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Measuring and modeling the dissolution of nonideally shaped dense nonaqueous phase liquid pools in saturated porous media

Chrysikopoulos Constantinos, Thomas C. Harmon, Brian K. Dela Barre

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Summary

A three-dimensional physical aquifer model was used to study the dissolution of adense nonaqueous phase liquid (DNAPL) pool. The model aquifer comprised a packing ofhomogeneous, medium-sized sand and conveyed steady, unidirectional flow.Tetrachloroethene (PCE) pools were introduced within model aquifers atop glass- andclay-lined aquifer bottoms. Transient breakthrough at an interstitial velocity of 7.2 cm/h,and three-dimensional steady state concentration distributions at velocities ranging from0.4 to 7.2 cm/h were monitored over periods of 59 and 71 days for the glass- and claybottomexperiments, respectively. Pool-averaged mass transfer coefficients were obtainedfrom the observations via a single-parameter fit using an analytical model formulated witha second type boundary condition to describe pool dissolution [Chrysikopoulos, 1995].Other model parameters (interstitial velocity, longitudinal and transverse dispersioncoefficients, and pool geometry) were estimated independently. Simulated and observeddissolution behavior agreed well, except for locations relatively close to the pool or theglass-bottom plate. Estimated mass transfer coefficients ranged from 0.15 to 0.22 cm/h,increasing weakly with velocity toward a limiting value. Pool mass depletions of 31 and43% for the glass- and clay-bottom experiments failed to produce observable changes inthe plumes and suggested that changes in pool interfacial area over the period of theexperiment were negligible. Dimensionless mass transfer behavior was quantified using amodified Sherwood number (Sh*). Observed Sh* values were found to be about 2–3times greater than values predicted by an existing theoretical mass transfer correlation,and 3–4 times greater than those estimated previously for an ideally configuredtrichloroethene (TCE) pool (circular and smooth). It appeared that the analytical model’sfailure to account for pore-scale pool-water interfacial characteristics and larger scale poolshape irregularities biased the Sh* estimates toward greater values

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