Natural source zone depletion (NSZD) insights from over 15 years of research and measurements: A multi-site study

Site-average Natural Source Zone Depletion (NSZD) rates measured from 40 petroleum light non-aqueous phase liquid (LNAPL) source zone sites were compiled from researchers, project reports, and scientific papers. At each site, the following data were compiled: i) general site location; ii) LNAPL fuel type; iii) measurement method, number of locations, and number of measurements per location; and iv) calculated site-average NSZD rate in liters per hectare per year (L/ha/yr) per site and the associated measurement method (i.e., Gradient Method, Carbon Traps, Dynamic Closed Chamber (DCC), or Thermal Monitoring). The resulting dataset showed site-average NSZD rates that ranged from 650 to 152,000 L/ha/yr (70 to 16,250 gallons per acre per year (gal/acre/yr)), with a median value of 9,540 L/ha/yr (1,020 gal/acre/yr). The median site-average NSZD rate by type of fuel spill did not show a statistically significant difference between fuel types. When comparing the different NSZD measurement methods applied to the same sites, the site-average NSZD rates differed by up to 4.8 times (i.e., ratio of faster rate to slower rate), with a median difference of 2.1 times. No clear bias was observed between NSZD rate measurement methods. At four sites with calculations of NSZD rates by season, NSZD rates were typically higher during summer and fall compared to winter and spring. For these sites, Q₁₀ values (a measure of the increase in NSZD rate associated with a 10 °C increase in temperature) ranged from 0.8 to 15.1, with a median of 2.2. The implications of this study suggest that increasing mean annual soil temperature at a site using engineered methods could potentially increase the biodegradation rate (e.g., an increase of 10 °C could double the NSZD rate). Finally, for five sites with site-average NSZD rates for multiple years, average NSZD rates varied by 1.1 to 4.9 times across years. Overall, the evaluation of NSZD rates measured at 40 LNAPL sites suggests that measurable NSZD occurs across a broad range of LNAPL sites. Although NSZD rates vary across sites, fuel type is not the primary factor explaining observed differences in rates.

Modeling a well-characterized perfluorooctane sulfonate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion

Two of the most important retention processes for per- and polyfluoroalkyl substances (PFAS) in groundwater likely are sorption and matrix diffusion. The objective of this study was to model concentration and mass discharge of one PFAS, perfluorooctane sulfonate (PFOS), with matrix diffusion processes incorporated using data from a highly chemically- and geologically-characterized site. When matrix diffusion is incorporated into the REMChlor-MD model for PFOS at this research site, it easily reproduces the field data for three key metrics (concentration, mass discharge, and total mass). However, the no-matrix diffusion model produced a much poorer match. Additionally, after about 40 years of groundwater transport, field data and the REMChlor-MD model both showed the majority (80%) of the measured PFOS mass that exited the source zones was located in downgradient low permeability zones due to matrix diffusion. As such, most of the PFOS mass is not available to immediately migrate downgradient via advection in the more permeable sands at this site, which has important implications for monitored natural attenuation (MNA). Plume expansion over the next 50 years is forecasted to be limited, from a 350-m plume length in 2017 to 550 m in 2070, as matrix diffusion will attenuate groundwater plumes by slowing their expansion. This phenomenon is important for constituents that do not degrade, such as PFOS, compared to those susceptible to degradation. Overall, this work shows that matrix diffusion is a relevant process in environmental PFAS persistence and slows the rate of plume expansion over time.

