Program Brief

Statement of Problem: Sorption is a critical part of groundwater contaminant transport modeling and risk assessment. In many cases, in-situ methods that rely on contaminant sorption and retention in the geosphere have been proposed for remediation of contaminated sites. Sorption in probabilistic risk assessment models typically is represented using sorption coefficients (K_D) derived from a limited number of laboratory experiments supplemented by expert judgment. Sorption is strongly influenced by the geochemistry of the system, however, and potentially important interactions are frequently neglected by this approach.

Approach and Accomplishments: CNWRA^® scientists use laboratory experiments and detailed modeling to identify key geochemical parameters controlling sorption. We have developed a uniform approach to ensure model consistency and have used risk assessment techniques to include critical aspects of detailed sorption models into transport simulations. Results of these models then can be used to build confidence in parameter distributions used in current risk assessment calculations.

We calibrate surface complexation models against laboratory experiments and calculate transport parameters based on site-specific water chemistry. We use model results to provide limits on K_D probability distribution functions as input into risk assessment. With this approach, we can estimate and represent sorption capacity as a function of spatial changes in the geochemical system. This approach also provides a means of correlating sorption behavior of different contaminants by explicitly considering the effects of geochemical variation.

Client Benefits: CNWRA^® scientists are working to develop a theoretical basis for representing sorption in contaminant transport models and probabilistic risk assessments. This approach provides a means to test assumptions used in risk assessment and is an important part of building confidence in the predictions of future behavior for long time periods.