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Environmental Pollution 137 (2005) 27e39
www.elsevier.com/locate/envpol
Trends in surface water chemistry of acidifiedUK Freshwaters, 1988e2002
J.J.L. Daviesa, A. Jenkinsa, D.T. Monteithb, C.D. Evansc, D.M. Coopera,*
aCentre for Ecology and Hydrology, Wallingford, Oxfordshire, OX10 8BB, UKbEnvironmental Change Research Centre, University College London, London, WC1H 0AP, UK
cCentre for Ecology and Hydrology, Bangor, Gwynedd, LL57 2UP, UK
Received 31 October 2004; accepted 17 December 2004
Acidified UK freshwaters show recovery but there are key uncertainties with regard to DOC and ANC.
Abstract
Analysis of water chemistry data from 15 years of monitoring at 22 acid-sensitive lakes and streams in the UK reveals coherent
national chemical trends indicative of recovery from acidification. Excess sulphate and base cations exhibit significant decline, oftenaccompanied by an increase in an alkalinity-based determination of acid neutralising capacity (AB-ANC) and, at fewer sites,a decline in hydrogen and labile aluminium. Acid neutralising capacity determined by ‘‘charge-balance’’ (CB-ANC) exhibits few
trends, possibly due to compound errors associated with its determination. Trend slopes in excess sulphate correlate with those forbase cations, hydrogen ion and AB-ANC, with between-site variability linked to catchment hydrology, sea-salt inputs and forestry.Nitrate concentrations have not changed significantly but show high sensitivity to varying climate. Trends in AB-ANC are
influenced by significant increases in dissolved organic carbon, the cause of which it is vital to establish before trends in the formercan definitively be attributed to decreasing acidic deposition.� 2005 Elsevier Ltd. All rights reserved.
Keywords: United Kingdom Acid Waters Monitoring Network; Acidification; Recovery; Trends; Acid neutralising capacity
1. Introduction
Over the period of monitoring covered by the UKAcid Waters Monitoring Network (AWMN), 1988e2002, there have been significant changes in sulphur(S) and nitrogen (N) deposition across the UK.Deposition of non-marine S (i.e., anthropogenic SO4
or xSO4) has declined by approximately 50% acrossmuch of the UK (NEGTAP, 2001), while significanttrends in N species are largely confined to the EnglishMidlands (Fowler et al., 2005, this issue). As levels of
* Corresponding author. Tel.: C44 1491 692259; fax: C44 1491
692430.
E-mail address: [email protected] (D.M. Cooper).
0269-7491/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.envpol.2004.12.029
atmospherically deposited S decrease, surface waterconcentrations of non-marine SO4 have fallen accord-ingly (Cooper and Jenkins, 2003; Cooper, 2005, thisissue).
While the decline in S in deposition and run-off isclearly substantial across much of the UK, biologicallyit is the outcome of chemical interactions betweendeposition and the receiving soils and geology of thewatershed that are of primary concern. Applications ofthe MAGIC model suggest that a reduction in xSO4
concentration should be partially balanced by a declinein base cations with proportionally larger declines inhydrogen ion (H) and inorganic, labile aluminium ions(Allab) resulting in a net increase in pH, alkalinity andacid neutralising capacity (ANC) (Evans et al., 1998).
28 J.J.L. Davies et al. / Environmental Pollution 137 (2005) 27e39
The chemical monitoring strategy within the AWMNwas specifically designed to test for such effects acrossa wide range of acid sensitive water-bodies. Trendanalyses were initially applied to AWMN data after 5years (Patrick et al., 1995). Few trends were found in thedataset at this early stage, although several rising trendsfor SO4 were observed. After 10 years of monitoring,Monteith and Evans (2000) found that most risingtrends in SO4 had disappeared and three sites of the 22in the network were showing evidence of significantdeclines in this ion. However, they found little evidenceof recovery from acidification; increases in pH andalkalinity were observed at six and four sites, re-spectively, but none of these showed concurrent declinesin acid anions. Rather, these increases were attributed tothe effects of variations in sea-salt inputs and/or rainfall.The lack of evidence for recovery at that time wasattributed to several factors, including relatively smallreductions in S deposition at AWMN sites; noise in thechemistry time-series generated by short-term variabilityin flow; decadal-scale climatic variations at coastal sites;and the release of S stored in catchment soils.
