Abstract: Selenium Remobilization Due to Destruction of Wetlands in the Irvine Subbasin, Orange County, California

HIBBS, BARRY, California State University, Los Angeles, Los Angeles, CA; MONICA LEE, California State University, Los Angeles, Los Angeles, CA; JAMES WALKER, California State University, Los Angeles, Los Angeles, CA

The Irvine Subbasin forms part of the coastal edge of Orange County, California. Prior to 1900, the central part of the Subbasin was marshland. After 1900, drainage ditches and channels were constructed to drain the marshes for agriculture. Today, the drainage ditches still exist in the Irvine Subbasin, which is undergoing massive urban growth. Shallow groundwater discharges into these channels, and the surface water eventually flows into Upper Newport Bay, a thriving habitat. Surface flows in these channels usually exceed the USEPA chronic criterion for selenium (Se) of 5 parts-per-billion (ppb) for protection of aquatic life. Se in surface flows is caused by groundwater inflows.

The highest concentrations of Se in shallow groundwater in the Irvine Subbasin coincide with the marshland areas that were displaced by agriculture. Concentrations of Se in groundwater in the former marsh areas often exceed 50 ppb, and are as high as 200 ppb. In areas where marshes were absent, concentrations of Se in groundwater are usually less than 10 ppb. The marshes are presumed to have been oxygen-deficient, following modern analogs in undisturbed habitats along the California coast. In oxygen deficient marshlands, Se is usually in the form of selenite or elemental Se, two relatively insoluble forms of Se that collect in the sediment at the bottom of the marsh. We hypothesize that the elevated concentrations of Se in groundwater where the marshes once existed are a direct result of the destruction of the marsh. Today, oxygenated groundwaters flow through the soils where the marshes once existed, remobilizing Se as selenate, a water soluble form of Se that is highly mobile in aquifer systems.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California