NEAR FIELD RECEIVING WATER MONITORING OF TRACE METALS IN CLAMS (MACOMA BALTHICA) AND SEDIMENTS NEAR THE PALO ALTO WATER QUALITY CONTROL PLANT IN SOUTH SAN FRANCISCO BAY: 2003

Trace element concentrations were analyzed on samples of fine-grained sediments and clams (Macoma balthica) collected from a mudflat one kilometer south of the discharge of the Palo Alto Regional Water Quality Control Plant in South San Francisco Bay. This report serves as a continuation of the Near Field Receiving Water Monitoring Study, which was started in 1994. The data for 2003, herein, are interpreted within that context. Metal concentrations in both sediments and clam tissue samples have been within the range of values produced by seasonal variability; however, copper and zinc, display continued decreases over the last decade. In 2003, copper in sediment was observed to drop below the ERL (Effects Range-Low) concentration for the third consecutive year and zinc concentrations never exceeded the ERL. Yearly average concentrations of copper, zinc and silver in Macoma balthica for 2003 are some of the lowest recorded since monitoring began in 1975. Mercury and selenium concentrations in sediments and clams at Palo Alto were similar to concentrations observed elsewhere in the San Francisco Bay. Introduction Sampling sediments and benthic organisms in an estuary is a common method used to determine spatial distributions and temporal trends of metal contamination. Sediment particles strongly bind metals, effectively removing them from solution. As a result, sediments may retain metals released to the environment. Thus, concentrations of metals in sediments serve as a record of metal exposure in an estuary, with some integration over time. Fluctuations in the record may be indicative of changes in anthropogenic releases of metals into the environment. Metals in sediments are also indicative of the level of metal exposure of benthic animals in contact with bottom sediments and suspended particulate materials. However, the route through which organisms assimilate bioavailable sediment-bound metal is not well understood. In order to better estimate bioavailable metal exposures, the tissues of the organisms themselves may be analyzed for trace metals. Benthic organisms concentrate most metals to levels higher than those that occur in solution, and therefore, the record of tissue metal concentrations can be a more sensitive indicator of anthropogenic metal inputs than the sediment record. Different species concentrate metals to different degrees. If one species is analyzed consistently, the results can be employed to indicate trace element exposures to the local food web. For example, silver (Ag), copper (Cu) and selenium (Se) contamination, originally observed in clams (Macoma balthica) at the Palo Alto mudflat, was later found in diving ducks, snails, and mussels also from that region (Luoma et al., USGS, unpublished data). 7 Because of the proven value of the above approaches for monitoring near field receiving waters, the California Regional Water Quality Control Board (RWQCB) has described a Self Monitoring Program, with its re-issuance of the National Pollutant Discharge Elimination System (NPDES) permits for South San Francisco Bay dischargers, that includes specific receiving water monitoring requirements. The RWQCB requires inshore monitoring of metals and other specified parameters using the clam M. balthica and fine-grained sediments. The monitoring protocols should be compatible with or complement the Board's Regional Monitoring Program. Monitoring efforts are to be coordinated with the 24 years of previous data collected by the U. S. Geological Survey (USGS) from the site south of the Palo Alto discharge site. Objectives The purpose of this study is to present trace metal concentrations observed in sediments and clams at an inshore location in South San Francisco Bay. These data and those collected in earlier studies (Luoma et al., 1991; 1992; 1993; 1995; 1996; 1997; 1998; Wellise et al., 1999; David et al., 2002; Moon et. al. 2003) will be used to meet the following objectives: • Provide data to assess seasonal and year-to-year trends in trace element concentrations in sediments and clams in receiving waters near the Palo Alto Regional Water Quality Control Plant (PARWQCP) as designated in the RWQCB's Self-Monitoring Program guidelines. • Present the data within the context of historical changes inshore in South San Francisco Bay and within the context of other locations in San Francisco Bay published in the international literature. • Coordinate inshore receiving water monitoring programs for PARWQCB and provide data compatible with relevant aspects of the Regional Monitoring Program. The near field data will augment the Regional Monitoring Program as suggested by the RWQCB. • Provide data, that could support other South San Francisco Bay issues or programs such as development of sediment quality standards.