Impact of matrix diffusion on the migration of groundwater plumes for Perfluoroalkyl acids (PFAAs) and other non-degradable compounds

Groundwater fate and transport modeling results demonstrate that matrix diffusion plays a role in attenuating the expansion of groundwater plumes of "non-degrading" or highly recalcitrant compounds. This is especially significant for systems where preferred destructive attenuation processes, such as biological and abiotic degradation, are weak or ineffective for plume control. Under these conditions, models of nondestructive physical attenuation processes, traditionally dispersion or sorption, do not demonstrate sufficient plume control unless matrix diffusion is considered. Matrix diffusion has been shown to be a notable emergent impact of geological heterogeneity, typically associated with back diffusion and extending remediation timeframes through concentration tailing of the trailing edge of a plume. However, less attention has been placed on evaluating how matrix diffusion can serve as an attenuation mechanism for the leading edge of a plume of non-degrading compounds like perfluoroalkyl acids (PFAAs), including perfluorooctane sulfonate (PFOS). In this study, the REMChlor-MD model was parametrically applied to a generic unconsolidated and heterogeneous geologic site with a constant PFOS source and no degradation of PFOS in the downgradient edge of the plume. Low levels of mechanical dispersion and retardation were used in the model for three different geologic heterogeneity cases ranging from no matrix diffusion (e.g., sand only) to considerable matrix diffusion using low permeability ("low-k") layers/lenses and/or aquitards. Our analysis shows that, in theory, many non-degrading plumes may expand for significant time periods before dispersion alone would eventually stabilize the plume; however, matrix diffusion can significantly slow the rate and degree of this migration. For one 100-year travel time scenario, consideration of matrix diffusion results in a simulated PFOS plume length that is over 80% shorter than the plume length simulated without matrix diffusion. Although many non-degrading plumes may continue to slowly expand over time, matrix diffusion resulted in lower concentrations and smaller plume footprints. Modeling multiple hydrogeologic settings showed that the effect of matrix diffusion is more significant in transmissive zones containing multiple low-k lenses/layers than transmissive zones underlain and overlain by low-k aquitards. This study found that at sites with significant matrix diffusion, groundwater plumes will be shorter, will expand more slowly, and may be amenable to a physical, retention-based, Monitored Natural Attenuation (MNA) paradigm. In this case, a small "Plume Assimilative Capacity Zone" in front of the existing plume could be reserved for slow, de minimus, future expansion of a non-degrading plume. If potential receptors are protected in this scenario, then this approach is similar to allowances for expanding plumes under some existing environmental regulatory programs. Accounting for matrix diffusion may support new strategic approaches and alternative paradigms for remediation even for sites and conditions with "non-degrading" constituents such as PFAAs, metals/metalloids, and radionuclides.

Time vs. Money: A Quantitative Evaluation of Monitoring Frequency vs. Monitoring Duration

The National Research Council has estimated that over 126,000 contaminated groundwater sites are unlikely to achieve low ug/L clean-up goals in the foreseeable future. At these sites, cost-effective, long-term monitoring schemes are needed in order to understand the long-term changes in contaminant concentrations. Current monitoring optimization schemes rely on site-specific evaluations to optimize groundwater monitoring frequency. However, when using linear regression to estimate the long-term zero-order or first-order contaminant attenuation rate, the effect of monitoring frequency and monitoring duration on the accuracy and confidence for the estimated attenuation rate is not site-specific. For a fixed number of monitoring events, doubling the time between monitoring events (e.g., changing from quarterly monitoring to semi-annual monitoring) will double the accuracy of estimated attenuation rate. For a fixed monitoring frequency (e.g., semi-annual monitoring), increasing the number of monitoring events by 60% will double the accuracy of the estimated attenuation rate. Combining these two factors, doubling the time between monitoring events (e.g., quarterly monitoring to semi-annual monitoring) while decreasing the total number of monitoring events by 38% will result in no change in the accuracy of the estimated attenuation rate. However, the time required to collect this dataset will increase by 25%. Understanding that the trade-off between monitoring frequency and monitoring duration is not site-specific should simplify the process of optimizing groundwater monitoring frequency at contaminated groundwater sites.

