Chunmiao Zheng

A dual-domain mass transfer approach for modeling solute transport in heterogeneous aquifers: Application to the Macrodispersion Experiment (MADE) site

A large-scale natural-gradient tracer test in a highly heterogeneous aquifer at the Macrodispersion Experiment (MADE) site on the Columbus Air Force Base in Mississippi is simulated using three-dimensional hydraulic conductivity distributions derived from borehole flowmeter test data. Two methods of hydraulic conduct(fBm), are used to construct the hydraulic conductivity distributions needed by the numerical model. Calculated and observed mass distributions are compared to evaluate the effectiveness of the dual-domain mass transfer approach relative to the single-domain advection-dispersion approach. The results show that the classical Fickian advection-dispersion model can reproduce reasonably well the observed tritium plume above a certain concentration limit but fails to reproduce the extensive spreading of the tracer at diluted concentrations as observed in the field. The alternative dual-domain mass transfer model is able to represent the rapid, anomalous spreading significantly better while retaining high concentrations near the injection point. This study demonstrates that the dual-domain mass transfer approach may offer a practical solution to modeling solute transport in highly heterogeneous aquifers where small-scale preferential flow pathways cannot be fully and explicitly represented by the spatial discretization of the numerical model.

Modelling the fate of oxidisable organic contaminants in groundwater

Subsurface contamination by organic chemicals is a pervasive environmental problem, susceptible to remediation by natural or enhanced attenuation approaches or more highly engineered methods such as pump-and-treat, amongst others. Such remediation approaches, along with risk assessment or the pressing need to address complex scientific questions, have driven the development of integrated modelling tools that incorporate physical, biological and geochemical processes. We provide a comprehensive modelling framework, including geochemical reactions and interphase mass transfer processes such as sorption/desorption, non-aqueous phase liquid dissolution and mineral precipitatation/dissolution, all of which can be in equilibrium or kinetically controlled. This framework is used to simulate microbially mediated transformation/degradation processes and the attendant microbial population growth and decay. Solution algorithms, particularly the split-operator (SO) approach, are described, along with a brief resume of numerical solution methods. Some of the available numerical models are described, mainly those constructed using available flow, transport and geochemical reaction packages. The general modelling framework is illustrated by pertinent examples, showing the degradation of dissolved organics by microbial activity limited by the availability of nutrients or electron acceptors (i.e., changing redox states), as well as concomitant secondary reactions. Two field-scale modelling examples are discussed, the Vejen landfill (Denmark) and an example where metal contamination is remediated by redox changes wrought by injection of a dissolved organic compound. A summary is provided of current and likely future challenges to modelling of oxidisable organics in the subsurface.

Natural Attenuation of BTEX Compounds: Model Development and Field-Scale Application

Benzene, toluene, ethyl benzene and xylene (BTEX) dissolved into ground water and migrated from a light nonaqueous phase liquid (LNAPL) source in a sandy aquifer near a petroleum, oil, and lubricants (POL) facility at Hill Air Force Base (AFB), Utah. Field observations indicated that microbially mediated BTEX degradation using multiple terminal electron-accepting processes including aerobic respiration, denitrification, Fe(III) reduction, sulfate reduction, and methanogenesis has occurred in the aquifer. To study the transport and transformation of dissolved BTEX compounds under natural conditions, a reactive flow and transport model incorporating biochemical multispecies interactions and BTEX was developed. The BTEX, oxygen, nitrate, Fe(II), sulfate, and methane plumes calculated by the model agree reasonably well with field observations. The first-order biodegradation rate constants, estimated based on model calibration are 0.051, 0.031, 0.005, 0.004, and 0.002 day(-1) for aerobic respiration, denitrification, Fe(III), sulfate reduction, and methanogenesis, respectively. The results of a sensitivity analysis show that the saturated aquifer thickness, hydraulic conductivity, and reaction rate constants are the most critical parameters controlling the natural attenuation of BTEX at this site. The hydraulic conductivity and aquifer thickness were found to be the key factors affecting the restoration of oxygen, nitrate, and sulfate after their interaction with the BTEX plume. The multispecies reactive transport modeling effort, describing BTEX degradation mediated by multiple electron-accepting processes, represents one of the few attempts to date to quantify a complete sequence of natural attenuation processes with a detailed field data set. Because the case study is representative of many petroleum-product contaminated sites, the results and insights obtained from this study are of general interest and relevance to other fuel-hydrocarbon natural attenuation sites.

