SELENIUM LOADING FROM GROUNDWATER TO NEWPORT BAY, ORANGE COUNTY, CALIFORNIA

Selenium concentrations are elevated above water quality criteria in several tributaries of Newport Bay. Two watershed-scale hydrogeologic studies were performed to identify processes driving selenium transport in different areas of the Newport Bay watershed: (1) the former “Swamp of the Frogs” area (cities of Santa Ana, Irvine and Tustin) and (2) the Upper Big Canyon subwatershed (City of Newport Beach). The 31 square-mile former Swamp of the Frogs area was a depositional environment that historically received regional surface water runoff. Naturally occurring selenium in surrounding Tertiary Monterey Formation (Tm) outcrops was eroded and deposited onto the low-oxygen floor of the swamp where it accumulated in reduced forms (Se⁰, Se^2-). Development beginning in the early 20^th century included surface water channel installation that lowered the water table and drained the swamp. Consequently, sediment was oxygenated and selenium was mobilized with groundwater into the channels as the more soluble Se⁺⁶ species. A comprehensive groundwater/surface water balance was conducted, including precipitation and irrigation percolation modeling and groundwater flow mapping from thousands of publically-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 swamp area with additional input supplied by deep percolation of precipitation within the former swamp 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). The Upper Big Canyon subwatershed is located in relatively higher elevation areas exhibiting outcrops of the high-selenium Tm formation and the primary draining creek (Big Canyon Wash) exhibits elevated selenium concentrations. Field investigation and quantitative groundwater/surface water balance estimation indicated that percolation from precipitation and a drinking-water reservoir drive groundwater flow and discharge to the creek.