GROUND WATER MONITORING FOR WELLHEAD PROTECTION, CASE STUDY IN A CONFINED TO SEMI-CONFINED AQUIFER, SALT LAKE COUNTY, UTAH

The 1986 Safe Drinking Water Act Amendments required states to develop wellhead protection (WHP) programs and wellhead protection areas (WHPAs). WHPAs often are delineated based on time-of-travel (TOT) criteria, usually using analytical or numerical models based on measured or assumed values for the hydraulic parameters. This study compares 250-day and 15-year TOT capture zones computed in a confined to semi-confined aquifer system in an alluvial basin using semi-analytical and two- and three-dimensional numerical ground water flow models, and evaluates the sensitivity of the models to various hydraulic parameters.

A numerical model should be used to delineate 15-year TOT capture zones for wells in confined to semi-confined aquifers in alluvial basins. A semi-analytical program may be acceptable for computing 250-day TOT capture zones, but 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 semi-confined aquifers.

Chemical data can be quite useful in confirming the modeling results. The major ion, C-13, and O-18 and D chemistries of water samples collected from two monitor wells installed for this study at a target well site are virtually identical to those found in all but two of the 14 other wells and one surface stream sampled. Carbon-14 dating of samples from the two monitor wells yielded age estimates of between 1,300 and 5,900 years, much older than of concern in a WHP study. Tritium values suggest that where tritium levels are low, the hydraulic conductivity of the aquifer probably is low, and where tritium levels are high, the hydraulic conductivity probably also is high. Thus, one sample from each well probably should be analyzed for tritium during the reconnaissance phase of every WHP investigation to identify wells that may be in direct communication with overlying or adjacent unconfined aquifers.