An analysis ofNO3-1NOr concentrations in ground and surface water samples taken from throughout the Santa Margarita, San Diego, Sweetwater, and Tijuana watersheds. The colorimetric method of analysis was used to measure N03-/ N02- levels in the samples. Grow1dwater samples were taken from the Tijuana Estuary, the Bonita Well Field, Lawson Valley, and from areas of Camp Pendleton including the Santa Mru:garita River near Basilone Rd., and a well site near Las Pulgas Rd. Surface water samples were taken from: the Tijuana River at two sites within the estuary, Barrett Lake, and Pine Creek; the Sweetwater River in Bonita, at the crossing of Highway 94, and along Highway 79; the San Diego River at the base of the El Capitan Dam, at the east and west ends of Mission Trails regional park, and a site behind Fashion Valley shopping center; Alvarado creek below San Diego State University; the Santa Margarita River on Camp Pendleton, at three sites within the Santa Margarita Ecological Preserve, a site just downstream, and two sites in Rainbow Creek. Analysis showed elevated relative concentrations in surface waters from the Santa Margarita watershed, particularly in the samples from Rainbow Creek. These levels were below what is required for drinking water standards, and consistent with results of an analysis done in 2000 by the Regional Water Quality Control Board. Higher than normal concentrations were also found in well waters in the Tijuana Estuary, the levels were also below what is considered safe for drinking water concentrations. All other levels ranged from 5 to 350 ppb for N03-, and 2-20 ppb for N02-. Comparisons with previous countywide surveys showed similar values. A comparison with a global surface water average of 810 ppb shows that county levels on the whole are relatively low.