REGIONAL MULTIYEAR STUDIES DEFINE NATURAL AND HUMAN FACTORS IN SOLUTE LOADING TO THE WATERSHEDS OF THE SANTA MONICA MOUNTAINS NATURAL RECREATION AREA

Fifteen years of our field and laboratory studies of source waters, nutrients, and trace elements in streams and shallow groundwater of the Santa Monica Mountains National Recreation Area (SMMNRA) have shed considerable insights on the water resources of the 60,000 hectare region. The SMMNRA, administered by the US National Park Service, is located in northwestern Los Angeles County and southeastern Ventura County, covering mostly hilly and coastal areas. The area has a Mediterranean coast climate with a dry season extending at least from May to December. It is a unique recreational area because small to medium sized cities occupy flanking highlands that drain the upper watersheds of SMMNRA. Cities supply some urban runoff to local streams, possibly increasing nutrient and trace element loads by several factors in local streams. Use of treated wastewater is escalating in these cities, increasing the potential for nutrient loading to watersheds. For many years, debate on the hydrology of the region focused on the following questions: 1) Are some of the streams perennial due to augmentation from dry weather urban runoff? 2) Is nutrient loading mostly due to dry weather urban runoff in the upper part of the watershed, and is treated wastewater a primary culprit? 3) Are elevated trace elements such as selenium and uranium in some streams natural, or are these constituents associated with land use activities in the developed areas? Our investigations using stable water isotopes, nitrate isotopes, sulfur isotopes, trace element and nutrient hydrochemistry have detected both imported water and locally derived waters in creeks, mostly anthropogenic sources of elevated nutrients, and mostly natural sources of trace elements. Land use activities have accentuated mobilization and release of trace elements from geologic formations however, by changing redox conditions in shallow aquifers feeding into local streams.