Stochastic Cost-Optimization and Risk Assessment of in situ Chemical Oxidation for Dense Non-Aqueous Phase Liquid (DNAPL) Source Remediation

This study involved development of a computer program to determine optimal design variables for in situ chemical oxidation (ISCO) of dense nonaqueous phase liquid (DNAPL) sites to meet site-wide remediation objectives with minimum life-cycle remediation cost while taking uncertainty in site characterization data and model predictions into consideration. A physically-based ISCO performance model computes field-scale DNAPL dissolution, instantaneous reaction of oxidant with contaminant and with readily oxidizable natural oxidant demand (NOD), second-order kinetic reactions for slowly oxidizable NOD, and time to reach ISCO termination criteria. Remediation cost is computed by coupling the performance model with a cost module. ISCO termination protocols are implemented that allow different treatment subregions (e.g., zones with different estimated contaminant concentrations) to be terminated independently based on statistical criteria related to confidence limits of contaminant concentrations estimated from soil and/or groundwater sampling data. The ISCO model was implemented in the program called Stochastic Cost Optimization Toolkit, which includes modules for additional remediation technologies that can be implemented serially or in parallel coupled with a dissolved plume model to enable design optimization to meet plume-scale cleanup objectives. This study focuses on optimization of ISCO design to meet specified source zone remediation objectives. ISCO design parameters considered for optimization include oxidant concentration and injection rate, frequency and number of soil or groundwater samples, and cleanup criteria for termination of subregion injection. Sensitivity studies and example applications are presented to demonstrate the benefits of proposed stochastic optimization methodology

Data Analysis and Modeling to Optimize Thermal Treatment Cost and Performance

The objective of in situ thermal treatment is typically to reduce the contaminant mass or average soil concentration below a specified value. Evaluation of whether the objective has been met is usually made by averaging soil concentrations from a limited number of soil samples.Results from several field sites indicate large performance uncertainty using this approach, even when the number of samples is large. We propose a method to estimate average soil concentration by fi tting a log normal probability model to thermal mass recovery data. A statistical approach is presented for making termination decisions from mass recovery data, soil sample data, or both for an entire treatment volume or for subregions that explicitly considers estimation uncertainty which is coupled to a stochastic optimization algorithm to identify monitoring strategies to meet objectives with minimum expected cost. Early termination of heating in regions that reach cleanup targets sooner enables operating costs to be reduced while ensuring a high likelihood of meeting remediation objectives. Results for an example problem demonstrate that significant performance improvement and cost reductions can be achieved using this approach

An Analysis of Horizontal Flow Treatment Well Applicability for the Treatment of Chlorinated Solvent Contaminated Groundwater at United States Forces Korea Installations

Past research has shown that there is a rising public concern with environmental issues in the Republic of Korea (ROK). As Korean government and public interest in the environment grow, there is likely to be increased pressure to remediate environmental contamination at United States Department of Defense (DoD) installations in Korea. Impacting DoD\u27s ability to remediate contaminated sites overseas is the fact that limited environmental funds must compete with high priority mission requirements. Thus, particularly at overseas bases, there is an urgent need for inexpensive and effective groundwater remediation technologies. Horizontal Flow Treatment Well (HFTW) systems have been demonstrated in the U.S. to be an effective technology for managing groundwater contamination. However, the problem of finding a technology that is appropriate for use in Korea is particularly challenging due to the fractured aquifer systems that are ubiquitous throughout the Korean peninsula. The model analyses conducted in this study found that HFTWs have the potential to be a cost effective alternative to conventional technologies for contaminant management in the fractured media found in Korea. This study focused on the containment of groundwater contaminated with chlorinated solvents in the fractured rock aquifers that are commonly encountered at DoD installations in the ROK. Horizontal Flow Treatment Wells were analyzed as a potentially cheaper, safer, and more effective technology for the containment of chlorinated solvent contaminated groundwater. In this study, an HFTW numerical model that was developed for porous media was applied to the fractured systems encountered in the ROK. It was concluded that at the scale of interest, use of a porous media model was appropriate. Both hydrogeologic and design parameters were varied to determine their effects on the technology performance

