Fate and transport modeling of BTEX from residual crude oil, validated by monitoring, supports remediation by natural attenuation (RNA): An update of a case study in Kern County, California
Authors: Klinchuch L.A. , Waldron J.M. 
Published: December 2007

Historical crude oil leaks from a pipeline affected unconsolidated alluvial sediments near a sensitive ground-water recharge area in Kern County, California. The residual crude oil occurs from about 10 ft below ground surface (BGS) to a maximum depth of 85 ft BGS. The site is irrigated regularly for agriculture. Ground-water recharge from nearby percolation ponds raised the water table to within the crude-oil-affected sediments. Fate and transport modeling using site-specific data shows that the existing hydrocarbons in the subsurface do not pose a significant risk to ground water quality.

The computer models selected for this project are incorporated as modules in the American Petroleum Institute's (API's) Exposure and Risk Assessment Decision Support System (DSS). Transport of benzene is modeled using AT123D. The model predicts that shallow ground water may be impacted for 3 to 5 years, however, health risk results are low (10.6). A sensitivity analysis shows that hydraulic gradient, which is affected by the percolation pond, is an important parameter. To date, monitoring wells validate that ground water is not significantly affected. Ground water geochemical trends demonstrate naturally-occurring biodegradation of petroleum hydrocarbons at this site. Remediation by natural attenuation (RNA) is the most favorable and cost-effective mechanism to remediate BTEX in the sediments.

Introduction

Historical crude oil leaks from a pipeline in Kern County, California affected unconsolidated alluvial sediments near property that is used to recharge ground water via percolation ponds. The pipeline right-of-way (Figure 1) contains a group of pipelines, including a threaded-collar line that dates back to as early as 1910. The most recent crude oil leaks were a 2000-barrel release in 1987 near the Southern Pacific railroad tracks and a 100-barrel release in 1991 or 1992 caused when farm equipment cut a line. Subsurface alluvial sediments affected by the residual crude oil contain low concentrations of benzene, toluene, ethylbenzene, and xylenes (BTEX). Total extractable hydrocarbons measured as high as 83,000 mg/kg in 1992 and as high as 28,000 mg/kg in 1994. The pipelines in the right-of-way are currently idle and were flushed with fresh water and drained by vacuum. The unconfined aquifer beneath the pipeline right-of-way is comprised of coalescing braided stream channels deposited by the Kern River alluvial fan system. The land overlying the pipelines, the NW Pioneer property, is used for agriculture. The primary crops grown are alfalfa and carrots, requiring irrigation of 2.5 to 4 ac-ft/acre. Adjacent to the property is a percolation pond used for ground water recharge (Figure 1).

Background

In 1995 it was believed that ground-water recharge would raise the water table to 50 ft below ground surface (BGS), within the lower limits of oil-stained sediments (Figure 2). Fate and transport modeling, using site-specific data, was used to determine if hydrocarbons present in the alluvial sediments pose a significant risk to ground-water quality in this sensitive recharge area. The models chosen are incorporated in the American Petroleum Institute's (API) Risk/Exposure Assessment Decision Support System (DSS). SESOIL was used for the unsaturated zone coupled with AT123D for the saturated zone. The models predicted that residual crude oil would not threaten ground water downgradient of the impacted sediments (Figure 3). Details of this initial modeling effort, including site data collected for the models and the sensitivity analysis, were documented in our first paper (Klinchuch and Waldron, 1995).

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