Clean Water Program

Analytical techniques are the same as previously described for in-drain sediments.

The techniques and sampling design rationale are identical to those stated for in-drain sediment sampling, with one exception. During periods of relatively low flow, sediment samples will be collected by wading in the sloughs. Personnel will stand downstream of the sampling location, facing upstream, to avoid spreading disturbed sediment over samples as they are collected.

Sediment samples will be collected quarterly.

Sites C, D, E, and F are located in the Salt Slough/Mud Slough system. Site C is located in Mud Slough upstream of the inflow from the SLD, and was selected for this reason. Site D is downstream of the SLD inflow. Influence of drain inflows, presence of a USGS gauging station, and an existing data base of sediment and water chemistry, were rationales for choosing this site. Site E is located on Mud Slough at Highway 140. A previous sampling site near this location was abandoned due to flooding and subsequent inaccessibility. Site F is located in Salt Slough near Highway 165.

Other locations will be sampled to determine whether changes in sediment chemistry are occurring as a result of the diverting of contaminated agricultural drain water through the SLD. Sediment samples will be analyzed for total selenium and TOC.

The in-drain sediment data will provide indication of any trends with respect to accumulation or depletion of selenium in the sediments, as well as distribution within the sediments. Coupled with water quality data from the inlet and outlet (A and B) stations, the sediment data will also help to indicate fate of selenium within the Drain.

The samples will be analyzed for total selenium and TOC by USBR contract laboratory. Selenium will be analyzed by continuous-flow hydride generation atomic absorption spectrophotometry, and TOC by combustion and automated carbon analyzer. All previous sediment data were analyzed by these analytical methods.

Samples at stations in the SLD will consist of three composite samples per sampling location using a precise volume detritus and bed sediment sampler developed by LBNL (Quinn and Clyde, 1996). One sample will be zero to three centimeters, another three to eight centimeters, and the third, greater than eight centimeters. At sampling sites in the sloughs, only the first two depth intervals are collected. During the first sampling episode, both discrete and composite samples will be taken for comparison purposes.

The Monitoring Plan provides for sediment chemistry samples on annual and quarterly sites in the SLD. The purpose of monitoring the annual sites is to provide an intensive sampling of the SLD to provide an indication of whether sediments are accumulating selenium towards hazardous waste criterion levels (minutes of Sediment Task Group meeting, February 22, 1996). The annual sites are located in four reaches of the SLD, with three sites (beginning, mid-point, and end defined by accessibility) chosen to characterize each of four selected reaches of the SLD between the inlet and the terminus.