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

1. The high volatilities of the chlorinated solvents led to a false sense of security regarding how these chemicals must be handled. Historically, it was believed that chlorinated solvent released to the unsaturated zone would easily volatilize to the atmosphere. Thus, when poured on dry ground, although a chlorinated solvent may appear to be lost entirely to the atmosphere, some will be transported into the subsurface by gaseous diffusion, by infiltration of contaminated water, and as a moving DNAPL phase. And, once contamination reaches the saturated zone (including direct releases to the saturated zone), high volatility is of little assistance in removing the solvents: transport across the capillary fringe can be exceedingly slow (McCarthy and Johnson, 1993).

2. The high densities of the chlorinated solvents (1.2 to 1.7 g/cm³ ) relative to that of water (1 g/cm³ ) mean that if a sufficient volume of a typical chlorinated solvent is spilled, then liquid solvent may be able to penetrate the water table. In the saturated zone, the unstable nature of the solvent flow causes the solvent to form thin "fingers" which can lead to the collection of large amounts of solvent in one or more "pools" on top of less permeable layers. Since a pool presents a very low cross section to on-coming ground-water flow, absolute removal rates of dissolved solvent from the pool will usually be very low (Johnson and Pankow, 1992).

3. The relatively low viscosities of the chlorinated solvents allow relatively rapid downward movement in the subsurface. Chlorinated-solvent mobility in the subsurface increases with increasing density/viscosity ratios (Cohen and Mercer, 1993).

4. The low interfacial tension between a liquid chlorinated-solvent phase and water allows a liquid chlorinated solvent to enter easily into small fractures and pore spaces, facilitating deep penetration into the subsurface. Low interfacial tension also contributes to the low retention capacities of soils for chlorinated solvents.

5. The low absolute solubilities of the chlorinated solvents (typically on the order of hundreds of mg/L) mean that when a significant quantity of such a compound is spilled on the ground surface, liquid solvent will be able to migrate as a DNAPL phase in the subsurface, potentially accumulating as one or more pools on the tops of low permeability layers. The low solubility will then permit such pools to persist for decades to centuries (Johnson and Pankow, 1992).

6. The high relative solubilities of the chlorinated solvents mean that a solvent spill can cause ground-water contamination at levels which are high relative to concentrations which appear harmful to human health.

7. The low partitioning to soil materials exhibited by the chlorinated solvents means that soil and rock materials will bind these compounds only weakly. This applies to both the unsaturated and saturated zones. Thus, sorption to soils will not significantly retard the movement of a chlorinated solvent, and zones of contamination can grow essentially as quickly as the ground water can move.

8. The low degradabilities of the chlorinated-solvent compounds, either by biological means, or by abiotic-chemical reactions, mean that subsurface lifetimes of these chemicals can be very long.