Comprehensive Groundwater Balance Development to Characterize Selenium Loading to Surface Water Channels in Orange County, California

Abstract

Selenium concentrations are elevated above water quality criteria in several tributaries of Newport Bay. A watershed-scale hydrogeologic study was performed to identify processes driving selenium transport in the former “Swamp of the Frogs” area (cities of Santa Ana, Irvine, and Tustin). The former Swamp of the Frogs area was a depositional environment that historically received regional surface water runoff and accumulated naturally occurring selenium-bearing sediments from the larger watershed. Development beginning in the early 20th century included surface water channel installation that lowered the water table and drained the wetland, mobilizing selenium. A comprehensive groundwater/surface water balance was conducted in order to constrain hydrologic processes that continue to drive selenium mobilization into the surface water channels. The groundwater budget included precipitation and irrigation percolation modeling and groundwater flow mapping from thousands of publicly-available groundwater level measurements. Groundwater balance results indicated that lateral groundwater flow from upgradient alluvial-fan recharge areas accounts for most of the groundwater input to the former wetland area with additional input supplied by deep percolation of precipitation within the former wetland area itself, deep percolation of irrigation, and sewer/water line leakage. Overlaying selenium concentration data from groundwater wells and surface water sampling locations confirmed that selenium concentrations are similar in groundwater and nearby groundwater-to-surface water discharge points (e.g., weepholes). Results supported a Best Management Practice (BMP) Strategic Plan developed in compliance with a regulatory order prescribing numeric limitations on groundwater-related discharges of selenium in the watershed.