Nonaqueous Phase Liquid Pool Dissolution Research Articles

Estimating transverse dispersivity based on hydraulic conductivity

Abstract The modeling of depletion timeframe for a non-aqueous phase liquid (NAPL) layer is partially dependent on transverse vertical dispersivity. This paper provides guidance for estimating this parameter over the wide range of soil textures in which NAPL resides at contaminated sites. Transverse dispersivity is re-calculated for several previous NAPL dissolution studies based on a consistent methodology for estimating model input parameters. Through the compilation of a number of case studies from the literature including multiple NAPL dissolution experiments, the trend of transverse vertical dispersivity appears to be inversely proportional to the effective grain diameter when groundwater velocity (v) is below a critical threshold ( v c ). A novel regression equation is derived for estimating transverse dispersivity based on hydraulic conductivity when v v c and the NAPL length is on the order of meters. The critical velocity for NAPL pool dissolution appears to range from 3 to 5 m/d based on a limited number of studies with NAPL pool lengths of 1 m or more. The method derived by Klenk and Grathwohl (2002) is validated for estimating a correction factor for dispersivity when v > v c .

Mass flux from a non-aqueous phase liquid pool considering spontaneous expansion of a discontinuous gas phase

The partitioning of non-aqueous phase liquid (NAPL) compounds to a discontinuous gas phase results in the repeated spontaneous expansion, snap-off, and vertical mobilization of the gas phase. This mechanism has the potential to significantly affect the mass transfer processes that control the dissolution of NAPL pools by increasing the vertical transport of NAPL mass and increasing the total mass transfer rate from the surface of the pool. The extent to which this mechanism affects mass transfer from a NAPL pool depends on the rate of expansion and the mass of NAPL compound in the gas phase. This study used well-controlled bench-scale experiments under no-flow conditions to quantify for the first time the expansion of a discontinuous gas phase in the presence of NAPL. Air bubbles placed in glass vials containing NAPL increased significantly in volume, from a radius of 1.0 mm to 2.0 mm over 215 days in the presence of tetrachloroethene (PCE), and from a radius of 1.2 mm to 2.3 mm over 22 days in the presence of trans-1,2-dichloroethene (tDCE). A one-dimensional mass transfer model, fit to the experimental data, showed that this expansion could result in a mass flux from the NAPL pool that was similar in magnitude to the mass flux expected for the dissolution of a NAPL pool in a two-fluid (NAPL and water) system. Conditions favouring the significant effect of a discontinuous gas phase on mass transfer were identified as groundwater velocities less than ~ 0.01 m/day, and a gas phase that covers greater than ~ 10% of the pool surface area and is located within ~ 0.01 m of the pool surface. Under these conditions the mass transfer via a discontinuous gas phase is expected to affect, for example, efforts to locate NAPL source zones using aqueous concentration data, and predict the lifetime and risk associated with NAPL source zones in a way that is not currently included in the common conceptual models used to assess NAPL-contaminated sites.

Comparison of Bioenhancement of Nonaqueous Phase Liquid Pool Dissolution with First- and Zero-Order Biokinetics

Dissolution is one of the key physiochemical processes controlling subsurface remediation of nonaqueous phase liquid (NAPL) pools. One process that can accelerate NAPL pool dissolution is in situ biodegradation. The bioenhancement of NAPL pool dissolution has been studied previously using an analytical solution for the case of first-order biodegradation kinetics. However, data suggest that at typical NAPL source zone concentrations, mathematical description of dissolution bioenhancement may require biokinetic models ranging from first to zero order. In this work, it was determined that an nth-order approximation could be derived using the first-order case and bioenhancement was compared under first- and zero-order conditions. Two example scenarios demonstrate that predictions of the potential magnitude of the bioenhancement effect are erroneously high when the first- and zero-order cases are incorrectly applied.

Nonaqueous Phase Liquid Pool Dissolution as a Function of Average Pore Water Velocity

Bench-scale reactor experiments were performed to study the dissolution of a binary naphthalene-in-nonane mixture nonaqueous phase liquid (NAPL) pool over a wide range of average pore water velocities, ν\dx(≈0.1–60 m/day). Experimental NAPL pool dissolution flux values were determined using a steady-state mass balance approach. The experimental flux data were compared to model predictions made assuming either local equilibrium or mass-transfer limited conditions. The local equilibrium model could describe the trends in the average effluent concentration and dissolution flux with 0.1 10 m/day. Data determined to be under mass-transfer limited conditions were fit to the nonequilibrium model to estimate values for an overall mass-transfer coefficient. The calculated overall mass-transfer coefficients had an average value of 0.407 m/day and showed no correlation with ν\dx, probably due to mass-transfer resistance becoming dominated by the diffusional resistance in the NAPL. These results suggest that the nonequilibrium approach is better suited for describing high velocity (ν\dx>10 m/day) dissolution of multicomponent NAPL pools, and that flushing of groundwater at very high velocities may not be an effective approach for enhancing NAPL-pool dissolution flux.

