Grain size analysis of California waterbodies to determine the spatial extent and temporal trends of depositional areas where hydrophobic organic compounds could accumulate

Abstract

The objective of this study was to summarize the grain size data from 1993 to 2016 from California water bodies and determine the number and percent of sites where sediment samples are primarily depositional sediment (more than 50% silt/clay) or non-depositional sediment (less than 50% silt/clay). Fine grain depositional areas were of interest because these are areas where hydrophobic organic compounds (HOCs) can accumulate if sources exist. Trends analysis of percent silt/clay data were conducted for all nine California Water Board Regions if adequate data for this type of analysis were available. Sediment sampling protocols were reviewed for all the grain size measurements to determine if depositional areas were targeted or random sampling was conducted. Twelve of the 13 sediment sampling protocols targeted depositional areas. Based on 23 years of data from 685 sites (1859 observations) in all 9 California Regions, the summary analysis showed that only 58% of the sites were considered depositional areas even when targeted sampling for depositional areas was used. Since only slightly more than half the sites were reported as depositional areas based on sampling that targets depositional areas, these results would certainly suggest that depositional areas are not dominant. In fact, for the one protocol where random sampling was used, depositional areas were not reported to be dominant. From an ecological risk perspective, the implication of this finding is that HOCs would not be expected to accumulate in the dominant type of sediment found in most California waterbodies. Trends analysis of percent silt/clay data showed mixed results for the various California Regions.