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Distribution, thickness, and volume of fine-grained sediment from precipitation of metals from acid-mine waters in Keswick Reservoir, Shasta County, California

Summary

High-resolution seismic-reflection data acquired in 1993 from Keswick Reservoir and its Spring Creek Arm show the thickness and distribution of fine-grained sediments in the reservoir. Three sediment accumulations are present in Spring Creek Arm. The western accumulation had a maximum sediment thickness greater than 2 m, and contained an estimated 14,000 cubic meters of sediment. The central accumulation had greater than 5 m of sediment thickness and contained an estimated 32,000 cubic meters of sediment. The eastern accumulation had sediment thickness of more than 8 m, and contained an estimated sediment volume of 105,000 cubic meters. The total volume of fine-grained sediment in the Spring Creek Arm related to precipitation of metals from acid-mine drainage was thus about 152,000 cubic meters. Sampling has shown that this sediment is composed predominantly (>90%) of chemically precipitated, metal-rich sediment with some thin (<1 cm thick) sand layers.

Additional fine-grained sediment derived from acid-mine drainage was seismically mapped in the main channel of Keswick Reservoir, with a maximum thickness of 3 m adjacent to the Spring Creek Arm, diminishing down-reservoir to less than 1 m thickness over much of the lower reservoir. This sediment has been sampled in the old Sacramento River channel near the Spring Creek Arm, and also consists dominantly of precipitated sediment, although more and thicker sand layers are present. The estimated volume of this material in the main channel based on the 1993 survey is 110,000 cubic meters. The total volume of fine-grained sediment in Keswick Reservoir and the Spring Creek Arm is therefore estimated to have been about 260,000 cubic meters at the time of the USGS survey in February 1993.

The reservoir bottom area in the Spring Creek Arm was presumably completely covered with acid-mine drainage sediments. In Keswick Reservoir, 42% to 50% of the reservoir area contiguous to Spring Creek Arm had mappable fine-grained sediment, decreasing to about 35% in the area to the south, and to about 12% near the dam. Substantially larger areas of the bottom could have had a thin, acid-mine drainage sediment cover that is not mappable using the seismic-reflection data, and probably much or most of the reservoir bottom was thinly covered.