Comparisons of Hydrogeologic Modeling Methods to Define Capture Zones for Public Water Supply Wells in Northern Arkansas

The usefulness, applicability, and practicality of more complex and resource consuming methods for groundwater modeling has been in question since computer based groundwater modeling was established (Anderson, 1992). In many situations, computer modeling of groundwater flow is a necessity and useful for extrapolating data where none exists or it is impossible or impractical to acquire. However, when delineating a recharge area around a public water well for protection purposes, it is unknown if more detailed computer modeling results are better than simpler hydrologic calculations and site study. In the case of public drinking water supply wells located in various aquifers and with differing hydrodynamic processes, it may be useful to examine specific supply wells that have reasonable data. Then utilize a variety of modeling methods to fully analyze well hydrodynamics. By utilizing a variety of models for a few wells that have the best available hydrologic data, it can be determined whether complex and in depth modeling methods are warranted. This more specific information can then be extrapolated to other similar water supply wells to provide the most practical and economical methodology for groundwater modeling. It was discovered in this study that computer modeling did prove to be useful and effective when surface water influence of the water supply well may be occurring. The computer model provided detailed information on how the aquifer responds when a pumping supply well is present in close proximity to a surface water body. The computer modeling was also able to indicate that a water supply well was not under influence of a nearby surface water body which is equally important when capture zones are being established for protection purposes. Computer modeling of deeper wells, without the potential of surface water influence, proved less useful. Results from computer modeling and analytical models were highly varied, even when similar input values were utilized. Results were more questionable and less accurate in determining proper capture zones. The delineation of capture zones can be established fairly well utilizing analytical models. Some analytical model results did give a much smaller radius of influence than was determined by the computer model. However, it ultimately falls to the regulatory agencies to determine what extent of protection for a wellhead is deemed appropriate and feasible