Parameter structure identification using tabu search and simulated annealing

In groundwater modeling the identification of an optimal flow or transport parameter that varies spatially should include both the values and structure of the parameter. However, most existing techniques for parameter identification only consider the parameter values. In this study, the problem of identifying optimal parameter structure is treated as a large combinatorial optimization problem. Two recently developed heuristic search techniques, simulated annealing and tabu search, are used to solve the large combinatorial optimization problem. The effectiveness and flexibility of these two techniques are evaluated and compared with simple grid search and descent search, using preliminary results from one-dimensional examples. Among the techniques examined in this paper, tabu search performs extremely well in terms of the total number of function evaluations required.

Lessons Learned from 25 Years of Research at the MADE Site

Field studies at well-instrumented research sites have provided extensive data sets and important insights essential for development and testing of transport theories and mathematical models. This paper provides an overview of over 25 years of research and lessons learned at one of such field research sites on the Columbus Air Force Base in Mississippi, commonly known as the Macrodispersion Experiment (MADE) site. Since the mid-1980s, field data from the MADE site have been used extensively by researchers around the world to explore complex contaminant transport phenomena in highly heterogeneous porous media. Results from field investigations and modeling analyses suggested that connected networks of small-scale preferential flow paths and relative flow barriers exert dominant control on solute transport processes. The classical advection-dispersion model was shown to inadequately represent plume-scale transport, while the dual-domain mass transfer model was found to reproduce the primary observed plume characteristics. The MADE site has served as a valuable natural observatory for contaminant transport studies where new observations have led to better understanding and improved models have sprung out analysis of new data.

An integrated global and local optimization approach for remediation system design

A global optimization (metaheuristic) method, tabu search, is integrated with linear programming to solve remediation design problems. This integrated approach takes advantage of the fact that the global optimization approach is most effective for optimizing discrete well location variables, while linear programming is much more efficient for optimizing continuous pumping rate variables. In addition, an efficient forward solution updating procedure is used to lessen the computational burden of the global optimization approach. With this procedure the new solution to a linear flow model perturbed by pumping is obtained as the sum of a nonperturbed base solution and the solution to the perturbed portion of the flow system, which can be derived directly without running the flow model. Numerical results, based on a two-dimensional capture zone design problem, show that the computation time can be reduced to a small fraction of that required by the conventional approach, in which a forward simulation model is run each time the objective function needs to be evaluated. It is also demonstrated that the maximum number of wells allowed in a given design has a significant effect on the total remediation costs. (The total remediation costs are nearly doubled when only one well is allowed instead of the optimal number of six for the test problem.) A Monte Carlo analysis, based on 200 realizations of a lognormally distributed random hydraulic conductivity field (the variance of lnT = 1.0), further reveals that the total remediation costs determined for the heterogeneous aquifer have a large uncertainty (the ratio of standard derivation over mean is 0.4). The total remediation costs and associated uncertainty are also shown to increase with the uncertainty of the hydraulic conductivity field.

Global change and the groundwater management challenge

With rivers in critical regions already exploited to capacity throughout the world and groundwater overdraft as well as large-scale contamination occurring in many areas, we have entered an era in which multiple simultaneous stresses will drive water management. Increasingly, groundwater resources are taking a more prominent role in providing freshwater supplies. We discuss the competing fresh groundwater needs for human consumption, food production, energy, and the environment, as well as physical hazards, and conflicts due to transboundary overexploitation. During the past 50 years, groundwater management modeling has focused on combining simulation with optimization methods to inspect important problems ranging from contaminant remediation to agricultural irrigation management. The compound challenges now faced by water planners require a new generation of aquifer management models that address the broad impacts of global change on aquifer storage and depletion trajectory management, land subsidence, groundwater-dependent ecosystems, seawater intrusion, anthropogenic and geogenic contamination, supply vulnerability, and long-term sustainability. The scope of research efforts is only beginning to address complex interactions using multiagent system models that are not readily formulated as optimization problems and that consider a suite of human behavioral responses.

