Charles J. Newell

Natural Attenuation of Fuels and Chlorinated Solvents in the Subsurface: Wiedemeier/Natural Attenuation of Fuels and Chlorinated Solvents in the Subsurface

Overview of Natural Attenuation. Attenuation of Source Zones and Formation of Plumes. Abiotic Processes of Natural Attenuation. Overview of Intrinsic Bioremediation. Intrinsic Bioremediation of Petroleum Hydrocarbons. Intrinsic Bioremediation of Chlorinated Solvents. Evaluating Natural Attenuation. Modeling Natural Attenuation. Case Studies: Fuel Hydrocarbons. Case Studies: Chlorinated Solvents. Design of Long-Term Monitoring Programs. Appendices. Index.

The New Potential for Understanding Groundwater Contaminant Transport: P.W. Hadley and C. Newell

The groundwater remediation field has been changing constantly since it first emerged in the 1970s. The remediation field has evolved from a dissolved-phase centric conceptual model to a DNAPL-dominated one, which is now being questioned due to a renewed appreciation of matrix diffusion effects on remediation. Detailed observations about contaminant transport have emerged from the remediation field, and challenge the validity of one of the mainstays of the groundwater solute transport modeling world: the concept of mechanical dispersion (Payne et al. 2008). We review and discuss how a new conceptual model of contaminant transport based on diffusion (the usurper) may topple the well-established position of mechanical dispersion (the status quo) that is commonly used in almost every groundwater contaminant transport model, and evaluate the status of existing models and modeling studies that were conducted using advection-dispersion models.

The new potential for understanding groundwater contaminant transport.

: The groundwater remediation field has been changing constantly since it first emerged in the 1970s. The remediation field has evolved from a dissolved-phase centric conceptual model to a DNAPL-dominated one, which is now being questioned due to a renewed appreciation of matrix diffusion effects on remediation. Detailed observations about contaminant transport have emerged from the remediation field, and challenge the validity of one of the mainstays of the groundwater solute transport modeling world: the concept of mechanical dispersion (Payne et al. 2008). We review and discuss how a new conceptual model of contaminant transport based on diffusion (the usurper) may topple the well-established position of mechanical dispersion (the status quo) that is commonly used in almost every groundwater contaminant transport model, and evaluate the status of existing models and modeling studies that were conducted using advection-dispersion models.

Support of Source Zone Bioremediation through Endogenous Biomass Decay and Electron Donor Recycling

ABSTRACT Enhanced bioremediation strategies employ intensive electron donor amendments that can be successful in generating high biomass concentrations within the targeted area, and this technology is increasingly being applied within source zones to address non–aqueous phase contaminants. An unintended consequence is potential electron donor recycling via the slow endogenous decay of these newly grown cells, which may persist in the source zone even after the enhanced bioremediation project is completed and the introduced electron donor is exhausted. This paper presents a conceptual model that outlines the endogenous decay process within source zones and identifies several key scenarios where it is an important contributor to long-term attenuation. A key concept is that this reservoir represented by decaying biomass is both potentially large and capable of several rounds of turnover before becoming exhausted. Thus, the slow decay of biomass and the recycling of these decay products within the source zone extended the duration of the treatment period. A recent survey on the performance of source depletion technologies strongly suggests that this process is being observed at sites where enhanced bioremediation has been implemented (McGuire et al., 2006, Ground Water Monitor. Remediat. 26:73–84). Concentrations continued to decline several years after treatment, providing a strong indication that there is an endogenous electron donor supply that is contributing to continued contaminant reduction over time. Little evidence of concentration rebound was observed relative to sites where other technologies were used, suggesting long-term benefits associated with enhanced bioremediation that appear to partially offset processes that can contribute to rebound (e.g., matrix diffusion). Because initial colonization can occur near the non-aqueous phase liquid (NAPL)-water interface when exogenous electron donor is readily available the endogenous cell decay occurs in an optimal location to continue to support reductive dechlorination. This electron donor recycling process is ideally suited to favor growth of dechlorinating organisms relative to competing populations because of the slow release rates associated with decay, and it should preferentially stimulate polychloroethylene (PCE) and trichloroethylene (TCE) source removal over metabolites. Endogenous decay can be directly employed as part of the remediation design through groundwater recirculation or by the construction of a carbon-based in situ biowall to generate large amounts of biomass. In the case of an in situ biowall, a key consideration is the placement, either as a permeable reactive barrier filled with fermentable carbon within the source zone, or as a barrier located upgradient of a source zone to ensure that groundwater is reduced before entering the targeted area. Regardless of the approach, the likely impact of electron donor recycling through endogenous decay is to extend low-level activity for up to several years, making enhanced bioremediation a promising technology in terms of initial performance and for long-term polishing.

