Selenium Information
We currently meet the California Department of Public Health drinking water standard maximum contaminant level (MCL) for selenium. The MCL is set at 50 parts per billion (ppb) and the range is 2.2 to 34 ppb in the intermediate aquifer and is below the reporting limit of 2.0 ppb in the deep aquifer. Selenium is more of an issue on the effluent at the Wastewater Treatment Plant.
Currently the 2007 National Pollutant Discharge Elimination System (NPDES) permit for Davis includes final limitations for selenium of 4.4 ppb monthly average and 7.1/7.2 ppb maximum day (different based on discharge point). We typically only sample once per month so the same result is reported as a daily value and a monthly average. Historically we have had difficulty meeting the final limitation of 4.4 ppb. So we asked for, and received, a Time Schedule Order (TSO) from the Regional Board with a compliance date of 1 February 2015. Our TSO limit is 7.1/7.2 ppb max day. As long as we do not exceed 7.1/7.2 ppb for selenium we are protected from mandatory minimum penalties (MMP) until 1 February 2015.
As of 1 February 2015 we will no longer have protection from MMP’s and if we exceed 4.4 ppb or 7.1/7.2 ppb we will incur an automatic $3,000 fine per violation. This is a minimum penalty that is required by law. The Regional Board can increase the fines above the minimum if circumstances warrant a higher level of penalty. Some circumstances considered include frequency of violations, degree of impact caused by the violation and efforts of the discharger to prevent the violation.
An additional permit requirement is to measure selenium levels in avian eggs taken from the wetlands. If the samples show a geometric mean above 4 ppb we are required to prepare a work plan to reduce the level of selenium in avian eggs. If the geometric mean reaches 8 ppb we are required to immediately cease discharge to the wetlands until we can show that concentrations in the wastewater effluent have been reduced to a level that is protective of wildlife. The 2010 geometric mean for avian eggs was 3.4 ppb.
