Modeling of transport of chloride in a sedimentary aquifer using MT3D: the case of a well field in Juazeiro do Norte (CE)

This paper presents a case study in which the solute transport model MT3D, associated with the MODFLOW flow model, was used in the assessment of contamination of an aquifer by surface water from a creek contaminated with urban wastewater, using chloride as a tracer. In that area of the aquifer, groundwater is exploited through a field of wells along the creek. The calibration of the transport model was implemented in order to reproduce the levels of chloride in water from wells observed in monitoring campaigns. The flow model sought to reproduce the hydraulic loads observed during pumping tests conducted in each well. The results obtained from these calibrations were consistent with observations. Based on the calibrated models, we carried out a "future simulation" (prognosis), in order to determine the time required for recovery of groundwater quality after stopping sewage discharge into the creek. Modeling results indicate that: drawdowns produced by pumping the wells are higher in the proximity of the creek; contaminant transport occurs, predominantly, vertically induced by pumping of the wells and by aquifer recharge from the creek; elevated levels of chloride in the waters from the wells may return to natural values in about one year, if the release of sewage into the creek is stopped. In this context, in order to prevent further damage to the aquifer area through persistent pollution, sewage must have a suitable destination and the area of permanent protection of the creek must be respected.

groundwater; transport of polluting; MODFLOW; MT3D