Previous research studies have shown that dissolved concentrations of trace elements such as boron and selenium occur at elevated levels in tile drainage water (Presser and Barnes, 1984, 1985; Shelton and Miller, 1988). Selenium, in particular, may accumulate to potentially toxic concentrations in fish and aquatic birds, primarily through bioaccumulation associated with contaminated prey (Ohlendorf et al., 1986; Saiki and Lowe, 1987; Ohlendorf, 1989; Saiki, 1989).  Baseline contaminant information on selenium, boron, and other chemical constituents is available for Mud Slough, Salt Slough, and the San Joaquin River from previous investigations (Saiki, 1985a, 1985b; 1986a, 1986b; Ohlendorf et al., 1987; White et al., 1987, 1988, 1989; Ardans et al., 1988; Saiki and May, 1988; Leland and Scudder, 1990; Saiki and Palawski, 1990; SWRCB, 1991; Saiki et al., 1992, 1993), as well as from ongoing investigations being conducted under the direction of the CVRWQCB as part of the routine water quality monitoring and biological body burden contaminant studies by USFWS and CDFG. Water, sediment, detritus, aquatic plants and invertebrates, and fish samples were analyzed for selenium and/or boron (Saiki, 1985a, 1985b, 1986b; Saiki and Lowe, 1987; Ohlendorf et al., 1987; Saiki and May, 1988; Hothem and Ohlendorf, 1989; Saiki and Palawski, 1990; Schuler et al., 1990; Saiki et al., 1992, 1993).

Biological monitoring programs have been carried out by the USFWS and CDFG since 1992 to ascertain environmental impacts of elevated selenium levels in water. The USFWS has five active biota sampling stations along or near Mud and Salt Sloughs.  Each site was sampled monthly from March through September 1992.  Types of samples taken include crayfish, non-gamefish, immature gamefish and water boatmen. These samples, together with water and sediment samples taken at the same sites, were analyzed for selenium and boron. The three USFWS sampling stations along Mud Slough and Salt Slough correspond to Stations C, D, and F . The USFWS monitoring had been conducted on Mud Slough at Gun Club Road, but this station was moved downstream to Station C at the outflow from S Lake.  The other two USFWS stations on Salt Slough at Wolfsen Road Bridge and on Kesterson National Wildlife Refuge at East Big Lake provide data that might be compared to GBP. Five USFWS sites previously sampled for water, sediment and biota correspond to primary stations or are located nearby (Stations C, D, F, G, and H).

The CDFG conducted quarterly sampling by electrofishing and hoop-netting at five stations along the San Joaquin River and Mud and Salt Sloughs from April 1992 to Spring 1993 as part of the Selenium Monitoring and Evaluation Program (F. Wernette, CDFG, pers. comm.).  The goal of the study was to measure selenium and other trace elements in biota from suspected problem areas and to determine whether these elements occurred at levels harmful to fish and wildlife. The fish species collected included channel catfish, white catfish, green sunfish, bluegill and crayfish. Two stations sampled by CDFG in the past in Mud and Salt Sloughs are sufficiently close to provide background data for proposed Stations D and F. In addition, CDFG has sampled fish from proposed Station E during 1991. The CDFG also has sampled fish and invertebrates from the San Joaquin River upstream of the Merced River confluence and downstream of the Newman Wasteway.

The USGS sampled freshwater clams (Corbicula flumina) at a number of sites within the project area during 1985 (Leland and Scudder, 1990). Several of the USGS sites (listed in Table 2.2) provide background toxicity information for comparative purposes.

The USBR has monitored sites in the vicinity of Mud Slough and the SLD as part of the Kesterson Reservoir Biological Monitoring Program. Samples have been collected each year since 1986 from various sites in the Reservoir and analyzed for selenium. These samples include bird eggs, small mammals, vegetation, invertebrates, and soil.