Using the Transition Assessment Teaching Assistant to Evaluate a Transition from Pump and Treat to MNA

Abstract

At many sites, chlorinated ethenes in some of the monitoring wells remain above cleanup goals, even though the water produced by the pumped wells of the pump-and-treat system meets the goals. Regulators and site risk managers need tools to determine whether it is appropriate to shut-down the pump-and-treat system and transition to monitored natural attenuation (MNA). A free decision support software (the Transition Assessment Teaching Assistant or TA²) is now available to determine whether bulk rate constants for natural attenuation in the aquifer (k_na) or rate constants for natural degradation (k_deg.) can be expected to bring the concentrations of chlorinated ethenes in monitoring wells below the cleanup goal before the groundwater reaches a point-of-compliance. TA² was evaluated at two study sites that had transitioned to MNA even though concentrations in a monitoring well were 86-fold higher than the cleanup goal at one site and 3.3-fold higher at the other. Tool 5 of TA² showed that k_na was adequate, and the risk was acceptable at both sites. At a third site, after pumping stopped, there was rebound of cis-dichloroethylene (cDCE) to concentrations that were 14-fold higher than the cleanup goal. A ¹⁴C-assay provided a rate constant for abiotic degradation of cDCE. Tool 5 of TA² showed that the rate constant for abiotic degradation of cDCE was adequate to control risk. TA² proved to be a useful addition to the toolkit of contaminant hydrology because it allowed a simple quantitative evaluation of benefits of natural attenuation processes at a site, which can inform a decision to transition to MNA.