COMPARISONS OF HYDROGEOLOGIC MODELING METHODS TO DEFINE CAPTURE ZONES FOR PUBLIC WATER SUPPLY WELLS IN NORTHERN ARKANSAS

The usefulness, applicability, and practicality of complex and resource consuming methods for groundwater modeling have been in question since the establishment of computer based groundwater modeling. In many situations, computer modeling of groundwater flow is a necessity and useful for extrapolating data where none exists, or it is impossible to acquire. However, when delineating a capture zone around a public water supply well for protection purposes, it is questionable whether more detailed computer modeling yields more appropriate results than simpler analytical modeling and site specific study of the groundwater hydrology. Two water systems in Northern Arkansas were selected for site study and comparison of capture zone analysis methodology. By modeling a few wells that have appropriate hydrologic data in addition to wells which are lacking data, and comparing model results, the potential extended benefits of numerical modeling were assessed. Numeric modeling did prove to be useful and effective when the wells were influenced by sources of surface water. In this situation, numeric models provided detailed information on aquifer response and provided the quantity of water withdrawn from various sources, in addition to establishing a physical connection to the surface water body which necessitates modification of the capture zone for wellhead protection. Numeric modeling was also useful in visually demonstrating that smaller wellhead protection zones arbitrarily established may be highly inadequate. Numeric modeling of deeper wells (600 feet deep) without the potential of surface water influence, proved less useful. The delineation of capture zones using analytical methods was quick, required minimal data, and resulted in a comparable output for several of the non-surface influenced wells. However, in several instances the analytical models resulted in smaller radius of influence than was determined by the numeric model. Ultimately, the appropriate and feasible extents of protection zone delineations are the responsibility of regulatory agencies, which are responsible for ensuring public health and safety.