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Abstract: Where is the DNAPL (Why Can’t I Find It)??
Camp, Dresser & McKee Inc.
The large number of sites contaminated by dense non-aqueous phased liquids (DNAPLs) such as chlorinated aliphatic hydrocarbons (CAHs) are the result of ineffective or improper disposal methods. The standard disposal method, during early applications of CAHs, involved the dumping or releasing of solvents onto dry ground or into nearby ditches or impoundments. This method was considered appropriate because it was assumed that various solvents would completely volatilize to the atmosphere before infiltrating the soil.
Field evidence clearly indicates that gravity and soil capillarity cause infiltration and vertical migration of the DNAPLs into the subsurface. CAHs released to the subsurface reach a steady-state condition in which the amount of CAHs adsorbed to the soil and dissolved in groundwater does not change, as long as the CAHs are present as a DNAPL. At this point, equilibrium exists between three phases (NAPL, solid/soil, and aqueous). A CAH will remain as a NAPL, adsorbed to soil, dissolved in groundwater, or volatilized into soil gas (if in the vadose zone) to the extent defined by the physical and chemical properties of the individual CAHs and the subsurface environment. Equilibrium changes and the higher specific gravity of DNAPL allow the CAH to continue migrating vertically through the saturated zone until the DNAPL volume is eventually exhausted to residual saturation or until it reaches a low-permeability formation where it begins to migrate laterally.
DNAPLs can be hard to locate for three reasons: most DNAPLs are at residual saturation levels and cannot flow to a sample point, in fractured media large amounts of dissolved contamination can diffuse into the porous rock matrix (this can happen in both hard rock and in the clays found along the Gulf Coast), and often sample locations are not ideal for locating the DNAPL.
The presentation will discuss the 1% Rule developed by Dr. John Cherry for chlorinated solvents. This rule was not intended to provide guidance for locating DNAPL. This rule was introduced (published by the US EPA ) in the late 1980s to counter the view that subsurface DNAPL occurrence is improbable anywhere at a site unless actual free product is found. Now the rule is commonly used to indicate DNAPL locations. This is an invalid application of the rule.
The importance of developing a conceptual site model (CSM), especially for evaluating the Technical Impracticability (TI) as a DNAPL remedial alternative, will also be discussed. The EPA’s 1993 “Guidance for Evaluating the Technical Impracticability of Groundwater Restoration” outlines three general factors that can inhibit groundwater restoration and justify the granting of a TI waiver:
1) Hydrogeologic factors;
2) Constituent-related factors; and
3) Remedial technology inadequacies.
Geologic complexities combined with chlorinated solvent behaviors are the most common reasons for determination of TI. To demonstrate TI requires 1) a conceptual site model and 2) an evaluation of restoration potential. The presentation includes case studies from CAH sites that support the discussion of DNAPL characterization and remediation.
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