INTRODUCTION
Two of the most problematic topics in contaminant hydrogeology are chlorinated solvents and karst (underlined terms can be found in Glossary) aquifers. Chlorinated solvents are one of the most widespread classes of ground-water contaminants in the United States (Pankow and Cherry, 1996). The physical and chemical properties of these organic chemicals make them difficult to find or remove once they have entered the subsurface (Schwille, 1988; Cohen and Mercer, 1993; Pankow and Cherry, 1996). Karst aquifers are units of water-soluble rocks, such as limestone, in which chemical dissolution has enlarged joints, bedding planes, and other water-transmitting openings (White, 1988; Ford and Williams, 1989; Quinlan and others, 1992). Chemical dissolution of water-transmitting openings gives karst aquifers hydraulic properties that differ radically from those of other aquifers (Ford and Williams, 1989; Quinlan, 1989; Field, 1993). The presence of either chlorinated solvents or karst development at a contaminated site increases the complexity, difficulty, and expense of site characterization and mitigation (Quinlan, 1989; Cohen and Mercer, 1993; Field, 1993; Pankow and Cherry, 1996), but the combination of these two factors is especially problematic. Because chlorinated DNAPL's are denser and less viscous than water, they migrate readily through subsurface openings, both above and below the water table (Cohen and Mercer, 1993). Karst aquifers are distinguished by an abundance of large subsurface openings and are therefore especially vulnerable to chlorinated-solvent contamination. The release of chlorinated solvents into karst aquifers presents a difficult challenge to environmental scientists, managers, and regulators.
Chlorinated solvents, such as perchloroethylene (PCE) and trichloroethylene (TCE), are widely used in manufacturing, cleaning and degreasing, and other industrial operations (Schwille, 1988; Pankow and Cherry, 1996). These compounds are generally produced and used as organic liquids but, when released into the environment, can also exist in other phases. Chlorinated solvents can sorb to organic matter or mineral surfaces, volatilize into vapor, or dissolve into aqueous solution. In aqueous solution, chlorinated solvents can enter drinking-water supplies and threaten human health. Drinking-water standards for chlorinated solvents are typically three to six orders of magnitude lower than their solubilities in water (Pankow and Cherry, p. 14, 1996). A relatively small amount of chlorinated solvent has the potential to contaminate ground water over a large area for decades or longer (Cohen and Mercer, 1993; Pankow and Cherry, 1996).
Until the 1970's, disposal of chlorinated solvents was routinely carried out by pouring the solvents onto the land surface in the expectation that they would quickly evaporate. Such practices resulted in large quantities of chlorinated solvents being introduced to the ground-water system. Since scientists first became aware of ground-water contamination from chlorinated solvents (Schwille, 1988), these compounds have been documented at hundreds of sites in the United States (Pankow and Cherry, 1996). Restoring ground-water quality in and around sites where chlorinated solvents have been released requires an understanding of how these contaminants behave in the subsurface. To date, the vast majority of research on the subsurface behavior of chlorinated solvents has focused on nonkarst settings (Cohen and Mercer, 1993).
The term "karst" has been defined many ways (Cvijic, 1893; Sweeting, 1972; Quinlan, 1978; Jennings, 1985; Quinlan and others, 1992). In this report, "karst" refers to a landscape underlain by rocks in which chemical dissolution has enlarged joints, fractures, bedding planes, or other openings through which water flows (Quinlan and others, 1992). In Tennessee, this definition applies to areas underlain by carbonate rocks--more than two thirds of the State (fig. 1). The carbonate rocks of Tennessee are generally of Paleozoic age--about 230 to 600 million years old (Miller, 1974). These rocks vary considerably in lithology and hydraulic characteristics, but in comparison with many younger carbonate rocks, they have high bulk density and low primary porosity (Brahana and others, 1988). Because primary pores, the spaces between the original sediment grains, have been filled through compaction and cementation during geologic time, the major flow paths for water and other fluids within these dense carbonate rocks are secondary openings such as fractures, joints, and bedding-plane partings. The frequency of such openings and the degree to which they have been enlarged by chemical dissolution are the major factors determining the hydraulic characteristics of dense carbonate rocks in Tennessee and in similar geologic settings elsewhere.
Karst aquifers within carbonate-rock units are important sources of water supply across most of Middle Tennessee and large areas of East Tennessee. In 1990, karst aquifers provided an average 1.84 x 105 cubic meters (48.6 million gallons) per day of water to Tennessee public water supplies--about 18 percent of ground-water withdrawals for public supply statewide and 87 percent of public-supply withdrawals in Middle and East Tennessee (Hutson, 1995). Approximately 15 percent of households in carbonate-rock areas of Tennessee, about half a million people, rely on private wells and springs for their water supply (estimate based on data from U.S. Bureau of Census, 1996). The importance of karst aquifers to water supply and their vulnerability to contamination by chlorinated solvents are reasons to seek improved understanding of how chlorinated solvents behave in karst aquifers.
The U.S. Geological Survey (USGS), in cooperation with the Tennessee Department of Environment and Conservation, Division of Superfund, is conducting a study of the occurrence, fate, and transport of chlorinated solvents in karst regions in Tennessee. One objective of this study is to develop conceptual models that describe the occurrence, fate, and transport of chlorinated solvents in karst aquifers of Tennessee.
This report presents preliminary results of the conceptual-model study of chlorinated solvents in karst settings. The scope of the report encompasses review and synthesis of relevant literature, examination and summary of data from 22 sites in Tennessee where chlorinated-solvent releases have occurred, and the development of conceptual models of chlorinated-solvent behavior in karst aquifers. The objectives of the report are to:
The body of the report is divided into five sections. The first section is a general overview of previous studies dealing with chlorinated-solvent contamination, contamination of karst aquifers, and the relatively few published studies of chlorinated-solvent contamination in karst. The second section provides a more detailed treatment of the physical and chemical properties of chlorinated solvents and how these properties affect the phase transformations and movement of chlorinated solvents in the subsurface. The third section reviews the characteristics of karst aquifers and their geological controls and presents a regionalization of karst settings in Tennessee with a description of each karst region. The fourth section integrates material from the previous sections to present five conceptual models of the accumulation of chlorinated solvents in different parts of karst aquifers and the implications of such accumulation for contaminant residence time, mitigation, and delivery to water supplies. Finally, the fifth section describes specific instances of chlorinated-solvent contamination in karst regions of Tennessee, with emphasis on characterization of site hydrogeology, contaminant-source-mass distribution, and the movement of dissolved contaminants into surrounding ground water.
The conceptual models presented in this report are preliminary in nature and are intended to serve as starting points in the site-specific analysis of chlorinated-solvent contamination in karst settings. Nothing in this report in any way reduces the critical importance of careful characterization of the environmental settings and contaminant distributions at specific sites. The geographic focus of this report is the karst regions of Tennessee. However, the concepts developed in the following sections are intended to be transferable to similar karst settings in dense carbonate rocks elsewhere.
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