Assessing the effects of land use on temporal change in well water qualityin a designated nitrate vulnerable zone

The nitrate concentration in discharge from the Balmalcolm borehole in Fife , Scotland, has steadily increased from 4.5 mg l(-1) NO3--N in the early 19 70s to 11.0 mg l(-1) NO3--N in 1998. Consequently the catchment of the bore hole, covering an area of 400 ha has recently been designated a Nitrate Vul nerable Zone under the EC Nitrate Directive [Commission of European Communi ties L375, (1991) 1]. The sandstone aquifer that supplies the borehole is r echarged by water draining from land that is intensively cropped to green v egetables. There is, therefore, a need to identify appropriate land managem ent techniques that will help to abate the nitrate losses from the land and to estimate the length of time that it is likely to take before the abatem ent is observed as a decrease in well-water concentrations. Estimates of ni trate leaching for the range of crops that have been grown in the catchment over the last 30 years have been made using a balance sheet approach, modi fied to allow for estimates of denitrification and in-field composting of v egetable crop residues. Integration over the whole catchment using a GIS ap proach, indicates a steady-state well water [NO3--N] of 23 mg l(-1) - a sit uation that has not yet been reached. Prediction of the time course of chan ge in well water quality from 1970 (when intensification began) has been ma de by calculating the travel time from different parts of the catchment bot h in the saturated and unsaturated zones. The results show good agreement b etween the measurements and simulation. Well water [NO3--N] under potential future management scenarios have also been investigated using the same app roach. The greatest reduction in steady-state concentration, to 9 mg l(-1), is achieved for the scenario of extensification to spring cereals with mod erately fertilised grassland. However, the temporal simulations suggest tha t it would take approximately 100 years before 80% of this change is observ ed in the well-water, starting from a concentration of 23 mg l(-1). (C) 200 1 Elsevier Science B.V. All rights reserved.