Implications of matrix diffusion on 1,4-dioxane persistence at contaminated groundwater sites

Management of groundwater sites impacted by 1,4-dioxane can be challenging due to its migration potential and perceived recalcitrance. This study examined the extent to which 1,4-dioxane's persistence was subject to diffusion of mass into and out of lower-permeability zones relative to co-released chlorinated solvents. Two different release scenarios were evaluated within a two-layer aquifer system using an analytical modeling approach. The first scenario simulated a 1,4-dioxane and 1,1,1-TCA source zone where spent solvent was released. The period when 1,4-dioxane was actively loading the low-permeability layer within the source zone was estimated to be <3years due to its high effective solubility. While this was approximately an order-of-magnitude shorter than the loading period for 1,1,1-TCA, the mass of 1,4-dioxane stored within the low-permeability zone at the end of the simulation period (26kg) was larger than that predicted for 1,1,1-TCA (17kg). Even 80years after release, the aqueous 1,4-dioxane concentration was still several orders-of-magnitude higher than potentially-applicable criteria. Within the downgradient plume, diffusion contributed to higher concentrations and enhanced penetration of 1,4-dioxane into the low-permeability zones relative to 1,1,1-TCA. In the second scenario, elevated 1,4-dioxane concentrations were predicted at a site impacted by migration of a weak source from an upgradient site. Plume cutoff was beneficial because it could be implemented in time to prevent further loading of the low-permeability zone at the downgradient site. Overall, this study documented that 1,4-dioxane within transmissive portions of the source zone is quickly depleted due to characteristics that favor both diffusion-based storage and groundwater transport, leaving little mass to treat using conventional means. Furthermore, the results highlight the differences between 1,4-dioxane and chlorinated solvent source zones, suggesting that back diffusion of 1,4-dioxane mass may be serving as the dominant long-term "secondary source" at many contaminated sites that must be managed using alternative approaches.

Evidence of 1,4-dioxane attenuation at groundwater sites contaminated with chlorinated solvents and 1,4-dioxane

There is a critical need to develop appropriate management strategies for 1,4-dioxane (dioxane) due to its widespread occurrence and perceived recalcitrance at groundwater sites where chlorinated solvents are present. A comprehensive evaluation of California state (GeoTracker) and Air Force monitoring records was used to provide significant evidence of dioxane attenuation at field sites. Temporal changes in the site-wide maximum concentrations were used to estimate source attenuation rates at the GeoTracker sites (median length of monitoring period = 6.8 years). While attenuation could not be established at all sites, statistically significant positive attenuation rates were confirmed at 22 sites. At sites where dioxane and chlorinated solvents were present, the median value of all statistically significant dioxane source attenuation rates (equivalent half-life = 31 months; n = 34) was lower than 1,1,1-trichloroethane (TCA) but similar to 1,1-dichloroethene (1,1-DCE) and trichloroethene (TCE). Dioxane attenuation rates were positively correlated with rates for 1,1-DCE and TCE but not TCA. At this set of sites, there was little evidence that chlorinated solvent remedial efforts (e.g., chemical oxidation, enhanced bioremediation) impacted dioxane attenuation. Attenuation rates based on well-specific records from the Air Force data set confirmed significant dioxane attenuation (131 out of 441 wells) at a similar frequency and extent (median equivalent half-life = 48 months) as observed at the California sites. Linear discriminant analysis established a positive correlation between dioxane attenuation and increasing concentrations of dissolved oxygen, while the same analysis found a negative correlation with metals and CVOC concentrations. The magnitude and prevalence of dioxane attenuation documented here suggest that natural attenuation may be used to manage some but not necessarily all dioxane-impacted sites.

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 $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.

Groundwater remediation: the next 30 years

Groundwater remediation technologies are designed, installed, and operated based on the conceptual models of contaminant hydrogeology that are accepted at that time. However, conceptual models of remediation can change as new research, new technologies, and new performance data become available. Over the past few years, results from multiple-site remediation performance studies have shown that achieving drinking water standards (i.e., Maximum Contaminant Levels, MCLs) at contaminated groundwater sites is very difficult. Recent groundwater research has shown that the process of matrix diffusion is one key constraint. New developments, such as mass discharge, orders of magnitude (OoMs), and SMART objectives are now being discussed more frequently by the groundwater remediation community. In this paper, the authors provide their perspectives on the existing "reach MCLs" approach that has historically guided groundwater remediation projects, and advocate a new approach built around the concepts of OoMs and mass discharge.

