Abstract
This study examined the depositional history of the Cache Creek Settling Basin, a sediment retention structure located at the modern terminus of Cache Creek that functions as a seasonal wetland. The settling basin receives sediment from areas of historical mercury mining in the Coast Ranges and is situated in a regional trough that historically received flood waters from the Sacramento and Feather rivers. In 1920 levees were constructed to direct floodwaters from the Sacramento and Feather rivers into the Yolo Bypass, and additional levees were constructed in 1937 forming the Cache Creek Settling Basin. In the post-1937 configuration, the Cache Creek Settling Basin receives inflow and sediment from only Cache Creek. Prior to this study, it was not known if the area that is now the settling basin also received inputs from the Sacramento River or the Feather River prior to levee construction. As a consequence, the historic fluvial setting within the basin and the provenance of historical basin sediments was not well understood. This study evaluated the sediment horizons corresponding to the 1920 and 1937 levee construction as well as the horizon corresponding to the onset of modern mercury mining, which occurred at approximately 1850. The objectives of this study were to (1) use legacy mercury contamination as a stratigraphic marker to establish a sediment horizon representing the onset of large-scale mercury and gold mining; (2) determine whether depositional changes occurred in the Cache Creek Settling Basin in response to anthropogenic changes related to mercury mining, the construction of the Yolo Bypass or the construction of the Cache Creek Settling Basin; (3) develop spatial linkages between basin deposits and contemporary fluvial sediment from three potential source areas; (4) characterize the depositional history of the Cache Creek Settling Basin by determining the provenance of pre-mining fine-grained sediment. Sixty-eight subsamples were collected from three sediment cores obtained during a 2011 and 2012 coring campaign by the U.S. Geological Survey; the samples were analyzed for major and trace element concentration, grain size distribution, organic content, total mercury, and mineralogy. Additionally, nine samples of recently deposited streambed sediment were collected from the upstream reaches of the Sacramento River, the Feather River, and Cache Creek. These samples were analyzed for the same constituents as the core samples. These data were used to develop a composite set of geochemical tracers that could link sediment from the settling basin to one of the potential source locations. Based on results from core subsamples, notable depositional changes took place within the basin in response to mercury mining and the construction of the Yolo Bypass. Significant changes in deposition include an increase in the concentration of many constituents in the post-mining horizon, including statistically significant increases in Ba, Ca, Cd, P, Pb, S, Sb, Se, and Sn. In addition, there is a significant decrease in the concentration of most constituents as well as a notable increase in grain size at the 1920 horizon. The concentrations of Ca, Cd, B, Li, Mg, and Ni, were used to create a composite sediment fingerprint which linked the target sediment in the Cache Creek Settling Basin to contemporary fluvial material in the Cache Creek watershed.