EFFECTS OF IMPROPER CHARACTERIZATION OF AQUIFER THICKNESS ON ESTIMATES OF HYDRAULIC CONDUCTIVITY FROM PUMPING TESTS

Pumping test data for surficial aquifers are commonly analyzed under t he assumption that the base of the aquifer corresponds to the bottom o f the test wells (i.e., the aquifer is truncated). This practice can l ead to inaccurate hydraulic conductivity estimates, resulting from the use of low saturated thickness values with transmissivity estimates, and not accounting for the effects of partially penetrating wells. The oretical time-drawdown data were generated at an observation well in a hypothetical unconfined aquifer for various values of saturated thick ness and were analyzed by standard curve-matching techniques. The base of the aquifer was assumed to be the bottom of the pumping and observ ation wells. The overestimation of horizontal hydraulic conductivity w as found to be directly proportional to the error in assumed saturated thickness, and to the (actual) ratio of vertical to horizontal hydrau lic conductivity (K(v)/K(h)). Inaccurately high estimates of hydraulic conductivity obtained by aquifer truncation can lead to overestimates of ground water velocity and contaminant plume spreading, narrow capt ure zone configuration estimates, and overestimates of available groun d water resources.