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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

Introduction

The purpose of the Keswick Reservoir project was to map the extent and depositional characteristics of fine-grained sediments deposited in association with acid-mine drainage that flows into Keswick Reservoir, on the Sacramento River near Redding, California (Figures 1 and 2). The reservoir receives drainage from the Iron Mountain mine area with elevated concentrations of iron, aluminum, copper, and other metals (Alpers and others, 2003). The dissolved metals are precipitated out as hydrous iron and aluminum oxides when the acid water of Spring Creek mixes with neutral water in Keswick Reservoir (Nordstrom and others, 1999). During times of high water flow, the fine-grained nature of the sediments makes them likely to be easily resuspended and moved further out into the reservoir or into the Sacramento River. High water flow in this area may result from high rates of water release from the upstream Shasta Dam, from the Spring Creek Debris Dam at the western end of Spring Creek Arm, or from the Whiskeytown Reservoir via the Spring Creek Power Plant, which discharges into the western end of the Arm below the debris dam (Figure 1). The resuspended metal-rich sediments, since they contain elevated concentrations of copper, zinc, and cadmium, could form a hazard to aquatic life in the Keswick Reservoir or the Sacramento River downstream of Keswick Dam.

The Iron Mountain area is the locus of an extensive array of cleanup activities by the U.S. Environmental Protection Agency (USEPA) under its Superfund Program. Between 1986 and 1997, the USEPA issued four Records of Decision (USEPA 1986, 1992, 1993, 1997) that have instituted remedial activities that include partial capping, surface-water diversions, tailings removal, and lime neutralization treatment of the most acidic, metal-rich flows, reducing copper and zinc loads in Spring Creek by 80% to 95%. In 2004, the USEPA issued a fifth Record of Decision (USEPA, 2004), addressing the contaminated sediments in the Spring Creek Arm of Keswick Reservoir. The selected remedy involves dredging, hydraulic removal, and chemical treatment of more than 120,000 cubic meters of sediment and placement of the treated, dredged material in an engineered disposal cell upstream of the Spring Creek Debris Dam (Figure 1).

Prior to 1993, the fine-grained, metal-rich sediments were known to be present in the Spring Creek Arm and in Keswick Reservoir; however, the thickness and extent of these sediments were unknown. In February 1993, the USGS undertook an experimental geophysical program to see if seismic-reflection data could be used to map the distribution and thickness of these sediments. We acquired high-resolution seismic-reflection data in the 1 km long Spring Creek Arm of the reservoir and in the main Keswick Reservoir from about 1.5 km above the Spring Creek Arm to the log boom near Keswick Dam 2 km below the arm (Figures 3A-E). These data show the distribution of the fine-grained sediments deposited in the reservoir, and have been used to construct maps of sediment location and thickness that are presented in this report.