 4.37 9.87 1.13

13 Chieveley . . . . 19.9 3.0 4.70 9.31 1.31 14 Cold Ash 106 0.28 0.653 1.30 . . . . . 17 Woolhampton 29 0.43 0.003 2.55 11.1 2.66 2.82 9.62 6.37 18 Shalford Farm 2,3 0.40 0.014 2.95 27.5 2.87 2.92 11.13 19 Old Mill Hotel 201 0.80 0.033 8.21 28.6 2.97 4.86 10.70 1.57 21 Ufton Nervet 50 0.40 0.011 3.25 29.6 2.71 3.44 10.62 1.40 22 Theale 61 0.43 1.068 1.33 60.8 2.29 4.36 10.10 1.85 23 Grazeley 77 0.41 0.019 3.29 143.4 - - 2.89 12.28 2.30 24 Shinfield 41 0.73 0.034 2.89 193.8 3.39 4.80 11.76 1.15 25 Arborfield Mill 46 0.82 0.023 2.58 . . . . . 26 Dairy Research Inst. 49 0.89 0.009 4,12 202.9 4.39 4.41 10.67 1.31 27 GrazeleyCourt Farm 59 0.90 0.018 2.06 162.9 3.44 3.80 - - 28 Missels Bridge 15 0.50 0.040 2.06 133.6 3.32 2.98 11.90 1.16 30 Little Park Farm 359 0.22 0.054 4.45 91.3 3.11 5.05 11.74 2.55 32 Stanford End 1067 0.87 0.035 2,32 275.8 2.79 2.92 1,1.70 33 Wellington Park 81 0.67 0.007 14,38 125.6 2.89 4.13 11.62 34 Wellhouse Farm House 72 1.07 0.004 10.01 116.4 2.98 5.39 12.15 4.65 35 Lawn Farm - - - - 0.132 1.54 . . . . . 36 Sherfield Manor 35 0.64 0.011 2,50 162.5 2.86 4.96 11.43 1.38 37 Redlands 42 0.44 0.192 1,45 43.4 4.15 4.67 11.51 1.95 38 Greywell 70 0.43 0 .3~ 1.27 10.4 2.90 4.56 - - - -

- - = not determined.

rather low Sr content of the Chalk implies a net loss of this element during freshwater diagenesis. This possibility is further discussed below.

Chemical evolution of Chalk groundwaters

Rainwater infiltrating the soil overlying the Chalk quickly becomes saturated with respect to calcite and remains so during passage through the unsaturated and saturated zones. The molar ratio of Ca 2+ to H C O 3- Of approximately 1 : 2 demonstrates that the source of both solutes in the unconfined aquifer is predominantly from the dissolution of calcite according to:

CaCO3(s ) + H2CO 3 ~.~ Ca 2+ + 2HCO3- .

Subtraction of the amount of Ca equivalent to SO4 (assuming gypsum to be the source of the SO4) from total Ca concentrations (Fig. 10) improves the apparent equivalence of Ca and HCO3 in the unconfined groundwaters still further. If stoichiometric dissolution of calcite is assumed, approximate Mg and Sr concentrations in the initial solution can be found from the known composition of the Chalk. Because Mg and Sr are approximately 1.4 and 0.7 mg/kg, respectively, in the Upper Chalk at Faircross, then 100 mg/l Ca in solution should be accompanied by 0.4 mg/1 Mg and 0.2 mg/l Sr. These values are close to those observed after allowing for rainwater inputs. The drop in Ca concentrations as the aquifer becomes confined is compensated for partly by increases in

H C O 3 and partly by changes in pH, thus maintaining calcite saturation.

Table 5 shows that SIcalcit e ranges from 0.15 to - 1.1 throughout the aquifer. The 613C becomes less negative during the evolution of the groundwater, approaching the isotopic composition of the bulk Chalk (+2 to +3%).

The overall trend can be discussed further with reference to the Na + K, Ca, Mg trilinear diagram (Fig. 11). Several points should be noted:

(1) Most groundwaters in the unconfined aquifer have very low Mg/Ca ratios close to, or slightly above, those in the solid Chalk.

1 ~ INTERaT ITI AL WATER8

A L ~ ~

FIG. 11. Trilinear cation plot of interstitial waters (Faircross and Winterbourne boreholes) and regional groundwaters

(pumped samples) from the Berkshire Chalk.


