Numerical Simulation Model for Nitrate Fate and Transport Processes in Groundwater around Lake Naivasha Area, Kenya

Abstract

Contamination of groundwater by nutrients originating from agricultural activities is an international environmental issue. Nitrogen leaches to groundwater in the form of nitrate, which is very mobile with negligible sorption. Field studies of groundwater quality around Lake Naivasha have indicated nitrate contamination from the agricultural activities in the area. In this study a numerical model simulating nitrate fate and transport processes in groundwater at this place was developed. The equation governing chemical fate and transport through a porous media in the saturated zone was used. The hydrologic equation, which combines the mass conservation and Darcy equation, was solved to yield the hydraulic head and consequently the velocity field that was used in the chemical transport and fate equation in accounting for the advection transport process of the chemical. Denitrification was regarded as the sole process of nitrate decay and was modeled by a first-order irreversible reaction. These equations were then discretised using the integrated finite difference method (IFDM). Various modules in solving the algorithm so obtained were implemented in object – oriented computer codes. Model calibration was done using isolated field data sets obtained from empirical results on nitrate concentration levels in wells and borehole water samples around Lake Naivasha area. The model results were quantitatively in agreement with the field results. This proves the usefulness of the model as a possible management tool in monitoring groundwater quality in the area.