The objective of this project was to equip the groundwater community with accessible, easy-to-use, and useful models to evaluate matrix diffusion effects. Results of the Air Force Civil Engineering Center (AFCEC) and SERDP research projects were developed into analytical matrix diffusion models. These models were used to create a decision support system that will enable users to enter site-specific parameters that can be used to predict matrix diffusion effects. The specific objectives were to (1) develop a decision support system equipped with planning level Mass Discharge Module and Concentration Module models; (2) create an interactive help guide and user’s manual; (3) test the decision support system using data from ESTCP and SERDP projects; and (4) finalize the decision support system and make it available as a free, easily downloadable software package.

Based on the Microsoft Excel™ platform, the Matrix Diffusion Toolkit is an easy-to-use, comprehensive, free software tool that can assist site personnel to effectively and efficiently estimate what effects matrix diffusion will have at their site, and transfer the results to stakeholders. Furthermore, the software can assist project managers in determining if remediation goals are achievable in the short term. The Toolkit can be applied to virtually any site with heterogeneity in the subsurface, dense non-aqueous phase liquid (DNAPL), and/or where persistent groundwater contaminant concentrations have been observed after source-zone remediation.

The Toolkit provides the following tools to calculate and evaluate matrix diffusion effects:

1. Square Root Model

   A module to provide planning-level estimates of the mass discharge (in units of grams per day) caused by release from a low-k diffusion-dominated unit (typically silt or clay) into a high permeability advection-dominated unit (typically sand or gravel). Estimates of concentration and mass remaining in the high permeability unit, after the source is removed, are also provided.    

2. Dandy-Sale Model

   A module allowing users to perform: 1) contaminant transport via advection and transverse diffusion in the transmissive layer, and 2) transport via transverse diffusion in the low-k zone. The module provides planning-level estimates of:  Low-k Zone:i) Aqueous, sorbed, and total concentration.ii) Aqueous, sorbed, and total mass.  Transmissive Zone:i) Aqueous, sorbed, and total concentration.ii) Aqueous, sorbed, and total mass.iii) Mass discharge. 

3. Matrix Diffusion Related Tools

   An additional feature that provides a review of theory and methods related to matrix diffusion:

   a. NAPL Dissolution Calculator: A module that estimates the transverse diffusion of contaminants into the groundwater passing over the top of a non-aqueous phase liquid (NAPL) pool and the transverse diffusion of contaminants into the low-k unit underlying the pool.

   b. Plume Magnitude Information: A summary of the Plume Magnitude Classification System (Newell et al., 2011) and its application to site investigation and remediation.

   c. Low-k Zone Remediation Alternatives: A summary of current alternatives for the remediation of low-k zones.

   d. 14-Compartment Model: A discussion on the quantitative application of the 14-compartment model. 

A comprehensive dynamic decision support tool can provide site stakeholders the information needed to characterize, understand, design, and predict the relevance and appropriateness of remediating sites affected by matrix diffusion. The Matrix Diffusion Decision Support System provides a valuable tool for supporting site characterization efforts and determining if matrix diffusion will affect remediation goals for Department of Defense (DoD) sites. The decision support system will provide an improved understanding of matrix diffusion and help DoD site stakeholders determine if matrix diffusion processes are significant enough to cause "rebounding" groundwater concentrations of downgradient plume concentrations above remediation goals after plume remediation or isolation is complete. The decision support system will have the immediate benefit of summarizing existing matrix diffusion information and experience into one source. Having this information readily available before a remedy is implemented could assist DoD site stakeholders select more appropriate remedies and improve effective risk communication with regulators and the public. Furthermore, the software will assist project managers in determining if remediation goals are achievable in the short term. Cost savings can be realized by addressing extended remediation time frames caused by matrix diffusion.