An Upscaled Approach for Transport in Media with Extended Tailing Due to Back-Diffusion Using Analytical and Numerical Solutions of the Advection Dispersion Equation

The mono-continuum advection-dispersion equation (mADE) is commonly regarded as unsuitable for application to media that exhibit rapid breakthrough and extended tailing associated with diffusion between high and low permeability regions. This paper demonstrates that the mADE can be successfully used to model such conditions if certain issues are addressed. First, since hydrodynamic dispersion, unlike molecular diffusion, cannot occur upstream of the contaminant source, models must be formulated to prevent “back-dispersion.” Second, large variations in aquifer permeability will result in differences between volume-weighted average concentration (resident concentration) and flow-weighted average concentration (flux concentration). Water samples taken from wells may be regarded as flux concentrations, while soil samples may be analyzed to determine resident concentrations. While the mADE is usually derived in terms of resident concentration, it is known that a mADE of the same mathematical form may be written in terms of flux concentration. However, when solving the latter, the mathematical transformation of a flux boundary condition applied to the resident mADE becomes a concentration type boundary condition for the flux mADE. Initial conditions must also be consistent with the form of the mADE that is to be solved. Thus, careful attention must be given to the type of concentration data that is available, whether resident or flux concentrations are to be simulated, and to boundary and initial conditions. We present 3-D analytical solutions for resident and flux concentrations, discuss methods of solving numerical models to obtain resident and flux concentrations, and compare results for hypothetical problems. We also present an upscaling method for computing “effective” dispersivities and other mADE model parameters in terms of physically meaningful parameters in a diffusion-limited mobile–immobile model. Application of the latter to previously published studies of systems that exhibit early breakthrough and extended tailing shows that the upscaled mADE model is able to describe the observed behavior with reasonable accuracy given only known physical parameters for the systems without any model calibration

Optimization-based Groundwater Modeling of Aqueous Phase Dnapl to Enhance Plume Remediation Management

Water Resources Engineering. Research was performed for the small military installation of Vance AFB to evaluate and optimize the current remediation Long-Term Monitoring Plan. Two shallow aquifer groundwater plumes contaminated with the DNAPL chemical trichloroethylene (TCE) were analyzed with two public domain software programs, Monitoring and Remediation Optimization Software (MAROS) and Geostatistical Temporal/Spatial (GTS) Algorithm, adopted by the Air Force Center of Environmental Excellence. The goal to reduce wells and testing utilized Mann-Kendall, linear regression, Delaunay triangulation, Modified Cost Estimating System weighting, thinning, locally-weighted quadratic equations, temporal variogram, and a comparison with Parsons Three-Tiered approach. Findings: Irregular test data was reformatted from several sources using Excel. Other remediation construction such as cutoff walls and extraction wells complicated definition of representative plume boundaries. Evaluation of a small portion of dissolved TCE did not account for residual contamination. Recommendations to reduce testing frequency and the number of monitoring wells offered a minimum cost saving of 52,000 per year. GTS could not operate with minimum data set for Vance. 1. MAROS can operate with minimal data sets with less information than required by other Long Term Monitoring Optimization Software as shown by the operation of the GTS software. 2. The effect of limited well test data from contaminated plumes demonstrates that analysis is primarily dependent upon temporal data and frequency. It does not matter how many wells are present if the data is not first organized into periods of testing and set at specific calendar dates. 3. The most efficient form for updating and inputting data into MAROS is Excel format. ERPIMS is extremely difficult to access from a layman's level and excellent support if provided by the Air Force help desk that sponsors the software. 4. In comparison to the higher level GTS statistical optimization program, MAROS is rated the simplest software to operate. Input data is minimal for the general site, geography, stratigraphy, and hydrogeology. 5. The attained results of MAROS remediation do not necessarily correlate with attainment criteria of the regulators. For instance, the MAROS recommendation for deletion of wells differed significantly from the simpler ODEQ criteria of recording less than MCL levels for six straight testing periods.School of Civil & Environmental Engineerin

An Upscaled Approach for Transport in Media with Extended Tailing Due to Back-Diffusion Using Analytical and Numerical Solutions of the Advection Dispersion Equation

