Matrix Diffusion Challenges & Potential Solutions Contamination that seeps into low permeability soils provides tough treatment conditions. There is now a treatment alternative. By CHARLES NEWELL, ROBERT BORDEN and ED ALPERIN T he history h h i is t to ry of o f ch chlorinated h l lo ri i na t te d dra-solvent cleanup has dra matically changed over the years. Prior to the 1970s, the problems caused by chlorinated solvent releases were basi-cally unknown  . Then in the 1970s and 1980s, the presence of dissolved solvents in mobile groundwater was recognized. This was followed by the “DNAPL Paradigm” in the 1990s, where the chlorinated solvent concep-tual model was expanded to include the presence of Dense Non-aqueous Phase Liquids (DNAPL) in source zones. Despite expensive efforts by site owners, remediation specialists and researchers, complete cleanup of chlorinated solvent sites to drinking water standards, such as the Maximum Concentration Level (MCL), remains the exception rather than the rule [2, 3] . The difficultly in remediating chlo-rinated sites led researchers and reme-diation specialists to reexamine the basic processes controlling site cleanup. In the 22 Pollution Engineering JUNE 2012 Figure 1. Depiction of matrix diffusion process. During the loading period, a solvent plume moves through the transmissive sand, and then diffuses into the non-flowing low-permeability silts immediately above and below the plume. When the contaminants are removed from the transmissive sand (either naturally or by active remediation), the back diffusion period begins and contaminants move from the low permeability silts and re-contaminate the transmissive sand.