Chongxuan Liu

A diffusion‐based interpretation of tetrachloroethene and trichloroethene concentration profiles in a groundwater aquitard

Analysis of subsurface soil cores from the site of a field-scale groundwater remediation experiment at Dover Air Force Base, Delaware, has revealed that tetrachloroethene (PCE) and trichloroethene (TCE) contamination extends into an aquitard underlying a groundwater aquifer. The site location is well downgradient of the locations of contaminant release, and the aquitard contamination is believed to have begun when contaminated groundwater first arrived in the overlying aquifer. Using independent estimates of sorption and diffusion properties in the aquitard layers, mathematical modeling based on diffusion in laminate slabs has been used to make inferences regarding the historical concentration conditions in the overlying aquifer. The results suggest that plume arrival occurred within the last two decades, with some important differences in the inferred TCE and PCE plume histories. The diffusion model was also applied toward predicting future aquitard concentrations and fluxes under scenarios based on the current condition as a starting point and hypothesized conditions of future groundwater cleanup. The results demonstrate how aquitard sampling and diffusion modeling can provide essential information relevant to forensic analysis, risk assessment, and subsurface cleanup.

Analytical modeling of diffusion-limited contamination and decontamination in a two-layer porous medium

Diffusion of dissolved contaminants in multilayer porous media is an important phenomenon affecting both contamination and remediation in natural aqueous environments, including diffusion in groundwater aquitards and contaminated bed sediments. This study presents a new analytical solution for solute diffusion in a semi-infinite two-layer porous medium for arbitrary boundary and initial conditions. The solution was obtained by using the Greens function approach in the Laplace domain with the application of the binomial theorem to facilitate inversion back to the real time domain. Results based on this solution were found to be simple both in form and ease of calculation and to be in good agreement with those obtained using numerical calculations based on the Crank-Nicolson finite difference method. Applications of the solution are presented in the context of a contaminated groundwater aquitard to demonstrate how different boundary and initial conditions can greatly affect the contamination and decontamination of porous media, and to illustrate how diffusion modeling might be used in a forensic sense.

An Analytical Solution to the One-Dimensional Solute Advection-Dispersion Equation in Multi-Layer Porous Media

An analytical solution to the one-dimensional solute advection-dispersion equation in multi-layer porous media is derived using a generalized integral transform method. The solution was derived under conditions of steady-state flow and arbitrary initial and inlet boundary conditions. The results obtained by this solution agree well with the results obtained by numerically inverting Laplace transform-generated solutions previously published in the literature. The analytical solution presented in this paper provides more flexibility with regard to the inlet conditions. The numerical evaluation of eigenvalues and matrix exponentials required in this solution technique can be accurately and efficiently computed using the sign-count method and eigenvalue evaluation methods commonly available. The illustrative calculations presented herein have shown how an analytical solution can provide insight into contaminant distribution and breakthrough in transport through well defined layered column systems. We also note that the method described here is readily adaptable to two and three-dimensional transport problems.

Multi-Scale Mass Transfer Processes Controlling Natural Attenuation and Engineered Remediation: An IFRC Focused on Hanford’s 300 Area Uranium Plume January 2010 to January 2011

The Integrated Field Research Challenge (IFRC) at the Hanford Site 300 Area uranium (U) plume addresses multi-scale mass transfer processes in a complex subsurface hydrogeologic setting where groundwater and riverwater interact. A series of forefront science questions on reactive mass transfer focus research. These questions relate to the effect of spatial heterogeneities; the importance of scale; coupled interactions between biogeochemical, hydrologic, and mass transfer processes; and measurements and approaches needed to characterize and model a mass-transfer dominated system. The project was initiated in February 2007, with CY 2007, CY 2008, and CY 2009 progress summarized in preceding reports. A project peer review was held in March 2010, and the IFRC project has responded to all suggestions and recommendations made in consequence by reviewers and SBR/DOE. These responses have included the development of “Modeling” and “Well-Field Mitigation” plans that are now posted on the Hanford IFRC web-site. The site has 35 instrumented wells, and an extensive monitoring system. It includes a deep borehole for microbiologic and biogeochemical research that sampled the entire thickness of the unconfined 300 A aquifer. Significant, impactful progress has been made in CY 2010 including the quantification of well-bore flows in the fully screened wells and the testing of means to mitigate them; the development of site geostatistical models of hydrologic and geochemical properties including the distribution of U; developing and parameterizing a reactive transport model of the smear zone that supplies contaminant U to the groundwater plume; performance of a second passive experiment of the spring water table rise and fall event with a associated multi-point tracer test; performance of downhole biogeochemical experiments where colonization substrates and discrete water and gas samplers were deployed to the lower aquifer zone; and modeling of past injection experiments for model parameterization, deconvolution of well-bore flow effects, system understanding, and publication. We continued efforts to assimilate geophysical logging and 3D ERT characterization data into our site wide geophysical model, and have now implemented a new strategy for this activity to bypass an approach that was found unworkable. An important focus of CY 2010 activities has been infrastructure modification to the IFRC site to eliminate vertical well bore flows in the fully screened wells. The mitigation procedure was carefully evaluated and is now being implementated. A new experimental campaign is planned for early spring 2011 that will utilize the modified well-field for a U reactive transport experiment in the upper aquifer zone. Preliminary geophysical monitoring experiments of rainwater recharge in the vadose zone have been initiated with promising results, and a controlled infiltration experiment to evaluate U mobilization from the vadose zone is now under planning for the September 2011. The increasingly comprehensive field experimental results, along with the field and laboratory characterization, are leading to a new conceptual model of U(VI) flow and transport in the IFRC footprint and the 300 Area in general, and insights on the microbiological community and associated biogeochemical processes.

