Natural Attenuation Software (NAS): Assessing Remedial Strategies and Estimating Timeframes

Natural Attenuation Software (NAS) was developed as a screening tool to estimate remediation timeframes for monitored natural attenuation (MNA) to lower groundwater contaminant concentrations to regulatory limits, and to assist in decision-making on the level of source zone treatment in conjunction with MNA using site-specific remediation objectives. In addition, NAS facilitates the combined use of MNA with engineered remedial actions (ERAs) so that the benefits of each technology can be maximized while minimizing costs of remediation. The primary expected benefit of NAS is to increase regulatory acceptance of MNA, thereby decreasing overall remediation costs.

NAS is designed for application to ground-water systems consisting of porous, relatively homogeneous, saturated media, and assumes that groundwater flow is uniform and unidirectional. NAS consists of a combination of analytical and numerical solute transport models 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 NAPL 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 advection, dispersion, sorption, non-aqueous phase liquid (NAPL) dissolution, and biodegradation. NAS determines redox zonation, and estimates and applies varied biodegradation rates from one redox zone to the next.

Recently, NAS was enhanced to include petroleum hydrocarbons, chlorinated ethenes, chlorinated ethanes, chlorinated methanes, and chlorinated benzenes, or any user-defined contaminants (e.g., heavy metals, radioisotopes), and has included the capability to model co-mingled plumes. To enable comparison of remediation timeframe estimates between MNA and specific ERAs, NAS was modified to incorporate an estimation technique for timeframes associated with pump-and-treat remediation technology for comparison to, or in conjunction with, MNA. NAS also expanded analysis tools for improved performance assessment, as well as the assessment of sustainability of natural attenuation processes over time.

A Department of Defense (DoD) Environmental Security Technology Certification Program (ESTCP) demonstration was undertaken to evaluate the capability of the NAS software to provide reasonable estimates of MNA cleanup timeframes in a variety of environments and sites throughout the United States. Overall, results suggest that NAS was satisfactory in meeting performance objectives set forth in the demonstration, and that because NAS is based on sound science, it can serve as an effective tool for decision-making and data analysis at a wide range of contaminated sites and is not limited to a small subset of “simple sites” because of its simplicity. At some sites, NAS-estimated timeframes were crucial for winning regulatory acceptance of MNA, with cost-benefit analyses providing estimates of savings associated with using MNA as a final remediation strategy.