Southwest Ocean Outfall Regional Monitoring Program
- Michael Kellogg
City & County of San Francisco
The City and County of San Francisco owns and operates the Oceanside Water Pollution Control Plant that collects, treats to secondary standards, and then discharges municipal wastewater and storm water into the Pacific Ocean approximately 3.75 miles offshore of Ocean Beach. The California State Regional Water Quality Control Board, San Francisco Bay Region and the U.S. Environmental Protection Agency, Region 9 regulate this discharge through a National Pollution Discharge Elimination System (NPDES) permit (no. CA 0037681). Since 1997, these regulators have recognized that a regional perspective is necessary to distinguish potential outfall effects from effects of outflow through the Golden Gate from the San Francisco Bay-Delta system.
The objectives of our monitoring, as mandated in the the NPDES permit, are to “…determine environmental effects from the discharged secondary treated effluent (18 MGD, average dry weather flow) from the City and County of San Francisco’s Oceanside Water Pollution Control Plant. The study plan characterizes the area outside San Francisco Bay between Rocky Point in Marin County and Point San Pedro in San Mateo County. Randomized sampling locations were determined using the EPA’s EMAP grid system within specified depth strata. The purpose of this effort is to: 1) evaluate gradient effects near the discharge pipe and gradient effects from San Francisco Bay; 2) characterize non-affected areas that can be combined to define reference conditions; and 3) provide information on sediment and infauna characteristics in the area between the discharge pipe and the Monterey Bay National Marine Sanctuary boundary."
Fifty to 55 stations are sampled annually for sediment quality and benthic infauna community analyses. Seven stations near the outfall have 15 or more years of data and of the EMAP randomly determined stations (sampled since 1997) five fall within the GFNMS and ten fall within the MBNMS. Comparison of outfall and reference sites is accomplished using multivariate techniques and correlation analyses. Hypotheses that conditions at the outfall are not statistically different from reference conditions are tested using BACIP (Before-After-Control-Impact-Paired) and Reference Envelope analyses.
Summary to DateThe sedimentary environment of the study area is dominated by input from the San Francisco Bay-Estuary system through the Golden Gate. Tidal and long-shore currents rework the sediments to form the barrier sand bars that surround the mouth of San Francisco Bay. Grain size distributions are consistent from year to year with coarse and medium sands predominating inside the sand bars. The sand bars are comprised of medium and fine sands and seaward of the sand bars fine to very fine sands predominate. The highest percentages of silt and clay (up to 28%) are measured just seaward of the sand bars. Silt and clay decrease to less than 10% well beyond the sand bars. Although sediment quality criteria do not currently exist for the San Francisco Bay area, both organic compounds and metals concentrations measured in sediments from the study area are generally below effects range median (ERM) sediment quality guidelines established from other studies.
Sediment grain size measures at the outfall have not changed since pre-construction and discharge periods. Benthic infauna community composition represents an assortment of native species common in central California near shore, sand-bottom environments. Sediment grain size is the most important factor structuring infauna communities. The area inside the sand bars with predominantly coarse and medium sands has a distinct community of mostly small, interstitial-like organisms. This coarse-grain community is dominated by the polychaetes Hesionura coineaui difficilis and Heteropodarke heteromorpha, nematodes, and the bivalve Tellina nuculoides. The sand bars, with predominantly well-sorted fine sands, are numerically dominated by the polychaete Spiophanes bombyx and characterized by a higher proportion of crustaceans than the other infauna communities. The higher proportion of crustaceans on the shallow sand bars is consistent with the pattern of wave disturbance zonation described for Monterey Bay sand bottoms by Oliver, et al. (1980).
A third community exists in the areas seaward of the sand bars where the sediments are predominantly very fine sands with a low and variable percentage of silt and clay. This latter community is numerically dominated by the polychaete Spiophanes berkeleyorum and the bivalve Tellina modesta and is the most diverse of the communities in the study area. More detailed results, including fisheries community and bioaccumulation analyses, are available on-line at: http://sfwater.org/main.cfm/MC_ID/4/MSC_ID/83.
- Substrate characterization
Study MethodsBenthic Infauna and Sediment
Field methods involve collecting three sediment grab samples at each station using a 0.1 M sq. Smith-McIntyre bottom sampler deployed from a research vessel. The first sample is sieved through nested 0.5 mm and 1.0 mm nested screens. The organisms retained on each screen are relaxed and preserved for enumeration and identification to the lowest practical taxon. The top five centimeters of the other two grab samples are homogenized (for compatibility with the San Francisco Estuary Regional Monitoring Program for Trace Substances) and analyzed for grain size, TOC, TKN, TS/TVS, and inorganic and organic contaminants. Detailed descriptions of field, laboratory, and analytical procedures including QA/QC criteria are available on-line at: http://sfwater.org/detail.cfm/MSC_ID/83/MTO_ID/NULL/MC_ID/4/C_ID/1547/holdSession/1.
Figures and Images
City & County of San Francisco Southwest Ocean Outfall Regional Monitoring Program Benthic Infauna Communities as illustrated by three polychaete species numerically dominate in each and showing their relationship to sediment grain size.
Download this map as a PDF (200 KB)
Download this map as a PDF (200 KB)
- Sediment and Benthic Infauna Sampling StationsCity and County of San Francisco Southwest Ocean Outfall Regional Monitoring Program Sediment and Benthic Infauna Sampling Stations (Not all stations sampled every year.)
- Average Sediment Grain SizeCity & County of San Francisco Southwest Ocean Outfall Regional Monitoring Program Average Sediment Grain Size 1997 - 2002 (not all stations sampled every year)
- 2005 Data Report2005 Data Report (1.8 Mb)