Development of Risk-Based Clean-Up Criteria at a Wellsite in Alberta
Authors: Brown G.L. , Knafla T. , Nerbas M. 
Published: December 2007

Drilling operations in the mid-1980s resulted in localized contamination of a well site lease in an agricultural area in central Alberta. The contamination was related to the on-site disposal of a drilling mud which contained diesel. No other source of contamination was present. Characterization of the contamination through target and non-target scans of soils, vegetation, and groundwater indicated the contaminants were inorganic salts and petroleum hydrocarbons in the C8 to C22 range. The petroleum hydrocarbon contamination consisted of primarily alkyl-substituted naphthalenes, alkyl-substituted benzenes, and linear and branched alkanes. Minor constituents of the contamination included benzene and certain non-carcinogenic polycyclic aromatic hydrocarbons.

A risk assessment was conducted to generate site-specific soil remediation criteria for chemicals present at concentrations which exceed regulatory criteria, or for which no regulatory criteria exist and which exceeded background concentrations. The assessment was conducted using a probabilistic stochastic model with ranges in model input parameters.

Although no residences currently exist on the site, remediation criteria were developed assuming a farming family moved onto the site. The hypothetical residents were assumed to spend all of their time at the site, consumed fruits and vegetables from a garden grown on-site, and to consume milk and beef from cattle which grazed on the site. Remediation criteria were also developed for cattle, as cattle raising is a predominant land use in the area. Cattle were assumed to be exposed through soil and vegetation, and also through drinking of water from a dugout assumed to be re-charged with groundwater.

Remediation criteria were established using surrogate chemicals to represent various fractions of the petroleum hydrocarbon contamination. surrogates were selected based on toxicological and physical/chemical parameters. Estimated exposures to background sources of the chemicals were also estimated in order to determine the maximum acceptable exposure from the site, which would not pose an unacceptable health risk when site and background exposures are combined. In addition, a site-specific total petroleum hydrocarbon (TPH) remediation criterion was developed based on the results.

Executive Summary

An environmental risk assessment was conducted on a well site lease located near Pigeon Lake, Alberta, Canada. The site was contaminated primarily by diesel-based drilling mud. The purpose of the risk assessment was to develop site-specific total petroleum hydrocarbon (TPH) remediation criteria.

Although the site is currently uninhabited, the risk assessment was based on a hypothetical future agricultural/residential land use. Potential human exposure pathways included inhalation of chemicals volatilized from soil, ingestion/inhalation/dermal intake of chemicals in soils and dust, ingestion of chemicals in garden produce, beef, and dairy products. Potential risks to cattle health were also evaluated based on inhalation of volatile chemicals, ingestion/inhalation/dermal uptake of chemicals in soils and dust, ingestion of chemicals in vegetation while grazing, and ingestion of water from a hypothetical dugout.

The risk assessment concluded that soil remediation criteria of 4,000 ppm (surface soils) and 3,500 ppm (subsurface soils) would be protective of human health. It was also concluded that a surface soil concentration of 4,000 ppm, in combination with a maximum dugout water substituted monocyclic aromatic hydrocarbon (MAH) concentration of 2.0 mg/L would be protective of cattle health.

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