The former Guadalupe Oil Field, in the central California coastal region, is the location of past and ongoing efforts to assess ecological risk. While there are a number of chemicals of concern, the main focus of the risk assessment is a crude oil thinning agent, known as diluent. Approximately 8 million gallons of diluent were accidentally spilled within the 3000 acre property during its 45 year history of use. The risk assessment must consider the hundreds of plant and animal species, both terrestrial and aquatic, that inhabit the site and its neighboring properties. Among these plants and animals are at least 40 threatened and endangered species.

The first efforts to evaluate ecological risk at the site were initiated during a Natural Resource Damages (NRD) lawsuit. These efforts focused on developing a case for biological injury and establishing a basis for financial damages. Since settlement of the NRD lawsuit in 1998, risk assessment has focused on providing input to decision-makers so that objective remediation goals can be established. This paper reviews the status of the ecological risk assessment, its role in site decision-making, and specific technical challenges that have been encountered.

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