Chlorinated ethene source remediation: lessons learned

Chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene (PCE) are widespread groundwater contaminants often released as dense nonaqueous phase liquids (DNAPLs). These contaminants are difficult to remediate, particularly their source zones. This review summarizes the progress made in improving DNAPL source zone remediation over the past decade, and is structured to highlight the important practical lessons learned for improving DNAPL source zone remediation. Experience has shown that complete restoration is rare, and alternative metrics such as mass discharge are often useful for assessing the performance of partial restoration efforts. Experience also has shown that different technologies are needed for different times and locations, and that deliberately combining technologies may improve overall remedy performance. Several injection-based technologies are capable of removing a large fraction of the total contaminant mass, and reducing groundwater concentrations and mass discharge by 1 to 2 orders of magnitude. Thermal treatment can remove even more mass, but even these technologies generally leave some contamination in place. Research on better delivery techniques and characterization technologies will likely improve treatment, but managers should anticipate that source treatment will leave some contamination in place that will require future management.

Relative contribution of DNAPL dissolution and matrix diffusion to the long-term persistence of chlorinated solvent source zones

The relative contribution of dense non-aqueous phase liquid (DNAPL) dissolution versus matrix diffusion processes to the longevity of chlorinated source zones was investigated. Matrix diffusion is being increasingly recognized as an important non-DNAPL component of source behavior over time, and understanding the persistence of contaminants that have diffused into lower permeability units can impact remedial decision-making. In this study, a hypothetical DNAPL source zone architecture consisting of several different sized pools and fingers originally developed by Anderson et al. (1992) was adapted to include defined low permeability layers. A coupled dissolution-diffusion model was developed to allow diffusion into these layers while in contact with DNAPL, followed by diffusion out of these same layers after complete DNAPL dissolution. This exercise was performed for releases of equivalent masses (675 kg) of three different compounds, including chlorinated solvents with solubilities ranging from low (tetrachloroethene (PCE)), moderate (trichloroethene (TCE)) to high (dichloromethane (DCM)). The results of this simple modeling exercise demonstrate that matrix diffusion can be a critical component of source zone longevity and may represent a longer-term contributor to source longevity (i.e., longer time maintaining concentrations above MCLs) than DNAPL dissolution alone at many sites. For the hypothetical TCE release, the simulation indicated that dissolution of DNAPL would take approximately 38 years, while the back diffusion from low permeability zones could maintain the source for an additional 83 years. This effect was even more dramatic for the higher solubility DCM (97% of longevity due to matrix diffusion), while the lower solubility PCE showed a more equal contribution from DNAPL dissolution vs. matrix diffusion. Several methods were used to describe the resulting source attenuation curves, including a first-order decay model which showed that half-life of mass discharge from the matrix-diffusion dominated phase is in the range of 13 to 29 years for TCE. Because the mass discharge rate shifts significantly over time once DNAPL dissolution is complete, a Power-Law model was shown to be useful, especially at later stages when matrix diffusion dominates. An assessment of mass distribution showed that while relatively small percentages of the initial source mass diffused into the low permeability compartment, this mass was sufficient to sustain concentrations above drinking water standards for decades. These data show that relatively typical conditions (e.g., 50-year-old release, moderate to high solubility contaminant) are consistent with late stage sources, where mass in low permeability matrices serves as the primary source, and fit the conceptual model that mass in low permeability zones is important when evaluating source longevity.

Contaminant plume classification system based on mass discharge

Estimation of mass discharge has become an increasingly valuable analysis technique at sites with contaminated groundwater plumes. We propose a simple plume magnitude classification system based on mass discharge comprised of 10 separate magnitude categories, such as a "Mag 7 plume." This system can be a useful tool for scientists, engineers, regulators, and stakeholders to better communicate site conceptual models, prioritize sites, evaluate plumes both spatially and temporally, and determine potential impacts.