In recent years, increasing evidence of long-termchemical recovery from acidification at AWMN siteshas been presented by Evans and Monteith (2001, 2002).As the monitoring period lengthens and the influence ofshort or medium term variability weakens relative to thecontinued decline in the deposition of acid anions, it islikely that trends in surface water chemistry will becomemore easily detected and assessed in terms of recovery.The current analysis examines data from 15 years ofmonitoring to (a) provide an updated assessment ofnational trends in the AWMN chemistry dataset; (b)examine the variation in ionic responses to significanttrends in pollutant SO4 at individual sites; and (c)evaluate the probable drivers of variation in ionicresponses and the implications of these for recoveryfrom acidification.
2. Methods
2.1. AWMN sites
Water chemistry from the 11 lakes and 11 streamsin the AWMN has been routinely sampled andanalysed since 1988. The majority of these sites aresituated on acid-sensitive geology and have experi-enced sufficient acid deposition over the last 200 yearsto have acidified significantly. Further informationrelating to the nature of sites, experimental design ofthe AWMN and field methods are provided by Patricket al. (1995) and Monteith and Evans (2005, thisissue). Analytical chemistry methods have been con-sistent at all laboratories throughout the monitoringperiod. Alkalinity is determined by gran-titration, total
monomeric aluminium by colourimetry (catechol violet)with labile and non-labile fractions separated by ionexchange. Major cations are determined by inductivelycoupled plasmaeoptical emission spectroscopy andanions by ion chromatography (Dionex). Full detailsof analytical methods are given in Patrick et al. (1995).
2.2. Statistical analysis of data
Variations in all chemical determinands indicative ofwater quality and acidification status were assessed.ANC was calculated two ways, and the variationsexamined for both methods. In Eq. (1), ANC is relatedto alkalinity (meq l�1), dissolved organic carbon (DOC,mmol l�1) and Allab (mmol l�1), while in Eq. (2), it isrelated to the charge balance of major ions (all speciesmeq l�1). In Eq. (1), F represents the charge density ofDOC and is assumed to be 4.5 where pH is 4.5e5.5 and5 where pHO 5.5 (Harriman and Taylor, 1999). Theimplications of the two methods are discussed later.
AB�ANCZAlkalinityCðF!DOCÞ � ð3!AllabÞ ð1Þ
CB�ANCZSO4CNO3CCl� Ca�Mg�Na�K
ð2Þ
In order to illustrate the extent of regional coherenceacross the AWMN, time-series summary plots ofstandardised median concentrations of each determi-nand for lakes and streams are presented. Standardisa-tion for each determinand at each site was performed bysubtracting the mean of the full sample set fromindividual concentrations and dividing by the standarddeviation, such that all standardised time series havea mean of zero and a standard deviation of one. Median,2nd lowest and 2nd highest values were determined andplotted for each sampling occasion to create dimension-less ‘regional time series’ (Evans and Monteith, 2001), inorder to give an indication of general trends across allAWMN monitoring sites. This method has been appliedpreviously by Evans and Monteith (2001, 2002), whocontended that such plots effectively demonstrate re-gional coherence in the temporal patterns of surfacewater chemistry.
The Seasonal Kendall Test (SKT) (Hirsch et al., 1982;Hirsch and Slack, 1984), described in detail by Evanset al. (2001a), was used to determine statisticallysignificant temporal trends in both the median stand-ardised data (as above) and the surface water chemistryat individual AWMN sites. The SKT is a non-para-metric test for detecting monotonic but not necessarilylinear change over the period of record. Trend analyseswere also undertaken for the 1992e2002 period in orderto reduce the influence of major precipitation and sea-salt events that characterised the early part of the
29J.J.L. Davies et al. / Environmental Pollution 137 (2005) 27e39
AWMN record, and so aid the examination of ionicresponses more specifically related to xSO4 decline.
