Kesterson Reservoir: 30 Years of Selenium Risk Assessment and Management

Severe effects of selenium (Se) occurred among birds feeding and nesting at Kesterson Reservoir (San Joaquin Valley, California, USA) in 1983 to 1985. This paper describes the integration of site monitoring, risk assessment, and management actions conducted after the effects of Se were discovered. Selenium contamination of the site occurred over just a few years, but actions to resolve the contamination issues required >20 y. The reservoir, a series of 12 ponds totaling about 1280 acres (518 hectares), served for storage and evaporation of subsurface agricultural drainage. Selenium concentrations in reservoir inflow in 1983 were about 300 µg/L, primarily as selenate; within the ponds it was biogeochemically reduced to other inorganic and organic forms and bioaccumulated by biota or deposited to sediments. An estimated 9000 kg of Se were delivered to Kesterson in 1981-1986. A 1985 order required cleanup and abatement of the reservoir, so the United States Bureau of Reclamation and the US Department of the Interior undertook actions and studies to reduce hazards to birds. In 1988, about 1 million cubic yards (764 500 m³ ) of soil were used to fill portions of the reservoir, transforming it into terrestrial habitat. Intensive monitoring began in 1989 to assess the impact of the reservoir on wildlife, provide a basis for adjusting site management, verify the effectiveness of cleanup actions, and provide a basis for modifying future monitoring. Monitoring continued until 2014, with modifications and management actions based on results of 2 risk assessments (1993 and 2000). Monitoring results in 2013-2014 showed that Se concentrations were relatively stable over time and risks to wildlife were low. From the initial problem discovery to the conclusion of actions taken to remediate the site, combining responsive, reactive, and adaptive monitoring; modeling; risk assessment; and mitigation actions proved effective in solving the problem so that risks to wildlife were reduced to minimal levels. Integr Environ Assess Manag 2020;16:257-268. © 2019 SETAC.