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A strategic programme for NERC Lowland catchment research
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Hydrogeochemical functioning of permeable lowland catchments

Title

Hydrogeochemical functioning of lowland permeable catchments: from process understanding to environmental management.

Overview of project

The water environment faces increased pressures such as changes in land-use, increased demand for water supplies, and uncertainties over climate change. These pressures create significant challenges for managing catchments and this is recognised in the EU Water Framework Directive. An improved understanding of catchment processes is needed to develop of appropriate decision-support tools. This project has sought to achieve that goal through the integration of an extensively monitored field site with hydrochemical and hydrological measurements of ground and surface waters, coupled with the development of a catchment-scale model. Natural geological features have been found to be very important in the complex movement and interaction of surface and groundwaters. A wetland site has been found to be a biogeochemical hotspot but contributes little to the net nutrient flux within the catchment. Modelling studies suggest that changes in land use practices will not lead to river water quality improvements for several decades.

Aims

This project addressed the scientific and management issues associated with surface and groundwater quality through integration of the LOCAR monitoring data, supplementary experiments, natural tracer analysis and numerical model development. It aimed to establish the main hydrological and hydrogeological controls on groundwater and surface water quality (including chemical transformations and flowpaths of the principal nutrients), and to produce an integrated modelling system describing the hydrochemical and nutrient functioning of lowland permeable catchments as a framework for sustainable river ecosystem management.

Main findings

The subsurface

A new hydrogeological conceptual model of the Pang and Lambourn catchments has been developed. Groundwater flow in the Berkshire Downs is dominated by the Rivers Kennet and Thames. Dry valleys act as collectors of groundwater and feed it to the rivers. The Pang groundwater catchment is almost impossible to define, except in the lower reaches. The Pang is best conceptualised as a high level ephemeral drain, under which for much of the year groundwater flows towards the Thames. The locations of springs in the Lambourn and Pang are controlled by lithological features (rock characteristics), intersection of fracture lineaments or faults with valleys and karstic development of the Chalk. Physical and chemical data indicate a patchy connection between the river, the gravels and chalk aquifer and suggest an important role for lateral flow along the river corridor in the gravels.

Detailed physically-based modelling of the Chalk unsaturated zone, representing flow in both matrix and fractures, and solute interactions between the two, has been used to explore water flow following rainfall and recharge. Results suggest that matrix flow predominates, but that fracture flow can contribute up to 30% of recharge water. Analysis of the LOCAR recharge site data has confirmed that the shallow soil zone strongly attenuates infiltration fluxes and supports the contention that fracture flow is of limited significance. Results suggest that the unsaturated zone storage can play a role in supporting groundwater levels; deep drainage of the unsaturated zone occurs even during prolonged dry periods.

Surface waters and riparian wetlands

The water quality of rainfall and runoff has been studied for sites across the Pang and Lambourn. Rainfall chemistry is variable, and shows impacts of local agricultural activity. Stream chemistry is dominated by calcium bicarbonate Chalk water. The major, minor and trace element chemistry is controlled by atmospheric and pollutant inputs from agricuture and sewage sources superimposed on a background signal linked to geological sources. Remarkably, in view of the quantity of agricultural and sewage inputs to the streams, the catchments appear to be retaining both phosphorus and nitrogen. Bed-sediment phosphorous-exchanges provide an important control on soluble reactive phosphorus (SRP) concentrations in Chalk streams exposed to point source discharges. Within-river processing provides an important 'self-cleansing' mechanism, removing most of the SRP within a few kilometres of effluent sources. At sites which have been heavily impacted by sewage effluent, phosphorus-stripping can result in a switch from bed sediments acting as net sinks to net sources of SRP, over a limited period (in this study, around six months).

Measurements of rates of growth of algal biofilms and P-uptake into biofilms have been carried out on the Pang and Lambourn. It is estimated that biofilms would remove up to about 5% of in-stream phosphorus load over a 4 km reach.

Detailed experimental studies have been undertaken in a riparian wetland on the Lambourn. The findings from geochemical analyses of soil porewaters and source waters suggest that the wetland is fed by groundwater. Attenuation of nutrient pollution appears to occur both within the hyporheic zone and the overlying wetland ecosystem. The primary pathway for modification of nutrient species is through plant uptake of inorganic nutrient species and microbiological breakdown of disolved organic matter (DOM), dissolved organic nitrate (DON) and soluble unreactive phosphorus (SUP) compounds. The primary mechanism for the export of nutrients accumulated in soil porewaters appears to be the flushing of the macro and micropores during storm events, with nutrient rich waters exported primarily via the stream channel, but also through lateral flow to the Lambourn and vertical drainage to the hyporheic zone.

