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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems, Water-Resources Investigation Report 99-4018B

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Reduced Phosphate Loading to South San Francisco Bay, California: Detection of Effects in the Water Column

by Laurence E. Schemel, Stephen W. Hager and David H. Peterson

This paper is available in pdf format: pdf Schemel.pdf

ABSTRACT

Dissolved phosphate has been a useful tracer of municipal wastewater in many studies of South San Francisco Bay (South Bay), in part because water column concentrations are higher than background levels even after appreciable dilution. Over the last decade, however, wastewater loading of phosphate to South Bay has been reduced. Our objective was to identify changes in phosphate concentrations in the water column of South Bay that were caused by recent reductions in wastewater loading. The changes proved difficult to detect in this estuary, primarily because of strong effects of interannual and seasonal variability in climate- and weather-dependent processes. This case study for phosphate identified factors that should be considered when attempting to detect effects in the water column for substances that are not completely removed from wastewater discharges and have potentially complex interactions with biogeochemical cycles.

Water-column measurements of phosphate concentrations did not provide clear evidence of a response to the reduced loading. The apparent change in water-column concentrations in recent years could have been caused by dry hydrolgic conditions before 1993 and very wet conditions over 1995-1998. Although interannual and seasonal variations in climate and weather influence water-column properties in many ways, the most apparent was greater effects of dilution by freshwater inflow that persisted for many months during the wet years. During early fall of most years, however, wastewater was the dominant source of freshwater to South Bay, and longitudinal gradients in both salinity and phosphate were directly related to wastewater inflow. At those times, a simple mixing model was effective in estimating average concentrations of phosphate in the wastewater from the longitudinal gradients in the bay. This technique detected a reduction in the phosphate gradient in recent years that was consistent with the decrease in wastewater concentration and the reduced loading.

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