Sources of Nitrate and Nitrate Removal by a Surface Flow Wetland in a Southern California Coastal Watershed

Abstract

Nitrate concentrations in shallow groundwater and surface water have been measured in San Diego Creek Watershed, Orange County, California. Over 1100 samples have been collected so far at 107 sampling stations in creeks, drainages, and shallow water wells. Nitrate concentrations are highest in drainage ditches that collect surface runoff from large commercial nurseries in the upper part of the watershed. Nitrate concentrations in these drainage ditches usually exceed 100 mg/L, and sometimes exceed 500 mg/L (as nitrate). Nitrate concentrations in drainage ditches that collect runoff from modern and technologically advanced agricultural lands are usually less than 30 mg/L. Samples collected from the shallow aquifer in the upper to middle portions of the watershed vary from 50 mg/L to as much as 250 mg/L nitrate, reflecting the historical effects of leaching from older agricultural plots that have been displaced by urban development. Water tables in much of the watershed are higher than the elevation of local streams, resulting in gaining stream conditions from groundwater seepage. Nitrate in groundwater is carried into these streams. All streams eventually discharge into upper Newport Bay, a thriving ecological habitat and ecological preserve. Nitrate in nursery discharge waters and in baseflows contribute to algal growth in the bay, which has an adverse impact on the ecology of the bay. A constructed surface flow wetland and bird sanctuary, the San Joaquin Wildlife Sanctuary and Marsh Preserve collects some of the nitrate-laden runoff waters before they are routed into Upper Newport Bay. The 300-acre site consists of coastal freshwater wetlands, duck ponds, tall grasses, and thousands of recently planted trees. The wildlife sanctuary is a habitat for the California least tern, light-footed clapper rail, and more than 200 other bird species. Studies indicate that the constructed wetland removes and converts into other nitrogen forms about 90% of the nitrate (concentration) in the wetland effluent waters before they flow into Upper Newport Bay. Total nitrogen concentration reduction is a more modest, but respectable 50%.