Contaminant transport modeling is crucial to the evaluation of remediation and waste management projects. Sensitivity analyses have shown that the partition (or distribution) coefficient (Kd) value chosen for a contaminant can be one of the most important variables in the prediction of a contaminant’s overall environmental impact.

Researchers at the Pacific Northwest National Laboratory, operated for the U.S. Department of Energy by Battelle, are providing technical support to the Environmental Protection Agency (EPA) on issues associated with the measurement and selection of Kd values. The technical guidance reports resulting from this project will aid site remediation managers and researchers to make more informed decisions regarding the selection of Kd values used for contaminant transport calculations.

The Kd model is the most common method for quantifying interactions of dissolved contaminants with soils, sediments, and in situ barrier materials for risk assessment calculations and remedial investigation evaluations. Contaminant concentrations in sediment pore water and ground water are controlled primarily by the amount of contaminant present at the source, rate of release from the source, hydrologic factors, and a number of geochemical processes. Contaminant transport models quantify the geochemical interactions that occur between each contaminant and the geologic material by combining the processes into one term, the Kd.

To assist those who conduct or evaluate contaminant transport calculations, Battelle has prepared for the EPA a multivolume primer that explains:

• Derivation of the Kd model construct
• Differences between the Kd model and other approaches used to predict contaminant adsorption onto geological materials
• Techniques used to measure Kd values
• How geochemical model calculations relate to Kd prediction and evaluation
• Geochemical processes that affect the attenuation and mobility of a selected list of inorganic contaminants
• Extent of published Kd information for these inorganic contaminants and the variation of these data with respect to key geochemical processes that affect the sorption behavior for these contaminants.

The two volumes published in 1999 are available from the EPA and a third volume will be completed in early 2001.

The importance of using Kd values determined with site-specific materials and conditions for contaminant transport calculations cannot be overly emphasized. The Kd data in the literature is limited, often because ancillary information allowing assessment of the values’ applicability is not provided. Currently there is no good substitute for measurement of Kd values using site-specific samples at the appropriate site-specific conditions.

Although other approaches provide more robust mechanistic approaches for predicting contaminant adsorption, the Kd model is an integral part of the current methodologies for modeling contaminant transport. As one reviewer of these volumes noted, “Kds are the coin of the realm in this business.”

For more information, please contact Jeff Serne (509) 376-8429, jeff.serne@pnl.gov, or Ken Krupka (509) 376-4412, ken.krupka@pnl.gov.