3. Results and discussion
3.1. National trends in AWMN chemistry
As for previous analyses (Evans and Monteith, 2001;2002), standardised median time series (Fig. 1) demon-strated that trends across the AWMN are fairlyconsistent; the ranges between the 2nd lowest and 2ndhighest values are generally narrow and extreme valuesexhibit similar behaviour to medians for all determi-nands. This effect is more marked for AWMN lakes(Fig. 1a) than for streams (Fig. 1b) due, in part, to theirlower short-term variability and more regionally clus-tered geographical distribution, which causes them toexperience a more similar deposition regime. Theregional coherence evident in standardised median timeseries indicates that this approach provides an effectivemethod of assessing the response to reduction in acidanion emission at a national scale.
The time series for lakes (Fig. 1a) demonstrate a post-1995 decline in xSO4 accompanied by a decline in excesscalciumCexcess magnesium (xCaCxMg), H and Allaband an increase in AB-ANC, although there is no cleartrend for CB-ANC. The large increase in DOC evidentat individual sites is also reflected in the combined trend,while chloride (Cl) clearly demonstrates elevated con-centrations at the beginning of the record, which havedeclined and stabilised somewhat since 1993. All15-year, median lake records exhibited significantmonotonic trends ( p!0.05) for all variables except Cl( pZ0.06), NO3 ( pZ0.27) and CB-ANC ( pZ0.31). ForNO3, the lack of trend mirrors observations at in-dividual sites, with the large pulse evident in 1996possibly related to a particularly cold winter in 1995e1996 (see below). For xSO4, it appears that concen-trations increased slightly during the first half of therecord. This has previously been attributed to a sea-saltdeposition effect, whereby surface water xSO4 concen-trations were apparently suppressed by elevated sea-saltinputs in the early part of the record (Evans et al.,2001b). However, given the tight temporal relationshipobserved between deposition and run-off fluxes, it nowseems more likely that a change in prevailing windsassociated with a shift in the North Atlantic oscillation(NAO) was the governing factor (Cooper, 2005, thisissue). The non-linear trend behaviour for xSO4 ismirrored by the time series for xCaCxMg, but the Htrends are relatively linear. Hydrogen concentrations inthe early part of the record may have been coinciden-tally enhanced by sea-salts through displacement fromthe ion exchange complex in catchment soils (Evans andMonteith, 2001).
For stream sites (Fig. 1b), higher sampling frequencyand flow-related episodicity have led to weaker trendsthan for lakes. Trends in standardised median variablesare all significant ( p!0.05) except for NO3 ( pZ0.85)and CB-ANC ( pZ0.39). Variations in xSO4 are lesspronounced than for lakes, except for a pulse in 1996,which reflects observations at many individual sites.A similar effect has been reported elsewhere (e.g.,Harriman et al., 2001) and has been argued to resultfrom effects of summer drought in 1995, with sub-sequent flushing of oxidised S on re-wetting. This effectwas compounded by particularly high deposition duringthe winter of 1995e1996 (Cooper and Jenkins, 2003),and elevated S deposition in 1995 is clear for severalAWMN sites (Cooper, 2005, this issue).
3.2. Patterns in chemical trends at individual sites
3.2.1. Acid anionsPerhaps the most important observation revealed by
the new analysis is that most sites (site identificationnumber in brackets, cf. figures) show a significantdownward trend in xSO4 (Fig. 2a). Of the five sites thatdo not, Loch Coire nan Arr (1) and Allt na Coire nanCon (3) have received relatively low levels of anthropo-genic S deposition since the onset of monitoring andtherefore we have no reason to expect strong trends. Themonitoring records for Scoat Tarn (10) and Llyn CwmMynach (16) exhibit recent declines in xSO4, althoughthe SK test does not show these to be statisticallysignificant over the full monitoring period. This may inpart be due to the effects of high sea-salt inputs towardsthe beginning of the record, which could have causedunder-estimates of xSO4 (discussed further below). Thisphenomenon may also be responsible for the small butsignificant positive xSO4 trend at Narrator Brook, a sitewhich is particularly prone to sea-salt inputs. However,Fowler et al. (2005, this issue) also indicate that anincrease in sulphur from shipping may have more thancompensated the small decline in terrestrial xSO4 in thisregion.
