Sediments represent one fate for contaminants that are discharged from urbanized areas and anthropogenic activities along the coast of the Southern California Bight (SCB). These contaminants have the potential to reside in the sediments for long periods of time, exerting acute and chronic effects at various levels of biological organization. Although tremendous effort and resources are expended annually to measure the inputs and effects of these contaminants, less than 10% of the area on the mainland shelf of the SCB is actually monitored. Recognizing the need for integrated regional monitoring to document, analyze, and understand large-scale ecological changes and cumulative effects from multiple contaminant sources, 12 agencies representing large wastewater dischargers, regulatory authorities, and monitoring facilitators joined together to assess the environmental condition of the SCB. Their objectives were: 1) to estimate the extent and magnitude of sediment contamination, 2) to assess whether contamination was similar throughout the SCB or more severe in particular areas, and 3) to associate the contamination with identifiable sources of pollution such as publicly owned treatment works (POTWs) or stormwater discharges.

In the summer of 1994, 248 sites were sampled from the SCB using a stratified-random study design. Sites were located between Point Conception and the U.S.-Mexico international border at depths ranging from 10 to 200 m. Five agencies collected the samples using standardized field protocols, and seven laboratories participated in the chemical analysis guided by a performance-based quality assurance project plan (QAPP). Sediment samples were analyzed for grain size, total organic carbon (TOC), total nitrogen (TN), 14 major and trace elements (aluminum, antimony, arsenic, beryllium, cadmium, chromium, copper, iron, lead, mercury, nickel, selenium, silver, and zinc), and 3 classes of organic compounds (total DDT, total PCB, total PAH).
Bight-wide data distributions for most sediment parameters including TOC, TN, 12 of the 14 major and trace elements, total DDT, and total PCB were positively skewed. Spatial patterns of these parameters indicated that the highest bulk sediment concentrations were located near Los Angeles either in Santa Monica Bay or nearby on the Palos Verdes Shelf. Concentrations of total PAH were uniformly below method reporting limits. Cumulative distribution functions (CDFs) for concentrations of sediment parameters provided in this report allow for direct comparison between individual programs and site-specific projects with Bight-wide results. We encourage resource managers to utilize this new data analysis tool.
Based on at least one constituent, 89% of the SCB area had some evidence of sediment contamination. Eighty-two percent of the SCB was contaminated with chlorinated hydrocarbons including total DDT (82%) and total PCB (46%). Utilizing iron as a conservative tracer of natural contributions for eight metals of interest, it was estimated that 50% of the SCB was anthropogenically enriched in at least one trace metal. Cadmium, chromium, copper, lead, silver, zinc, and possibly mercury showed the greatest degree of anthropogenic enrichment; arsenic and nickel showed the least enrichment.
Sediment quality screening level thresholds developed by NOAA (effects range low [ERL], effects range median [ERM]) (Long et al. 1995) were used to assess the areal extent of predicted biological effects for 13 constituents. Approximately 12% of the SCB exceeded thresholds where biological effects would likely occur (433 km2 > ERM); 55% of the SCB exceeded screening thresholds where biological effects may occasionally occur (ERL < 1,919 km2 < ERM); and 33% of the SCB had sediments where biological effects would be rare (1,169 km2 < ERL). The constituent that exceeded these screening level thresholds most often was total DDT. Unfortunately, the correspondence of the screening level threshold for total DDT and toxic effects is poor. The unreliability of the screening level threshold for total DDT was exacerbated by the lack of correlation between sediment chemistry and other biological indicators measured as part of this study including sediment toxicity and benthic infaunal assemblages.
Since total DDT was the most widespread pollutant of concern, and the predictability of its screening level threshold was low, a sensitivity analysis was conducted to compare 11 biological effects thresholds for this constituent. As expected, when the threshold of concern changed, so did the estimate of areal extent. Thresholds for total DDT based upon bulk sediment concentrations were generally higher than the ERL. Hence, the estimate of areal extent decreased, sometimes by as much as 20% of the SCB. However, thresholds based upon TOC normalization (analogous to an equilibrium partitioning approach) resulted in areal extents of biological concern that were greater than the ERL. Based upon the data available, we are unable to assess which threshold is most accurate since these values do not measure biological impairment directly, but are used as screening values to predict potential impairment. As the science regarding sediment toxicity evolves and predictability improves, we can apply new threshold values to our data set retroactively and more accurately assess biological impairment.

The magnitude of sediment contamination inside Santa Monica Bay was greater than other geographic regions of the SCB. Although grain size was similar between these two regions, mean sediment concentrations of all constituents were higher inside of than outside of the Bay; 12 of 14 constituents were significantly higher. The relative extent of sediment contamination inside of the Bay was also greater than outside of the Bay. Approximately 50% of the area inside of the Bay exceeded screening level thresholds of likely biological impairment for at least one constituent, compared to only 7% of the area outside of the Bay.

Although the proportion of area with sediment contamination near anthropogenic sources of pollutants was similar to the proportion of area distant from these sources, areas near pollutant sources generally had a greater magnitude (i.e. higher levels) of contamination. For example, areas near POTW outfall discharges and distant from these discharges were relatively similar in the extent of contamination; anthropogenic enrichment was evident in approximately 90% of the sediments from both subpopulations. However, 36% of the sediments near POTW outfall discharges exceeded screening levels for likely biological impairment, whereas 13% of the sediments distant from POTW outfalls exceeded this threshold. Similarly, the areal extent of sediments exceeding screening level thresholds for occasional biological impairment near stormwater discharge areas was comparable to the proportion of area distant from stormwater discharges (50 versus 43%, respectively). The mean concentrations of those trace metals near stormwater discharges, however, were approximately double the mean concentrations in sediments distant from stormwater discharges.

To view the SCB in the context of a larger spatial scale, Bight-wide sediment concentrations were compared with the results of the National Status and Trends Program (NS&T, Daskalakis and O?Conner 1995), which summarizes sediment contamination throughout the country. Nationally, concentrations of arsenic, chromium, copper, lead, mercury, selenium, and zinc were comparable to concentrations observed in the SCB. Concentrations of cadmium, silver, and total DDT were three-fold higher in the SCB than other areas of the nation. Concentrations of antimony, total PCB, and total PAH were three-fold higher nationwide than in the SCB.