WaterSubject

Although it is well documented that ground water extraction (i.e., pump and treat) has limited potential for restoring aquifers to pristine conditions at most sites, it is still the most widely used technology for controlling plume migration and protecting potential receptors. Much of the early work in remediation hydrogeology focused on ground water capture problems, and various methods using analytical solutions and numerical models have been developed for capture design and assessment. One of the most common approaches for capture analysis is to compute the steady-state flow field with conventional ground water modeling software like MODFLOW followed by backward or forward advective particle tracking software such as MODPATH. Although this approach is straightforward and efficient, it does not provide information regarding the uncertainty about the delineated capture zone that could be associated with data scarcity or model discretization. Further, the pathline analysis method does not readily identify the capture zones associated with specific extraction wells, and therefore determining well-specific minimum flow values necessary to maintain capture is difficult.

In this paper, we present the tool MODular ALLocation (MODALL), which is designed to aid in capture zone analysis. As opposed to other conventional methods, MODALL directly and quantitatively determines capture by computing the fraction of flow in an individual cell that will eventually contribute to the flow removed by specific extraction wells. Therefore, the primary advantages with MODALL are that areas of uncertain capture in the modeled capture zone are readily communicated and that the hydraulic influences of specific injection and extraction wells are explicitly calculated. This greatly aids in comparing the relative merits of different designs (e.g., well locations, pumping rates), thereby streamlining the optimization process. Another advantage is that MODALL interfaces with the widely used and accepted numerical flow code MODFLOW. Additionally, we have found that the results from MODALL are easy to communicate to hydrogeologists and engineers, as well as nontechnical stakeholders.

MODALL was initially developed at ARCADIS U.S. Inc. (formerly Geraghty & Miller Inc.) more than a decade ago and has been applied at numerous remediation projects throughout the United States. MODALL was only recently first presented at a conference to the general ground water modeling community; this method note is the first appearance of the tool in the refereed literature. The tool is available free of charge for unrestricted use from the authors. No warranties are stated or implied regarding the accuracy or reliability of this software, and the user assumes all liability and responsibly associated with its use and application.

The original concept of MODALL was developed in the early 1990s in the context of a unique water supply problem. Four different ground water supply wells provided water to a town through a closed pipe system, and each water supply well had its own associated trichloroethene (TCE) plume. The client was responsible for one of the contamination sources and needed to determine what percentage of the contamination at various points in the system was associated with the plume it was responsible for as a basis for responsibility allocation and compensation programs. The approach was simple, assuming that all the water and TCE in the pipe system were completely mixed and a division of flow at a system junction resulted in an equal division of contaminant and water. A flow allocation code was then developed based on complete mix theory. That tool evolved to the current version of MODALL that is presented in this paper.

MODALL uses the MODFLOW-calculated cell-by-cell flow terms to evaluate internodal flow balances to determine the percentage of flow in a particular cell that has either originated from a given source or flows to a specified sink. The flow allocation is based on complete mix theory; the water entering or exiting a block from a specified source can be represented as a function of the ratio of flow rates exiting the block. The general method and steps are described subsequently:

The output from MODALL is formatted as a MODFLOW head-save file that can be easily imported into any graphing software for display and interpretation. Isopleths of CF are generated to evaluate the capture zone and identify the strength and certainty of capture in different areas. Based on these plots, the capture system can be quickly revised to improve efficiency (i.e., wells locations and pumping rates are modified) by minimizing overextraction in areas of high capture certainty and increasing capture certainty in key areas of interest. The plots can also readily identify areas of potentially high-value data for future field characterization efforts.