Estimates of hydraulic conductivity from aquifer-test analyses and specific-capacity data, Gulf Coast Regional Aquifer Systems, south-central United States

Abstract

Hydraulic conductivities were estimated from more than 1,500 aquifer-test analyses and more than 5,000 specific-capacity data from wells drilled into Tertiary and younger sediments of the Gulf Coast region in the south-central United States. The values are assumed to represent the coarser-grained sediments in the aquifer systems. The purpose of estimating hydraulic conductivities for this area is to compare these estimates to hydraulic conductivities determined from the simulation of regional groundwater flow as part of the Gulf Coast Regional Aquifer-System Analysis project. In the simulation model, hydraulic conductivities are separated into two groups: coarse-grained sediments (sands) and fine-grained sediments (silts and clays). Values for hydraulic conductivity range from less than 1 foot per day to more than 1,000 feet per day. The values are log normally distributed; thus, the geometric mean was used to represent a typical hydraulic conductivity. The geometric mean hydraulic conductivity for the entire study area was 55 feet per day from aquifer-test analyses and 71 feet per day from specific-capacity data. A two-way analysis of variance was performed on the combined estimates of hydraulic conductivity that were grouped into 10 model layers and 9 areas within the overall study area. Results of this analysis indicate that area, layer, and the interaction of area and layer were all significant in explaining the variation of hydraulic conductivity at a probability level of 0.001. Thus, comparisons of means were done for each area and layer combination. Overall, the highest geometric means generally were in model layer 11 which corresponds to the upper Pleistocene and younger deposits along the coast of the Gulf of Mexico and the alluvium of the Mississippi River. Within each model layer, the geometric mean increased from areas along the western part of the study area to the eastern part, which indicates that the deposits near the Mississippi River might be more permeable than elsewhere. Two separate analysis of covariance were performed on the estimates of hydraulic conductivity to determine if variations within each area and layer combination could be explained by depth of the well or by the thickness of sand beds throughout the perforated interval of the well. Results of these analyses indicate that depth to the middle of the perforated or screened interval was significant at the probability level of 0.02 and that sandbed thickness was not significant at the probability of 0.10. In the analysis with depth, hydraulic conductivity decreased as a function of depth in a majority of area and layer combinations.