Understanding our groundwater systems – insights from the UK Lowland

Catchment Research Programme (LOCAR)

Howard Wheater, Imperial College London, - with acknowledgements to colleagues at BGS, CEH, Lancaster, Reading & Imperial

LOCAR – a £10 million programme

Science and Policy driversLimitations of UK experimental hydrology base- uplands/small scaleManagement pressures - lowlands/ larger scaleLowland permeable catchments- linking surface water and groundwaterWater Framework Directive - ecological qualityNeed for new interdisciplinary science

Scientific Aims

To develop an improved understanding of hydrological, hydrogeological, geomorph-ological and ecological interactions within permeable catchment systems, and their associated aquatic habitats, at different spatial and temporal scales and for different land uses;

To develop improved modelling tools to inform and support the integrated management of lowland catchment systems.

InstrumentsRecharge site

Experimental groundwater arrays Water quality

The HYDRA: actual evaporationin thePang/Lambourn

LOCAR SCIENCE PROJECTS I

Evaporation • Assessment of new methods to estimate grid or catchment

evaporation using satellite and ground-based measurements. • The influence of woodlands on recharge in the Pang

catchment Soils • Flow and transport of water in the Chalk unsaturated zone

Groundwater

• Characterising permeability and groundwater flow in Chalk catchments using tracer techniques

• Investigation of groundwater flow heterogeneity in the Chalk aquifer using detailed borehole arrays and stochastic modelling techniques

• Assessing stream-groundwater interactions in lowland chalk catchments using hydrogeophysical characterisation of the riparian zone

LOCAR SCIENCE PROJECTS II • Towards a methodology for determining the pattern and

magnitude of recharge through drift deposits Sediments • The fine sediment budgets of lowland permeable

catchments and their implications for nutrient and contaminant transfer

• Fine sediment and nutrient dynamics in lowland permeable streams: establishing the significance of biotic processes for sediment modification

• Vegetation management influences on fine sediment and propagule dynamics within groundwater-fed rivers: implications for river management, restoration and riparianbiodiversity.

LOCAR SCIENCE PROJECTS III Nutrients • Hydrogeochemical functioning of lowland permeable

catchments: from process understanding to environmental management

Ecology • Utilisation of off-river habitats by lowland river fishes:

influences of flow regime and land-use change • Ecological significance of surface and subsurface

exchange in lowland channels

Hydrology of the Pang and Lambourn

The Chalk

Pang and Lambourn topography and river system

Solid Geology

Streamhydrographs

1997 1998 1999 2000 2001 2002 2003 20040

1

2

3

4

5

6

7

Cum

ecs

Lambourn: daily flow 1997-2004

1997 1998 1999 2000 2001 2002 2003 20040

1

2

3

4

Cum

ecs

Pang: daily flow 1997-2004

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

1000

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

1000

10

20

30

40

50

60

70

80

90

100

0102030405060708090100

0

10

20

30

40

50

60

70

80

90

100

90

80

70

60

50

40

30

20

10

0

PangLambournPalaeogene

Ca Na + K HCO3 Cl

Mg SO4

Hydrochemistry of the Pang, Lambourn and Palaeogene

Seasonal variation in catchment area

Chalk groundwater

Pang-Lambourn catchment

PL10Trumpletts Farm

Site layout

Packer Test Results

0

10

20

30

40

50

60

70

0.01 0.1 1 10 100

K (m/day)

m a

OD Mar-03

Nov-03Mar-04

Borehole A K/Kmin1.0c

D

dwt

du

z

Videoscan logs for Borehole A

K = 60 m/d K = 0.2 m/d K = 0.2 m/d

Interval: 59.5 – 56.6 mAOD Interval: 15.0 – 12.1 mAOD Interval: 37.5 – 34.6 mAOD

Discontinuous tabular flint

Dispersed small flints

Developed tabular flint

Nodular flint

Marl seams

Steep fault with ~ 2cm displacement

1) Inflections in temperature profile are indicative of flow horizons;

3) Steep temperature gradient indicates low vertical flow;

7) Except the ‘Chalk Rock’, indicated by the Gamma log.

6) Most horizons are supported by features in the calliper log;

2) Flow horizon not seen in temp. log;

Insight from geophysics(ambient conditions)

4) But in the same region there is a large diluting feature;

5) Impeller and heat-pulse flow-meters show discrepancy;

0.09m3/hr

0.03m3/hr

0.51m3/hr

0.27m3/hr

0.12m3/hr

Solid lines show fits to the FEC data using an advection dispersion model of the borehole

Red line indicates the Upflow obtained by inverse modelling of the FEC data

Note that these flows are very high!!

Inversion of in/outflows from FEC logs

Note that the temperature gradient has now been lost

Insight from geophysics(pumped condition)

Vertical flow rates have markedly increased

Dilution test complete in just over 1 hour

0.96m3/hr

1.08m3/hr

0.84m3/hr

2.4m3/hr

1.2m3/hr

Solid lines show fits to the FEC data using an advection dispersion model of the borehole

Red line indicates the Upflow obtained by inverse modelling of the FEC data

1) Pumping has increased flows by an order of magnitude.2) Flow has actually reversed at 74mAOD.