Geographically, the magnitudes of trends in xSO4
roughly match those in S deposition, with sites showingthe highest initial levels and largest declines in Sdeposition (see Fowler et al., 2005, this issue) alsodemonstrating the largest decreases in surface waterxSO4. The River Etherow (12, north-central England)and Old Lodge (13, south-east England) exhibit verylarge, highly significant negative xSO4 trend slopes,reflecting positions close to significant emission sourcesof S, while the most remote sites in northwest Scotland(1, 3) show very small and non-significant trends.Between these extremes, the magnitude of downwardtrends in xSO4 generally decreases in a northwesterlydirection. This pattern is mirrored in Northern Irelandwhere Blue Lough (21) in the southeast exhibits
−2
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(ix) Dissolved Organic Carbon
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4
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(x) Chloride
(a)
Fig. 1. Standardised median time series for (a) lakes and (b) streams (black lines). Grey lines represent the 2nd highest and 2nd lowest standardised
concentration for each time step.
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02
46
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(i) non−marine Sulphate (b)
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(ix) Dissolved Organic Carbon
−4
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810
1988 1990 1992 1994 1996 1998 2000 2002
(x) Chloride
Fig. 1 (continued )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
−8
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µeq
l−l yr
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(a) Excess Sulphate
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(b) Nitrate
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8(f) Charge balance ANC
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Fig. 2. Slopes and significance levels of trends for a range of determinands at individual sites (site numbers are as specified in text). Black bars
represent trends significant at p!0.01, grey bars those significant at p!0.05 and hatched bars those that are insignificant.
33J.J.L. Davies et al. / Environmental Pollution 137 (2005) 27e39
a relatively large, highly significant trend and ConeyglenBurn (22) in the northwest one of the smallest of allsignificant xSO4 trends. This observation is supportedby the work of Cooper and Jenkins (2003); Cooper(2005, this issue), who have demonstrated the respon-siveness of freshwater run-off chemistry to declining Sdeposition.
No site shows a significant trend in NO3 concentra-tion (Fig. 2b). It has been hypothesised that as catch-ments become saturated with N, increased NO3 leachingwill occur even at stable levels of deposition (Aber et al.,1989; Stoddard, 1994). Stoddard (1994) proposed a fourstage classification of N saturation with year-round soilretention of N progressing to pronounced seasonality,and further to year-round leaching. Monteith and Evans(2000) tentatively linked AWMN sites to each of thesestages but there is little evidence of a temporal pro-gression in N saturation for AWMN or other long-termdatasets (Wright et al., 2001). N deposition in Englandand Wales is at a comparably high level to areas ofcentral Europe, but is lower in Scotland and N. Ireland(NEGTAP, 2001). Examination of time series showsshort term increases in NO3 at Lochnagar (4), LochChon (5), the Round Loch of Glenhead (7) and LochGrannoch (8) (Fig. 3) after the 1995 summer droughtand the unusually cold winter of 1995e1996 (see below).At these sites, a shift from some seasonality in the earlieryears of monitoring to more elevated year-roundleaching is observed, although all sites except Lochnagaralso exhibit evidence of a subsequent decline to pre-1995concentrations. The cause of the extreme NO3 peak atLoch Grannoch in 1999e2000 is not known. Thispattern is unlikely to represent a breach of a saturationthreshold, the timing of which is likely to be site-specific,but elevated or increasing levels of NO3 leaching haveimportant implications for recovery from acidification,potentially reducing or even nullifying the impact ofxSO4 decline on acidity. This has been suggested as thecause of the apparent slight increase in acidity observedat Lochnagar after 10 years of monitoring (Monteithand Evans, 2000), and this site still shows no significanttrend in either pH or ANC despite a significant decreasein xSO4. Clearly, NO3 leaching may be an importantconfounding factor in the recovery pattern at all sites.
