Sediment and Mercury Distribution in the Dow Wetlands, San Francisco Bay-Delta
The main goal of this study was to assess distribution of sediment sizes and mercury concentrations in the vicinity of Dow Wetlands area and to correlate distribution of sediment sizes and mercury concentrations to different types of depositional environments in this area. The area of study is located to the north of the town of Antioch. Sediment samples were collected from the following sites in the Bay Delta: Dow Wetlands, Brown Island, Sherman Island, Antioch Marina, and Big Break Bay. Sampling was performed in August 2003 and November 2004. Sediment samples were collected both from shore and from a boat using handheld coring devices and an Eckman grab sampler from the upper 10 cm of sediments. The grain size analyses were performed at Moss Landing Marine Laboratory using a laser diffraction grain size analyzer Beckman Coulter Model LS 13 320, and by pipette method at the Department of Geological Sciences at California State University East Bay. The grain size distribution was related to distribution of total mercury concentration. The statistical parameters (mean size, standard deviation, skewness, and kurtosis) were calculated using the method of moment. The samples show characteristics of multi-source (poorly sorted) sediments with pronounced skewness and kurtosis, or large deviation from normal distribution curves. The curves derived from grain size distributions are bimodal to polymodal, which could mean that samples consist of two or more subpopulations with different means and standard deviations. The average size of all samples is approximately 103 μm, which is in the range of very fine sand. The majority of samples are extremely leptokurtic and strongly fine skewed. They have peaks of values within the range of fine sand or coarse silt with significant amounts of fines (silt and clay). The data suggest that samples with larger specific surface areas have higher mercury concentrations due to the increased amount of fine fractions, particularly clay. The average mercury concentration at the area of study is 385 ppb. The highest concentration of mercury occurred in the Dow Wetlands at location S-30 (1929 ppb), which exceeds the Total Maximum Daily Load target for mercury in sediments by almost 10 times. There are three different depositional types of environments in the area of study: the open channel (sandy bottoms with traces of fines and low mercury concentrations), stream banks (sandy and silty bottoms with more clay and higher mercury concentrations), and enclosed areas with low current velocities (dominant fine fractions, particularly clay, and highest mercury concentrations). In general, the data suggest that mercury concentrations are higher in areas with denser vegetation, low water currents, dominant fine sediment fractions, and away from open water.