Behavior of Dense, Immiscible Solvents in Fractured Clay-rich Soils - Final Report - 09/10/1996 - 09/14/2000

Abstract

This project investigated the behavior of chlorinated solvent DNAPLs (mainly TCE) in two fractured clay-rich materials: highly weathered shale saprolite; and weathered glacial till. Over the 4-year course of this project researchers: (1) Investigated the potential for biodegradation of chlorinated solvents in fractured and weathered shales and in microcosms and undisturbed columns of fractured shale saprolite, which indicates that anaerobic degradation of TCE and its daughter products is occurring in the upper portion of the bedrock, and possibly in the overlying saprolite, suggesting that natural attenuation may be a viable option for organic contaminants. It also confirms that the TCE degradation is microbially mediated, and that microbial communities can rapidly shift to facilitate biodegradation, even in fractured clay-rich materials. (2) Investigated the factors controlling migration of chlorinated solvent DNAPLs in fine-grained, highly structured soils and weathered shale bedrock , which indicated that DNAPL is likely to enter both the fractures and the matrix at many sites, where it would be virtually impossible to remove with DNAPL recovery wells. The study also shows that after only 2-3 weeks, almost all of the TCE has dissolved and spread into the fine-grained matrix pores adjacent to the fractures and other macropores, indicating that it would not be feasible to use methods such as pump and treat to remove DNAPL contaminants from this type of material. (3) Investigated the influence of ''matrix diffusion'' on the dissolution and apparent disappearance of residual DNAPL, which indicates that there is a wide range of fracture and matrix pore types and sizes in the saprolite, and that there is much greater variability in the matrix lithology and pore size distribution than previously expected. (4) Compared DNAPL behavior in different types of fractured clay-rich materials, which showed that TCE can enter very small fractures at relatively low pressure heads, an d confirmed that matrix diffusion plays an important role in controlling the rate of dissolution of TCE residuals in fractures, which largely dissolve within a month or two of the initial contaminant ''spill''.