Influence of Mass Transfer on Bioavailability and Kinetic Rate of Uranium(VI) Biotransformation

This research is investigating the influence of mass transfer process on the rate and extent of microbial reduction of U(VI) associated with intragrain domains in the Hanford subsurface sediments. The project will develop instrumental techniques to characterize microscopic mass transfer process at the sediment grain scale and to develop kinetic data and process models that describe microbial reduction of intragrain U(VI). Scientific knowledge and process models developed from this research will enhance our understanding on the future behavior of in-ground U(VI) at Hanford and other DOE sites where sediments contain U(VI) in intragrain domains or fracture-matrix systems.

Mineralogic Residence and Desorption Rates of Sorbed 90Sr in Contaminated Subsurface Sediments: Implications to Future Behavior and In-Ground Stability

The project is investigating the adsorption/desorption process of 90Sr in coarse-textured pristine and contaminated Hanford sediment with the goal to define a generalized reaction-based model for use in reactive transport calculations. While it is known that sorbed 90Sr exists in an ion exchangeable state, the mass action relationships that control the solid-liquid distribution and the mineral phases responsible for adsorption have not been defined. Many coarse-textured Hanford sediment display significant sorptivity for 90Sr, but contain few if any fines that may harbor phyllosilicates with permanent negative charge and associated cation exchange capacity. Moreover, it is not known whether the adsorption-desorption process exhibits time dependence within context of transport, and if so, the causes for kinetic behavior.

Diffusion-Limited Contamination and Decontamination in a Layered Aquitard: Forensic and Predictive Analysis of Field Data

Subsurface zones of low hydraulic conductivity serve as sinks and sources of chemical contamination for adjacent regions of comparatively high groundwater flow, and may often control the overall rate of remediation at sites contaminated by hazardous substances. At Dover Air Force Base, DE, we have extensively characterized the concentrations and transport of chlorinated organic contaminants in a silty aquitard that underlies an unconfined aquifer, as part of a field-scale investigation involving the installation of sheet-pile enclosures to hydraulically isolate a portion of a long-extant contamination plume. Vertical contaminant concentration profiles within the test cells were obtained by means of subsurface coring, high-resolution sub-sampling and a rigorous hot-methanol extraction of total (sorbed plus aqueous) contaminants. Through independent characterization of the sorption properties and porosity in the various strata of porous media, we have explored and validated a hypothesis that the chlorinated organic chemicals have behaved conservatively and that their transport within the aquitard was dominated by a process of sorption-retarded vertical diffusion. A new analytical solution to a multi-layer diffusion model has been developed and was used both to make inferences about the historical conditions of contamination in the overlying aquifer and to make predictions regarding future rates of release. Our predictions of on-going diffusion, including back transport to the aquifer, were tested by re-coring at the site following a 15-month period during which no-flow conditions were maintained within the test cells. Overall, the project illustrates how the aquitard will be a long-term limiting factor to contaminant remediation at this site, and that rates of release will be affected both by the geologic stratification within the aquitard and by the initially complex initial conditions within this zone. These characteristics of the problem could not have been understood without the given combination of high-resolution sampling, extensive laboratory characterization, and mathematical modeling analysis.