Estimating Cleanup Times Associated With Combining Source-Area Remediation With Monitored Natural Attenuation

Abstract : Under suitable conditions, monitored natural attenuation (MNA) can be a cost-effective strategy for restoring contaminated aquifer systems either as a stand-alone technology or in combination with other engineered remedial actions. However, the Environmental Protection Agency (EPA) guidance specifically requires MNA to achieve site-specific cleanup objectives within a reasonable time frame. Thus, it is necessary to provide estimates of cleanup times whenever MNA is proposed as part of a cleanup strategy. In response, a screening tool NAS was developed for estimating time of remediation (TOR) for MNA with varying degrees of source area remediation. NAS is designed to make complex analytical and numerical solutions of the TOR problem accessible to remedial project managers (RPM) and their contractors using site-specific remediation objectives. Conventional screening tools for MNA are not designed to address source zone remediation options or simulation plume reduction. The Natural Attenuation Software tool was co-developed by the U.S. Navy, USGS, and Virginia Tech. NAS consists of a combination of computational tools implemented in three main interactive modules to provide estimates for: (1) target source concentration required for a plume extent to contract to regulatory limits, (2) time required for contaminants in the source area to attenuate to a predetermined target source concentration, and (3) time required for a plume extent to contract to regulatory limits after source reduction. Natural attenuation processes that NAS models include are advection, dispersion, sorption, non-aqueous phase liquid (NAPL) dissolution, and biodegradation of petroleum hydrocarbons, chlorinated solvents, or any userspecified contaminants or mixtures. The objective of this demonstration is to evaluate the NAS capability to provide reasonable estimates of MNA cleanup time frames in a variety of environments and sites throughout the United States.