Modelling flow and nitrate transport in groundwater for the prediction of water travel times and of consequences of land use evolution on water quality

Abstract

AbstractA two‐dimensional model representing flow and nitrate transport in groundwater was developed and applied to a hillslope of the Kervidy catchment. The objective of the modelling was: (1) to characterize better the flow and nitrate transport in the groundwater and to determine the characteristic times of the system; (2) to explore the consequences of changes in nitrate leaching to groundwater on nitrate concentration in stream water. The finite‐difference code MODFLOW was used to simulate the distribution of hydraulic head within the groundwater. Nitrate transport was described by the convection equation solved using MT3D. MODPATH was also used to analyse flow paths and travel times in the groundwater. Four units were considered in the model: (i) the plough layer, (ii) the soil, (iii) the weathered shale and (iv) the fissured shale. Autotrophic denitrification in the shale and partly in the weathered shale was represented, as well as heterotrophic denitrification in the upper horizon of bottom land. A steady‐state average flow was assumed with a spatially uniform groundwater recharge of 2 mm day−1, corresponding to the winter mean daily recharge observed in the Kervidy catchment. Nitrate recharge rate was fixed at 100 mg l−1, which is equivalent to a nitrogen flux of 165 kg ha−1 year−1. Six scenarios of nitrate leaching changes were analysed with the model. The first two correspond to spatially uniform decreases of the nitrate recharge rate to 80 and 60 mg l−1, respectively. In the other four scenarios, nitrate recharge rate was spatially distributed along the hillslope so that the average nitrogen flux remained equal to 165 kg ha−1 year−1. Simulated hydraulic heads were similar to observed values along the hillslope, except for the summit. The transport model reproduced the spatial pattern of nitrate concentrations observed in the weathered shale groundwater, a deviation appearing only in the nearest piezometer to the stream. Travel times within the groundwater appeared to be highly variable, from a few days up to 3 years. Scenario analysis showed that a significant decrease of stream nitrate concentration can be expected following a global decrease in nitrate leaching along the hillslope. However, the fall could be very gradual in time. Copyright © 2002 John Wiley & Sons, Ltd.