There are a number of state and federal regulatory documents that prescribe the use of solute transport models to support site investigation and remediation activities, along with guidelines on the types of analyses that need to be performed, models that should be utilized, and in some instances, specific input parameters that must be incorporated into analyses. Common applications of solute transport models in state and federal guidance documents include analyses to estimate the expected extent and duration of groundwater plumes, whether sensitive receptors may be impacted, and as a line of evidence to support Monitored Natural Attenuation (MNA) evaluations. In this first post in the “State and Federal Guidance Spotlight” series, we take a look at the New Jersey Department of Environmental Protection’s (NJDEP’s) Classification Exception Area Guidance, and how TS-CHEM can be used to estimate plume extent and duration to support the delineation of Classification Exception Areas for sites where impacts to groundwater may be present.
Overview of NJDEP CEA Guidance
In the state of New Jersey, groundwaters of the state are classified according to a combination of natural characteristics and actual or potential uses, and groundwater quality standards (GWQS) have been established for these classification areas to ensure that the characteristics and/or actual and potential uses are protected. In instances where GWQS may not be met in a particular area (e.g., an area where impacted groundwater may be present as a result of a discharge of contaminants into the subsurface), the NJDEP requires the establishment of a Classification Exception Area (CEA), which provides notice that the constituent standards for a given aquifer classification are not (or will not) be met over a particular localized area, and that designated aquifer uses in the affected area are suspended for the duration of CEA term.
CEAs have three main components, including 1) delineation of the horizontal and vertical boundaries of the affected exception area; 2) identification of all groundwater constituents of concern (COCs) to which the exception applies; and 3) an estimate of the longevity of the CEA (i.e., the duration in which COCs will remain above GWQS within the exception area). In the event that the designated use of groundwater within the CEA includes potable use, the NJDEP will identify the CEA as a Well Restriction Area (WRA), which functions as an institutional control by which potable use restriction can be effected (though the NJDEP will not typically prohibit the installation of wells in WRAs).
Appendix A of the NJDEP CEA Guidance provides an overview of the methods that may be used for CEA delineation, and in particular, how to estimate the amount of time required for COCs to reach the GWQS, and the distance in which COCs are anticipated to migrate. With regard to the latter, as long as a sufficient amount of sampling data have been collected, the NJDEP recommends that a “best-fit” methodology to estimate COC attenuation rates (such as the methodology described in the 2003 USEPA issue paper “Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies”), which can be used to calculate the anticipated time to reach the GWQS. With regard to the estimation of the maximum distance a plume is expected to travel, the NJDEP identifies several different numerical models that may be used, as well as a more simple analytical solution, which is often utilized by environmental practitioners responsible for the delineation of CEAs at sites in the state of New Jersey. As discussed in a recent TS-CHEM Blog Post, however, a 1-dimensional analysis of contaminant transport (such as the one described in Appendix A of the NJDEP CEA Guidance) often greatly overestimates plume extent, resulting in an overly-conservative assessment. Fortunately, TS-CHEM provides a library of analytical and semi-analytical solute transport modeling solutions that can be used to estimate both the length and duration of groundwater plumes more accurately, without the steep learning curve typically associated with more sophisticated numerical models.
CEA Delineation Using TS-CHEM
As noted above, TS-CHEM, with a library of more than 30 analytical solutions, is perfectly suited to analyze the length and duration of groundwater plumes to assist with CEA development. The library of solutions allows for the selection of model that is best suited to conditions at a particular site. For example, at a site where source concentrations appear to be attenuating exponentially, a model like BIOSCREEN-AT may be a good fit. Or, if source concentrations are variable over time (e.g., as a result of site remediation activities), a model like ATRANS4 may provide the best solution. And since these models are semi-analytical solutions that analyze the transport of COCs in three dimensions, they are not overly conservative, and as such, will not result in an estimated plume length (and CEA delineation) that is much larger than it is realistically likely to be.
In addition to allowing for the selection of a model solution that best fits conditions at a site, TS-CHEM’s built-in charting and mapping tools can be used to generate all of the necessary output for documenting your analyses (as required when submitting a CEA application), including:
- Charts showing concentration vs. distance and concentration vs. time
- Plots showing plume extents through time
- Mapping tools that allow for the overlay on an interactive digital map of the maximum plume extent in the direction of groundwater flow; or the mapping of plume extent through time to support an application at some point in time for a reduction of the CEA area
|Figure 1 - Use of TS-CHEM mapping tool to overlay depiction of maximum plume extent in the direction of groundwater flow|
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