Sediment samples were taken for trace metals analysis from the lower North and South Forks on August 13, 1987. In aquatic systems the concentration of trace metals in suspended sediment and the top few centimeters of bottom sediment is generally greater than the concentration of dissolved trace metals in the water column (Horowitz, 1985). Trace metals are ubiquitous and persistent in the environment. Although many trace metals are essential to plant and animal life, all are potentially toxic to aquatic life at varying threshold exposure levels. Short-term exposure to relatively high metals concentrations can cause immediate acutely toxic effects, whereas long-term low level exposure can result in chronic effects such as impaired growth and reproduction as well as cancer.
Bottom sediments often act as a sink or reservoir for many trace metals. Dissolved metals in the water exhibit a strong tendency to bind to sediments through adsorption processes. Many physical and chemical factors affect a sediment's capacity to collect and concentrate trace metals. Increases in metal concentrations are strongly correlated with decreasing grain size and increasing surface area, surface charge, cation exchange capacity, as well as increasing concentrations of iron and manganese oxides, organic matter, and clay minerals (Horowitz). Bottom sediments are the primary source of suspended sediment in the water column. During peak flow periods in Strawberry Creek, the streambed is scoured and the bottom sediments are flushed downstream. In this manner a localized pollution problem may become widespread and result in significant environmental impact. Under certain conditions, trace metals may dissolve into the water column, possibly enter the food chain, and have an adverse impact in this manner as well. The tendency for trace metals to bioaccumulate and biomagnify in aquatic organisms can result in toxic concentrations higher in the food chain even though initial concentrations in water are at or below apparently safe levels.
Both forks were sampled as far downstream as possible to assess the impacts of upstream sources of heavy metals. Sampling locations are shown on Figure 12. The South Fork sample was taken upstream of the LSB Annex construction to avoid siltation from that site. The top few centimeters of silt and organic material along an approximately 50 foot stretch of each fork were collected to make up a composite sediment sample at each location. The sediment samples were brought immediately to a state certified laboratory for analysis. Results of the analyses are presented in Table 16.
The trace metal concentrations found in Strawberry Creek sediments were generally within expected naturally-occurring ranges. For comparative purposes, Table 17 presents USGS sediment analysis data from the South Bay and the Clarke Number (CN) for each trace metal. The USGS analyses were the latest available comprehensive trace metal data for freshwater sediments close to the East Bay. On the other hand, the CN is a value calculated by geochemists that represents the average abundance of a trace element throughout the earth's crust. Since the CN reflects an average of all geologic formations within the earth's crust, some deviation can be expected at any given location. However, metal concentrations higher than the CN may indicate that pollution has occurred.
Lead and zinc concentrations exceeded the CN in both forks of Strawberry Creek and mercury exceeded the CN in the North Fork. In addition, mercury and zinc concentrations were higher than the average levels observed in urban Santa Clara County. Water quality experts from the USGS Water Resources Division and the S.F. Bay Regional Water Quality Control Board agreed that the mercury level found in the North Fork sediment was high and that the lead concentrations were also somewhat elevated. A 1972 USGS study reported mercury levels in various tributaries and margins of central San Francisco Bay ranging from 0.08 - 0.75 ppm based on dry weight. The average mercury concentration in the Bay sediments was 0.26 ppm.
Lead is a very common constituent in urban runoff, and zinc is also commonly found in urban environments. However, mercury does not commonly occur in the environment which suggests a source of significant pollution in the North Fork subwatershed. Highly toxic methylmercury can be formed by bacterial action in bottom sediments under anaerobic conditions. Further investigation into the source or sources of mercury pollution in the North Fork is warranted due to its significant environmental impacts.