Table 5. Mineral saturation indices for Chalk groundwaters calculated using WATEQF (PLUMMER et al., 1976)

Site No. Location Sicalcite Sldolomite Slbarite SItluorite Slchalcedony

1 Ashbury Spring -0.01 -1.51 -1.46 - - 0.28 2 Letcombe Regis Spring -0.38 -2.27 -1.62 - - 0.27 3 Mile End -0.17 -1.87 - - -1.65 0.04 4 Eastbury Grange -0.20 -2.15 - - -2.62 0.03 5 Henley Farm -0.13 -1.71 - - -1.98 0.07 7 Brightwalton -0.14 -1.81 -1.73 -1.76 -0.02 8 Welford -0.21 -1.88 -1.54 -2.00 0.10 9 Brightwalton Holt -1.14 -1.89 - - -2.05 -0.14

10 Chapel Farm -0.06 -1.68 - - -1.88 -0.09 11 Peasemore -0.29 -2.07 - - -1.89 -0.03 12 Winterbourne North -0.08 -1.88 - - -1.86 -0.06 13 Chieveley -0.09 -1.43 -1.11 -1.34 0.05 14 Cold Ash -0.13 -1.51 -0.16 -1.72 -0.02 15 Bradfield Valley -0.04 -1.46 -0.95 -2.02 0.15 16 Colthrop Board Mills -0.03 -0.83 -0.24 -0.63 -0.20 17 Woolhampton -0.06 -1.14 -0.23 -0.92 0.23 18 Shalford Farm -0.03 -0.58 -0.27 -0.09 0.20 19 Old Mill Hotel 0.03 -0.41 -0.52 -0.80 0.11 20 Aldermaston Court -0.23 -0.52 -0.33 -0.18 0.21 21 Ufton Nervet -0.10 -0.75 -0.20 -0.32 0.18 22 Theale -0.21 -1.47 0.26 -1.07 -0.05 23 Grazeley -0.07 -0.07 0.11 -0.39 0.19 24 Shinfield -0.05 -0.17 0.06 -0.27 0.04 25 Abortield Mill -0.17 -0.34 - - -0.48 -0.05 26 Dairy Research Inst. -0.13 -0.22 -0.25 -0.19 0.08 27 Grazeley Court Farm -0.01 -0.08 - - -0.13 0.18 28 Missels Bridge -0.05 -0.32 -0.18 -0.10 0.22 29 Mortimer -0.08 -0.62 -0.26 -0.37 0.21 30 Little Park Farm -0.08 -0.07 -0.18 -1.07 -0.11 32 Stanford End 0.11 -0.02 -0.22 0.19 -0.02 33 Wellington Park -0.30 -0.72 - - -0.47 -0.02 34 Weilhouse Farm House 0.15 0.25 -0.11 -0.53 -0.16 35 Lawn Farm -0.09 -0.33 - - 0.51 0.14 36 Sherlield Manor -0.03 -0.35 - - 0.05 0.20 37 Redlands -0.36 -1.12 -0.38 0.25 0.18 38 Greywell -0.14 -1.79 -1.16 -1.83 -0.08

(2) Along the flow path from outcrop the groundwaters undergo a significant increase in Mg/Ca ratio, which takes place before any increase in salinity is indicated by increased Na.

(3) Because the increase in Mg/Ca ratio is apparent even in unconfined groundwaters, it cannot be related to overlying strata, and therefore it must, in some way, be related to the active freshwater diagenesis of the Chalk.

(4) In the Faircross and Winterbourne interstitial waters, a constant Mg/Ca ratio of ~1 is found in the saline groundwaters, possibly indicating that this is an equilibrium value.

It is apparent that some of the pumped groundwaters, are approaching this equilibrium value within relatively short time scales (e.g. < 104 a) in view of the significant number of samples which have detectable 14C. Evidence that Mg is increasing progressively as a result of mineral diagenesis, and not for example as a result of mixing with ancient pore fluids, is also provided from the shallow unsaturated zone of the Berkshire Chalk (K1r~r~muR6ri et al . , in prep.) where Mg increases gradually over a depth interval of 8 m.

The increase in Sr is even more marked than that shown by Mg (Fig. 12); Sr increases from 0.1 mg/l to 5 mg/1 while Mg only increases from 1 to 20 mg/l. The comparative gain in Sr is thus some 3 times greater, suggesting that Sr is more strongly rejected during recrystallisation than is Mg.

The carbonate hydrochemistry provides strong evidence of reaction between relatively dilute groundwater and the solid Chalk. This implies an ongoing and progressive diagenesis of the Chalk towards lower Mg and Sr contents and a lighter 613C composition. However, the reservoir of these constituents in the Chalk is so great that little, if any, change can be expected in the bulk rock chemistry. Thus, in the Faircross profile, it is likely that the host rock chemistry still closely reflects its original composition.

It is likely that Mg ~- Ca exchange takes place during progressive recrystallization of microcrystalline calcite, or by incongruent dissolution of the carbonate. This reaction can be viewed as the equilibrium readjustment of a mineral formed biogenically under marine conditions, to a composition which is more stable in the freshwater environment. One


unknown factor is whether the coccoliths or other carbonate microcrystalline aggregates are homogeneous or zoned, in which case the relative stabilities of the calcites would differ. Thermo- dynamics predict that the less pure calcite, having Mg, Sr and other structural impurities present, would undergo incongruent reaction to produce purer calcite. Nevertheless, it is significant that such marked water-rock interactions are occurring and producing major changes in groundwater chemistry when it is considered that the Chalk is among the purest of carbonate sediments.

