Uncertainty in wellhead protection area delineation due to uncertainty in aquifer parameter values

Abstract

The importance of modeling in hydrogeologic investigations has been stressed in a number of studies. Since the applicability of any model is dependent on the type of need and availability of hydrogeological information, a parametric analysis is essential to determine its applicability. This paper presents the results of a parametric analysis conducted to evaluate the effect of data uncertainty on wellhead protection area (WHPA) delineation. The precision of aquifer parameter values is the most important factor in a WHPA delineation process in relation to the model itself. To demonstrate the effect of variable values in a wellfield, we tested modified version of time-related analytical groundwater flow concept using a RESSQC model, which is used for capture zone delineation and to delineate contaminant front for injection wells. The RESSQC model is a modified version of the RESSQ code which was initially designed to only delineate contaminant fronts for injection wells. Field measured aquifer parameters for a shallow aquifer were used in the analysis. The results indicate that great caution must be taken when aquifer parameters such as hydraulic gradient, thickness, transmissivity and porosity are being used for WHPA delineation, since over or under-protection of WHPA can jeopardize the public health. An extended aquifer test and installation of various observation wells in or near different lithologic and depth zones may increase reliability of parameter values.