Statistical Evaluation of Variables Affecting Occurrence of Hydrocarbons in Aquifers Used for Public Supply, California

Abstract

The variables affecting the occurrence of hydrocarbons in aquifers used for public supply in California were assessed based on statistical evaluation of three large statewide datasets; gasoline oxygenates also were analyzed for comparison with hydrocarbons. Benzene is the most frequently detected (1.7%) compound among 17 hydrocarbons analyzed at generally low concentrations (median detected concentration 0.024 μg/l) in groundwater used for public supply in California; methyl tert‐butyl ether (MTBE) is the most frequently detected (5.8%) compound among seven oxygenates analyzed (median detected concentration 0.1 μg/l). At aquifer depths used for public supply, hydrocarbons and MTBE rarely co‐occur and are generally related to different variables; in shallower groundwater, co‐occurrence is more frequent and there are similar relations to the density or proximity of potential sources. Benzene concentrations are most strongly correlated with reducing conditions, regardless of groundwater age and depth. Multiple lines of evidence indicate that benzene and other hydrocarbons detected in old, deep, and/or brackish groundwater result from geogenic sources of oil and gas. However, in recently recharged (since ~1950), generally shallower groundwater, higher concentrations and detection frequencies of benzene and hydrocarbons were associated with a greater proportion of commercial land use surrounding the well, likely reflecting effects of anthropogenic sources, particularly in combination with reducing conditions.

Life Cycle of Methyl tert-Butyl Ether in California Public Water Supply Wells

“…For comparison to MTBE, we also evaluated PCE, a volatile organic chemical widely used for dry cleaning and other industrial purposes. We selected PCE rather than benzene, a petroleum constituent associated with gasoline, because benzene has rarely been detected in public water supply wells in California and the infrequent detections have been attributed primarily to natural sources rather than gasoline releases . Using the same selection criteria (i.e., raw and untreated active, standby, and inactive wells), we identified 15963 locations tested for PCE.…”

“…We selected PCE rather than benzene, a petroleum constituent associated with gasoline, because benzene has rarely been detected in public water supply wells in California and the infrequent detections have been attributed primarily to natural sources rather than gasoline releases. 11 Using the same selection criteria (i.e., raw and untreated active, standby, and inactive wells), we identified 15963 locations tested for PCE. Approximately 80% of the locations tested for either MTBE or PCE have been analyzed for both of these constituents; 92% of the detection limits reported for PCE analyses were 0.5 μg/L.…”

CAFOs and Surface Water Quality: Evidence from Wisconsin

“…Previous studies in the in the ecological literature have used these water quality measures to examine individual surface waterbodies (e.g., Landon et al 2014). We are aware of only three published studies in the economics literature that use these water quality measures to answer economic questions over a large spatial scale (Grant and Langpap 2019; Keiser and Shapiro 2019; Paudel and Crago 2020).…”

Metrics for Assessing the Quality of Groundwater Used for Public Supply, CA, USA: Equivalent-Population and Area

“…Evaluation of groundwater quality, and the factors affecting groundwater quality, have been presented in 25 USGS Scientific Investigations Reports (SIRs) and 35 USGS Fact Sheets (FSs; Table S1, SI). Important results for selected individual constituents have also been presented at statewide and regional scales. − The data for this assessment are provided in the supplement to this paper (Table S2, SI).…”