The mono-continuum advection-dispersion equation (mADE) is commonly regarded as unsuitable for application to media that exhibit rapid breakthrough and extended tailing associated with diffusion between high and low permeability regions. This paper demonstrates that the mADE can be successfully used to model such conditions if certain issues are addressed. First, since hydrodynamic dispersion, unlike molecular diffusion, cannot occur upstream of the contaminant source, models must be formulated to prevent “back-dispersion.” Second, large variations in aquifer permeability will result in differences between volume-weighted average concentration (resident concentration) and flow-weighted average concentration (flux concentration). Water samples taken from wells may be regarded as flux concentrations, while soil samples may be analyzed to determine resident concentrations. While the mADE is usually derived in terms of resident concentration, it is known that a mADE of the same mathematical form may be written in terms of flux concentration. However, when solving the latter, the mathematical transformation of a flux boundary condition applied to the resident mADE becomes a concentration type boundary condition for the flux mADE. Initial conditions must also be consistent with the form of the mADE that is to be solved. Thus, careful attention must be given to the type of concentration data that is available, whether resident or flux concentrations are to be simulated, and to boundary and initial conditions. We present 3-D analytical solutions for resident and flux concentrations, discuss methods of solving numerical models to obtain resident and flux concentrations, and compare results for hypothetical problems. We also present an upscaling method for computing “effective” dispersivities and other mADE model parameters in terms of physically meaningful parameters in a diffusion-limited mobile–immobile model. Application of the latter to previously published studies of systems that exhibit early breakthrough and extended tailing shows that the upscaled mADE model is able to describe the observed behavior with reasonable accuracy given only known physical parameters for the systems without any model calibration

Electrical Resistivity Tomography for Mapping Subsurface Remediation

Cleaning up sites contaminated with dense non-aqueous phase liquids (DNAPLs) remains a challenging geoenvironmental problem. The performance of site remediation methods is difficult to assess without a practical, non-destructive technique to map where and how quickly DNAPL mass is being reduced. The promise of electrical resistivity tomography (ERT) in this context has not been realized, in part because traditional ERT methods were used to solve the near-impossible problem of mapping the initial DNAPL outline. However, new developments in ERT have emerged that focus on resolving subsurface changes over time. The objective of this work was to evaluate the potential of time-lapse ERT for mapping DNAPL mass reduction during remediation. A new numerical model was developed to explore this potential at the field scale, generating realistic DNAPL scenarios and predicting the response of an ERT survey. Central to the model was the development of a novel linkage between hydrogeological and geoelectrical properties. Sensitivity studies conducted at a variety of scales demonstrated that the linkage routine is robust and the DNAPL-ERT model is a valuable research tool. Moving forward to consider site applications, a new time-lapse method, four-dimensional (4D, three spatial dimensions plus time) ERT, was identified as highly promising. A laboratory experiment was conducted that demonstrated, for the first time, the effectiveness of 4D ERT applied at the surface for mapping an evolving DNAPL distribution. Independent simulation of the experiment demonstrated the reliability of the DNAPL-ERT model for simulating real systems. The numerical model was then used to explore the 4D surface ERT approach at the field scale for monitoring a range of realistic DNAPL remediation scenarios. The approach showed excellent potential for mapping shallow DNAPL changes but deeper changes were not as well resolved. To overcome this limitation, a new surface-to-horizontal borehole (S2HB) ERT configuration was proposed. The potential benefit of this innovation was first demonstrated by using the numerical model to compare surface ERT to S2HB ERT for a realistic, field scale DNAPL scenario with remediation at depth. A second laboratory experiment then demonstrated that this new configuration does better resolve changes in DNAPL distribution relative to surface ERT, particularly at depth. Independent simulation of the experiment showed that S2HB ERT is reliably modelled. Overall, this research has substantially advanced ERT in the context of DNAPL sites, with novel contributions to theory, modelling, demonstrations with physical systems, and simulations of realistic field scenarios. As a whole, this work demonstrates that, with these innovations, ERT exhibits significant potential as a DNAPL remediation site monitoring tool