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 state 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 capture zones computed in a confined to semiconfined aquifer system in an alluvial basin rising semianalytical and two- and three-dimensional numerical ground-water flow models, and evaluates the relative importance df several sources of uncertainty, such as aquifer hydraulic conductivity, aquitard leakance, vertical transit time, hydraulic gradients, transient pumping effects, well interference, and three-dimensional aquifer geometries. A numerical model should be used to delineate 15-year TOT capture zones for wells in confined to semiconfined aquifers in alluvial basins. A semianalytical program may be acceptable for computing 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 zones computed in confined to semiconfined aquifers. Small, circular capture zones are computed in low hydraulic conductivity areas, More complex geometries should be expected where hydraulic conductivities are higher and pumping wells are in close proximity to each other. Aquifers with horizontal hydraulic conductivities that are greater than 1,000 times the vertical hydraulic conductivity of the overlying aquitard are effectively fully confined, and larger capture zones would be computed for these aquifers than for semiconfined aquifers where significant leakage is induced by pumping. In addition, relatively large drawdowns are computed in low hydraulic conductivity areas, resulting in short vertical transit times. Vertical transit times are longer where aquifer hydraulic conductivities are higher.