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Wellhead Protection Area Delineation (459 Wells)

Idaho Department of Environmental Quality

WHPA delineated 3, 6 and 10 year time-of-travel capture zones for 459 wells located in a 1,500 square mile region of southwestern Idaho. The purpose of the project was to develop wellhead protection areas for Idaho Department of Environmental Quality.

Our colleagues at Barr Engineering constructed a hybrid MODFLOW model based on two previous models of the region and updated it to reflect current conditions in the area. Because of the large grid cells in those models, it was not possible to use the MODFLOW based MODPATH for the delineations.

WHPA made use of analytic element (AEM) models, using boundary conditions derived from the MODFLOW model. The advantage of this strategy was it allowed for accurate pathline tracing, while including all the information in the original model. ModAEM, developed by Vic Kelson of WHPA, was used. ModAEM was the computational heart of the EPA WhAEM for Windows wellhead protection model, and was well-suited to this task. Enhancements were made to ModAEM and a set of tools were developed which automated the process of executing the delineation runs and postprocessing the results.

All the delineations were performed in AEM 'subdomains' of the regional MODFLOW model. The modeler uses an automated script to create each subdomain, using GIS to specify the region of interest. The use of subdomains was necessary because the size and complexity of an AEM version of the complete MODFLOW domain would have resulted in extremely long runtimes for the AEM model. The locations of the subdomains were chosen in a manner that balanced the runtimes for models and the number of subdomain models required.