CONTROLS ON WATER QUALITY IN SMALL, URBAN WATERSHED-RESERVOIR SYSTEMS: EXAMPLES FROM THE SAN FRANCISCO BAY AREA, CA

The largest sources of contamination to the coastal ocean are associated with storm and dry season terrestrial runoff. In San Francisco Bay (SFB), riverine inputs of nutrient and metal pollutants can be orders of magnitude greater than other sources. In California, many coastal creeks drain small, upstream reservoirs. These small urban reservoirs provide opportunities for a unique field investigation of the environmental controls on water quality in urban areas because of their ubiquity and varied management strategies.

We present results from an ongoing investigation of three urban watershed-reservoir systems in the SFB area including: [1] Lake Aliso-Leona Creek (acid mine drainage influence), [2] Lake Don Castro-San Lorenzo Creek (residential and freeway influence), and [3] Lake Anza-Wildcat Creek (parkland). Water quality monitoring protocols established in all three of study watersheds include tributary and inlet/outlet sampling and in-lake depth profile measurements of temperature, pH, conductivity, and dissolved oxygen. Additionally, samples are collected for laboratory general chemical and trace element analysis. We utilize the results of this watershed monitoring as calibration points for sediment cores collected from each reservoir to investigate changes over time.

Results from water quality measurements indicate that oxygen content is one of the primary controls on water quality in these systems. These small reservoirs are seasonally stratified, which can lead to the development of an oxygen-depleted layer near the sediment - water interface. Therefore, whether these reservoirs are sources or sinks of pollutants to the watershed, and ultimately SFB, largely depends on how they are managed. Geochemical results from the sediment cores suggest that these reservoirs’ role in controlling water quality has evolved in response to changes in the landscape. Analysis of microbial communities at the sampling locations suggests a correlation with water quality.