Catchment-scale nutrient modelling

The integrated catchment model of nitrogen (INCA-N) and the phosphorus model (INCA-P) were applied to the Pang and Lambourn to explore catchment response and investigate long-term performance. Simple mixing of source waters, moderated by in-stream processes explains the dominant response, but stochastic analysis shows the dominant effect of Chalk nitrogen concentrations. The Chalk is characterised by deep unsaturated zones, of up to 70 m, and solute migration is expected to occur at a rate of around 1 m per year. The resulting travel times for nitrate can therefore be of the order of many decades. Clearly models to represent nitrate transport in the Chalk and the response to management interventions must recognise this. A new conceptualisation of unsaturated zone processes has been developed (INCA-Chalk), and incorporated within a GIS-based framework. This uses digital topography to characterise a distribution of unsaturated zone solute travel times. Model tests show that the characteristics of the system are well reproduced, and INCA-Chalk has been run to evaluate long term response for different scenarios of nutrient management and for different scenarios of future climate. This represents a significant development in methodology for the management of nutrients at catchment scale.

Description of activities

Extensive hydrochemical monitoring of groundwater was undertaken over three years at the LOCAR site in Westbrook Farm. Boreholes were regularly sampled using natural chemical and isotopic tracers including residence time indicators to determine dominant flowpaths from rainwater-soil/surface water-groundwater-stream. Chemical tracers included trace elements such as boron, transition metals, as well as the major ions (sodium, potassium, magnesium, calcium, chloride, sulphate) and minor components such as strontium and barium. Piezometric levels were measured at the same time. In-stream water quality was determined through weekly sampling with a particular emphasis placed on nutrients and in particular phosphate species. In addition intensive observation campaigns were mounted to capture episodic responses. River bed sediment phosphorus concentrations were also sampled on an intensive basis. A comprehensive set of piezometers in the adjacent wet land were regularly sampled and analysed for nitrogen and phosphorus species. Modelling activities focused on development of numerically consistent unsaturated zone flow models and improving the INCA model. The improved INCA model (INCA-Chalk) was then used for scenario testing under a range of climate and land use change scenarios.

Areas of application

Implementation of the Habitats Directive and the Water Framework Directive.

Related and future work

Concepts developed in this study are being further explored under the NERC FREE thematic programme (Flood Risk from Extreme Events) with the project "Modelling groundwater flood risk in the Chalk aquifer from future extreme rainfall events" (NERC Reference: NE/E002307/1).

Researchers' details

Principal Investigator:

Co-investigators:

Other researchers:

  • Dr P Shand, British Geological Survey
  • Dr D Gooddy, British Geological Survey
  • A Gallagher, British Geological Survey
  • Dr C Abesser, British Geological Survey
  • D Lapworth, British Geological Survey
  • Dr H Jarvie, Centre for Ecology and Hydrology
  • E Sutton, Centre for Ecology and Hydrology
  • Dr M Kennedy, University of Reading
  • Dr A Wade, University of Reading
  • P Franklin, University of Reading
  • B Jackson, Imperial College London
  • S Mathias, Imperial College London
  • A Ireson, Imperial College London

Publications

All publications from this and other LOCAR projects are listed in the publications database.

Refereed journal papers

Evans, DJ and Johnes, PJ. 2004. Physico-chemical controls on phosphorus cycling in two lowland streams: I - Water column. Science of the Total Environment, 329, 145-163.

Evans, DJ, Johnes, PJ and Lawrence, D. 2004. Physico-chemical controls on phosphorus cycling in two lowland streams: II - Bed sediment. Science of the Total Environment, 329, 165-182.

Gooddy D C, Darling W G, Abesser, C and Lapworth D J. 2006. Using Chlorofluorocarbons (CFCs) And Sulphur Hexafluoride (SF6) To Characterise Groundwater Movement And Residence Time In A Lowland Chalk Catchment. Journal of Hydrology, 330, 44-52.

Ireson, A M, Wheater, H S, Butler, A P, Finch J, Cooper J D and Mathias S. Hydrological processes in the Chalk Unsaturated zone - inferences from an intensive monitoring programme. Journal of Hydrology, 330, 29-43.

Jackson B M, Wheater H S, Mathias S A, McIntyre N and Butler A P. 2006. A simple model of variable residence time flow and nutrient transport in the Chalk. Journal of Hydrology, 330, 221-234.

