Abstract
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.
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.
| Original language | English |
|---|---|
| Publication status | Published - 2006 |