Relatively elevated peaks in NO3 have been observedin spring samples for most sites in 1991 and 1996 andthese are linked with variations in the winter NorthAtlantic Oscillation Index (NAOI) (Monteith et al.,2000), with the highest leaching occurring after thewinters with the most negative winter NAOI. A similarrelationship between NO3 and the NAOI has beendescribed for Lake Windermere by George et al. (2000).Although the mechanism is still unclear it has beenargued that NO3 leaching is enhanced during cold(negative NAO) winters when a greater duration andintensity of soil freezing enhances biocidal effects,
releasing more N for mineralisation, while low soiltemperatures may also retard assimilation of N by soilbiota. It is also possible that this relationship may beinfluenced by inter-annual variability in prevailing winddirection and hence supply of pollutant N. However,given the absence of any apparent link between de-position and run-off fluxes of N (Cooper, 2005, thisissue) such an effect is not likely to be strong. With fivemore years of data than were available for the analysisof Monteith et al. (2000), the NO3eNAOI relationshipstill holds (Fig. 4). Negative winter NAOI scores havebeen relatively rare over the past two decades andunfortunately, the most recent, in the winter 2000e2001,coincided with foot and mouth access restrictions whenwater samples could not be taken from several sites.
3.2.2. Base cationsMany sites show a significant decrease in xCaCxMg
(Fig. 2c). This is a likely consequence of xSO4 decreaseand is sometimes referred to as a ‘‘confounding factor’’in the recovery of catchments from acidification(Stoddard et al., 1999), although a decline will occurat any site where the base cation supply from ionexchange or weathering has not been completelyexhausted. In general, it might be expected that thewell-buffered sites will show the largest proportionaldecreases in base cation concentrations, and conse-quently the effects of xSO4 decline on ANC will be smallat these sites (Evans et al., 2001a). Conversely, in poorlybuffered catchments where more run-off xSO4 isbalanced by export of H and Allab, a proportionallylarger decline in these ions is expected relative to basecations, leading to more pronounced effects on pH andANC. This effect is seen at neighbouring lakes in theEnglish Lake District. At the poorly buffered ScoatTarn (10), the xCaCxMg trend slope is smaller than atthe well-buffered Burnmoor Tarn (11), and pH andANC trends are highly significant at the former and notsignificant at the latter, despite similar reductions inxSO4.
3.2.3. Measures of acidityIt is anticipated that xSO4 decline will be accompa-
nied by a decline in the acidity of acidified systems andthis is crucial for their biological recovery. Aquatic biotamay be directly sensitive to H or Allab toxicity, orrespond to variations in the availability and form ofdissolved inorganic carbon (represented by alkalinity) oran excess of a toxic agent such as Allab over potentialbuffering from Ca or DOC (which may be broadlyreflected by ANC). Eight sites show a significantdecrease in H (Fig. 2g), and of these all but two, ScoatTarn (10) and Afon Gwy (18), also show a significantincrease in alkalinity (Fig. 2d). Two sites showa significant trend for alkalinity only: Llyn CwmMynach (16), which actually shows a very slight decline
34 J.J.L. Davies et al. / Environmental Pollution 137 (2005) 27e39
010
2030
4050
1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
LochnagarLoch ChonRound Loch of GlenheadLoch Grannoch
Fig. 3. Time series for NO3 (meq l�1) at Lochnagar, Loch Chon, Round Loch of Genhead and Loch Grannoch.
of 0.07 meq l�1 yr�1, and Narrator Brook(14). The steepdecline in alkalinity at Coneyglen Burn (22) is notstatistically significant and the time-series at this site isextremely noisy suggesting that the trend may be anartefact related to flow variation. With the exception ofOld Lodge (13) the recovering sites are contained in anarea of central western UK, some 300 km from north tosouth and 150 km from east to west, where the strongestdownward trends in xSO4 have also been recorded. Thiszone also contains two relatively well-buffered sites(Loch Tinker, 6; Burnmoor Tarn, 11), two lakes withafforested catchments (Loch Grannoch, 8; Llyn CwmMynach, 16) and three acidic and strongly episodicstreams (River Etherow, 12; Bencrom River, 20; Beagh’sBurn, 19). Despite the absence of overall trends thelatter three sites (and several others which do not exhibitsignificant H trends) do show a tendency for declininghydrogen ion maxima (Fig. 5a). With the exception ofthe two forested catchments, sites where xSO4 concen-
December − March NAO Index
Mea
n st
anda
rdis
ed M
arch
nitr
ate
conc
entr
atio
n
−2 −1 0 1 2 3
−0.
