Establishing background arsenic in soil of the urbanized San Francisco Bay region

Abstract

Soil analysis data within the nine-county San Francisco Bay Area was compiled from the State Water Quality Control Board's Geotracker online database to determine the background levels and variability of arsenic concentrations across four Quaternary geologic units. Arsenic analyses of 1,454 soil samples across 77 sites were screened from Geotracker for inclusion in a JMP 7.0 database. Mean arsenic concentrations within Holocene alluvium (5.10 mg/kg) were determined to be statistically greater than those within Pleistocene alluvium (3.65 mg/kg) and "other" Quaternary units (3.30 mg/kg); and no significant relationship was found between arsenic concentrations and sampling depth. The proposed upper estimate for background arsenic (99th percentile) within undifferentiated flatland soils of the study area—11.00 mg/kg—is markedly lower than commonly cited sources in the literature. These findings present the first regional estimates of background arsenic concentrations in the San Francisco Bay Area.