Bioenhancement of NAPL pool dissolution: experimental evaluation

Experiments were conducted to quantify nonaqueous phase liquid (NAPL) pool dissolution and its enhancement by in situ biodegradation. The experiments were performed using square cross-section, glass-bead packed column reactors with a small pool of a toluene-in-dodecane mixture (toluene mole fraction, X tol≈0.02 or 0.09). Experimental quasi-steady-state toluene dissolution fluxes were determined using a 14C-carbon mass-balance approach during water flushing with and without biodegradation. The experiments demonstrated a statistically significant bioenhancement of the toluene dissolution flux of up to roughly twofold at average pore water velocities of approximately 0.1 and 1 m/day when the toluene mole fraction was low (≈0.02); however, little or no bioenhancement was observed with the higher mole fraction (≈0.09). Although it cannot be determined conclusively, the weight of evidence based on biomass measurements and model analyses suggests that the reduced bioenhancement for the high mole fraction was due to higher dissolved toluene concentrations, which may have caused toxicity effects. Importantly, even though NAPL dissolution was not bioenhanced in every case, the biodegradation reduced toluene concentrations to low levels in the reactor effluents.

Modeling the Transport of Contaminants Originating from the Dissolution of DNAPL Pools in Aquifers in the Presence of Dissolved Humic Substances

A two-dimensional finite difference numerical model was developed to describe the transport of dissolved organics originating from nonaqueous phase liquid (NAPL) pool dissolution in saturated porous media in the presence of dissolved humic substances. A rectangular NAPL pool was considered in a homogeneous porous medium with unidirectional interstitial groundwater velocity. It was assumed that dissolved humic substances and aqueous phase contaminants may sorb onto the solid matrix under local equilibrium conditions. The contaminant in the aqueous phase may undergo first-order decay. Also, the dissolved contaminant may sorb onto humic substances. The transport properties of dissolved humic substances are assumed to be unaffected by sorbing contaminants, because dissolved humic macromolecules are much larger than dissolved contaminants and sorp- tion of nonpolar contaminants onto humic substances do not affect the overall surface charge of humic substances. The sorption characteristics of dissolved humic substances onto clean sand were determined from column experiments. An effective local mass transfer rate coefficient accounting for the presence of dissolved humic substances was developed. Model simulations indicate that dissolved humic substances substantially increase NAPL pool dissolution, and consequently reduce the required pump-and-treat aquifer remediation time.

Local Mass Transfer Correlations for Nonaqueous Phase Liquid Pool Dissolution in Saturated Porous Media

Local mass transfer correlations are developed to describe the rate of interface mass transfer of single component nonaqueous phase liquid (NAPL) pools in saturated subsurface formations. A three-dimensional solute transport model is employed to compute local mass transfer coefficients from concentration gradients at the NAPL-water interface, assuming that the aqueous phase concentration along the NAPL-water interface is constant and equal to the solubility concentration. Furthermore, it is assumed that the porous medium is homogeneous, the interstitial fluid velocity steady and the dissolved solute may undergo first-order decay or may sorb under local equilibrium conditions. Power-law expressions relating the local Sherwood number to appropriate local Peclet numbers are developed for both rectangular and elliptic/circular source geometries. The proposed power law correlations are fitted to numerically generated data and the correlation coefficients are determined using nonlinear least squares regression. The estimated correlation coefficients are found to be direct functions of the interstitial fluid velocity, pool dimensions, and pool geometry.

A critical evaluation of the local-equilibrium assumption in modeling NAPL-pool dissolution

An analytical modeling analysis was used to assess when local equilibrium (LE) and nonequilibrium (NE) modeling approaches may be appropriate for describing nonaqueous-phase liquid (NAPL) pool dissolution. NE mass-transfer between NAPL pools and groundwater is expected to affect the dissolution flux under conditions corresponding to values of Sh′St (the modified Sherwood number ( L x k l/ D z ) multiplied by the Stanton number ( k l/ v x ))<≈400. A small Sh′St can be brought about by one or more of: a large average pore water velocity ( v x ), a large transverse dispersivity ( α z ), a small pool length ( L x ), or a small mass-transfer coefficient ( k l). On the other hand, at Sh′St>≈400, the NE and LE solutions converge, and the LE assumption is appropriate. Based on typical groundwater conditions, many cases of interest are expected to fall in this range. The parameter with the greatest impact on Sh′St is k l. The NAPL pool mass-transfer coefficient correlation of Pfannkuch [Pfannkuch, H.-O., 1984. Determination of the contaminant source strength from mass exchange processes at the petroleum–ground-water interface in shallow aquifer systems. In: Proceedings of the NWWA/API Conference on Petroleum Hydrocarbons and Organic Chemicals in Ground Water—Prevention, Detection, and Restoration, Houston, TX. Natl. Water Well Assoc., Worthington, OH, Nov. 1984, pp. 111–129.] was evaluated using the toluene pool data from Seagren et al. [Seagren, E.A., Rittmann, B.E., Valocchi, A.J., 1998. An experimental investigation of NAPL-pool dissolution enhancement by flushing. J. Contam. Hydrol., accepted.]. Dissolution flux predictions made with k l calculated using the Pfannkuch correlation were similar to the LE model predictions, and deviated systematically from predictions made using the average overall k l=4.76 m/day estimated by Seagren et al. [Seagren, E.A., Rittmann, B.E., Valocchi, A.J., 1998. An experimental investigation of NAPL-pool dissolution enhancement by flushing. J. Contam. Hydrol., accepted.] and from the experimental data for v x >18 m/day. The Pfannkuch correlation k l was too large for v x >≈10 m/day, possibly because of the relatively low Peclet number data used by Pfannkuch [Pfannkuch, H.-O., 1984. Determination of the contaminant source strength from mass exchange processes at the petroleum–ground-water interface in shallow aquifer systems. In: Proceedings of the NWWA/API Conference on Petroleum Hydrocarbons and Organic Chemicals in Ground Water—Prevention, Detection, and Restoration, Houston, TX. Natl. Water Well Assoc., Worthington, OH, Nov. 1984, pp. 111–129.]. The results of the modeling analyses were evaluated by comparing pool dissolution fluxes from the literature to each other and to the corresponding LE and NE model predictions. The LE model described most of the pool dissolution flux data reasonably well, given the uncertainty in some of the model parameter estimates, suggesting that the LE model can be a useful tool for describing steady-state NAPL pool dissolution under some conditions. However, a conclusive test of the LE assumption was difficult due to the limited range of experimental conditions covered and the uncertainties in some of the model input parameters, including the mass-transfer coefficient correlation required for the NE model.

