Preliminary Conceptual Models of the Occurrence, Fate, and Transport of Chlorinated Solvents in Karst Regions of Tennessee

SUMMARY AND CONCLUSIONS

Two of the most problematic topics in contaminant hydrogeology are chlorinated solvents and karst. Chlorinated solvents have physical, chemical, and biological properties that make this class of compounds particularly likely to cause ground-water contamination. The high densities and low viscosities of chlorinated solvents allow them to move readily downward as a DNAPL through the subsurface due to gravity. The same properties that make chlorinated solvents potent ground-water contaminants make them difficult to locate or remove once they enter the ground-water system. Nowhere is this more true than in karst settings.

Many publications deal with chlorinated-solvent contamination and contamination of karst aquifers, but relatively few published studies address chlorinated-solvent contamination in karst. Previous studies relevant to this report include (1) studies of chlorinated-solvent properties, (2) studies of karst properties and karst hydrogeology, and (3) studies of aqueous-phase contamination of karst.

The physical and chemical properties of chlorinated solvents govern their behavior in the environment. The exact pathway of downward migration is influenced by such factors as interfacial tension, capillary pressure, hydraulic gradients, structural controls, and the type and nature of openings in an aquifer. Effects of capillary pressure explain much of the distribution and movement of subsurface DNAPL. Dissolution-enlarged fractures in carbonate aquifers have apertures of a scale sufficiently large that entry pressures will be easily overcome by even a thin pool of DNAPL. In large fractures (greater than 1 cm), capillary forces will be insignificant and DNAPL will drain freely under the influence of gravity.

Carbonate rocks underlie most of Middle Tennessee and large areas of East Tennessee. Karst aquifers contain a variety of flow regimes, ranging from rapid turbulent flow in freely draining conduits to slow laminar flow through bedrock-fracture networks, cave sediments, or residuum. The carbonate areas of Tennessee can be divided into regions based on geologic structure, stratigraphy, relief, unconsolidated cover, and karst landforms. Six karst regions are considered in this report: (1) the inner Central Basin, (2) the outer Central Basin, (3) the Highland Rim, (4) the coves and escarpments of the Cumberland Plateau, (5) the Valley and Ridge, and (6) the western toe of the Blue Ridge. Thickness of karst regolith across the State ranges from less than 1 m in the inner Central Basin to greater than 38 m in the Valley and Ridge. Topographic relief varies from nearly flat in the inner Central Basin to steeply sloping along the escarpments of the Cumberland Plateau.

The extensiveness of karst aquifers and their distinctive hydraulic properties makes these aquifers vulnerable to contamination by chlorinated solvents. DNAPL-accumulation areas within an aquifer are important because they are source zones for dissolved-phase contamination. The five conceptual models described in this report emphasize DNAPL accumulation in different compartments of the subsurface environment. The models were developed with the karst environments of Tennessee in mind, but are intended to be transferable to karst settings in adjacent states and broadly applicable elsewhere.

The five models of DNAPL accumulation in karst settings are

1. trapping in regolith,
2. pooling at the top of bedrock,
3. pooling in bedrock diffuse-flow zones,
4. pooling in karst conduits, and
5. pooling in isolation from active ground-water flow.

The conceptual models presented in this report are preliminary. They are intended to be starting points for analysis of chlorinated-solvent contamination in karst settings and site-specific studies. Nothing in this report in any way reduces the critical importance of careful characterization of the environmental settings and contaminant distributions at specific sites. Every contaminated site is unique and requires an individual characterization to successfully understand the site and to plan for mitigation. Under the best of circumstances, chlorinated solvents are difficult to find and recover once they have entered the subsurface. The complexity and variability of karst aquifers greatly compound this difficulty.

Twenty-two case studies were selected from the Tennessee Division of Superfund files to evaluate chlorinated-solvent contamination in the karst regions of Tennessee. Only 4 of the 22 case studies document visual evidence of DNAPL although the presence of DNAPL can be inferred from historical and field data for many of the sites. Five case studies are presented in detail to illustrate the relevance of their specific site characteristics to the conceptual models. DNAPL pools are documented at the top of rock 6 m below land surface at one site; in regolith, less than 11 m below land surface at another site; and in fractured rock, 72 m below land surface at a third site.