USING MODFLOW AS AN ANALYTIC ELEMENT PREPROCESSOR

When beginning a new groundwater modeling project, it is common to discover that the area of interest has been modeled as part of a previous project. Our firm often wishes to make use of models that were developed by the U.S. Geological Survey or a state or local agency. Because existing models document the substantial efforts and insights derived from the study of groundwater flow systems, it is prudent to make use of them where possible as a starting point for new modeling.

The author has developed a software tool that constructs local analytic element models from regional MODFLOW models. The preprocessor is analogous to a traditional "telescoping mesh"; however, local analytic element models provide advantages over local MODFLOW models for some problems. This strategy is particularly useful when the region to be modeled is large, but accurate velocities are required in particular regions, e.g. near wells and streams.

This strategy was successfully applied to the problem of wellhead protection area delineation for 549 wells in the Treasure Valley of southwestern Idaho. Two MODFLOW models were available, but their large cell sizes made it impossible to compute accurate streamlines using MODPATH. Boundary conditions, aquifer properties, and perimeter flux values for over 30 analytic element "subregions" were extracted from MODFLOW, using the custom software tool. The analytic element models were then used to trace accurate streamlines and to delineate wellhead protection areas. This strategy made it possible to efficiently delineate wellhead protection areas while making best use of hydrogeological knowledge derived from the existing modeling studies.