50.
00.
51.
01.
52.
0
••
•
•••
•
•
• •
• •
•
•
R−squared = 0.85
Fig. 4. Relationship between AWMN median standardised NO3 and
the North Atlantic Oscillation Index.
trations have declined most have also shown improve-ments in pH and alkalinity. For Allab, ten sites showsignificant decreases (Fig. 2h), again broadly mirroringdeclines in xSO4 and H, and again with a tendency fordeclining maxima even at stream sites where nostatistically significant trend is detected (Fig. 5b). Thisis likely to have considerable biological significance, asepisodically H and Allab peaks may be more importantthan average levels of acidity in determining speciescomposition and diversity of surface waters.
For ANC, the two methods of calculation describedabove yield very different trend results. ANC isa calculated determinand, and as such is subject toa number of errors related to the accuracy of constituentdata and the method of calculation. CB-ANC is widelyused (e.g., Stoddard et al., 1999), has been utilised inmost previous analyses of AWMN data, and isparticularly suited for modelling purposes where chargebalance is an underlying requisite. However, Evans et al.(2001c) have argued that CB-ANC is sensitive to smallerrors in individual ion determinations, which cumula-tively may lead to considerable inaccuracies in ANCcalculations where ion concentrations are high, as inwaters with a high marine ion input. They also suggestthat this sensitivity is particularly acute when examiningtemporal changes in ANC which may be only a fewmeq l�1 year�1. The use of AB-ANC reduces inaccura-cies due to the cumulative error of individual constitu-ents, but is subject to a number of generalisationsrelated to the charge density of DOC and the dissocia-tion of Allab which may not hold for all surface waters.
For CB-ANC (Fig. 2f), few significant trends wereidentified with the exception of a large increase at OldLodge (13), consistent with the large decrease in xSO4 atthis site. and small increases at Dargall Lane (9) andAfon Gwy (18). The River Etherow (12), despite a largexSO4 decrease, showed no significant CB-ANC trend.With both xSO4 and base cation concentrations de-creasing at many sites, the noise generated by compounderrors may be too great to allow the detection of
35J.J.L. Davies et al. / Environmental Pollution 137 (2005) 27e39
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Fig. 5. Time series for (a) H (meq l�1) and (b) Allab (mmol l�1) at AWMN stream sites.
36 J.J.L. Davies et al. / Environmental Pollution 137 (2005) 27e39
underlying trends in CB-ANC. By contrast, AB-ANCexhibits many large and significant positive trends, mostnotably at the River Etherow and Old Lodge. Trendsshow a similar distribution to those in H and alkalinity,but the influence of DOC is clear; for those sites wheretrends in alkalinity and H are weaker than those for AB-ANC (e.g., River Etherow), this suggests that a shift hasoccurred from strong mineral to weaker organic acidity,rather than a straightforward reduction in acidity(Evans et al., 2001c).
Given the clear relationship between changes in DOCand AB-ANC it is essential that the cause of the largeand highly significant DOC increase at all sites beestablished, before trends in AB-ANC can be attributedto decreasing acid deposition. The widespread existenceof DOC trends was first noted at AWMN sites(Monteith and Evans, 2000), and similar trends havenow been noted elsewhere in the UK and elsewhere.Potential drivers, which include both climate- and aciddeposition-related factors, are considered in detail byEvans et al. (2005, this issue).
3.3. Ionic response to declining xSO4
Relationships between the magnitude of xSO4 trendslopes and xCaCxMg, H, AB-ANC and CB-ANCslopes since 1992 are strongly linear for those sitesexhibiting significant trends in these species (Fig. 6).Regression analyses reveal that the variability inmagnitude of significant xSO4 trend slopes can accountfor 93%, 89% and 78%, respectively, of the variation inthe slopes of xCaCxMg, H and AB-ANC (for CB-ANConly one site exhibits a significant trend for both thisvariable and xSO4). This indicates that the pattern ofcatchment recovery from acidification in response toxSO4 decline is relatively consistent across these sites,but there is considerable variability among sitesexhibiting non significant trends.
