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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Where is the DNAPL (Why Can’t I Find It)??
By
Camp, Dresser & McKee Inc.
Houston, Texas
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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