Dense Nonaqueous Phase Liquids (DNAPLs)

Modeling

Abstracts of Journal Articles

Decision Support System for Evaluating Source Depletion at Chlorinated Solvent DNAPL Sites
Newell, C.J., T. McGuire, J. McDade, P. Newberry, D. Adamson, and I. Cowie, Groundwater Services, Inc., Houston, TX. Partners in Environmental Technology Technical Symposium & Workshop, 29 November - 1 December 2005, Washington, DC: Abstracts. Poster presentation No 61, p F-41, 2005

A decision support system was developed to aid the evaluation of source zone remediation at sites impacted with chlorinated solvent dense nonaqueous phase liquid (DNAPL). The decision support system consists of four modules: (1) a performance database for source depletion sites; (2) a performance database for untreated sites; (3) a cost database for source depletion sites; and (4) a literature summary of other source depletion projects. The project was one component of a project funded by SERDP focusing on DNAPL source zone remediation. The performance database for treated sites includes temporal groundwater concentration data from before and after active remediation for 144 wells at 59 chlorinated solvent sites. Sites in the database used one of the following source depletion technologies: enhanced bioremediation, chemical oxidation, thermal treatment, and surfactant/cosolvent treatment. The decision support system allows the user to generate custom concentration versus time curves using various site characteristics as selection criteria. For each custom data set, summary statistics of concentration reductions from before and after treatment are provided. The performance database for untreated sites includes temporal groundwater concentration data for 52 wells at 23 natural attenuation sites. Only sites with at least five years of monitoring data were included in the database; the length of the monitoring record for the sites in the database ranged from five to 15 years. The decision support system allows the user to generate custom concentration versus time curves using various site characteristics as selection criteria. For each custom data set, summary statistics of concentration change over the monitoring period are provided. The cost database summarizes treatment cost data for 36 sites where source depletion technologies were used to treat chlorinated solvent DNAPL source zones. The decision support system allows the user to generate custom cost data using various site characteristics as selection criteria. Cost data is summarized in terms of US dollars per cubic yard treated for each site. The literature summary module includes recent data from three other SERDP-funded DNAPL remediation research projects on the following topics: development of assessment tools for evaluating benefits of source zone treatment, mass transfer of entrapped DNAPL from sources undergoing remediation, and assessment of the benefits of partial source removal through experimentation and modeling.

Evaluating and Applying Site-Specific NAPL Dissolution Rates During Remediation
Stewart, L., M. Widdowson, J. Chambon, R. Deeb, M. Kavanaugh, and J. Nyman.
ESTCP Project ER19-5223, 251 pp, 2023

The objective of this project was to establish a practical and cost-effective method to assess source control at NAPL sites using site- and technology-specific NAPL dissolution rates in a volume-averaged source zone. The project implemented a volume-averaged source zone model based on upscaling complex NAPL dissolution processes to characteristic dimensions of multiple NAPL accumulations. Mathematical description of complex processes was combined into a system of coupled, nonlinear, first-order differential equations through volume-averaging. The output is an order-of-magnitude estimate for source zone discharge masses and concentrations over time subject to varied remedial processes applicable to design and performance evaluation. This effort resulted in beta versions of practical tools for multi- and single-component NAPL. The calculation tools estimate site-specific remedial impact given a modest amount of site characterization data. The approach was validated and demonstrated through comparisons with numerical, lab, and field-scale data. The technology was demonstrated at two sites with various NAPL architectures exposed to multiple remedial processes. The outputs were compared to observed data and results from calibrated numerical transport models for validation. The technology was also used to evaluate past and future remedial strategies. Qualitative performance objectives for the project were ease of use and utility for supporting remedial decisions. Remedial project managers confirmed the utility of the results for remedial decisions. During beta version testing, users cited the main implementation and utility issue as developing the conceptual source zone model and identifying input parameters. Based on this feedback, additional guidance was developed on estimating input parameters for the modeling. Additional information: Executive Summary , User Guide

Modeling Tools for Assessing the Benefits of DNAPL Source Zone Remediation
Abriola, L.M. (Tufts Univ., Medford, MA); J.A. Christ (U.S. Air Force Academy); K. Li and C.A. Ramsburg (Tufts Univ.). Partners in Environmental Technology Technical Symposium & Workshop, 29 November - 1 December 2005, Washington, DC: Abstracts. Poster presentation No 62, p F-42, 2005

