Natural Attenuation Software (NAS) is 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.

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NAS is designed for application to ground-water systems consisting of porous, relatively homogeneous, saturated media such as sands and gravels, and assumes that groundwater flow is uniform and unidirectional. NAS consists of a combination of analytical and numerical solute transport models. 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.

NAS models are implemented in three main interactive modules to provide estimates for:

1. Required Source Reduction: target source concentration required for a plume extent to contract to regulatory limits (i.e. Distance of Stabilization (DOS)),   
    
2. Time of Stabilization (TOS): time required for a plume extent to contract to regulatory limits after source reduction, and
    
3. Time of Remediation (TOR): time required for NAPL contaminants in the source area to attenuate to a predetermined target source concentration.

With the development of the initial version of NAS and application of NAS to several contaminated sites, several critical needs had been identified. Software enhancements have been implemented to improve NAS and maximize its utility for site managers. Major ugrades to NAS include:

1. General operational and functional revisions.
    
2. NAS has expanded source contaminant specification options to include chlorinated ethanes, chlorinated methanes, and chlorinated benzenes, and to allow for the analysis of any other user-defined contaminants that may be subject to microbially-mediated transformations (e.g., heavy metals, radioisotopes, etc.). Included is the capability to model co-mingled plumes, with constituents from multiple contaminant categories.
    
3. To enable comparison of remediation timeframe estimates between MNA and specific engineered remedial actions (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 is now able to model sites with existing or proposed pumping wells, based on user-specified site hydrogeologic and source data.

    
4. NAS also includes expanded analysis tools for improved performance assessment. Sustainability of natural attenuation processes over time may also be assessed. Users may analyze historically successive, site-measured, contaminant concentration and redox indicator data, to allow for comparisons of NAS-predicted versus observed trends, and to monitor changes in natural attenuation capacities and redox conditions over time.

NAS development is funded by Southern Division, Naval Facilities Engineering Command (NAVFAC) and Naval Facilities Engineering Service Center (NFESC) , and has resulted from a continuing collaboration between Virginia Tech, United States Geological Survey (USGS), and NAVFAC.

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