Abstract

Historical crude oil leaks from a pipeline affected unconsolidated alluvial sediments near a sensitive groundwater recharge area in Kern County, California. The residual crude oil is confined to the vadose zone and occurs from ~3 m below ground surface (BGS) to a maximum depth of 24 m BGS. The water table beneath the affected sediments is currently 46 m BGS. The site is irrigated regularly for agriculture. To date, the residual crude oil has not impacted groundwater quality. Future groundwater recharge plans may raise the water table to 15 m BGS in the area affected by the crude oil. Fate and transport modeling using site-specific data shows that the existing hydrocarbons in the subsurface do not pose a significant risk to groundwater quality.

The computer models selected for this project are incorporated as modules in the American Petroleum Institute’s Exposure and Risk Assessment Decision Support System. Transport of benzene, toluene, ethylbenzene, and xylencs (BTEX) is modeled using Seasonal Soil (SESOIL) for the unsaturated zone coupled with AT123D for the saturated zone. The SESOIL model is calibrated using actual soil moisture measurements and groundwater recharge estimates based on applied irrigation. Peak BTEX concentrations in groundwater predicted for the site are well below maximum contaminant levels. A sensitivity analysis confirms that aerobic biodegradation significantly reduces BTEX compounds. Due to the high availability of dissolved oxygen in groundwater at this site, natural attenuation may be the most favorable mechanism to remediate BTEX in the subsurface.