For xCaCxMg, the degree of decline at a particularsite is likely to be a function of both xSO4 decline andthe degree of base cation buffering, while there is alsolikely to be a positive influence from the rise in DOC.The large significant decrease in xCaCxMg at the River
−10 −8 −6 −4 −2 0
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xSO4 µeq l−l yr−l
xSO4 µeq l−l yr−1xSO4 µeq l−l yr−l
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ess
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g µe
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ity−
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Fig. 6. Relationship between trend slopes for xSO4 and the response variables xCaCxMg, H, CB-ANC and AB-ANC. Filled circles represent sites
which exhibit significant trend slopes for both determinands, open circles those where either or both trends are insignificant. Trend lines apply to
significant sites only.
37J.J.L. Davies et al. / Environmental Pollution 137 (2005) 27e39
Etherow (81% of the decrease in xSO4) is consistentwith the relatively well-buffered nature of this site; alarge proportion of xSO4 is accompanied by basecations and therefore decline in the former will bemirrored to a large extent by a decline in the latter.Other significant trends for xCaCxMg exhibit moremodest declines (47e67% of the decline in xSO4) andthis may reflect lower base cation buffering at these sites.Variation may also be related to base cation uptake atafforested sites.
The decline of H as a percentage of the declinein xSO4 ranges from 7% at Dargall Lane (9) to 25%at Old Lodge (13). Most of the non-significant H trendshave a slope close to zero. Many of these sites arenot acidic and exhibit weak trends in xSO4 (Allta’Mharcaidh, 2; Coneyglen Burn, 22; Loch Coire nanArr, 1; Allt na Coire nan Con, 3) and for othersconfounding factors such as forestry (Afon Hafren, 17)or declines in xCaCxMg (River Etherow, 12; ConeyglenBurn, 22; Beaghs Burn, 19; Loch Tinker, 6) may beimportant.
3.4. Other factors influencing chemical variation
Trends of declining xSO4 in run-off mirror thepattern of decrease in S deposition and thereby indicatea rapid catchment response. The apparent sensitivity ofxCaCxMg, H and AB-ANC to xSO4 decline isencouraging, although there is some variability in thestrength and slope of these relationships, even withinregions. A number of factors may affect catchmentresponses to inputs of acid anions and therefore‘interfere’ with the desired pattern of chemical recovery.For example, variation in catchment geology and soils,internal sources of SO4 (e.g., at the River Etherow), highevaporative losses (at Old Lodge) and altitudinal effects(including enhanced deposition rates and effects ofwinter ice cover) may all be significant.
Inter-annual variability in the NAO has at least threesignificant influences on run-off chemistry. In additionto the observation discussed earlier, that peak annualNO3 concentrations are inversely related to the averagewinter state of the NAO, strong positive correlations arealso apparent between the winterespring NAO Indexand both precipitation and sea-salt inputs. Precipitationduring winter and spring is particularly tightly linked tothe state of the NAO in western regions of the UK.Hydrological variation affects parameters such as pHand alkalinity by influencing catchment flow paths.During periods of low rainfall, runoff is dominated byrelatively well-buffered base-flow. High precipitation orsudden snow-melt leads to a greater run-off contributionfrom poorly buffered surface or through flow, likely tobe high in H, Allab and organic acids. The rather weaktemporal relationship between xSO4 and H mayprimarily reflect of this process. Occasional sea-salt
episodes, mainly arising during energetic westerlystorms associated with a high winter NAO Index, canalso influence on run-off chemistry. During periods ofhigh sea-salt inputs, short-term retention of SO4 relativeto chloride may reduce the xSO4 estimate. Concurrently,marine cations will temporarily displace H and Allabfrom the soil, thus raising the acidity of run-off,sometimes to extreme levels (Evans et al., 2001b).