Quantifying the benefits of partial mass removal from chlorinated dense nonaqueous phase liquid (DNAPL) source zones has emerged as a key issue in the decision to implement active source-zone treatment technologies. Researchers at Tufts University and Georgia Tech are collaborating under SERDP Project ER-1293 to develop assessment tools to quantify these benefits. To evaluate the utility of incorporating upscaled mass transfer coefficients into simplified transport models for the prediction of source-zone mass discharge and source longevity, predictions of flux-averaged concentrations and source longevity obtained with an upscaled model were compared to 3-D multiphase numerical simulations of tetrachloroethene (PCE)-NAPL dissolution for an ensemble of nonuniform initial PCE saturation distributions. Incorporation of a limited amount of site-specific information (e.g., initial flux-averaged concentration and source-zone ganglia-to-pool mass ratio) enhanced the accuracy of the upscaled approach, facilitating its application to remedial alternative evaluation at a broad range of sites. Modification of a compositional multiphase simulator to include metabolic reductive dechlorination kinetics has facilitated the simulation of enhancements in DNAPL dissolution due to source-zone dechlorination processes. This modeling tool has been validated against laboratory batch and 1-D column data and is being used to determine the sensitivity of bioenhanced dissolution to source zone characteristics (e.g., NAPL contaminated length, saturation), flushing conditions (e.g., ground water velocity, electron donor amendment), and biomass distribution. Results from this effort will facilitate the refinement of biostimulation and bioaugmentation strategies for enhanced source zone dechlorination. Geostatistical approaches are being used to develop and refine a tool which uses local concentration and permeability measurements to estimate downstream dissolved mass flux and to quantify the uncertainty in these estimates. The refined tool employs a multi-stage sampling strategy to overcome the limitations on estimate precision created by the presence of spatial discontinuities (hot spots) in the concentration field commonly found at DNAPL sites. Results indicate that the refined tool can rapidly improve the accuracy and precision of the derived mass flux probability distribution. Examples demonstrate that a 25% increase in sampling frequency can improve estimate accuracy and precision by more than 50%.

Monitoring the Progress of Remedial Technologies Using a Rate-limiting NAPL Dissolution Model
Benni, William C. and Mary Ann Parcher, Environmental Systems & Technology. NGWA 2006 Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Assessment, and Remediation Conference, 6-7 November 2006

Application of a rate-limiting nonaqueous phase (NAPL) dissolution model has successfully monitored the progress and effectiveness of remedial activities on NAPL sources. The approach relies on the principles of NAPL dissolution, measured groundwater concentration data, and estimates of the mass removal from each phase. When a multi-component NAPL mixture dissolves, higher soluble constituents are preferentially removed, which results in noticeable and predictable changes in the NAPL's composition. The equations for the rate-limiting mechanisms have been developed through review of laboratory studies and application of the model to several sites. The model uses the observed recovery data to calibrate an initial effective NAPL mass to the same level of certainty as the recovery data. The model estimates the amount of source mass remaining and provides real-time monitoring of the remedy's progress toward achieving the remedial objectives. The model's use of data that are readily available and frequently obtained makes it applicable to a wide range of remedial technologies and sites. Case studies for a recovery system and an innovative technology are presented to show the modeling approach for monitoring the progress of these remedial activities.

Spatial Variations of Residual NAPL Zone Concentration in Subsurface
Sun, D. and J. Zhu, Texas A&M Univ., College Station. 2004 Joint Assembly of the Canadian Geophysical Union, American Geophysical Union, Society of Exploration Geophysicists, and Environmental and Engineering Geophysical Society, 17-21 May 2004, Montreal, Canada. Eos Trans. AGU, Vol 85 No 17, Jt. Assem. Suppl., Abstract H21B-13, 2004

Improved models are presented for the prediction of the solute concentration in the zone of residual nonaqueous phase liquid (NAPL) as a result of dissolution. The models enable the pseudo-equilibrium formulation to be used and therefore the numerical simulations for field application problems can be simplified compared to the non-equilibrium counterpart. The models are especially useful for situations of small residual NAPL saturation, which are typical for many field applications. The previous screening models lumped the entire NAPL source zone together without considering the spatial distribution of NAPL source zone concentrations. The new models incorporate the fact that the NAPL mass in the upstream will dissolve first, creating a dissolution zone that will travel downstream at certain velocity and thus demonstrating that the NAPL zone concentration is dependent on both the time and the distance into the NAPL zone.