Nitrate transport modeling to evaluate source water protection scenarios for a municipal well in an agricultural area

Abstract

Fertilizers that are spread on agricultural fields can leach into aquifers and contaminate groundwater sources for drinking water particularly with nitrate. Modeling this phenomenon can help in evaluating the impact of current or future agricultural practices on nitrate content within an aquifer. The three-dimensional Water flow and Nitrate transport Global Model ( WNGM), that was previously developed and applied to a well-capture zone, is actually used to simulate future land management scenarios over the same zone. The studied area is located in a region with intensive agricultural practice in Southern Québec (Canada). The sandy unconfined aquifer underlying the agricultural fields is highly vulnerable to nitrate contamination from ground surface and nitrate concentrations around critical threshold of 45 mg NO 3 - /L were observed in the municipal well. Using the calibrated WNGM, a number of land management scenarios were simulated in order to predict future trends of nitrate concentrations in the pumping well from 2008 through 2040 that could be induced by changes in land management practices over this well-capture zone. More specifically, these scenarios simulate implementation of improved or ideal agricultural practices within the agricultural portion of the well-capture zone, and modification of the pumping rate at the municipal well. The results show how a decrease and stabilization of nitrate concentration should occur at the municipal well after a period varying from 5 to 30 years depending on the implemented agricultural modifications. It is also shown that, in this particular case, an increase of the pumping rate at the municipal well from 700 to 2800 m 3/d would lead to an increase of the well-capture zone from 0.8 to 2.5 km 2 and to a decrease of nitrate concentrations at the municipal well from 43 to 34 mg NO 3 - /L. Furthermore, it is predicted that application of improved agricultural practices in the vicinity of this municipal well could lead to a reduction of nitrate concentrations at the well of about 8 mg NO 3 - /L.