Can China Cope with Its Water Crisis?—Perspectives from the North China Plain

by Chunmiao Zheng1,2,3, Jie Liu3, Guoliang Cao2, Eloise Kendy4, Hao Wang5, and Yangwen Jia5 1Corresponding author: Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487; (205) 348-0579; fax: (205) 348-0818; czheng@ua.edu 2Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487. 3Center for Water Research, Peking University, Beijing 100871, China. 4Environmental Flow Program, The Nature Conservancy, Helena, MT 59601. 5Department of Water Resources, China Institute of Water Resources & Hydropower Research, Beijing 100038, China.

Effects of Density and Viscosity in Modeling Heat as a Groundwater Tracer

Decoupled simulation of groundwater flow and heat transport assuming constant fluid density and viscosity is computationally efficient and simple. However, by neglecting the effects of variable density and viscosity, numerical solution of heat transport may be inaccurate. This study investigates the conditions under which the density and viscosity effects on heat transport modeling can be neglected without any significant loss of computational accuracy. A cross-section model of aquifer-river interactions at the Hanford 300 Area in Washington State was employed as the reference frame to quantify the role of fluid density and viscosity in heat transport modeling. This was achieved by comparing the differences in simulated temperature distributions with and without considering variable density and viscosity, respectively. The differences between the two sets of simulations were found to be minor under the complex field conditions at the Hanford 300A site. Based on the same model setup but under different prescribed temperature gradients across the simulation domain, a series of heat transport scenarios were further examined. When the maximum temperature difference across the simulation domain is within 15 degrees C, the mean discrepancy between the simulated temperature distributions with and without considering the effects of variable density and viscosity is approximately 2.5% with a correlation coefficient of above 0.8. Meanwhile, the speedup in runtime is roughly 225% when the maximum temperature difference is at 15 degrees C. This work provides some quantitative guidelines for when heat transport may be simulated by assuming constant density and viscosity as a reasonable compromise between accuracy and efficiency.

Controlling processes in a CaCO3 precipitating stream in Huanglong Natural Scenic District, Sichuan, China

Abstract Huanglong Scenic District is well known for its unusual and diversified landforms such as travertine pools, travertine falls and travertine flows. These landforms, resulting from high-altitude surface cold-water CaCO 3 precipitation, were chosen by UNESCO in 1994 as an entry in The Worlds Nature Heritage. Huanglong is a pristine region where there are limited human activities. Water analyses and thin section (glass slide) precipitation experiments were conducted to determine the aqueous processes controlling CaCO 3 precipitation and travertine landform formation. Results from the travertine flow indicate that the concentrations of HCO 3 − , Ca +2 , and H + decrease regularly along the flow paths. Chemical equilibrium modeling results demonstrate the importance of CO 2 out-gassing and CaCO 3 precipitation processes. CO 2 out-gassing and CaCO 3 precipitation increase with increasing flow velocities. In the pool area, varying hydrodynamics are the primary factors which determine the extent of processes such as advection and diffusion, and hence also control CaCO 3 precipitation and CO 2 out-gassing. When the pool water circulation is very slow, the pH of water flowing over the travertine dams increases significantly (approximately 0.15 pH units) downstream. When the circulation is relatively fast, the pH of stream water initially decreases followed by an increase of approximately 0.21 pH units as it flows past the travertine pool dams. In both cases, the pH rise is caused by sudden changes in the hydrodynamics of the pools, despite the different initial flow conditions. Pool development is a consequence of spatial variations in pH which provide different conditions for CaCO 3 precipitation inside the travertine dam, where less precipitation or even dissolution occurs, compared to conditions at the top and downstream side of the dams. Precipitation experiments demonstrate that the top and downstream side of travertine dams are the locations of the most active precipitation, particularly for pools having faster circulation. Precipitation experiments also reveal that vaterite, a rare polymorph of CaCO 3 , co-precipitates with calcite in milky opalescent water near the upstream input portion of the pool groups. Thin sections covered by algae at the bottom of pools have 40% less CaCO 3 precipitation than those not covered by algae. SEM photographs of the surface of natural travertine deposits show that biofilms with diatom minimize CaCO 3 precipitation and that diatom-adhered calcite surfaces show signs of etching, suggesting that calcite dissolution may be aided by diatoms.