How do groundwater flow systems effect the impact of on-farm mitigation measures on river receptor pollution?
The challenge
Agricultural diffuse pollution, particularly from nitrate and phosphate, is a significant problem in the UK and is the focus of the national Demonstration Test Catchments (DTC) study — a UK Government initiative.
The solution
In general in catchments the timescales of water flow can vary significantly - from possibly hours for overland flow to tens of decades for deep groundwater movement.
The DTC catchments are the Eden, the Avon and the Wensum and groundwater flow is a significant component of main river flow in all of the catchments, ranging in overall terms from around 50% of river flow in the Eden to 90% in the Avon. BGS has contributed to the creation of DTC groundwater conceptual models in all three study catchments.
Within the Avon DTC sub-catchments, groundwater is often considered to be an important water flow mechanism. By looking at the nature of the geological materials underlying the catchments, the groundwaters that they contain and how these waters interact with the surface, the BGS work is helping to build up a picture of how groundwater moves from areas of recharge on high ground to lowland discharge zones - for example as springs or seeps to rivers.
In the Eden DTC, the nature of glacial deposits (till) can have a significant effect on groundwater flow routes to streams. Drilling in one of the DTC sub-catchments has shown that while the till sequence mainly consists of clay it also contains layers of sand and gravel. These sands are likely to be permeable and are known to provide subsurface drainage pathways to a stream.
The Chalk aquifer in Norfolk is overlain and confined by a thick and complex sequence of Quaternary
deposits laid down by glacial activity. Analyses of the pore waters provides information on the processes occurring in these deposits which assists understanding of groundwater movement through the sequence into the underlying Chalk aquifer.
Resulting benefits
BGS is helping to construct conceptual models of the groundwater flow systems of DTC subcatchments. As a result of geological variations, groundwater is significant in many of the sub-catchments and the subsurface flow systems are commonly complex, with a spectrum of travel times. This result is important for understanding both pollutant behaviour and for catchment management, because it implies that groundwater flow systems can strongly influence both the degree and the timing of the impact of on-farm mitigation measures on river receptor pollution.
Future directions
These models will be used to improve knowledge of the rates of movement in groundwater flow systems within the catchments and this in turn should help with predicting the likely timescales before measures at the surface will become effective at the watercourses.
Add Pingback