Jarvie H P, Neal C, Jürgens M D, Sutton E J, Neal M, Wickham H D, Hill L K, Harman S A, Davies J J L,Warwick A, Barrett C, Griffiths J, Binley A, Swannack N and McIntyre N. 2006, Within-river nutrient processing in the Pang and Lambourn Chalk streams, UK. Journal of Hydrology, 330, 101-125.

Mathias S A, Butler A P, Jackson B M. and Wheater H S. 2006. Transient simulations of flow and transport in the Chalk unsaturated zone, Journal of Hydrology, 330, 10-28.

Mathias S A, Butler A P, McIntyre N, Wheater H S. 2005. The significance of flow in the matrix of the Chalk unsaturated zone. Journal of Hydrology, 310, 62-77.

Neal C, Neal M, Leeks G J L, Old G, Hill L. and Wickham H. 2006. Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK, Journal of Hydrology, 330, 142-154.

Neal C, Jarvie, H P, Neal M, Hill L and Wickham H. 2006. Nitrate concentrations in river waters of the upper Thames and its tributaries. Science of the Total Environment, 365 (1-3): 15-32.

Neal C, Jarvie H P, Neal M, Love A J, Hill L and Wickham H. 2005. Water quality of treated sewage effluent in a rural area of the upper Thames Basin, southern England, and the impacts of such effluents on riverine phosphorus concentrations. Journal of Hydrology, 304 (1-4), 103-117.

Neal C, Jarvie H P, Wade AJ, Neal M, Wyatt R, Wickham H, Hill L and Hewitt N. 2004. The water quality of the LOCAR Pang and Lambourn catchments. Hydrology and Earth System Sciences, 8(4), 614-635.

Neal C, Skeffington, R, Neal M, Wyatt R, Wickham H, Hill L and Hewett N. 2004. Rainfall and runoff water quality of the Pang and Lambourn, tributaries of the River Thames, south eastern England. Hydrology and Earth System Sciences, 8(4), 601-613.

Wade A J, Whitehead P G, Jarvie H P, Neal C, Prior H and Johnes P J. 2004. Nutrient Monitoring, Simulation and Management within a Major Lowland UK River System: The Kennet., Journal of Mathematics, 64,307-317.

Wade A J, Butterfield D and Whitehead P G. 2006. Towards an improved understanding of the nitrate dynamics in lowland, permeable river-systems: applications of INCA-N. Journal of Hydrology, 330, 185-203.

Whitehead P G, Wilby R L, Butterfield D and Wade A J. 2005. Impacts of Climate Change on Nitrogen in Lowland Chalk Streams: Adaptation Strategies to Minimise Impacts, Science of the Total Environment, 365 (1-3): 260-273.

Wilby R L, Whitehead P G, Wade A J, Butterfield D, Davis R J and Watts G. 2005. Integrated Modelling of climate change impacts on water resources and quality in a lowland catchment: River Kennet, UK. Journal of Hydrology, 330 (1-2): 204-220.

Conference abstracts and papers

Ireson A M, Wheater H S, Butler A P, Finch J, Cooper J D, Wyatt R G and Hewitt, E J. 2005. Field monitoring of matric potential and soil water content in the Chalk unsaturated zone. In: Advanced experimental unsaturated soil mechanics, Experus 2005, p 511-517, AA Balkema Publishers.

Jackson B M, Wheater H S, McIntyre N, Butler A P, Whitehead P and Wade A. 2004. Calibration and uncertainty issues arising from a process-based integrated nitrogen model (INCA) placed within a subject probability framework. In: Hydrology: Science and Practice for the 21st Century, Proceedings of the British Hydrological Society International Conference, Imperial College London, July, Vol II, 123-129.

Mathias S A, Butler A P, McIntyre N and Wheater H S. 2004. Applicability of box models to dual porosity systems. In Hydrology: Science & Practice for the 21st Century, Vol 1, British Hydrological Society, 315-321.

Recommended reading

Wheater H S and Peach D. 2004. Developing inter-disciplinary science for integrated catchment management - the UK Lowland CAtchment Research (LOCAR) programme. International Journal of Water Resources Development, 20 (3), 369-385.

Wheater H S, Neal C and Peach D. 2006. Hydro-ecological functioning of the Pang and Lambourn catchments, UK: An introduction to the special issue. Journal of Hydrology, 330, 1-9.

Wheater H S, Peach D and Binley A. 2007. Hydrology and Earth System Sciences. Characterising groundwater-dominated lowland catchments: the UK Lowland Catchment Research programme (LOCAR). Hydrology and Earth System Sciences, 11, 108-124.

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