Comparison of Bioeffect Screening Results for Hydrocarbons and Hydrocarbon Oxidation Products: Implications for Total Petroleum Hydrocarbon Analyses

Abstract:

At petroleum hydrocarbon spill sites, analyses of total petroleum hydrocarbons in the diesel range (TPHd) also quantify some polar hydrocarbon oxidation products and natural organic matter. A debate about whether these polar compounds should be removed prior to TPHd analyses prompted this study. Groundwater samples were collected from a crude-oil contaminant plume in July 2018 from a clean well and five wells along a flowline. Nonvolatile dissolved organic carbon (NVDOC) was quantified at each well. Two samples were extracted with dichloromethane (DCM) and analyzed for TPHd with and without silica gel cleanup (SGC) to remove polar compounds. A third sample was extracted with a solid-phase Oasis HLB cartridge. These three extracts were dried under nitrogen gas and resuspended in dimethysulfoxide (DMSO). A commercial cell bioassay was used to evaluate activation of 46 toxicologically relevant molecular targets. A fourth water sample was extracted with a solid-phase Oasis HLB cartridge and analyzed for aryl hydrocarbon receptor (AhR) activity using a chemically activated luciferase expression (CALUX) human cell line. Results for the CALUX assay are in bioanalytical equivalents (BEQ) relative to the activity of a known AhR agonist (β-naphthoflavone; ng βNF/L).

TPHd decreased from 8.1 mg/L near the source to 0.15 mg/L at 254 m downgradient. NVDOC values were 3-20 times TPHd. Hydrocarbons isolated with SGC were 6-7% of TPHd in the first three wells, then below detection. Of the 46 targets tested, bioeffects were highest for AhR and the pregnane X receptor (PXR). The SGC fraction showed activity comparable to the background well. In contrast, PXR was higher than background in all plume wells for the HLB and DCM extracts; with HLB 1.5-1.2 times higher than DCM. Similarly, for AhR, extracts using HLB were 2.7-1.2 times DCM values. The CALUX-based AhR activity of the HLB fraction yielded average BEQs ranging from 790±140 near the source to 340±65 and 130±17 ng βNF/L for the plume wells where TPHd-SGC concentrations were below detection. Together these data indicate the biological effects of the hydrocarbon fraction isolated with SGC were like the background site. In contrast, the biological effects of the polar fraction support previous observations of biological activity suggesting further study of this fraction is needed.