Description
The Mammoth Meadows groundwater basin provides a municipal water supply to the town of Mammoth Lakes by production from 3 wells. The groundwater basin is characterized by interbedded basalt flows and glacial tills. The basin is recharged by precipitation, flow from a municipally controlled stream, and possibly from groundwater inflow across basin boundaries. Water level contour maps indicate that horizontal groundwater flow is mainly to the northeast at a gradient of 0.02 to 0.05 in the deep aquifers and 0.02 in the shallow aquifers. Vertical hydraulic gradients are upward to neutral in the southwest part of the basin and downward in the northeast area. Aquifer tests were conducted on Mammoth County Water District production wells 1, 6, and 10. Transmissivities were determined to be 2179, 5443, and 7652 feet squared per day for wells 1, 6, and 10 respectively. The corresponding hydraulic conductivities are 12.7, 25.3, and 22.4 feet per day for wells 1, 6, and 10, respectively. The best estimate of storage coefficient is 0. 00093. Of the deep observation wells, only wells 6 and 10 demonstrated mutual hydraulic connection during the aquifer tests despite the fact that the Theis equation predicts drawdown in all of the monitored observation wells. Hydrographs indicate that two of the deep monitoring wells (5 and 11) respond to the production of wells 6 and 10 over longer periods of time. Analytic models, based on the Theis equation were used to determine if basin responses could be explained by simple anisotropy or whether heterogeneities are necessary. The principal direction of anisotropy was assumed to be N58E, parallel to the line between wells 6 and 10. The models indicate that deep well responses in most of the basin may be explained by an anisotropy ratio approaching 100. The models did not explain the responses in the northeastern part of the basin (wells 1 and 14). These wells, which have lower water levels, may be controlled by heterogeneities or low head zones which are not continuous westward to the rest of the basin. Isotropic flow models indicate that, if aquifers are continuous over sufficient distances, springs which supply the local fish hatchery, located 33,000 feet from the production field, could be affected by well production. Similarly, a heterogeneous model which simulated a 200 foot wide channel from wells 6 and 10 to the springs, indicates that the springs could be affected.
