| The purpose of this research was to identify highly probable locations of recharge features to semi-confined aquifers for use in wellhead protection. Geochemical and hydrogeologic data were used in combination with environmental isotopes (tritium and tritium/helium-3), to develop a better understanding of the flow regime at a wellhead through inverse modeling of age distributions. Defining wellhead protection zones for semi-confined aquifers is difficult when the leakage is highly localized and only limited data is available. Typically, very large data sets, which are difficult and expensive to obtain, are required for other techniques. A successful calibration of models developed in this method was achieved with a very limited data set, which is a benefit of the approach. Other methods of delineating wellhead protection areas do not take into account areas where young water may be entering a semi-confined aquifer system, which may lead to ineffective protection zones. The use of environmental tracer signals at a wellhead can aid in the location of recharge features and provide a more realistic wellhead protection strategy.; A test case for a semi-confined aquifer system was developed to determine the number and type of calibration targets necessary for a unique convergence of the lumped parameter model, as well as the type of prior information required. The technique was capable of identifying the location of a recharge source to a well screened in a semi-confined aquifer, provided both tritium and helium-3 data were available. Prior information was required only for the fraction of the submodern water contribution.; The technique was used for three production wells within the Memphis Light, Gas, and Water Sheahan wellfield in Memphis, Tennessee. A common recharge source was identified for all three wells, and a recharge rate ranging between 20.3 and 29.0 feet per year was predicted. It is recommended that the area identified as being the most probable location of a confining unit breach be incorporated within the Zone 1 area, required under the Environmental Protection Agency's Safe Drinking Water Act Amendments of 1986, of the wellhead protection plan for the Sheahan wellfield. |