Aerobic bioremediation of chlorobenzene source-zone soil in flow-through columns: performance assessment using quantitative PCR

Flow-through aquifer columns were operated for 12 weeks to evaluate the benefits of aerobic biostimulation for the bioremediation of source-zone soil contaminated with chlorobenzenes (CBs). Quantitative Polymerase Chain Reaction (qPCR) was used to measure the concentration of total bacteria (16S rRNA gene) and oxygenase genes involved in the biodegradation of aromatic compounds (i.e., toluene dioxygenase, ring hydroxylating monooxygenase, naphthalene dioxygenase, phenol hydroxylase, and biphenyl dioxygenase). Monochlorobenzene, which is much more soluble than dichlorobenzenes, was primarily removed by flushing, and biostimulation showed little benefit. In contrast, dichlorobenzene removal was primarily due to biodegradation, and the removal efficiency was much higher in oxygen-amended columns compared to a control column. To our knowledge, this is the first report that oxygen addition can enhance CB source-zone soil bioremediation. Analysis by qPCR showed that whereas the biphenyl and toluene dioxygenase biomarkers were most abundant, increases in the concentration of the phenol hydroxylase gene reflected best the higher dichlorobenzene removal due to aerobic biostimulation. This suggests that quantitative molecular microbial ecology techniques could be useful to assess CB source-zone bioremediation performance.

Remediation of RDX- and HMX-contaminated groundwater using organic mulch permeable reactive barriers

Organic mulch is a complex organic material that is typically populated with its own consortium of microorganisms. The organisms in mulch breakdown complex organics to soluble carbon, which can then be used by these and other microorganisms as an electron donor for treating RDX and HMX via reductive pathways. A bench-scale treatability study with organic mulch was conducted for the treatment of RDX- and HMX-contaminated groundwater obtained from a plume at the Pueblo Chemical Depot (PCD) in Pueblo, Colorado. The site-specific cleanup criteria of 0.55 ppb RDX and 602 ppb HMX were used as the logical goals of the study. Column flow-through tests were run to steady-state at the average site seepage velocity, using a 70%:30% (vol.:vol.) mulch:pea gravel packing to approach the formation's permeability. Significant results included: (1) Complete removal of 90 ppb influent RDX and 8 ppb influent HMX in steady-state mulch column effluent; (2) pseudo-first-order steady-state kinetic rate constant, k, of 0.20 to 0.27 h(-1) based on RDX data, using triplicate parallel column runs; (3) accumulation of reduced RDX intermediates in the steady-state column effluent at less than 2% of the influent RDX mass; (4) no binding of RDX to the column fill material; and (5) no leaching of RDX, HMX or reduction intermediates from the column fill material. The results of the bench-scale study will be used to design and implement a pilot-scale organic mulch/pea gravel permeable reactive barrier (PRB) at the site.

Long-Term Sustainability of Reductive Dechlorination Reactions at Chlorinatedsolvents Sites

The sustainability of biodegradation reactions is of interest at Type 1 chlorinated solvent sites where monitored natural attenuation is being considered as a remedial alternative. Type 1 chlorinated solvent sites are sites undergoing reductive dechlorination where anthropogenic substrates (such as landfill leachate or fermentable organics in the waste materials) ferment to produce hydrogen, a key electron donor. A framework is provided that classifies Type 1 chlorinated solvent sites based on the relative amounts and the depletion rates of the electron donors and the electron acceptors (i.e., chlorinated solvents). Expressions are presented for estimating the total electron donor demand due to the presence of solvents and competing electron acceptors such as dissolved oxygen, nitrate, and sulfate. Finally, a database of 13 chlorinated solvent sites was analyzed to estimate the median and maximum mass discharge rate for dissolved oxygen, nitrate, and sulfate flowing into chlorinated solvent plumes. These values were then used to calculate the amount of hydrogen equivalents and potential for lost perchloroethylene (PCE) biodegradation represented by the inflow of these competing electron acceptors. The median and maximum mass of PCE biodegradation lost due to competing electron acceptors, assuming 100% efficiency, was 226 and 4621 kg year(-1), respectively.

