The Modified TPH values in the Look Up Table have different values in each of the three columns sub-titled gasoline, diesel/#2 fuel oil and #6 oil (heavy oil). Modified TPH is Total TPH minus the BTEX compounds.
Use the column that describes the type of petroleum source that has created the contaminated site. For example, if the source is gasoline, the lab report is compared to the Modified TPH criteria under the gasoline column. The Look Up Table value in the gasoline column is the not the allowable level of C6-C10 hydrocarbon as reported by a laboratory. The Look Up Table value is to be compared to the total petroleum hydrocarbon measured at the site less the concentration of the BTEX parameters.
If two products have spilled and mixed (i.e. gasoline and diesel) use the lowest value under either the gasoline or diesel columns. If two contaminants are present on the same site but clearly separated, they may be treated as two different source areas using the appropriate column criteria.
At present, Version 2 of the Atlantic RBCA software is sanctioned for use only with TPH and BTEX. The physical/chemical properties and toxicological data has been reviewed and updated for these compounds. Although the current release of the software contains similar data for many other compounds, these data have not been reviewed to ensure that they are up-to-date and appropriate for use in Canada. As a result, the Atlantic RBCA software model cannot be generally sanctioned for use with chemicals other than TPH and BTEX.
However, the concepts of risk assessment and management apply to non-petroleum contaminants under the Atlantic RBCA process adopted by the Atlantic Provinces. Site Professionals calculate such risks for a Tier 3 assessment.
The use of cumulative risks in developing remediation objectives for contaminated sites is appropriate for chemicals that have the same biological end-points. The current Atlantic RBCA model uses cumulative risks for dealing with TPH and BTEX. However, for chemicals that have differing biological end-points, the summation of exposures or risks is not toxicologically sound and has little practical meaning.
The User Guidance manual outlines how cumulative risks are considered in the RBCA process. It also clearly indicates that the use of the Atlantic RBCA model for compounds other than BTEX and TPH can only be done in consultation with local regulators. Such consultation would also likely require input from a toxicologist or similar professional who can properly address the issues relating to interactive effects between chemicals.
The Site Professional must also consider the potential for cumulative risks from exposure to the same chemical through multiple pathways (e.g., indoor air and potable water). At many sites, one exposure pathway will be the critical pathway; however, the Site Professional should consider all possible exposure pathways in the site assessment process. Version 2 of Atlantic RBCA contains a Cumulative Risk Worksheet to facilitate this type of evaluation.