Modelling non-point sources of nitrate pollution of groundwater in the Great Ouse Chalk, U.K.

Abstract

A numerical distributed transport model, is described which was used to simulate nitrate concentrations in groundwater. The model takes into account the nitrate input from the soil zone and the movement of nitrate in the unsaturated and saturated zones. The model was developed for an area of 600 km 2 of the Cretaceous Chalk, near Cambridge, England. The model area is described in terms of geology, hydrogeology, land use and agricultural practice. The Chalk aquifer was represented by rectangular cells which are divided into an upper and lower layer to represent the unsaturated and saturated zone. The procedure adopted to calculate the nitrate input from the soil zone is outlined. A simplified equation was used to account for the delay for nitrate to pass through the unsaturated zone. Nitrate concentrations in groundwater were determined using a mass-balance approach. The model results in the simulation mode and in the forecast mode are presented and discussed. The sensitivity of the model is examined.