Test holes for monitoring surface-water/ground-water relations in the Cottonwood Creek area, Shasta and Tehama Counties, California, 1984-85

Abstract

This report provides a summary of data collected at 10 test holes drilled to obtain hydrogeologic information for an investigation of stream-aquifer interaction in the area of proposed damsites on Cottonwood Creek and South Fork Cottonwood Creek. Test holes were drilled at three sites adjacent to Cottonwood Creek and one site adjacent to South Fork Cottonwood Creek. At each site, one deep well was completed below the first confining clay encountered in the upper Tehama Formation. These wells ranged from 100 to 185 feet deep, and were constructed of 6-inch diameter casing with 5-to 10-foot screened intervals set in a sand pack. At three sites along Cottonwood Creek, two shallow wells were drilled at each site on a line perpendicular to the stream channel. These wells ranged from 43 to 85 feet deep and were of similar construction. The deep wells were constructed to help determine the hydraulic gradients between the water-bearing deposits in the Tehama Formation and the overlying channel deposits. The shallow wells were constructed to determine if ground water in the channel deposits is moving toward or away from the stream channel and to monitor water levels. A lithologic log of each well was compiled from analyses of drill cuttings. Geophysical logs (natural gamma, spontaneous potential, single-point resistance, and caliper) were correlated with lithologic logs to determine the depths for setting the well screens. Selected samples of drill cuttings were analyzed for grain-size distribution. In addition, selected core samples were analyzed for grain-size distribution and vertical permeabilities. After preliminary pumping at each well site to confirm hydraulic connection between the well and the Tehama Formation, water levels were monitored monthly from June 1984 to June 1985. Water-level data indicate that at two of the four sites the water levels were above the altitude of the stream channel bottom during all streamflow conditions. Results of tritium dating indicate that two wells have water more than 100 years old, and one well has either a mixture of old and new water or an intermediate-aged water.