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

Abstract

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 1970s to 11.0 mg l−1 NO3−N in 1998. Consequently the catchment of the borehole, covering an area of 400 ha has recently been designated a Nitrate Vulnerable Zone under the EC Nitrate Directive [Commission of European Communities L375, (1991) 1]. The sandstone aquifer that supplies the borehole is recharged by water draining from land that is intensively cropped to green vegetables. There is, therefore, a need to identify appropriate land management 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 abatement is observed as a decrease in well-water concentrations. Estimates of nitrate 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, modified to allow for estimates of denitrification and in-field composting of vegetable crop residues. Integration over the whole catchment using a GIS approach, indicates a steady-state well water [NO3−N] of 23 mg l−1 — a situation that has not yet been reached. Prediction of the time course of change in well water quality from 1970 (when intensification began) has been made by calculating the travel time from different parts of the catchment both in the saturated and unsaturated zones. The results show good agreement between the measurements and simulation. Well water [NO3−N] under potential future management scenarios have also been investigated using the same approach. The greatest reduction in steady-state concentration, to 9 mg l−1, is achieved for the scenario of extensification to spring cereals with moderately fertilised grassland. However, the temporal simulations suggest that it would take approximately 100 years before 80% of this change is observed in the well-water, starting from a concentration of 23 mg l−1.