Western
Geo-EngineersWestern
Geo-EngineersAs described in ASTM E-1739 "Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites", the RBCA process represents a streamlined approach for assessment and response to subsurface contamination associated with hydrocarbon releases.
The RBCA process uses a tiered approach to assessments. There are three tiers or levels of assessment: Tier 1, Tier 2, and Tier 3. As the tier level increases, so does the amount of data needed to perform the evaluation along with the complexity of the models used.
Tier 1 is the initial screening level in RBCA. During a Tier 1 assessment information is collected about the site and a determination is made whether the site represents an acute or chronic health hazard. If the site posses an acute hazard, immediate steps are taken to lower the danger. If the site posses a chronic risk or the acute risk has been reduced, a Tier 1 screening model is executed.
The Tier 1 screening evaluates the risk associated with onsite exposure to the constituents of concern.
*Soil carbon and porosity values are often estimated.
Under normal circumstances enough data can be collected during the Soil Probe Survey TM phase, of the Rapid Site Characterization, to perform a Tier 1 assessment. If prior site investigations have been performed, all or a portion of the necessary data may already be available.
If the Tier 1 screening indicates that the site posses a possible unacceptable risk, then a Tier 2 assessment is usually completed.
A Tier 2 assessment calculates the probable impact of the constituents of concern on offsite receptors. In calculating these probable impacts, analytical modeling is used to estimate the fate and transport of the constituents under the influence of Natural Attenuation.
The above information is available usually only after groundwater monitor wells have been installed, sampled and Slug/Bail tests have been performed.
If the site is found to posses an offsite hazard under Tier 2, it may be advisable to begin some form of active remediation. If it is felt that further site information might allow closure of the site then a Tier 3, study may be warranted.
During a Tier 3 study, numeric models are developed to determine the fate and transport of the constituents of concern. These models are used to estimate the probable concentration of contaminant at offsite Points of Exposure (POE) and to calculate the health risk.