Abstract: Nitrate Contamination in Surface Water and Shallow Groundwater, San Diego Creek Watershed, Orange County, California

HIBBS, BARRY, California State University, Los Angeles, Los Angeles, CA; JAMES WALKER, California State University, Los Angeles, Los Angeles, CA; MONICA LEE, California State University, Los Angeles, Los Angeles, CA

Nitrate concentrations were measured in the San Diego Creek Watershed, Orange County, California. From April 1999 until January 2000, we collected and analyzed surface water and groundwater for nitrate and index parameters (specific conductance, Eh, pH, dissolved oxygen, and temperature). Over 900 samples were collected at 82 sampling stations in creeks, drainages, and shallow water wells. Nitrate concentrations were 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 exceeded 200 mg/L, and sometimes were as high as 700 mg/L. Nitrate concentrations in drainage ditches that collect runoff from modern and technologically advanced agricultural lands were usually less than 20 mg/L. However, samples collected from the shallow aquifer in the upper to middle portions of the watershed varied 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. Urban runoff was usually less than 15 mg/L nitrate.

Water tables in much of the watershed are higher than the elevation of local streams, resulting in gaining stream conditions from groundwater seepage. These streams eventually discharge into upper Newport Bay, a thriving ecological habitat. Nitrate in discharge waters contributes to algal growth in the bay, which has an adverse impact on the bay's ecology.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California