Sources of Excess Phosphorous and Nitrogen in a Coastal Watershed

G. Desselle, E. Chavez, and B. Hibbs
Department of Geological Sciences, California State University-Los Angeles, Los Angeles, CA

The San Diego Creek Watershed extends over part of Orange County, California. It is bound to the north by the Santa Ana Mountains, to the southeast by the San Joaquin Hills, and on the south by Upper Newport Bay. This area was a marshland up until the early 1900s, at which time it was drained for irrigated agricultural use. An extensive drainage system consisting of concrete-lined and unlined channels was constructed for this purpose. Currently, the surrounding areas are experiencing tremendous urban growth. Field studies are focusing on sources of excess nutrient contamination (nitrogen and phosphorus) in the San Diego Creek Watershed. Ground water (seeps, weepholes) and surface water samples are taken from the drainage channels and analyzed for these constituents. The upper part of the watershed contains nursery and agricultural operations that contribute significant amounts of N and P to the watershed through the application of fertilizers. Preliminary investigations have detected levels of nitrate as high as 359 mg/l as NO₃, and phosphate levels of 14.96 mg/l PO₄ in surface water runoff from nurseries. In the lower watershed, preliminary data suggest ground water seepage into drainage channels contributes large amounts of nitrate (up to 120 mg/l NO₃) into the watershed. High levels of PO4 in the lower watershed are most likely contributed by construction activities causing sediment loading through runoff, and also by the use of “reclaimed” water for irrigation of greenbelts and parks. This irrigation water has tested as high as 2.93 mg/l PO₄.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California