Other reactions taking place in the aquifer need to be considered in relation to the dominance of carbonate reactions. An overall increase in salinity, as indicated by C1, is related to the slight displacement of connate water (sensu stricto). Cation exchange and/ or leakage from overlying strata have been suggested by INESON and DOWNING (1963) and DOWNXNG et al. (1979) to explain the change in water chemistry from a Ca-HCO3 to a Na(Mg)-HCO3 type in the deeper, central London Basin. The present evidence strongly suggests that the increase in Mg here and elsewhere in the Chalk can be explained solely on the basis of carbonate reactions. Significant cation exchange of Mg 2+ for Ca > would demand very high initial Mg/Ca ratios in the solid phase exchanger, because Ca 2+ and Mg 2+ have similar affinities for most clay ion exchange sites.

Some increase in Na + probably occurs as a result of cation exchange, although this is rather small. The increase in Na + in the direction of flow occurs after the increase in Mg (due to carbonate reaction) but before that of Cl (due to mixing with connate water) (Table 4). The Na/Cl ratio increases by up to a factor of 2, amounting to an excess of some 30 mg/l Na which can be attributed to ion exchange.

Sulphate concentrations in the unconfined aquifer are very similar to input concentrations from the unsaturated zone (=35 mg/l), and these Concen- trations persist across the redox boundary and well into the confined aquifer. The increase in SO4 to above 50 rag/1 is probably the result of reaction with gypsum which occurs as a trace mineral in Chalk hardgrounds (MORGAN-JONES, 1977). It is important to note that some sulphate dissolution occurs in a reducing environment and this has been discussed elsewhere in relation to other U.K. aquifers (EDMUNDS et al., 1984). Despite the presence of traces of dissolved sulphide in the reducing zone of the Chalk, sulphate reduction is considered to be negligible. This is borne out also by the change to more positive 6~3C values because a lighter isotopic composition would be expected (TrmRSTENSEN et al. , 1979) if significant sulphate reduction was taking place. The release of 13C during carbonate recrystallization, however, might override any contrary effect of sulphate reduction. Nitrate is present at levels between 18-35 mg/l NO 3 in all but one of the samples from the unconfined aquifer. Levels of NO3 are close

to or at detection limits in all groundwaters in the confined aquifer, and where traces occur, some tritium is also present. The abrupt disappearance of NO 3 is related to the reaction of oxygen. It is likely that NO 3 becomes the main oxidising agent once oxygen has been consumed. The evidence from this and other aquifers (EDMUNDS et al., 1984) is that denitrification takes place rapidly after the onset of reducing conditions associated with the removal of oxygen. The anomalies found probably relate to vertical mixing during pumping.

The behaviour of minor elements (Si, ST, Ba, F, Br, I, K, Li, NH4, B and Mn) is considered below in the light of the major element chemistry and diagenetic processes.

Silicon

Silicon in groundwaters from the unconfined zone is typically around 6.0 mg/1 and increases abruptly at the onset of confined conditions (Fig. 6). These values are too high to be controlled by equilibrium with quartz; equilibrium calculations suggest that chalcedony (Slcha|ce d values generally O + 0.1) is the most likely control (Table 5) although only quartz and cristobalite are detected in the Chalk by XRD. The Si concentrations are at least double those in the soil porewaters and upper unsaturated zone. Pro- gressive dissolution of silica is taking place mainly during percolation through the soil and unsaturated zone, and little reaction occurs in the saturated zone of the unconfined aquifer. Silicon concentrations increase again in the confined aquifer and may be partly temperature controlled, but also related to clay mineral diagenesis.

Stront ium

The concentration of Sr across the aquifer is closely related to the congruent and incongruent reactions of calcite, described above. The highest concentration found in the Chalk groundwaters (6.3 mg/l) is well below celestite saturation. Higher values (up to 13 mg/l) are found in the interstitial waters of the Faircross borehole (Fig. 3). Interstitial waters saturated with celestite have, however, been recorded from the Chalk in East Anglia (BATH and EDIdUNDS, 1981). In the Berkshire Chalk groundwaters, there appear to be two modes of Sr occurrence which are illustrated in Fig. 12.

(1) Very high values in pore fluids which may be related to early diagenetic reactions of aragonite and high Mg calcite (both now no longer present). Stron- tium concentrations so derived are represented by the Faircross interstitial waters and some deep groundwaters lying on the line linked to evolved connate water.

(2) Continuous diagenesis of the low Mg calcite in


10 4 -

• ,o 3.

m z

10 2,

10 I-

lOS.

10 2

IO)

101

I00

INTERSTITIAL WATERS ~A WATn~,dk o Upper Chalk "/ Q / • Middle Chelk ) F~RREG~O E" /

Lower Chalk ) /

REGIONAL " ~ / GROUNDWATERS • ~ Y • (Upper, Middle

• e e ~

• •

161 162 CI i6~ 16'

• . . wll COI~IAT|

. . . "t" t •

101 IOZ CI I03 I04

,o, I 81A WAlrE•

.o,v,,

10 2 .

v o O o

101" • 4O e41•e ~ I:Z] _ _

10 2 ,

101 .

(~10 o.

16, 162 C I 1~ 16,

EVOLVED

~ ~ A / EVOLVED GONNATE WATER 102 • REA WATER

e QQ • / .

