Modelling the Impacts of the WAgriCo Measures on Wessex Water's Groundwater Supplies - Summary Nick Rukin, Hydrogeologist (Associate Director, Entec UK Ltd) WAgriCo Project Dissemination Workshop 29 th September 2008

Background Modelling undertaken to evaluate the impact of WAgriCo Measures on Wessex Water's groundwater supplies To ensure security of supply, Wessex Water need to: –provide treatment or blending capacity (expensive and not sustainable) –reduce nitrate leaching through catchment management Important to consider the legacy from historical nitrate leaching Important to understand how soon any benefits from changes to farming practice will be realised – when will we see the results of our hard work.

Typical Trend Drinking Water Standard Short Term Spike Long Term Trend Seasonal Variation ?? ? ? ? ? The Big Question Can the observed trends be simulated to gain confidence in controlling factors and forward predictions?

Modelling Objectives Constrain Historically Leached Nitrate Constrain Travel Times from Soil to Abstraction Factor in Seasonality and 'Spikiness'

Historically Leached Nitrate Derived by Considering: - historical fertiliser use - historical land use changes - historical stocking densities - observed nitrate in observation boreholes - optimised model fit for NO 3 at the PWS Arable Managed Grassland Note: Concentration in 440 mm/yr recharge

Travel Times from Soil to Water Table Longer in interfluves Shorter near rivers

Excel processed GIS outputs of travel times for different land uses in a selected catchment area

Seasonal Variations in NO 3 Related to Water Level Variation NO 3 at PWS Water Level at OBH

Spikes – Bypass Recharge is a plausible explanation Infiltration recharge drives slow movement of historically leached NO 3 Slow plug flow through the Chalk matrix Rapid flow through Chalk fissures Bypass recharge sends through spikes of this months (?) leaching

Achieved Good Fits for A Number of Catchments Modelled Nitrate in Groundwater including Seasonality & Spikes Modelled Long Term Trend Measured Nitrate in Groundwater Eagle Lodge PWS Good model fit gives confidence to predict future nitrate with WAgriCo Measures

WAgriCo Measures Modelled

Eagle Lodge Forward Prediction of Nitrate at PWS Future leaching with baseline rates

Eagle Lodge Forward Prediction of Nitrate at PWS Future leaching with best case rates Benefits of Measures will be seen in <10-20 years in some catchments

Friar Waddon Forward Prediction of Nitrate at PWS Future leaching with baseline rates

Friar Waddon Forward Prediction of Nitrate at PWS Future leaching with best case rates Benefits will take much longer to be seen in other catchments

Shorter Term Effects – Bypass Flow 10-20% of recharge may be from this week's / month's rainfall This is bypass recharge which happens when rainfall intensity is higher Spikes in nitrate cause problems for many water supplies Bypass recharge can carry other substances such as pesticides Soil nitrate is variable through the year so bypass recharge may carry this variability to the water table A small piece of the modelling work has looked at this and is currently not conclusive, but….

Variability in Soil Nitrate

Effect of Nitrate Vulnerable Zones ? Is this trend less spiky after 2002? NVZ Start

Results Summary Historically leached nitrate dominates the quality of water at the water table for the next years In some catchments, the measures will help make a difference between Wessex Water needing to treat or blend the water In others, the measures will have a longer term benefit Much shorter term (within months) benefits may be achieved by reducing the variability of soil leached nitrate in bypass recharge (further work required). NVZ measures may have already made a difference in some catchments (further work required)