Evaluation of Hydraulic Conductivity of Carson County Well Field, Amarillo, Texas

Abstract Environmental assessment, ground‐water management, and aquifer remediation designs rely on comprehension of the hydraulic parameters of aquifers. Because of the heterogeneity of most aquifers, a number of pumping tests are commonly required to provide a reasonable hydraulic parameter distribution. Data for 11 pumping tests, conducted at the Carson County well field, Amarillo, Texas in the 1950s to 1970s were analyzed by the Theis solution and the Cooper and Jacob solution to provide information for optimal ground‐water management. The unconfined aquifer at the well field consists of sands, sandstone, gravels, and clay. A delayed gravity response was observed in the drawdown curves from pumping tests conducted in the unconfined aquifer. Because the Theis solution and the Cooper and Jacob solution do not take this delayed response, or other unsaturated effect into account, these two solutions may overestimate hydraulic conductivity of an unconfined aquifer. Therefore, the Neuman solution which considers the delayed gravity response was used to estimate the hydraulic parameters using the software AQTESOLV (by Geraghty & Miller, Inc., 1990). This paper presents the results of a study of the unconfined aquifer at Carson County well field, Amarillo, Texas. The analysis shows that the results obtained by the Neuman method are more reasonable than those obtained by the Theis solution and the Cooper and Jacob solution. According to results from the Neuman solution, the hydraulic conductivity of the unconfined aquifer of Carson County well field varies from 2.55 to 5.97 × 10 −5 m/s (weighted average hydraulic conductivity is 4.13 × 10 −5 m/s). The small difference of the estimated hydraulic conductivity between wells shows that the unconfined aquifer under Carson County well field is relatively uniform. The effects of infinitesimal borehole assumption, accuracy of individual parameters, skin effects, and spatial distribution of hydraulic conductivity are discussed as well. It is also shown that benefits from obtaining accurate results should be weighted against budget requirements.