What are the Biological Effects of Reopening the San Luis Drain?
Historically, farmers in the Grasslands area of the western San Joaquin Valley have discharged subsurface agricultural drainwater through wetland channels in the San Luis National Wildlife Refuge Complex (Refuges) to the San Joaquin River. This drainwater contains extraordinarily elevated concentrations of selenium, boron, chromium, molybdenum, and extremely high concentrations of various salts that disrupt the normal ionic balance of the aquatic system. State health advisories for consumption of fish and waterfowl have been put into effect for the Grasslands area, and warnings not to eat fish have been posted at the Refuges. In 1985, the San Luis Drain was closed due to selenium poisoning of waterbirds at a reservoir in the Kesterson National Wildlife Refuge at the terminus of the Drain. The Drain, constructed by the U.S. Bureau of Reclamation (USBR), had been conceived as the solution to valley-wide problems of disposal of agricultural drainwater. Due to environmental concerns and budget constraints, the Drain had never been completed as originally planned. The constructed portion of the Drain had been used only to convey subsurface agricultural drainwater from the Westland Water District in the western San Joaquin Valley. Farms in the adjacent Grasslands area never used the Drain, but discharged toxic subsurface drainwater through wetland channels in the Refuges to the San Joaquin River. Discharge from Grasslands area farmers continued to contaminate Refuge water delivery channels after the closure of Kesterson Reservoir.
Methods: To address this problem, a proposal to reopen the San Luis Drain and extend it to Mud Slough, a natural waterway in the Refuges, was implemented by the USBR in September 1996 with support from other federal and state agencies. An interagency group is cooperatively monitoring and evaluating the effects of the Project to insure that the Project does not result in a net degradation of the ecosystem, and to determine whether the Project should be continued beyond the first 2 years of operation. The responsibility of the U.. Fish and Wildlife Service (Service) in this program is to collect samples of potentially affected fish and wildlife for analysis of levels of selenium and boron and provide an interpretation of environmental risk of the project within the Refuge and beyond. The primary contaminant of concern in this study is selenium because of the propensity of this element to bioaccumulate in aquatic organisms and because of the well-known history of poisoning and terratogenesis caused by selenium in drain water in this area. Nearly a dozen inorganic constituents in drainage water, however, are of potential toxicological interest, especially boron.
Baseline samples were collected from two sites on Salt Slough, four sites on Mud Slough, two sites in the San Luis Drain, and from one reference site at East Big Lake, which receives little selenium contaminated drainwater. Experience gained in the early years of baseline sampling for this Project led to the identification of four sampling times based on historic water use and drainage practices and on seasonal use of wetland resources by fish and wildlife. They are in November, March, June, and August. Matrices sampled and parameters measured Samples of biota, water, and sediment were collected concurrently at each site and analyzed for selenium and boron. At each site, standard water quality parameters including temperature, dissolved oxygen, pH, and electrical conductance (as specific conductance) were also measured with a Sonde hydrolab. Aquatic specimens, water and sediment were collected at various sites.
Conclusions: Analyses completed to date indicate that selenium concentrations have begun to decline in the fish and invertebrates of Salt Slough, the principal wetlands channel from which drainwater has been removed by the Project. After the Drain began discharging drainwater into Mud Slough, selenium concentrations in the most common fish (mosquitofish and inland silversides) both upstream and downstream of the discharge point rose substantially during the first six months of operation of the Project. These concentrations have more recently declined somewhat, but are still above pre-project levels. Continued biological monitoring is critical to determine whether trends continue downward sufficiently to justify continuation of this Project. Within a month after the reopening of the San Luis Drain to carry Grasslands area agricultural drainwater, the worst affected Refuge ecosystem (Mud Slough) exhibited bioaccumulation of selenium to well above hazardous concentrations. In the last six to nine months, selenium concentrations in the biota have begun to trend downward, but remain sufficiently elevated to be of ecological concern. While drainwater was formerly discharged to Mud Slough it is now the predominant source of water and this dominance persists year round. In the portions of the Refuges that were to benefit from the Project, principally Salt Slough, reductions in selenium have thus far been slight to moderate but statistically and toxicologically significant. It remains to be seen whether the overall effect of the Project on the wetland ecosystems in the area will prove to be significantly negative or positive. Continued monitoring of the biota is essential to a sound evaluation of the project impact. On this assessment rests the fate of the Grasslands Bypass Project, a project which has combined all nonpoint sub-surface drainage sources into one monitorable and manageable source. Within the year this project will be "regulated" by a waste discharge requirement under state law. It tentatively appears that the first year result of the project on selenium loads was to reduce the annual selenium load from 10,000 lbs in 1996 to 7,300 lbs in 1997.
Learn more by reading the following full report: Beckon, W.N. , Henderson,J.D. , Maurer,T.C. , and Schwarzbach, S.E. , Biological Effects of the Reopening of the San Luis Drain (Grasslands Bypass Project) to Carry Subsurface Irrigation Drainwater, USFWS, Div. of Env. Contaminants, Sacramento, CA., Sept. 1997.
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