Selenium in Kesterson Reservoir Ephemeral Pools Formed by Groundwater Rise: I. A Field Study

Abstract

AbstractOne of the principal initial plans for managing Se contamination at Kesterson Reservoir (Merced County, California) entailed excavation to a depth of at least 0.15 m in order to remove soils with Se concentrations higher than 4 mg kg−1. Prior to implementing this plan, surface waters were collected from numerous shallow ephemeral pools that formed during the winters of 1986 to 1988. Elevated concentrations of Se were measured in all of these pools. Pools formed during this period from a combination of overland flow from adjacent nonseleniferous pond waters, rainfall ponding, and emergence of the fluctuating shallow water table above the soil surface. Selenium in these ephemeral pool waters originated primarily from dissolution of surface salt crusts and from displacement of seleniferous soil solutions up to the surface. Through the monitoring of a test plot where 0.30 m of the surface soil had been removed, the potential was demonstrated for shallow water table rise to displace seleniferous soil solutions to the surface, resulting in highly seleniferous ephemeral pools. In the test plot, ephemeral pool Se concentrations often exceeded 103µg L−1. This study clearly demonstrated (i) that pools formed over soils with total Se concentrations in the range of 0.5 to 1.7 mg kg−1 could still provide a highly seleniferous aquatic environment, (ii) that the total Se concentration in a soil can be a misleading parameter to use in determining target goals for remediation, and (iii) that the excavation strategy was inappropriate for the Kesterson Reservoir setting.