Facilitated transport of toluene and naphthalene with humic acid in high- and low-permeability systems: Role of ionic strength and cationic type.

The occurrence of colloids on pollutants transport in groundwater has attracted more attention. However, the research on the regulation mechanism of colloids on combined pollutants transport in heterogeneous aquifers is limited. In this study, a series of tank experiments were conducted to systematically investigate the effects of ionic strength, and cation type on humic acid (HA) facilitated transport of toluene (TOL), and naphthalene (NAP) in high- and low-permeability systems. The results showed that HA facilitated pollutants transport in low Na+ solution. In Ca2+ solution, the presence of HA hindered pollutants transport, and the inhibition increased with the increase of ionic strength. Both in Na+ solution and low Ca2+ solution, the influence of heterogeneous structure on pollutant transport played a dominant role, and TOL and NAP had a greater transport potential in the high permeability zone (HPZ) due to the preferential flow. Whereas, deposition of HA aggregates, and electrostatic attractive interaction had negative effects on transport than groundwater flow in high Ca2+ solution. Pollutants were prone to accumulate at the bottom of the HPZ, and the top of the low permeability zone (LPZ). These new findings provide insights into the mechanism of colloids influence on the pollutants transport in heterogenous aquifer.

Reduction capacity in the transmissive zones fueled by the embedded low-permeability lenses: Implications for contaminant transformation in heterogeneous aquifers.

Redox conditions play a decisive role in regulating contaminant and nutrient transformation in groundwater. Here we quantitatively described and interpreted the temporal and spatial variations of aquifer reduction capacity formation in lens-embedded heterogeneous aquifers in 1-D columns. Experimental results indicated that the aquifer reduction capacity exported from the low-permeability lens permeated into the downstream sandy zones, where it subsequently accumulated and extended. Reactive transport modeling suggested that reduction capacity within the lens preferentially diffused to the transmissive zones around the lens-sand interface, and was then transported via convection to downstream transmissive zones. A low-permeability lens of the same volume, but more elongated in the flow direction, led to less concentrated reduction capacity but extended further downgradient from the lens. The increased flow velocity attenuated the maintenance of aquifer reduction capacity by enhancing mixing and diluting processes in the transmissive zones. The reduction zones formed downstream from the low-permeability lens were hotpots for resisting the oxidative perturbation by O2. This study highlights the important role of low-permeability lenses as large and long-term electron pools for the transmissive zones, and thus providing aquifer reduction capacity for contaminant transformation and remediation in heterogeneous aquifers.

Exploration of processes governing microbial reductive dechlorination in a heterogeneous aquifer flow cell.

Although microbial reductive dechlorination (MRD) has proven to be an effective approach for in situ treatment of chlorinated ethenes, field implementation of this technology is complicated by many factors, including subsurface heterogeneity, electron donor availability, and distribution of microbial populations. This work presents a coupled experimental and mathematical modeling study designed to explore the influence of heterogeneity on MRD and to assess the suitability of microcosm-derived rate parameters for modeling complex heterogeneous systems. A Monod-based model is applied to simulate a bioremediation experiment conducted in a laboratory-scale aquifer cell packed with aquifer material from the Commerce Street Superfund site in Williston, VT. Results reveal that (uncalibrated) model application of microcosm-derived dechlorination and microbial growth rates for transformation of trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), and vinyl chloride (VC) reproduced observed aquifer cell concentration levels and trends. Mean relative errors between predicted and measured effluent concentrations were quantified as 6.7%, 27.0%, 41.5%, 32.0% and 21.6% over time for TCE, cis-DCE, VC, ethene and total volatile fatty acids (fermentable electron donor substrate and carbon source), respectively. The time-averaged extent of MRD (i.e., ethene formation) was well-predicted (4% underprediction), with modeled MRD exhibiting increased deviation from measured values under electron donor limiting conditions (maximum discrepancy of 14%). In contrast, simulations employing a homogeneous (uniform flow) domain resulted in underprediction of MRD extent by an average of 13%, with a maximum discrepancy of 45%. Model sensitivity analysis suggested that trace amounts of natural dissolved organic carbon served as an important fermentable substrate, providing up to 69% of the reducing equivalents consumed for MRD under donor-limiting conditions. Aquifer cell port concentration data and model simulations revealed that ethene formation varied spatially within the domain and was associated with regions of longer residence times. These results demonstrate the strong influence of subsurface heterogeneity on the accuracy of MRD predictions, and highlight the importance of subsurface characterization and the incorporation of flow field uncertainty in model applications for successful design and assessment of in situ bioremediation.

Performance of polymer-enhanced KMnO4 delivery for remediation of TCE contaminated heterogeneous aquifer: A bench-scale visualization.

The uniform migration of remedial amendments in an aquifer was negatively influenced by medium heterogeneity and the density effect of amendment. This study sought to use a polymer (xanthan) to enhance the uniformity of amendment distribution in contamination zones. Visible tank experiments were conducted to investigate the feasibility and performance of xanthan-enhanced KMnO4 delivery in the simulated aquifer. The results showed that the addition of xanthan improved fluid movement into the lower-permeability stratum, so the overall sweeping efficiency was remarkably increased compared to the fluid control test without polymer using. In two layered aquifer systems, the smaller the thickness of the low-permeability layer is, or the greater the permeability contrast between layers is, the more obvious the enhancement of the uniform distribution of remedial fluid by xanthan. The sinking of KMnO4 solution in medium and coarse sand aquifers was obvious, and the effect of KMnO4 concentration and aquifer medium size on the density effect was evaluated. The fluid viscosity increase caused by xanthan addition could stabilize the displacement front and reduce the density effect. Xanthan-KMnO4 applications were more effective at penetrating finer-grained lenses and played a more obvious role in TCE oxidation removal.