Sediment samples have been collected quarterly by USBR since August 1993 for selenium and boron analysis from two monitoring locations within the SLD (Check 2 and Check 10); from each of the primary stations in Mud Slough (Stations C, D, and E); from Salt Slough (Station F), and annually from the backwater site (Station I). Separate samples of the top and bottom sediments were taken at three locations across the channel. The samples taken on the channel transect were composited. After September 1994, separate sediment samples were taken at each of three depth ranges (0 - 30, 30 - 80 and > 80mm) with a precise core sampler designed for submerged sediments to help answer questions about selenium transport within the sediments and selenium bioavailability. Samples were composited for each of the three channel sampling points on the transacts as before.
In September 1994, and again in March 1996, a Sediment Task Group, comprised of GBP Monitoring Plan Team Members, conducted a sediment survey at the upper, middle and lower end of five channel reaches along SLD. The sampling protocol was the same for both surveys, although in the 1996 survey the deepest sample was taken from 80mm to make contact with the SLD lining in order to obtain a total sediment depth and minimize sampling error. In the September 1994 survey, the chosen channel reaches were considered to be representative of the range of conditions observed along the SLD. The reaches were between a. Checks 1 and 2; b. Checks 10 and 11;
c. Checks 15 and 16; and d. Checks 27 and 28. The second survey, completed in March 1996, eliminated the reach between Checks 27 and 28 and added sampling points a. Between Checks 17 and 18, b. upstream of Check 18, and c. at Site B, which is the compliance point on the Grassland Bypass.
Sediment surveys were conducted in the SLD in 1985, 1987, 1995 and, most recently, in 1996. These surveys, together with sediment depth measurements at these and three other stations within the SLD, as well as monitoring of Total Suspended Solids (TSS) in water at Stations A and B, allow for the estimation of selenium and boron movement within the sediments and provide information on sediment deposition and movement. This data will assist in the evaluation of the sediment as an exposure medium for aquatic organisms. Selenium concentrations were measured as total recoverable selenium consistent with previous chemical analysis performed on SLD sediments.
A laboratory experiment to determine mass transfer rates and potential mobilization of selenium from the SLD sediments to fresh water flowing in the drain was conducted by Oleh Weres (1993). Weres' experiment showed that selenium was immobilized within the sediment by geochemical processes, and that soluble species accounted for a small fraction of the selenium in the SLD. Weres hypothesized that a large fraction of boron would be extracted rapidly in the water added to the SLD, but very little selenium. The initial inventory of soluble selenium and boron would be flushed from the SLD in the first full volume discharged, equal to approximately 900 acre-ft, which would require 36 hours to drain at a flow rate of 300 cfs. Weres concluded that after the initial discharge of selenium and boron, the amount of these constituents transferred to the water from the sediments might not be detectable analytically.
The anticipated flow rate in the SLD will not exceed 150 cfs, which is about one half the design flow of the SLD. The Weres experiments used fresh water rather than agricultural drainage water and hence did not measure selenium flux from the water column into the sediments. Loss of selenium to the sediments would reduce the load in the drainage water but increase the inventory in the SLD sediments.
Selenium and boron concentrations in organic detritus were much higher than those in sediments from stations in or near the project area in an experiment conducted by Saiki et al., 1993. Research conducted by LBNL (Quinn et al., 1995) has shown that the organic detritus which appears to accumulate in the stagnant SLD does not build up in a flowing system such as the Agatha Canal. Quarterly sediment samples taken from the Agatha Canal showed no visible organic detrital layer. Sediment size analyses were predominantly silty-clay in composition. A similar sediment size analysis conducted by the USBR Laboratory in Denver on the SLD sediments was inconclusive. The high organic fraction prevented the laboratory from using standard methods. The Laboratory classified the SLD sediment samples as a "muck".