Engineering Natural Filtration Systems to Remove Selenium, Nitrate, and Bacteria from Impaired Surface Waters — Foundational Studies

Selenium is among the pollutants of concern in a growing number of areas within California. TMDLs proposed by USEPA (USEPA, 2002) for the San Diego Creek watershed in Orange County, California (and in Ballona Creek and Los Angeles River in Los Angeles County) calls for an in-stream standard of 5 ppb total selenium. Selenium sources and treatment methods, however, are significantly different than those typically utilized as stormwater BMPs for metals such as copper, lead and zinc. Studies on the sources of selenium in the San Diego Creek Watershed indicated that a major potential source is groundwater seepage, particularly in areas where the groundwater table is shallow. Thus dry-weather concentrations and loads are of the most interest. Within the San Diego Creek Watershed, flows appear to correspond to a location where historically, naturally occurring selenium became sequestered under anaerobic conditions in the peat soils within a swamp. Urbanization and draining of the swamp appears to have created aerobic conditions, releasing selenium from these sediments into groundwater in a soluble phase. Selenium concentrations in the surface water discharge have been observed in the 50 ppb range, and local groundwater selenium concentrations commonly exceed 100 ppb. Several technologies potentially reduce selenium (Se) concentrations to low levels (