Using mass balance in risk assessment

Quantitative risk assessments are often conducted with constant steady state source concentrations and an unlimited mass available for potential exposures. For many exposure scenarios and available site data these conservative assumptions are appropriate and can be useful when data are available to understand the limits of mass. Accounting for mass balance minimises the unnecessary conservatism in risk assessments. The National Environment Protection Measure (NEPM, 1999) has already adopted this approach in in the 2013 updated inclusion of the soil Health Screening Levels (HSLs). In developing the soil HSLs protective of vapour intrusion, a finite source model was used to reduce the disparity in finite vs. infinite source modelling from typical residual soil impacts (Friebel and Nadebaum, 2011). An evaluation of a finite mass balance vs. infinite source approach was applied to vapour intrusion assessment for a scenario of petroleum hydrocarbon impacted groundwater seepage into a car park basement. This provided an example where simple mass balance evaluation can refine the risk assessment process and reduce the conservatism (author)