In many vadose zone hydrological studies, it is imperative that the soil's unsaturated hydraulic conductivity is known. Frequently, the Mualem-van Genuchten model (MVG) is used for this purpose because it allows prediction of unsaturated hydraulic conductivity from water retention parameters. For this and similar equations, it is often assumed that a measured saturated hydraulic conductivity (K s ) can be used as a matching point (K o ) while a factor S L e is used to account for pore connectivity and tortuosity (where S e is the relative saturation and L = 0.5). We used a data set of 235 soil samples with retention and unsaturated hydraulic conductivity data to test and improve predictions with the MVG equation. The standard practice of using K o = K, and L = 0.5 resulted in a root mean square error for log(K) (RMSE K ) of 1.31. Optimization of the matching point (K o ) and L to the hydraulic conductivity data yielded a RMSE K of 0.41. The fitted K 0 were, on average, about one order of magnitude smaller than measured K s . Furthermore, L was predominantly negative, casting doubt that the MVG can be interpreted in a physical way, Spearman rank correlations showed that both K 0 and L were related to van Genuchten water retention parameters and neural network analyses confirmed that K 0 and L could indeed be predicted in this way. The corresponding RMSE K was 0.84, which was half an order of magnitude better than the traditional MVG model. Bulk density and textural parameters were poor predictors while addition of K s improved the RMSE K only marginally. Bootstrap analysis showed that the uncertainty in predicted unsaturated hydraulic conductivity was about one order of magnitude near saturation and larger at lower water contents.

Improved Prediction of Unsaturated Hydraulic Conductivity with the Mualem-van Genuchten Model

A review of NAPL source zone remediation efficiency and the mass flux approach.

In recent years it has become increasingly clear that most remedial technologies fail to completely remove dense nonaqueous phase liquid (DNAPL) from subsurface source zones. Recognition of this limitation leads to the question of what benefit can be achieved through partial removal of DNAPL. To address this issue, a mathematical technique referred to as the multiple analytical source superposition technique (MASST) has been developed. MASST is based on a conceptualization of a DNAPL source zone as a grouping of discrete subzones containing DNAPL (e.g., fingers and/or pools) separated by portions of the aquifer that are entirely free of DNAPL. Using analytical techniques, spatial superposition of responses to multiple sources is used to estimate aqueous mass transfer rates from individual subzones. This procedure accounts for multiple DNAPL subzones with different volumes, geometries, and locations within an overall source zone that is otherwise free of the nonaqueous liquid. The mass transfer rate from a particular subzone is affected by mass transfer from all other subzones in the vicinity. Groundwater flow is assumed to be uniform, and transport processes are considered to be at a steady state. Comparison of MASST results with exact analytical solutions and laboratory data confirms the validity of MASST. Sensitivity analyses indicate that source-zone architecture is a primary factor governing bulk mass transfer and source longevity. Analysis of rate-limited mass transfer within DNAPL subzones and advective-dispersive transport about DNAPL subzones indicates that advective-dispersive transport is the primary factor controlling mass transfer rates. Finally, results indicate that removal of the vast majority of the DNAPL will likely be necessary to achieve significant near-term improvements in groundwater quality.

Steady state mass transfer from single‐component dense nonaqueous phase liquids in uniform flow fields

[1] This study investigates field-scale DNAPL dissolution kinetics using high-resolution numerical simulations of DNAPL releases and dissolved phase transport. A percolation model is employed to simulate the distribution of TCE within 10 × 10 × 10 m source zones with spatially heterogeneous aquifer properties following a release event. Distributed aquifer properties and DNAPL saturations are utilized to simulate coupled groundwater flow and long-term dissolved phase transport. Grid-scale dissolution rates are computed based on published bench-scale relationships. Effective field-scale mass transfer coefficients are computed from simulated TCE fluxes at the downstream source zone boundary. Heterogeneity in groundwater velocity and DNAPL distributions leads to field-scale mass transfer coefficients that are much lower than laboratory-scale values. Field-scale mass transfer coefficients are observed to vary in direct proportion to the mean groundwater velocity, in contrast to laboratory studies that indicate proportionality with velocity to a power of ∼0.7. Computed field-scale mass transfer coefficients vary approximately in proportion to relative DNAPL mass raised to an empirical depletion exponent, which is 1 for more randomly oriented residual DNAPL regions. The former DNAPL geometries exhibit slow reductions in source concentration and contaminant flux with time as mass depletion proceeds. The latter DNAPL geometries exhibit significant and steady declines in source concentration and contaminant flux with time as depletion occurs.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003WR002807

Modeling field-scale dense nonaqueous phase liquid dissolution kinetics in heterogeneous aquifers

DNAPL source depletion: linking architecture and flux response.

Experiments quantifying rates of non–aqueous phase liquid (NAPL) dissolution from heterogeneous media are presented and compared with model simulations. This work specifically addresses the overall dissolution of NAPL entrapped in a coarse sand lens at a high saturation. To explore the mechanisms governing dissolution rates, mathematical models describing the hydrodynamics of flow through the heterogeneous system were developed and coupled with a mass balance equation and a local equilibrium assumption (LEA) to quantify interphase mass transfer processes. Variations in the effective permeabilities as a function of NAPL saturation and the intrinsic permeabilities of the sands were employed to characterize the hydrodynamic aspects of flow through the heterogeneous system. Relative to errors generated by the ill-defined aqueous phase relative permeabilities at high NAPL saturations, the model incorporating the system hydrodynamics as the sole rate-limiting process provided a reasonable first estimate of effluent concentrations. With the representative elemental volume defined here, rate-limited dissolution becomes important for low-NAPL saturations (Sn < ∼0.05–0.15) causing tailing in the observed dissolution data and deviations between these data and the LEA model.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98WR02471

Garrey W. Curry

Papers

Non-aqueous phase liquid dissolution in heterogeneous systems: Mechanisms and a local equilibrium modeling approach

Dissolution of NAPLs Entrapped in Heterogeneous Porous Media