.J io. ~ o • c~ io ~ f

FIG. 12. Chloride plotted against Na, K, Mg, Sr, Li and Br to show the evolution of groundwater and the links between regional and interstitial waters. All concentrations in mg/I.

&G 2 = 3-~,


the Chalk by groundwater flow which is continuing at the present day. This leads to the significant increase in Sr at nearly constant values of CI.

B a ~ u m

Interstitial water immediately beneath the Chalk soil (Table 1) contains around 20/zg/1 Ba probably related to congruent solution of calcite from the Chalk. This concentration is similar to the values found in most saturated zone groundwaters in the Chalk aquifer at outcrop. There is an overall increase in Ba with residence time and depth, and concentrations rise sharply at the onset of confined conditions. Mineral equilibrium calculations using WATEQF (PLtlMMER et al., 1976) indicate that barite solubility controls the Ba concentration towards the centre of the confined aquifer (Table 5).

Fluoride

The mean value for F in rainfall at Chilton is 0.096 mg/1 (Table 1) which is similar to saturated zone values of 0.1-0.3 mg/1 F. Fluoride shows an increase downgradient similar to Sr indicating progressive reaction with the Chalk matrix. The most likely source of F is the reaction of apatite (or fluorapatite) which is present as an accessory mineral, especially in the Upper Chalk (MORGAN-JONES, 1977). Saturation with respect to fluorite (Table 5) is reached in a few groundwaters in the confined aquifer at values of F of around 1.4 mg/1.

Bromide

Iodate levels are below the detection limit of 1 tzg/l in all of the reducing groundwaters. Total I increases with salinity in the deeper confined aquifer, accompanied by an increase in the I/C1 ratio in the shallower section of the confined aquifer suggesting reaction during downgradient flow which is releasing I. This behaviour corresponds with that observed in the Lincolnshire Limestone (EDMuNDS, 1973) where I increases before any increase in Br or C1 is observed, possibly due to redox reactions. Iodine may be released by the oxidation of organic matter or during the mobilization of Fe 2÷, and further study is needed to resolve the processes involved. It is also possible that I is released during the reaction of fluorapatite which may be a source of I.

Potassium

Baseline values for K in the unconfined aquifer are probably near 1.0 mg/l, and Na/K ratios are >10. Following the natural course of K evolution is made difficult because of the possible addition of K from fertilisers and other man-made sources. It is noted from Figs 5 and 6, however, that higher K and NO 3 concentrations generally do not correlate. The further increase to near 10 mg/1 K in the confined aquifer is geochemically controlled and links with the trend observed in the interstitial waters from Fair- cross (Fig. 12). The most saline formation water in the Chalk is significantly depleted in K relative to sea water, suggesting that K was incorporated into clays during early diagenesis. The observed trend in the interstitial waters suggests that in the freshwater system, clay mineral diagenesis is presently releasing K into solution.

The behaviour of Br in the aquifer closely follows that of CI. Bromide in rainfall at Chilton is 26/zg/l and values in the Chalk unsaturated zone are typically 20--60 t~g/1, thus the variation and increase can be accounted for by evapotranspiration. The relation between Br and C1 is maintained throughout most of the aquifer. Interstitial waters from the Faircross borehole (Fig. 12) are slightly enriched in Br relative to sea water and this indicates a small gain in Br by the interstitial water during diagenesis. The influence of this higher Br/CI ratio can be seen in the most saline groundwater at Stanford End.

Total iodine

Total iodine in groundwaters from the unconfined aquifer is around 5 v,g/l which is similar to, or slightly higher than, that in rainfall (Table 1). Iodine in oxidizing waters exists mainly as iodate (IO 3- ) but is reduced to iodide ( I - ) along the flow path, the reaction coinciding with the reduction of NO 3 (Fig. 6).

Lithium

Lithium in rainfall (Table 1) is at or below 1 tzg/l. In the unconfined aquifer the Li content and Li/Na ratio both increase significantly (Fig. 12). Lithium undergoes a tenfold enrichment in shallow groundwater and undergoes a similar increase along the fow path (EDMUNDS et al., 1986) in the low salinity groundwaters. The evidence from the Faircross borehole (Fig. 12) suggests that during early diagenesis the pore fluids were enriched in Li (unlike K) and that the solid phase might therefore be expected to be relatively depleted in Li. Lithium is being preferentially released from clays or carbonate surfaces during freshwater diagenesis and reaches a maximum Li/Na ratio similar to that in the saline interstitial waters.

A m m o n i u m

Ammonium is found above the detection limit of 10 /~g/1 only in the reducing section of the Chalk


aquifer. Relatively high NH4 values are found immediately after the onset of reducing conditions in the aquifer (Fig. 6). In addition, there is a further general increase in NH4 to over 1 mg/l which parallels the rise in salinity.

