COMPARISON OF MODELS FOR DELINEATING WELLHEAD PROTECTION AREAS IN CONFINED TO SEMICONFINED AQUIFERS IN ALLUVIAL BASINS

The 1986 Safe Drinking Water Act Amendments include provisions for sta te wellhead protection (WHP) programs that address wellhead protection areas (WHPAs). In many states, WHPAs are delineated based on time-of- travel (TOT) criteria. This study compares 250-day and 15-year TOT cap ture zones computed in a confined to semiconfined aquifer system in an alluvial basin rising semianalytical and two- and three-dimensional n umerical ground-water flow models, and evaluates the relative importan ce df several sources of uncertainty, such as aquifer hydraulic conduc tivity, aquitard leakance, vertical transit time, hydraulic gradients, transient pumping effects, well interference, and three-dimensional a quifer geometries. A numerical model should be used to delineate 15-ye ar TOT capture zones for wells in confined to semiconfined aquifers in alluvial basins. A semianalytical program may be acceptable for compu ting the 250-day TOT capture zones; however, such codes can be applied only under a very narrow range of conditions. Hydraulic conductivity plays a critical role in controlling the sizes and shapes of capture z ones computed in confined to semiconfined aquifers. Small, circular ca pture zones are computed in low hydraulic conductivity areas, More com plex geometries should be expected where hydraulic conductivities are higher and pumping wells are in close proximity to each other. Aquifer s with horizontal hydraulic conductivities that are greater than 1,000 times the vertical hydraulic conductivity of the overlying aquitard a re effectively fully confined, and larger capture zones would be compu ted for these aquifers than for semiconfined aquifers where significan t leakage is induced by pumping. In addition, relatively large drawdow ns are computed in low hydraulic conductivity areas, resulting in shor t vertical transit times. Vertical transit times are longer where aqui fer hydraulic conductivities are higher.