Summary

Description

This report discusses the affects of irrigation induced contaminants on fish, wildlife, and human health near Stillwater Wildlife Management Area. In 1988, the U.S. Geological Survey and the U.S. Fish and Wildlife Service initiated detailed investigations of the extent of irrigation induced contaminants affecting fish, wildlife, and human health in and near Stillwater Wildlife Management Area, in the Carson River drainage basin in west central Nevada. Study objectives included the determination of the extent of trace element contamination and wetland deterioration, the definition of contaminant pathways, and the identification of contaminant source areas, in preparation for remediation. Approximately 10 percent of historic wetlands at the terminus of the Carson River remain and the quality of water now reaching the wetland has been degraded. Concentrations of dissolved solids in marsh inflow water is approximately 5 times greater than is believed to have existed prior to agricultural diversion and water consumption beginning in the late 1800s. Degraded water quality has contributed to decreases in the abundance and diversity of wetland vegetation, fish, and wetland dependent wildlife. Acute toxicity was shown in water of some agricultural drains and wetlands through bioassays using fish and invertebrates. Toxicity was attributed to a combination of arsenic, boron, lithium, and molybdenum. Rapidly fluctuating levels of dissolved solid and ionic composition may also be contributing to additional stress to aquatic organisms. Evidence suggests that mercury and selenium are bioaccumulated and biomagnified. Selenium, mobilized by agricultural activities, is transported to wetlands within organic material through the agricultural drains, particularly during peak discharge events. The selenium hazard to migratory birds was greatest in wetlands receiving plant and invertebrate drift directly from drains. However, evidence linking migratory birds mortality in the study area to selenosis was not found, and selenium concentrations in wetland biota were not cumulative in response to decades of irrigation drainage. High levels of mercury, introduced in the late 1800s by mining activities, were found at sites associated with the historic floodways of the Carson River. Mercury continues to bioaccumulate in organisms from these previously contaminated areas. Although waterfowl production was reduced in the study area, contaminants did not appear to effect hatching success. Concentrations of potentially toxic trace elements found in waterfowl eggs were below those reported to adversely affect development or hatching. Likewise, no evidence of teratogenesis in migratory bird embryos or juveniles was found. Poor waterfowl production was attributed to extended drought conditions, absence of water, sparse nesting vegetation, and predation. Mercury and selenium levels in tissues of adult waterfowl harvested in the study area were sufficient to warrant human health concerns. Levels varied among waterfowl species with highest levels found in shoveler ducks, where mercury levels were great enough to warrant a Nevada State Health Advisory for the consumption of shovelers taken at Carson Lake. Evidence was found that waterfowl are accumulating mercury and selenium within the wetlands.