Ammonium, like Li, is likely to be associated with clay minerals in the Chalk and, if released in the oxidizing zone, would undergo nitrification. The main reason for the increased NH 4 concentrations at depth is considered to be desorption from clay minerals under reducing conditions (TOWLER, 1982). Another possibility is its formation from N 2 gas under strongly reducing conditions, but this seems unlikely because En values below -200 mV are required and no En values below - 100 mV were observed.

Boron

Values for B in groundwater from the unconfined aquifer are in the range 3 to 50/zg/1. The increase of B in the confined aquifer is related to that of C1, but it would appear that little boron is added by water-rock interaction.

Manganese

Manganese concentrations in the unconfined aquifer are extremely low, usually below the detection limit of 0.03/tg/1. Detectable Mn is found only beyond the redox boundary where concentrations are typically between 3 and 20 /zg/l and show no correlation with salinity. The few higher values recorded at sites 19, 34 and especially 22, where Mn reaches 280 v.g/l, correlate with high concentrations of Fe.

Manganese, which occurs at concentrations of 100- 200 mg/kg in the Chalk, must be liberated during the dissolution of the bulk carbonate but, in view of the very low dissolved concentrations observed, Mn 2+ is evidently removed by oxidation. The removal of Mn and Fe from solution gives rise to the Mn dendrites and Fe 3÷ precipitates seen on Chalk surfaces.

GROUNDWATER RESIDENCE TIME

The stable isotope, radiocarbon, radiogenic 4He and inert gas results all provide evidence on the residence time of groundwater in the deeper parts of the basin. The tritium results indicate that relatively little groundwater of contemporary origin has moved into the confined zone.

The oxygen and hydrogen stable isotope ratios in the Berkshire groundwaters are similar to those reported for groundwater (SMrrn et al., 1976) in the centre of the London Basin. However, their interpretation of radiocarbon data for the London Basin Chalk aquifer was subsequently revised by WmLEY

(1979) and by DowmsG etal. (1979), and the original estimates of residence time were reduced towards Holocene (post-glacial) ages. The relative uniformity of the entire set of stable oxygen and hydrogen isotope data for this Chalk aquifer suggests that the bulk of the pumped water either comprises Holocene recharge, or is a homogeneous mixture of groundwater with different ages. The data do not support the concept of Pleistocene groundwater movement towards the centre of the London Basin syncline in a "piston-type" flow system.

The micropores (approximately 1/.tm in diameter) of the chalk matrix contain water which is hydrauli- cally distinct from that contained in the fissures and might therefore differ in both age and solution chemistry. Diffusive exchange between the matrix and fissures (BARKER and FOSTER, 1981) probably ensures a homogeneous distribution of natural isotopic and chemical compositions in the upper, fissured and transmissive part of the chalk sequence. However, with increasing depth and decreasing fissure frequency and water flux, there is less prob- ability of diffusive exchange between interstitial matrix water and the active groundwater system.

Connate interstitial sea-water has been identified at the base of a 500 m thick Chalk sequence in East Anglia (BATH and EDMONDS, 1981). In Berkshire, the presence of higher C1 concentrations in the deeper pumped groundwaters indicates that a component of residual connate water remains in the deeper chalk matrix and is influencing the chemistry of fissure water by diffusive mixing. Residual saline interstitial water is also found in the deeper section of the unconfined Chalk at Winterbourne (EDMONDS et al., 1973). The increasing interstitial water salinity with depth in the Faircross borehole is primary evidence for this component of connate water, although no isotopic data are available to confirm its marine origin.

The chemical and hydrogeoiogical evidence therefore points to there being varying degrees of mixing between interstitial and fissure water components with different residence times, stretching right back to depositional age. The isotopic data must be interpreted in this context, and they offer a means of estimating the proportion of components of different ages. The question arises as to whether, because of the potentially massive "buffering" reservoir offered by the deeper chalk matrix, the Chalk groundwater available for development is a broad distribution of components of ages stretching back to depositional age, or whether the pumped water reflects a partition of the chalk into an upper transmissive zone and a deeper, thicker, generally unfissured zone with only very limited exchange between the two reservoirs.

The radiocarbon data in the present study have been adjusted for dilution by "dead" carbonate dissolved from the chalk matrix, according to the three models most widely used and considered most appropriate. All of these assume that total carbon


isotopic exchange equilibrium is not achieved, because if that were to occur the 613C of dissolved carbonate would no longer be an indication of the extent of 14C dilution by rock carbonate. Two of the models (REARDON and FRITZ, 1978; WIGLEY et a l . ,

1978) assume that the carbonate system becomes closed to further isotopic exchange with soil-atmosphere COz at an early stage of evolution. This assumption is supported by the rapid achievement of saturation with respect to CaCO3 and the decreasing computed values for equilibrium pCO2 away from the outcrop aquifer. The FOr~TES and GARNIER (1979) model treats the entire evolution as an open system allowing partial exchange with one of two potential buffering media, either the solid or the gas phase. The budget with respect to gains or losses of CaCO 3 or CO2, and therefore the net amount of "dead" carbon added to solution, is computed from cation balance or total alkalinity and from the shift in 613C values. The salient features of each model are summarised below, together with the particular values for variables which were specified in the computed adjustments to the set of ~4C data:

(a) All calculations assume:

Soil atmosphere: •13C(CO2) = -25%o; A14C(CO2) = 100 pmc

Rock carbonate: 613C(CaCO3) = +2.3%O; A14C(CaCO3) = 0 pmc

Isotopic fractionation factor e13, CaCO3 precipitation + 3%o.

