Boron Contamination of Waterfowl at the Salton Sea and Implications for Avian Impacts
This study focused on the potential for toxicity and reproductive impacts from boron on waterfowl at the Salton Sea. It was prompted by the fact that boron was identified as a contaminant of concern for wildlife under the Department of the Interior's National Irrigation Water Quality Program's Reconnaissance Investigation, which found increased boron concentrations in ruddy duck livers during the winter. Sediments and vegetation samples were collected from three sites to evaluate boron accumulation in water of different types: 1) The Alamo River which is a mixture of water from several sources including Colorado River water from irrigation canals, subsurface irrigation drainage, and surface run-off from agriculture and other sources; 2) The Bruchard drain which carries predominantly subsurface drainage and surface run-off from agriculture; and 3) The Hazard 6 pond which receives Colorado River water purchased from the irrigation district and delivered through the agricultural delivery system. Three sediment samples were collected from each of the three sites. They were collected in October and December of 1991 and February of 1992. Vegetation samples of Alkali bulrush and aquatic invertebrates were collected from the same sites. Waterfowl (snow geese, northern shovelers, northern pintails, and ruddy ducks) were also collected at several locations around the south end of the Salton Sea beginning in February 1991 to April 1992.
Results: The highest mean boron concentration in sediments was found in the Hazard 6 pond while the least was found in Bruchard drain. This was probably due to the fact that the Hazard 6 pond is a very low-flow system, so when water evaporates, the concentration of boron increases. At all three sites, the highest concentrations were found in October and the lowest occurred in December. This is probably due to evaporation rates which would be highest in October and lowest in December. The same temporal pattern was observed in the bulrushes, invertebrates, and waterfowl.
Discussion: The results indicate that overall boron concentrations at the Salton Sea are relatively low. The highest sediment concentration measured was well within the range of naturally occurring boron in soils. From the perspective of vegetation as a food item for waterfowl, there may be cause for concern. It has been found that dietary concentrations similar to the high concentrations found in the Salton Sea do cause reductions of duckling weight gain. However, boron is eliminated quickly and would pose little reproductive risk to species that only winter at the Salton Sea such as snow geese and northern shovelers. Overall the risk presented by boron to waterfowl wintering at the Salton Sea appears to be low, so efforts to control waterfowl exposure to boron do not appear to be necessary. Based on the dietary concentrations measured in vegetation and invertebrate samples, boron does not appear to pose a major risk for resident or breeding bird species at the Salton Sea at this time. However, analysis of eggs for their boron concentrations would be needed to confirm this. The large scale water conservation currently being planned may increase boron concentrations throughout the system and at all levels of the food chain, so this should be monitored and addressed as appropriate through future management actions in the Salton Sea system.
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