Groundwater NAPL dissolved

High airflow rates may result in unintended fracturing leading to nonuniform flow or short-circuiting of injected air in the subsurface, or may result in unintended mobilization of contaminants as nonaqueous phase liquids (NAPL), dissolved in groundwater, or in soil gas. 

It is assumed that the contaminant enters the water table or aquifer at a concentration near its solubility limit, although there is no practical means to verify this. This method is more favorable when the release occurred as a single, short-term episode. A long-term release from a continuing source would result in a date that more closely represents the last date upon which the contaminant entered the aquifer at or near its solubility limit. Should the contaminant enter the aquifer below its solubility limit, then a date earlier than the actual event would result. Conversely, should the contaminant enter the aquifer as NAPL for a period of time, a date in which all the NAPL dissolved in groundwater would result. If NAPL was present when measurements were obtained, then the zone of highest concentration would... 

Free-phase NAPL refers to NAPL that exists as an independent phase, not as a dissolved component in the pore water or pore atmosphere. The environmental concerns associated with sites affected with free-phase NAPLs revolve around hydrocarbon-impacted soil (residual hydrocarbon), the NAPL itself (which can serve as a continued source for groundwater contamination), dissolved hydrocarbon constituents in groundwater, and hydrocarbon vapors. The detection of free-phase NAPLs in the subsurface presents many challenges. Two questions frequently arise at sites impacted by NAPLs how much is there and how long will it take to clean up. Before one can address these two questions, assessments of the type and subsurface distri-... 

Environmental issues associated with the subsurface release of petroleum hydrocarbons and other organics fall into four areas (1) vapors (Figure 1.5), (2) impacted soils, (3) the presence of nonaqueous phase liquids (NAPLs), and (4) dissolved constituents (i.e., benzene, toluene, ethylbenzene, and xylenes (BTEX), and other components) in groundwater. 

The practice of free-phase NAPL recovery, soil vapor extraction, and hydraulic containment at remediation sites almost always generates some volume of water contaminated with dissolved fractions. Depending on the size of the facility and the scale of the recovery and restoration project, the amount of groundwater coproduced can possibly exceed 1000 gal/min. Handling of these volumes of contaminated water can be very expensive if treatment is required prior to disposal or reinjection. Treatment of water derived from sites contaminated by other organic chemicals will involve adaptations of these procedures to the specific situation. 

In Illustrative Example 19.4, the dissolution of a non-aqueous-phase liquid (NAPL) into groundwater was discussed. Here we consider a similar (although somewhat hypothetical) case. Assume that a mixture of chlorinated solvents totally covers the flat bottom of a small pond (maximum depth zmax = 4 m, surface area Asurface = 104 m2) forming a dense non-aqueous-phase liquid (DNAPL). The DNAPL is contaminated by benzene which dissolves into the water column and is vertically transported by turbulent diffusion. The pond is horizontally well mixed. The vertical turbulent diffusion coefficient is , = 0.1 cm2s l and approximately constant over the whole water column. 

The concentration of a dissolved NAPL in groundwater is governed mainly by interface mass-transfer processes that often are slow and rate-limited [7,8]. There is a relatively large body of available literature on the migration of NAPLs and dissolution of residual blobs [6,9-22], and pools [5,23-34]. Furthermore, empirical correlations useful for convenient estimation of NAPL dissolution... 

Mathematical models for mass transfer at the NAPL-water interface often adopt the assumption that thermodynamic equilibrium is instantaneously approached when mass transfer rates at the NAPL-water interface are much faster than the advective-dispersive transport of the dissolved NAPLs away from the interface [28,36]. Therefore, the solubility concentration is often employed as an appropriate concentration boundary condition specified at the interface. Several experimental column and field studies at typical groundwater velocities in homogeneous porous media justified the above equilibrium assumption for residual NAPL dissolution [9,37-39]. 

Numerous empirical correlations for the prediction of residual NAPL dissolution have been presented in the literature and have been compiled by Khachikian and Harmon [68]. On the other hand, just a few correlations for the rate of interface mass transfer from single-component NAPL pools in saturated, homogeneous porous media have been established, and they are based on numerically determined mass transfer coefficients [69, 70]. These correlations relate a dimensionless mass transfer coefficient, i.e., Sherwood number, to appropriate Peclet numbers, as dictated by dimensional analysis with application of the Buckingham Pi theorem [71,72], and they have been developed under the assumption that the thickness of the concentration boundary layer originating from a dissolving NAPL pool is mainly controlled by the contact time of groundwater with the NAPL-water interface that is directly affected by the interstitial groundwater velocity, hydrodynamic dispersion, and pool size. For uniform... 

Tier 3 - Process Models This tier employs more detailed fundamental process-based equations to determine the time and amount of naturally flowing groundwater required to flush out dissolved-phase and NAPL dominated constituents from the source zone. 

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