Junqi Huang

Development and application of an analytical model to aid design and implementation of in situ remediation technologies

Innovative in situ remediation technologies using injection/extraction well pairs which recirculate partially treated groundwater as a method of increasing overall treatment efficiency have recently been demonstrated. An important parameter in determining overall treatment efficiency is the fraction of flow that recycles between injection and extraction points. Numerical and semi-analytical methods are currently available to determine this parameter and an analytical solution has been presented for a single injection/extraction well pair. In this work, the analytical solution for fraction of recycled flow for a single injection/extraction well pair is extended to multiple co-linear well pairs. In addition, a semi-analytical method is presented which permits direct calculation of fraction recycle for a system of arbitrarily placed injection/extraction wells pumping at different rates. The solution is used to investigate the behavior of multiple well pairs, looking at how well placement and pumping rates impact capture zone width and fraction recirculation (and, therefore, overall treatment efficiency). Results of the two-well solution are compared with data obtained at a recent field-scale demonstration of in situ aerobic cometabolic bioremediation, where a pair of dual-screened injection/extraction wells was evaluated. A numerical model is then used to evaluate the impact of hydraulic conductivity anisotropy on the overall treatment efficiency of the dual-screened injection/extraction well pair. The method presented here provides a fast and accurate technique for determining the efficacy of injection/extraction systems, and represents a tool that can be useful when designing such in situ treatment systems.

Validation of two innovative methods to measure contaminant mass flux in groundwater.

The ability to quantify the mass flux of a groundwater contaminant that is leaching from a source area is critical to enable us to: (1) evaluate the risk posed by the contamination source and prioritize cleanup, (2) evaluate the effectiveness of source remediation technologies or natural attenuation processes, and (3) quantify a source term for use in models that may be applied to predict maximum contaminant concentrations in downstream wells. Recently, a number of new methods have been developed and subsequently applied to measure contaminant mass flux in groundwater in the field. However, none of these methods has been validated at larger than the laboratory-scale through a comparison of measured mass flux and a known flux that has been introduced into flowing groundwater. A couple of innovative flux measurement methods, the tandem circulation well (TCW) and modified integral pumping test (MIPT) methods, have recently been proposed. The TCW method can measure mass flux integrated over a large subsurface volume without extracting water. The TCW method may be implemented using two different techniques. One technique, the multi-dipole technique, is relatively simple and inexpensive, only requiring measurement of heads, while the second technique requires conducting a tracer test. The MIPT method is an easily implemented method of obtaining volume-integrated flux measurements. In the current study, flux measurements obtained using these two methods are compared with known mass fluxes in a three-dimensional, artificial aquifer. Experiments in the artificial aquifer show that the TCW multi-dipole and tracer test techniques accurately estimated flux, within 2% and 16%, respectively; although the good results obtained using the multi-dipole technique may be fortuitous. The MIPT method was not as accurate as the TCW method, underestimating flux by as much as 70%. MIPT method inaccuracies may be due to the fact that the method assumptions (two-dimensional steady groundwater flow to fully-screened wells) were not well-approximated. While fluxes measured using the MIPT method were consistently underestimated, the methods simplicity and applicability to the field may compensate for the inaccuracies that were observed in this artificial aquifer test.

Transport issues and bioremediation modeling for the in situ aerobic co-metabolism of chlorinated solvents

For aerobic co-metabolism of chlorinated solvents to occur, it isnecessary that oxygen, a primary substrate, and the chlorinated compound all be available to an appropriate microorganism – that is, a microorganism capable of producing the nonspecific enzyme that will promote degradation of the ontaminant while the primary substrate is aerobically metabolized. Thus, the transport processes that serve to mix the reactants are crucial in determining the rate and extent of biodegradation, particularly when considering in situ biodegradation. These transport processes intersect, at a range of scales, with the biochemical reactions. This paper reviews how the important processes contributing to aerobic co-metabolism of chlorinated solvents at different scales can be integrated into mathematical models. The application of these models to field-scale bioremediation is critically examined. It is demonstrated that modeling can be a useful tool in gaining insight into the physical, chemical, and biological processes relevant to aerobic co-metabolism, designing aerobic co-metabolic bioremediation systems, and predicting system performance. Research needs are identified that primarily relate to gaps in our current knowledge of inter-scale interactions.