(b) REARDON and FRITZ (1978), W A T E Q F - ISOTOP model: calculates the extent of ~4C dilution

for a range of initial open system conditions to gener- ate the observed total alkalinity and the final 6L3C

value. (c) WIGLEY et al. (1978) model: calculates the

extent of ~4C dilution for a system which undergoes congruent carbonate dissolution followed by incongruent dolomite dissolution and CaCO 3 precipitation. The evolution of di13C alone is used as a guide to 14C dilution, but prior knowledge or some assumption about the stoichiometry of the incongruent dissolution reaction is required.

613C (open system DIC) = -18%o

613C (at congruent equilibrium) = 13.1%o (mean outcrop aquifer value)

F (dissolution : precipitation) = 1.1 (stoichiometry of incongruent reaction).

(d) FONTES and GARNIER (1979) model: calculates the amount of "dead" carbon added either by (1) cation balance or (2) total alkalinity balance, and allows for further exchange according to 6t3C changes using an exchange-mixing model:

El3 (CO2-CaCO3) = - 11.4%o (isotopic fractionation).

Only the FOr~TES and GARNIER model attempts to make an interpretation which is isotopically and chemically consistent. The WIGLEV et al. model assumes that the stoichiometry of the incongruent reaction is constant and known, which is not the case in this Chalk aquifer (the value for the reaction ratio, 1.1, is therefore an estimate only). Dolomite does not occur in the matrix of this aquifer, and it is not clear

Table 6. Summary carbon isotope results and radiocarbon ages derived from computer models of WIGLEY et al. (1978), REARDObl and FRITZ (1978) and ForqxEs and GARNIER (1979)

613C(DIC)°~ Modelled "ages"

Site ~4C WIGLEY REARDON and ForcrEs and No. Location Year Specific Bulk (pmc) etal. FRITZ GARNIER

7 Brightwalton 1982 -14.8 -13.9 62.1 2000 0--2000 modern 13 Chieveley 1982 - 12.5 - 12.3 56.3 2000 0-2000 modern 16 Colthrop Board Mills 1982 -6.3 -9.6 7.0 17,000 14,000-16,000 13,000/13,000 17 Woolhampton 1979 -8.1 -8.3 10.9 12,000 10,000-13,000 7000/7000 19 Old Mill Hotel 1979 -6.7 -6.2 5.8 15,000 14,000-16,000 7000/7000 20 Aldermaston Court 1982 -3.9 -2.1 2.0 13,000 16,000-20,000 negative 21 Ufton Nervet 1979 -1.7 -2.5 2.6 13,000 15,000-18,000 negative 22 Theale 1982 - 13.4 - 11.7 55.4 2000 0-2000 modern 23 Grazeley 1979 -2.1 1.1 18,000 21,000-24,000 negative 24 Shinfield 1982 -9.0 -4.6 2.5 19,000 19,000-21,000 1000/2000 26 Dairy Research Inst. 1979 -4.7 -4.4 1.9 21,000 21,000-24,000 neg/7000 29 Mortimer 1982 -5.6 -8.6 11.7 12,000 10,000-12,000 8000/8000 30 Little Park Farm 1979 -13.3 22.9 1000 7(1(10-10,000 8000/8000

1982 -15.9 -13.3 23.9 10,000 71XI0-9000 8000/8000 32 Stanford End 1979 -6.4 -5.4 4.8 15,000 14,000-17,000 neg/6000 33 Wellington Park 1979 -5.6 -7.3 10.2 12,000 10,000-13,000 6000/5000 34 Wellhouse Farm House 1979 -8.6 -8.6 7.3 16,000 14,000-17,000 12,000/12,000 36 Sheffield Manor 1979 -3.1 -2.5 3.4 11,000 14,000-16,000 negative 37 Redlands 1979 -2.1 -2.4 5.9 6000 8000-11,000 negative 38 Greywell 1982 -14.2 -13.2 68.7 1000 0-1000 modem


whether there is a bulk recrystallization of CaCO 3 or whether it is only a surficial exchange process which leads to concomitant increases in 6 ]3C and dissolved Mg and St. The application of the models which incorporate the effect of incongruent dissolution in driving the 613C value towards isotopic equilibrium caused a very significant reduction of model ages when applied to groundwaters in the London Basin Chalk (WIGLEY, 1976; DOWNING et al., 1979).

The values for model ages derived from the three cases are shown in Table 6. The results of calculations by the REARDON and FRITZ (1978) and WIGLEY et al. (1978) models are very similar. As expected, calculations according to FONTES and GARNIER (1979) give significantly younger ages because this model assumes only partial isotopic exchange. These "ages" are also very sensitive to the precision of chemical data and to the assumptions inherent in the chemical balance calculations. The ForcrEs and GARrqtER model becomes unreliable for the heavier 613C values because it uses an algebraic correction term which generates unrealistically young or even negative "model" ages.