MAROS: a decision support system for optimizing monitoring plans

The Monitoring and Remediation Optimization System (MAROS), a decision-support software, was developed to assist in formulating cost-effective ground water long-term monitoring plans. MAROS optimizes an existing ground water monitoring program using both temporal and spatial data analyses to determine the general monitoring system category and the locations and frequency of sampling for future compliance monitoring at the site. The objective of the MAROS optimization is to minimize monitoring locations in the sampling network and reduce sampling frequency without significant loss of information, ensuring adequate future characterization of the contaminant plume. The interpretive trend analysis approach recommends the general monitoring system category for a site based on plume stability and site-specific hydrogeologic information. Plume stability is characterized using primary lines of evidence (i.e., Mann-Kendall analysis and linear regression analysis) based on concentration trends, and secondary lines of evidence based on modeling results and empirical data. The sampling optimization approach, consisting of a two-dimensional spatial sampling reduction method (Delaunay method) and a temporal sampling analysis method (Modified CES method), provides detailed sampling location and frequency results. The Delaunay method is designed to identify and eliminate redundant sampling locations without causing significant information loss in characterizing the plume. The Modified CES method determines the optimal sampling frequency for a sampling location based on the direction, magnitude, and uncertainty in its concentration trend. MAROS addresses a variety of ground water contaminants (fuels, solvents, and metals), allows import of various data formats, and is designed for continual modification of long-term monitoring plans as the plume or site conditions change over time.

Groundwater monitoring plans at small-scale sites--an innovative spatial and temporal methodology

An innovative methodology for improving existing groundwater monitoring plans at small-scale sites is presented. The methodology consists of three stand-alone methods: a spatial redundancy reduction method, a well-siting method for adding new sampling locations, and a sampling frequency determination method. The spatial redundancy reduction method eliminates redundant wells through an optimization process that minimizes the errors in plume delineation and the average plume concentration estimation. The well-siting method locates possible new sampling points for an inadequately delineated plume via regression analysis of plume centerline concentrations and estimation of plume dispersivity values. The sampling frequency determination method recommends the future frequency of sampling for each sampling location based on the direction, magnitude, and uncertainty of the concentration trend derived from representative historical concentration data. Although the methodology is designed for small-scale sites, it can be easily adopted for large-scale site applications. The proposed methodology is applied to a small petroleum hydrocarbon-contaminated site with a network of 12 monitoring wells to demonstrate its effectiveness and validity.

More realistic soil cleanup standards with dual-equilibrium desorption

The desorption of contaminants from soils/sediments is one of the most important processes controlling contaminant transport and environmental risks. None of the currently adopted desorption models can accurately quantify desorption at relatively low concentrations; these models often overestimate the desorption and thus the risks of hydrophobic organic chemicals, such as benzene and chlorinated solvents. In reality, desorption is generally found to be biphasic, with two soil-phase compartments. A new dual-equilibrium desorption (DED) model has been developed to account for the biphasic desorption. This model has been tested using a wide range of laboratory and field data and has been used to explain key observations related to underground storage tank plumes. The DED model relates the amount of a chemical sorbed to the aqueous concentration, with simple parameters including octanol-water partition coefficient, solubility, and fractional organic carbon; thus, it is the only biphasic model, to date, that is based on readily available parameters. The DED model can be easily incorporated into standard risk and transport models. According to this model, many regulatory standards of soils and sediments could be increased without increasing the risks.

An ethics core curriculum for Australasian medical Schools

Teaching ethics incorporates teaching of knowledge as well as skills and attitudes. Each of these requires different teaching and assessment methods. A core curriculum of ethics knowledge must address both the foundations of ethics and specific ethical topics. Ethical skills teaching focuses on the development of ethical awareness, moral reasoning, communication and collaborative action skills. Attitudes that are important for medical students to develop include honesty, integrity and trustworthiness, empathy and compassion, respect, and responsibility, as well as critical self-appraisal and commitment to lifelong education.