Solutions to equations incorporating the effect of rate‐limited contaminant mass transfer on vadose zone remediation by soil vapor extraction

Soil vapor extraction (SVE) is a technique that is frequently used to remediate unsaturated soils in the vadose zone that are contaminated with volatile organic compounds. Laboratory research and field experience have demonstrated that rate-limited mass transfer of a contaminant from aquifer solids and soil water to the gas phase significantly impacts the efficacy and speed of the remediation. An exact analytical solution to equations describing SVE controlled by rate-limited mass transfer is presented. Processes modeled include contaminant advective-dispersive transport in a gas phase converging on an extraction well and rate-limited mass transfer of dissolved and sorbed contaminants into the gas phase, with the rate limitation modeled assuming first-order kinetics. In addition to the exact solution an approximate solution, as well as an exact solution for plug flow (no dispersion), is presented. These analytical solutions may be useful in verifying numerical codes that are being developed to model SVE of volatile organic compounds.

Use of tandem circulation wells to measure hydraulic conductivity without groundwater extraction.

Conventional methods to measure the hydraulic conductivity of an aquifer on a relatively large scale (10-100 m) require extraction of significant quantities of groundwater. This can be expensive, and otherwise problematic, when investigating a contaminated aquifer. In this study, innovative approaches that make use of tandem circulation wells to measure hydraulic conductivity are proposed. These approaches measure conductivity on a relatively large scale, but do not require extraction of groundwater. Two basic approaches for using circulation wells to measure hydraulic conductivity are presented; one approach is based upon the dipole-flow test method, while the other approach relies on a tracer test to measure the flow of water between two recirculating wells. The approaches are tested in a relatively homogeneous and isotropic artificial aquifer, where the conductivities measured by both approaches are compared to each other and to the previously measured hydraulic conductivity of the aquifer. It was shown that both approaches have the potential to accurately measure horizontal and vertical hydraulic conductivity for a relatively large subsurface volume without the need to pump groundwater to the surface. Future work is recommended to evaluate the ability of these tandem circulation wells to accurately measure hydraulic conductivity when anisotropy and heterogeneity are greater than in the artificial aquifer used for these studies.

An assembly model for simulation of large-scale ground water flow and transport.

When managing large-scale ground water contamination problems, it is often necessary to model flow and transport using finely discretized domains--for instance (1) to simulate flow and transport near a contamination source area or in the area where a remediation technology is being implemented; (2) to account for small-scale heterogeneities; (3) to represent ground water-surface water interactions; or (4) some combination of these scenarios. A model with a large domain and fine-grid resolution will need extensive computing resources. In this work, a domain decomposition-based assembly model implemented in a parallel computing environment is developed, which will allow efficient simulation of large-scale ground water flow and transport problems using domain-wide grid refinement. The method employs common ground water flow (MODFLOW) and transport (RT3D) simulators, enabling the solution of almost all commonly encountered ground water flow and transport problems. The basic approach partitions a large model domain into any number of subdomains. Parallel processors are used to solve the model equations within each subdomain. Schwarz iteration is applied to match the flow solution at the subdomain boundaries. For the transport model, an extended numerical array is implemented to permit the exchange of dispersive and advective flux information across subdomain boundaries. The model is verified using a conventional single-domain model. Model simulations demonstrate that the proposed model operated in a parallel computing environment can result in considerable savings in computer run times (between 50% and 80%) compared with conventional modeling approaches and may be used to simulate grid discretizations that were formerly intractable.