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

Ten test holes were drilled to obtain hydrogeologic information for an investigation of stream-aquifer interaction near proposed damsites on Cottonwood Creek and South Fork Cottonwood Creek, California. At each site, one deep well was completed below the first confining clay encountered in the upper Tehama Formation to determine hydraulic gradients between water-bearing deposits in the Tehama Formation and overlying channel deposits. At three sites along Cottonwood Creek, two shallow wells were drilled at each site on a line perpendicular to the stream channel to determine if groundwater in channel deposits is moving toward or away from the stream channel and to monitor water levels. Geophysical logs were correlated with lithologic logs compiled from analyses of drill cuttings to determine depths for setting well screens. After pumping to confirm hydraulic connection between each well and the Tehama Formation, water levels were monitored monthly from June 1984 to June 1985; at two sites, water levels were above the altitude of the stream channel bottom during all streamflow conditions. Tritium dating indicates two wells have water more than 100 years old; one well has either a mixture of old and new water or an intermediate-aged water. (USGS)