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.
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:
Required Source Reduction: target source
concentration required for a plume extent to contract to regulatory limits
(i.e. Distance of Stabilization (DOS)),
Time of Stabilization (TOS): time required for a
plume extent to contract to regulatory limits after source reduction, and
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:
General operational and functional
revisions.
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.
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.
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.