There is great uncertainty over the reliability of any single "model age". Ages in the ranges indicated in Table 6 are suggested as being the best estimates of the mean residence times. It is unlikely that the mean residence times are greater than these values because the first two models would tend to underestimate the age if there had been only partial isotopic exchange.

The balance of evidence from 1 4 C and from 6180 and 62H data suggests that the bulk of the groundwater being abstracted from the Chalk aquifer orig- inated as recharge during the Holocene, or at least since the last glacial maximum (around 18,000 a ago). There is no major component of pre-glacial Pleis- tocene recharge even in the deepest pumped water. This dominance of Holocene water, with only small components of very old connate water, is clear evidence that the Chalk behaves as a system hydrogeologically parti t ioned into an upper transmissive zone and a lower, virtually non-conductive zone. The concentration gradient along which diffusive exchange occurs between these two zones has pre- sumably been modified both by natural flow and also by abstraction over the last 100 years or so, with the transmissive zone being progressively flushed. Groundwater abstraction has also lowered the pressure head in the confined chalk aquifer. This may have increased the proportion of recharge to the chalk originating as leakage through the overlying Tertiary clays and sands.

The distribution of '*He in the aquifer may be used as an indicator of groundwater migration. The downgradient increase in the 4He content of groundwaters (Table 4) may result from the addition of He from deeper formations (ANDREWS etal., 1985; TORGERSEN and IVEV, 1985). A depth profile governed by diffusion develops over the geological age of the formation (ANDREWS, 1985). The 4He and C1 data (Figs 4

~ UNCONFINED. - - e . - : - ~ C O N F I N E D m ~ " U N C . " ,q AEROBIC - ~ , - - - - - - - ~ J'~*"---- ANAEROBIC ~ : - AERO, 9

~ S r y

of•

o~ g

r r

(~o lb

~a

t

$

Distance along line of section (kin)

Fsc. 13. Recharge temperatures, derived from inert gas (Ne, Ar, Kr, Xe) ratios, across the Berkshire syncline. Distance along line of section is measured from the base of the Chalk on the northern outcrop shown in Fig. 1. Open symbols denote unconfined groundwaters and solid symbols

indicate confined groundwaters.

and 8) show that the highest 4He concentrations correspond with areas of high Cl near the centre of the syncline. The rate of 4He generation in the Chalk may be calculated from its U and Th contents. The modal radioelement contents are low; the U content is ~<1 mg/kg and Th is ~<1.5 mg/kg (larger amounts may locally be present in phosphatic zones). For a typical porosity of 30% for the Middle Chalk, the generation time for the groundwater 4He content at Stanford End ABH is about 2.5 Ma if all of the He produced were dissolved in the pore fluids. This is much greater than the 14C groundwater ages and can be explained only as a consequence of there being large amounts of stored 4He in the Chalk matrix, possibly dissolved in very old pore fluids, or as a result of 4He diffusion from deeper strata. Measurement of the 3He/4He ratio, together with estimates of this production ratio based on the geochemistry of the different strata, could identify the He source; such data are not presently available.

Recharge temperatures have been estimated by comparison of the groundwater inert gas contents with known values of Ne, Ar , and Kr solubilities (Fig. 13). A good correlation exists between decreasing recharge temperature and increasing groundwater temperature (also a measure of residence time and depth). This suggests that there is a zone of groundwater in the centre of the basin that has been recharged under cooler climatic conditions than the present. These results, if confirmed, offer the only evidence of periglacial water in the Chalk aquifer, but they conflict with the 6180 evidence. This conflict cannot presently be resolved.

SUMMARY AND CONCLUSIONS

The dominant chemical characteristics of Chalk groundwater are acquired during initial percolation through the soil and upper unsaturated zone. Stoichiometric dissolution of calcite is taking place in

272 w.M. Edmunds et al.

the unsaturated zone and, once calcite saturation is reached, very little further increase in total mineralization takes place for some distance. However, as groundwater moves downgradient the chemistry is modified by several processes---redox reactions, incongruent solution of carbonates, exchange reactions, and mixing with saline water. The overall evolution may be considered as the adjustment of a marine sedimentary water-rock system, formed under relatively high free energy conditions, to the lower free energy environment of the freshwater system. The products of these reactions serve as a means of identifying the maturity of water pumped from individual boreholes:

(1) Oxygen concentrations are reduced to zero, mainly by the oxidation of Fe 2÷ over a period of 25 or more years. Reducing conditions allow Fe 2÷ concentrations to increase and, beyond the redox boundary, N O 3 is quite rapidly reduced following the disappearance of oxygen. Sulphate concentrations increase and the reduction of sulphate to sulphide is quanti- tatively unimportant.

(2) Along the flow path, there is a progressive increase in Mg and Sr concentrations, and more positive 613C values indicate that incongruent recrystallisation of carbonate is taking place, resulting also in a "purer" solid phase.

