USING ANALYTIC ELEMENT MODELS TO DELINEATE DRINKING WATER SOURCE PROTECTION AREAS IN OHIO

The 1996 Amendments to the Safe Drinking Water Act require the delineation of drinking water source protection areas for all public water systems by May, 2003. The Amendments also specify that delineations should be completed with existing data. Ohio EPA wanted to select a delineation method that was technically defensible, but also attainable given the time and resource constraints (Ohio has > 6,000 public water systems). The most commonly used delineation programs, WHPA and MODFLOW, were impractical since WHPA is not capable of simulating many of Ohio's more complex aquifers and the information necessary to develop an appropriate MODFLOW model is unavailable for most of the state. Ohio EPA decided that analytic element modeling, specifically GFLOW, was the best delineation option. GFLOW has an easy to use Graphical User Interface, GIS compatibility, and the ability to model the inhomogeneities common in Ohio's complex buried valley aquifers. In addition, the lack of a model grid eliminates much of the guesswork of determining appropriate model size, and makes expanding the modeled area much simpler. Many Ohio EPA staff, however, were skeptical about analytic element models and did not believe GFLOW could provide an accurate delineation. To evaluate GFLOW's effectiveness, Ohio EPA compared delineations completed by consultants using other delineation methods with models Ohio EPA staff created using GFLOW. In many cases the GFLOW models were created for the same public water system by multiple staff to determine the potential variability of modeling results. The results of the analysis changed many opinions, as the GFLOW-modeled protection areas often closely resembled the delineations completed using other methods. In one case, the GFLOW model was considered to produce a more appropriate protection area, since the model grid for the MODFLOW model was too small and did not encompass the entire GFLOW-derived protection area. Ohio EPA has utilized GFLOW to successfully delineate protection areas for hundreds of public water systems and will continue to use GFLOW for future modeling in Ohio.