Evaluation of Specific-storage Values

Calibrated models of aquifer-system compaction developed by Helm (1975, 1976, 1977, 1978) and by Williamson and others (1989) indicate that a narrow range of both aquitard inelastic skeletal specific storage values and aquitard elastic skeletal specific storage values have been used to successfully simulate aquifersystem compaction despite differences in depth, thickness, lithology, and stratigraphy among the San Joaquin Valley sites that were modeled (table 4). The aquitard inelastic skeletal specific storage values derived by Helm (1978) for seven sites in the San Joaquin Valley ranged from about 1.4×10–4 to 6.7×10–4 ft–1, and have a mean of about 3.2×10–4 ft–1 and a standard deviation of about 1.8×10–4 ft–1. The aquitard elastic skeletal specific storage values that Helm (1978) derived ranged from about 2.0×10–6 to 7.5×10–6 ft–1, and have a mean of about 4.5×10–6 ft–1 and a standard deviation of about 2.1×10–6 ft–1. Aquitard elastic and inelastic skeletal specific storage values used by Williamson and others (1989) fall within the ranges of values derived by Helm. The narrow range of aquitard inelastic and elastic skeletal specific storage values derived by Helm (1978) at these seven sites also has been found elsewhere in the United States, including areas of California other than the San Joaquin Valley. For seven sites in the Santa Clara Valley, California, model-derived values of aquitard inelastic skeletal specific storage ranged from 1.4×10–4 to 4.0×10–4 ft–1 (Helm, 1978) and have a mean of 2.8×10–4 ft–1 and a standard deviation of 8.9×10–5 ft–1. The model-derived values of aquitard elastic skeletal specific storage for the seven Santa Clara Valley sites ranged from 2.2×10–6 to 1.6×10–5 ft–1 (Helm, 1978) and have a mean of 6.7×10–6 ft–1 and a standard deviation of 5.2×10–6 ft–1. At an extensometer site in Antelope Valley in the Mojave Desert of southern California, the modelderived value of aquitard inelastic skeletal specific storage for aquitards that were actively compacting inelastically was 3.5×10–4 ft–1 (Sneed and Galloway, 2000), which is strikingly close to the mean value of 3.2×10–4 ft–1 for the 7 sites in San Joaquin Valley. At this same Antelope Valley site, the model-derived value of aquifer-system elastic skeletal specific storage was 1.7×10–6 ft–1 (Sneed and Galloway, 2000). This value is slightly smaller than those reported for the San Joaquin and Santa Clara Valleys, perhaps because the elastic skeletal specific storage value for the Antelope Valley site represents an average value for the aquifer system, rather than an elastic skeletal specific storage value that explicitly represents the aquitard component of the aquifer system.