(3) The concentrations of several cations increase downgradient as a result of the reaction of "marine" clay minerals with the lower salinity groundwaters, thereby releasing K, NH4 and Li.

(4) Cation exchange reactions are relatively unimportant and amount, at most, to the addition of 30 mg/l Na to the groundwaters.

(5) The salinity increases observed in the deepest pumped groundwaters can be related to exchange with static or slowly moving residual connate water.

Many groundwaters contain tritium indicating a contribution from water having an origin in the thermonuclear era, but at the same time several boreholes, even some in the outcrop Chalk, have zero tritium values suggesting that locally there are considerable reserves of older water. The combined evidence suggests that nowhere in the basin are readily accessible waters likely to be older than Holocene. This is shown mainly by the widespread occurrence of 14C which, in view of the carbonate reactions taking place, indicates that most groundwaters are likely to be quite young. Several conclusions may be drawn which have implications for groundwater quality management on a regional scale.

(1) The natural "baseline" conditions in the Chalk

Reactions between water and rock dominate the chemistry of the Chalk groundwater. The baseline conditions vary mainly according to the position along the flow path within the aquifer. Important changes in water chemistry may be interpreted purely

on geochemical grounds and these patterns need to be understood before any artificial influences may be recognised. For many elements, baseline variations of an order of magnitude or more can be expected. Magnesium or Sr, along with 613C, are considered to be the best guide to maturity and residence time along flow paths in the Chalk.

(2) Contamination of the Chalk aquifer

The effect of man's activity upon the aquifer is most vividly seen by the presence of thermonuclear tritium in many groundwaters. This serves as a marker to monitor the progress of any other pol- lutants into the aquifer. Tritium typically persists beyond the redox boundary and shows that oxygen consumption can occur within a few decades. Nitrate, the main indicator of agrichemicai pollution, persists where oxygen is present but is reduced to N2 gas either inorganically or bacteriologically in the reducing section of the aquifer, so that nitrate-free and tritium-containing waters are sometimes found together. Thus, significant denitrification can and does occur in the Chalk. Oxygen reaction must occur before significant nitrate removal can take place. The removal of oxygen appears to be mainly inorganically controlled and is thought to be linked to the oxidation of Fe 2÷, present in accessory minerals, to Fe 3÷.

Apart from nitrate, there is no evidence so far of significant regional contamination of the Chalk aquifer despite the rather intensive agricultural development in recent decades. Locally, there may be anthropogenic SO4 and K, but this has not yet attained regional significance.

(3) Residence time of groundwater and solutes

Various lines of geochemical evidence may be used to infer the residence time of the water within the aquifer. It is clear that mixing of water of different ages is taking place and that there is stratification of age as well as of chemical "maturity". The bulk of the water in the basin, at least that which is readily available on pumping, has had a residence time of less than 10,000 a; many of the solutes, on the other hand, are derived from the Chalk which has an age of around 60 Ma.

The Chalk has a large capacity for modifying the chemistry of natural and contaminated waters, as well as the isotopic composition. This is demon- strated by the 613C results which show that within the timescales of flow, the ~3C which enters the aquifer as HCO3 has almost completely been exchanged with Chalk mineral carbonate.

With the improved understanding of the Chalk hydrogeochemistry, it is possible to use a few simple parameters such as Mg, Sr, Ci or 613C, to indicate residence times.


(4) Future management and development of the Chalk aquifer

The Chalk still contains the largest reserves of fresh, unpolluted water of any British aquifer. The Chalk is also important in several countries in northern Europe and so the results of this study apply more widely. The main current p roblem is one of nitrate pollution in the aerobic sections of the aquifer. This can be minimized by abstraction of water downgradient of redox boundaries because natural denitrification is relatively rapid. Al though, as a side effect, this may increase the concentra t ion of Fe, this is likely to be at or below 300/~g/l and may not require t reatment .

Future contaminat ion of the aquifer by other agrichemicals is a possibility, but the huge capacity of the Chalk for acid neutralising and cleansing by geochemical processes renders it highly resistant to pollution.

The results of the present study can form the basis for design of moni tor ing programmes for the Chalk aquifer. It is likely that fairly infrequent (e.g. yearly) moni tor ing will suffice and that only a few key elements are requi red as indicators of change, once the geochemistry is understood. The results of this study also provide a basis for est imating the "matur i ty" of groundwater in any new wellfield development .

Acknowledgements--We wish to thank staff of Thames Water Authority for facilitating access to many of the boreholes mentioned in the text. We are most grateful to Janice Trafford for assisting with the sampling and carrying out the bulk of the analytical work, to Nick Walton for organising the 1979 fieldwork. Dr D. Harkness of Scottish Universities Research Reactor Centre (SURRC), East Kil- bride, carried out 14C determinations and is also thanked for helpful discussion of the results. Phillipa Towler carried out the sampling and analysis for inert gases. We are grateful to Brian Hitchon for his thoughtful, detailed and extremely valuable comments. This paper is published with permission of the Director, British Geological Survey, Natural En- vironment Research Council.

Editorial handling: Brian Hitchon.

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