Winter NAO Index has been less positive since themid-1990s, and this is reflected in generally lower Clconcentrations (at least for west-coast sites south of theTrossachs). This, together with the extended duration ofmonitoring, has provided a clearer picture of xSO4
decline, and the true response to this reduction in termsof acidity, at most sites. However, with future globalwarming anticipated to cause an intensification of theNAO (Hulme et al., 2002), it is important that therelationship between the NAO and water chemistry isquantified sufficiently to allow its various effects to beincorporated within predictive recovery models. Inaddition to possible long-term effects of an increase inthe NAO, it will also be important to consider theimplications of changes in episodic intensity andfrequency on recovering aquatic biota.
Forested catchments are likely to be more impactedby acidic pollutants than non-forested ones through‘scavenging’ of air-borne pollutants by the forest canopy(Mayer and Ullrich, 1977), increased uptake of basecations (Miller, 1981), and reduced dilution of pollu-tants (Neal et al., 1986). This appears to be the case forAWMN sites, where examination of forested/non-forested ‘pairs’ of catchments suggests that forestedsites have higher acid anion concentrations and aremore acidic (Monteith and Evans, 2000). It is alsopossible that forest growth could offset recovery ofsurface waters (Jenkins et al., 1997). Of the fivecatchments which contain significant forest cover, threedo not show significant trends in pH or alkalinity (Alltna Coire nan Con, Loch Grannoch and Afon Hafren)and one a small but significant decrease in alkalinity(Llyn Cwm Mynach). However, all except the AfonHafren show a positive trend for AB-ANC. A study often forest stream monitoring sites in Wales revealssimilar results, with few trends detected in the datasetafter 10 years of monitoring (Forest Research, un-published data). However, time series at forestedAWMN sites are not clearly dissimilar to those forother sites, and it is possible that trends at forestedsites are simply obscured by variability caused byforest growth and felling. The felling and replantingexpected at some forested sites in the next few yearswill reduce scavenging of atmospheric pollutants butalso increase base cation uptake, the net effect ofwhich is difficult to anticipate. Again, continued moni-toring will be necessary to determine the longer termimpact.
38 J.J.L. Davies et al. / Environmental Pollution 137 (2005) 27e39
4. Conclusions
The AWMN dataset reveals encouraging signs ofsurface water recovery from acidification. Significantdeclines in xSO4, with accompanying increases in pHand alkalinity and declines in labile Al, provide clearevidence of chemical improvements at many sites. Thegeographical location of these sites is closely related tothe spatial pattern of decreasing acid deposition acrossthe AWMN, with the strongest recovery trends in areaswhere SO4 deposition has declined most, i.e., central andsoutheast England. The consistency of trends amongsites suggests that this analysis should be representativeof the wider population of acid deposition impactedcatchments across the UK.
Despite a general trend towards recovery, there issubstantial between-site variation in acidity-relatedvariables such as H and ANC. Local factors such ashydrology, sea-salt inputs, forestry, soil bufferingcapacity and N saturation may affect patterns ofrecovery in response to xSO4 decline. The behaviourof ANC in particular remains unclear. Trends in CB-ANC are currently only detectable at a minority of sites,although more significant trends are expected as thesignal to noise ratio increases with further monitoring.However, widespread significant trends in AB-ANC,coupled with the between-site correlation in the trendstrength of xSO4 and AB-ANC, are consistent witha (biologically beneficial) shift from strong mineral toweaker organic acidity. It is vital that the driversincreases in DOC, which strongly influence AB-ANCtrends, are better understood in order to establishwhether these represent evidence of recovery fromacidification, or a ‘confounding factor’ which is effec-tively delivering additional acidity. Although there isnow strong evidence for recovery from acidification atAWMN, these key uncertainties illustrate the need forcontinued monitoring in order to confirm the long-termextent of recovery, and to improve understanding of theprocesses involved.
Acknowledgements
This work was funded by the Department forEnvironment, Food and Rural Affairs (contract EPG1/3/160) and the Department for Environment, Foodand Rural Affairs (Northern Ireland) (contract CON4/4 (38)).
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