Elevation Ambient Pumped(mAOD) (m3/hr) (m3/hr)79 0.12 1.2074 0.27 -2.4057 -0.51 -0.8427 0.03 1.0810 0.09 0.96

Inversion of in/outflows from FEC logs(pumped conditions)

Discrete inflows

Cone of depression develops

Implication of drilling a borehole and pumping an abstraction well

Stream-aquifer interactions

Flow accretion in the Pang and Lambourn

Structure and control on springs

Springs and sinks on Pang/Lambourn

Boxford Site borehole transects

Water levels at Westbrook,

River Lambourn

Date

3/04 4/04 5/04 6/04 7/04 8/04 9/04 10/04 11/0412/04 1/05 2/05 3/05

Wat

er le

vel (

aOD

)

90.8

90.9

91.0

91.1

91.2

91.3

91.4

91.5N 4N 7N 15A1A2C1C2D1D2

Date

3/04 4/04 5/04 6/04 7/04 8/04 9/04 10/0411/0412/04 1/05 2/05 3/05

Wat

er le

vel (

aOD

)

90.8

90.9

91.0

91.1

91.2

91.3

91.4

91.5

91.6N4N7N 15river upstreamriver downstreamriver at site

N15 N4

N7

Flow mechanisms in the river corridor

The Chalk unsaturated zone

10

20

30

40

50

60

70

80

1989 1990 1991 1992 1993 1994 1995 1996

GW

L (m

AO

D)

0

1

2

3

4

5

6

7

EP

(mm

/d)

Fast water table response

From Chilgrove

GWL

EP

Slow solute migration

acquired from Oakes et al. (1981)

Conceptual dual permeability model

fast

flow

thro

ugh

fract

uressolute

diffuses into matrix

slow flow through matrix

Simulation of unsaturated zone matrix-fracture interactions

0 50 100 150 200 250 300 350 4000

10

20

30

40

Flu

x (m

m/d

ay)

0 50 100 150 200 250 300 350 4000

5

10

15

20

Flu

x (m

m/d

ay)

0 50 100 150 200 250 300 350 4000

5

10

15

20

Flu

x (m

m/d

ay)

0 50 100 150 200 250 300 350 4000

5

10

15

20

Flu

x (m

m/d

ay)

0 50 100 150 200 250 300 350 4000

5

10

15

Flu

x (m

m/d

ay)

Time (days)

matrixfracturetotal

matrixfracturetotal

matrixfracturetotal

Effective precipitation

Beneath soil layer

z = 100cm

z = 500cm

z = 1000cm (water table)

a)

b)

c)

d)

e)

Recharge site instrumentation

Chalk unsaturated zone and groundwater- recharge period

Effective rainfall and groundwater rise

Modelling nutrients at catchment-scale

INCA nitrogen pathways and transformations

Organic N

N2N2O NH4

NO3

mineralisation

immobilisation

nitrification

NH3

volatilisation

denitrification

fixationNO3

Groundwater Zone

NH4

Urban wasteto River

NitrogenFixation

Ammonium +Nitrate deposition

Ammonium +Nitrate fertiliser

NO3 NH4

nitrificationOrganic N

Netmineralisation

NitrateAddition

Plantuptake

AmmoniumAddition

Plantuptake

Reactive Soil Zone

denitrification

Leachingto river

Leachingto river

Nitrogen transformations

INCA-Chalk

soil zone

unsaturatedzone

groundwater zone

soil zone

groundwater zone

soil zone

groundwater zone

a) Original INCA b) INCA-unsat c) INCA-CHALK

Initial trial: Lambourn catchment

Travel time distributions through unsaturated zone

Hindcast simulation, INCA-Chalk, Lambourn: simulated versus observed flow and nitrate at Welford (EA data)

Groundwater with A2HADCM3 scenario, nitrate loading cut to zero from 2003

Some conclusions (1)

• Hydrological science has some way to go before understanding these Chalk systems can be thought of as sufficient to adequately manage water quality, wetlands, and individual abstractions without significant risk of error

• The management of riverine and riparian ecology requires a more detailed understanding of flow regimes than we currently have

• Interpretation of pumped borehole data can be misleading – the construction of a borehole and perturbation by pumping can change flow paths

• Unsaturated processes are important in solute transmission, in maintaining evaporation and potentially in maintaining drought flows

Some conclusions (2)

• At present regional numerical models simulate very few of the mechanisms discussed. The development of appropriate models to represent this sort of complexity is needed to help understand, predict behaviour of and design monitoring for catchments under changing land use and climate

• However, numerical models are the only way forward to integrate diverse data sets and provide a unified interpretation of hydrological and hydrogeological response – the EA plans for progressive development of regional models are an important step in this direction

• Simplified models can represent important characteristics of catchment-scale flow and solute transport, and provide useful support for policy and catchment scale management

• Integrated multidisciplinary research is essential to improve our understanding of groundwater systems; current momentum should be maintained