Mass transfer correlations for nonaqueous phase liquid pool dissolution in saturated porous media

Correlations describing the rate of interface mass transfer from singlecomponent nonaqueous phase liquid (NAPL) pools in saturated subsurface formations are developed. A three‐dimensional contaminant transport model is employed to obtain overall mass transfer coefficients computed from concentration gradients at the NAPLwater interface. The model assumes that the porous medium is homogeneous, the interstitial fluid velocity is steady, and the dissolved solute may sorb under local equilibrium conditions. Furthermore, it is assumed that the dissolved concentration along the NAPL‐water interface is equal to the solubility concentration. Power law correlations relate the overall Sherwood number to the appropriate overall Peclet numbers. Both rectangular and elliptic/circular source geometries are considered. The proposed relationships are fitted to numerically determined mass transfer coefficients, and the correlation coefficients are determined by nonlinear least squares regression. Good agreement between predicted and available experimentally determined overall mass transfer coefficients is observed.

Contaminant transport resulting from multicomponent nonaqueous phase liquid pool dissolution in three-dimensional subsurface formations

A semi-analytical method for simulating transient contaminant transport originating from the dissolution of multicomponent nonaqueous phase liquid (NAPL) pools in three-dimensional, saturated, homogeneous porous media is presented. Each dissolved component may undergo first-order decay and may sorb under local equilibrium conditions. The NAPL pool dissolution process is envisioned to occur in a series of consecutive short intervals (pulses). The mole fraction, nonaqueous phase activity coefficient and aqueous solubility of every pool constituent are estimated before the initiation of each pulse, and they are assumed to remain constant for the duration of each interval. Individual component aqueous phase concentrations resulting from each dissolution interval are estimated by existing analytical solutions applicable to single component NAPL pools, and total concentration distributions of the same component are obtained by direct superposition. The semi-analytical method is more efficient and less computationally demanding than a finite-difference approximation. Furthermore, it is shown that neglecting the changes in nonaqueous phase activity coefficients that occur during multicomponent NAPL pool dissolution may result in erroneous predictions. The model presented in this work is useful for the design and interpretation of experiments in laboratory bench scale aquifers, certain homogeneous subsurface formations, and for the verification of complex numerical codes.

Three-dimensional analytical models of contaminant transport from nonaqueous phase liquid pool dissolution in saturated subsurface formations : C. V. Chrysikopoulos, Water Resources Research, 31(4), 1995, pp 1137–1145

Three-dimensional analytical models of contaminant transport from nonaqueous phase liquid pool dissolution in saturated subsurface formations : C. V. Chrysikopoulos, Water Resources Research, 31(4), 1995, pp 1137–1145

Three‐Dimensional Analytical Models of Contaminant Transport From Nonaqueous Phase Liquid Pool Dissolution in Saturated Subsurface Formations

Closed form analytical solutions are derived for three‐dimensional transient contaminant transport resulting from dissolution of single‐component nonaqueous phase liquid pools in saturated porous media. The solutions are suitable for homogeneous porous media with unidirectional interstitial velocity. The dissolved solute may undergo first‐order decay or may sorb under local equilibrium conditions. The solutions are obtained for rectangular and elliptic as well as circular source geometries, assuming that the dissolution process is mass transfer limited, by applying Laplace and Fourier transforms. Although the solutions contain integral expressions, these integrals are easily evaluated numerically. These solutions are useful for verifying the accuracy of numerical solutions to more comprehensive models and for design and interpretation of experiments in laboratory‐packed beds and possibly some field studies. The results of several simulations indicate that for short downstream distances, predictions of contaminant concentrations are sensitive to the source structure and orientation with respect to the direction of interstitial flow.