GEOCHEMISTRY AND GRAIN SIZE ANALYSIS OF SEDIMENTS RECOVERED FROM GOLD RAY RESERVOIR AND IMPLICATIONS FOR DAM REMOVAL ON THE ROGUE RIVER, JACKSON COUNTY, OREGON

The abandoned hydroelectric Gold Ray Dam of Jackson County, Oregon forms a 0.1 km² reservoir containing sediments derived from the Upper Rogue River and Bear Creek watersheds that have accumulated over the last 90 years. These sediments may provide insights into land use changes and urban development associated with the Medford-Ashland metropolitan area. Sources of metallic contaminants may include contributions from leaded gasoline, industrial activities, and natural resource extraction.

Two sediment cores, 50 and 70 cm in length, were recovered from Gold Ray Reservoir at water depths of 1.0 to 1.5 m and ~100 meters above the dam. Both sediment cores contained four sand layers with normal grading interbedded with mud; these sand layers may correspond to historic flooding events on the Rogue River, such as the 1964 and 1997 events. Sediments for geochemical analysis were sampled at 3 cm increments from each core and processed using EPA Method 3050B. Concentrations of Al, As, Ca, Cd, Cu, Fe, K, Mg, Na, Pb, and Zn were determined using a Perkin Elmer Optima 2100 Inductively Coupled Plasma Optical Emission Spectrometer. In both sediment cores, metal content for Pb ranged between 13 to 107 mg/kg dry weight, Cu between 354 to 860 mg/kg dry weight, and Zn between 163 to 567 mg/kg dry weight. As and Cd in both cores were below detection limits, with the exception of a mud layer between 57 and 63 cm depth with metal contents of 4.4 mg/kg dry weight for Cd and 5.4 mg/kg dry weight for As. All concentrations in the sediment cores are within the threshold limits for persistent substances as outlined by the EPA. In general, Al, Ca, Fe, K, Mg, Na, and Pb content increased with depth while Cu and Zn decreased with depth in the sediment cores. Cu, Pb, and Zn concentrations are covariant and correlate with grain size, with increased concentrations associated with mud layers. Ca, Fe, K, Mg, and Na concentrations did not correspond to grain size variations in the sediment cores.

The sedimentological and geochemical data collected in this study will be used to develop a comprehensive plan for the removal of Gold Ray Dam. Although the metal concentrations are below EPA limits for contaminated sediments, the disruption during dam removal may result in the remobilization of toxic metals potentially harmful to aquatic organisms downstream, specifically salmonids.