Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies

This issue paper explains when and how to apply first-order attenuation rate constant calculations in monitored natural attenuation (MNA) studies. First-order attenuation rate constant calculations can be an important tool for evaluating natural attenuation processes at groundwater contamination sites. Specific applications identified in U.S. EPA guidelines (U.S. EPA, 1999) include use in characterization of plume trends (shrinking, expanding, or showing relatively little change), as well as estimation of the time required for achieving remediation goals. However, the use of the attenuation rate data for these purposes is complicated as different types of first-order rate constants represent very different attenuation processes: Concentration vs. time rate constants (kpoint) are used for estimating how quickly remediation goals will be met at a site. Concentration vs. distance bulk attenuation rate constants (k) are used for estimating if a plume is expanding, showing relatively little change, or shrinking due to the combined effects of dispersion, biodegradation, and other attenuation processes. Biodegradation rate constants ( λ ) are used in solute transport models to characterize the effect of biodegradation on contaminant migration. Correct use of attenuation rate constants requires an understanding of the different attenuation processes that different first-order rate constants represent.