Progress in Remediation of Groundwater at Petroleum Sites in California

Quantifying the overall progress in remediation of contaminated groundwater has been a significant challenge. We utilized the GeoTracker database to evaluate the progress in groundwater remediation from 2001 to 2011 at over 12,000 sites in California with contaminated groundwater. This paper presents an analysis of analytical results from over 2.1 million groundwater samples representing at least

Membrane Interface Probe Protocol for Contaminants in Low‐Permeability Zones

100 million in laboratory analytical costs. Overall, the evaluation of monitoring data shows a large decrease in groundwater concentrations of gasoline constituents. For benzene, half of the sites showed a decrease in concentration of 85% or more. For methyl tert-butyl ether (MTBE), this decrease was 96% and for TBE, 87%. At remediation sites in California, the median source attenuation rate was 0.18/year for benzene and 0.36/year for MTBE, corresponding to half-lives of 3.9 and 1.9 years, respectively. Attenuation rates were positive (i.e., decreasing concentration) for benzene at 76% of sites and for MTBE at 85% of sites. An evaluation of sites with active remediation technologies suggests differences in technology effectiveness. The median attenuation rates for benzene and MTBE are higher at sites with soil vapor extraction or air sparging compared with sites without these technologies. In contrast, there was little difference in attenuation rates at sites with or without soil excavation, dual phase extraction, or in situ enhanced biodegradation. The evaluation of remediation technologies, however, did not evaluate whether specific systems were well designed or implemented and did not control for potential differences in other site factors, such as soil type.

Strategies and Decision-Support Tools for Optimizing Long-Term Groundwater Monitoring Plans—MAROS 2.0

Accurate characterization of contaminant mass in zones of low hydraulic conductivity (low k) is essential for site management because this difficult-to-treat mass can be a long-term secondary source. This study developed a protocol for the membrane interface probe (MIP) as a low-cost, rapid data-acquisition tool for qualitatively evaluating the location and relative distribution of mass in low-k zones. MIP operating parameters were varied systematically at high and low concentration locations at a contaminated site to evaluate the impact of the parameters on data quality relative to a detailed adjacent profile of soil concentrations. Evaluation of the relative location of maximum concentrations and the shape of the MIP vs. soil profiles led to a standard operating procedure (SOP) for the MIP to delineate contamination in low-k zones. This includes recommendations for: (1) preferred detector (ECD for low concentration zones, PID or ECD for higher concentration zones); (2) combining downlogged and uplogged data to reduce carryover; and (3) higher carrier gas flow rate in high concentration zones. Linear regression indicated scatter in all MIP-to-soil comparisons, including R(2) values using the SOP of 0.32 in the low concentration boring and 0.49 in the high concentration boring. In contrast, a control dataset with soil-to-soil correlations from borings 1-m apart exhibited an R(2) of ≥ 0.88, highlighting the uncertainty in predicting soil concentrations using MIP data. This study demonstrates that the MIP provides lower-precision contaminant distribution and heterogeneity data compared to more intensive high-resolution characterization methods. This is consistent with its use as a complementary screening tool.

Evaluation of Source-Zone Attenuation at LUFT Sites with Mobile LNAPL

ABSTRACT Existing long-term groundwater monitoring programs can be optimized to increase their effectiveness/efficiency with the potential to generate considerable cost savings. The optimization can be achieved through an overall evaluation of conditions of the contaminant plume and the monitoring network, focused spatial and temporal sampling analyses, and automated and efficient management of data, analyses, and reporting. Version 2.0 of the Monitoring and Remediation Optimization System (MAROS) software, by integrating long-term monitoring analysis strategies and innovative optimization methods with a data management, processing, and reporting system, allows site managers to quickly and readily develop cost-effective long-term groundwater monitoring plans. The MAROS optimization strategy consists of a hierarchical combination of analysis methods essential to the decision-making process. Analyses are performed in three phases: 1) evaluating site information and historical monitoring data to obtain local concentration trends and an overview of the plume status; 2) developing optimal sampling plans for future monitoring at the site with innovative optimization methods; and 3) assessing the statistical sufficiency of the sampling plans to provide insights into the future performance of the monitoring program. Two case studies are presented to demonstrate the usefulness of the developed techniques and the rigor of the software.

Natural Attenuation Of Chlorinated Solvent Source Zones

The objective of this study is to better understand the effect of mobile LNAPL on source-zone attenuation at sites using a statistical evaluation of 3,523 leaking underground fuel tank (LUFT) sites from GeoTracker, an extensive database of chemical release sites in California. Our analysis indicates that sites with mobile LNAPL (i.e., sites with measurable LNAPL thicknesses in one or more groundwater monitoring wells (LNAPL sites)) have higher maximum dissolved groundwater constituent concentrations and significantly slower source-zone attenuation rates (i.e., changes in maximum concentrations over time) compared to sites with a history of no measurable LNAPL thickness (non-LNAPL sites). However, the evaluation indicates that, for mobile LNAPL sites, physical recovery (skimming and bailing) does not increase source attenuation rates. The results suggest a need for more careful evaluation of the potential benefits of physical LNAPL technologies.

Modeling Remediation of Chlorinated Solvent Plumes

At a site where groundwater is contaminated by chlorinated solvents, the spatial distribution of contamination and the persistence of the contamination over time is controlled by the interaction between the rate of natural attenuation in groundwater and the rate of natural attenuation of the source. Recent advances make it possible to describe and forecast the rate of natural attenuation of the source. This information makes it possible at many sites to include attenuation of the sources as part of a natural attenuation remedy. At other sites it may be possible to optimize the extent of active remediation of the source that is necessary to achieve a final remedy at a site.