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, U.S. Environmental Protection Agency 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 (Natural Attenuation Software, NAS) was developed for estimating time of remediation (TOR) for MNA with varying degrees of source area remediation. The objective of this project was to demonstrate and validate NAS at eight sites representative of different hydrologic settings where different contaminant removal actions had been used in the recent past using a consistent set of procedures and metrics for comparing predicted and observed times of remediation.

The Natural Attenuation Software screening tool was co-developed by the U.S. Navy, U.S. Geological Survey (USGS), and Virginia Tech. NAS is designed to make complex analytical and numerical solutions of the TOR problem accessible to remedial project managers (RPM) and their contractors. NAS consists of a combination of computational tools implemented in three main interactive modules to provide estimates for target source concentration required for a plume extent to contract to regulatory limits; time required for contaminants in the source area to attenuate to a predetermined target source concentration; and time required for a plume extent to contract to regulatory limits after source reduction. NAS estimates TOR through consideration of multiple natural attenuation processes, including advection, dispersion, sorption, nonaqueous phase liquid (NAPL) dissolution, and biodegradation of petroleum hydrocarbons, chlorinated solvents, or any user-specified contaminants or mixtures. Simulations also account for variable rates of biodegradation based on specific redox zone conditions.

Demonstration of NAS was conducted using long-term performance monitoring data from eight sites: Seneca Army Depot and a USGS study site in New York, NAES Lakehurst in New Jersey, Hill AFB in Utah, NSB Kings Bay in Georgia, NAS Cecil Field and NAS Pensacola in Florida, and a USGS study site in Alaska. NAS was effective in predicting the time of stabilization of concentrations at downgradient monitoring wells located relatively close to the source (within 100 to 700 ft) following source remediation and a reduction in groundwater contaminant concentrations in the source zone. Accuracy of the solution relative to the observations was impacted by dynamic behavior of the plume over time. NAS also was effective in capturing concentration time trends of natural source depletion of a multi-component NAPL, providing a prediction that was superior to a comprehensive numerical model that was not based on source zone mass balance.

This project led to improvements to the NAS code and a guidance document that will help DoD RPMs and their contractors better utilize NAS to estimate remediation time frames and improve decision-making to achieve remedial action objectives. The applicability and versatility of NAS was demonstrated at chlorinated solvent-contaminated sites representing a wide range of hydrogeologic conditions and remediation scenarios. By documenting the effectiveness of NAS as a tool to predict time of remediation and through integration with a National Ground Water Association short course on NAS, regulatory acceptance will be facilitated. Furthermore, management decisions regarding source zone treatment (including cost-effective implementation of MNA strategies) can be enhanced through appropriate data collection activities driven by model input requirements followed by NAS simulations and cost benefit analyses. (Project Completed - 2007)