Evaluation of Exploration and Monitoring Methods for Verification of Natural Attenuation Using the Virtual Aquifer Approach

Though natural attenuation (NA) is increasingly considered as a remediation technology, the methods for proper identification and quantification of NA are still under discussion. Here the "Virtual Aquifer" approach is used to demonstrate problems which may arise during measurement of concentrations in observation wells and for interpolation of locally measured concentrations in contaminated heterogeneous aquifers. The misinterpretation of measured concentrations complicates the identification and quantification of natural attenuation processes. The "Virtual Aquifer" approach accepts the plume simulated with a numerical model for a heterogeneous aquifer as "virtual reality". This virtual plume is investigated in the model with conventional methods like observations wells. The results of the investigation can be compared to the virtual "reality", evaluating the monitoring method. Locally determined concentrations are interpolated using various interpolation methods and different monitoring set-ups. The interpolation results are compared to the simulated plume to evaluate the quality of interpolation. This evaluation is not possible in nature, since concentrations in a heterogeneous aquifer are never known in detail.     

Modeling the Impact of Oxygen Reaeration on Natural Attenuation

Based on studies of leaking petroleum storage tank (LPST) sites in Texas and California, the average plume of benzene, toluene, ethylene, and xylenes (BTEX) is between 61 and 132 m (200 and 400 ft) long. Standard modeling of BTEX plumes produces plumes well in excess of observed plume lengths. The amount of oxygen carried into the plume zone with clean upgradient water often is insufficient to account for the levels of biodegradation observed in these studies. Traditional recharge of oxygen-containing water into an aquifer adds insufficient oxygen to the system and cannot account for the observed plume lengths. Research has shown that anaerobic processes can contribute to biodegradation in certain cases; however, anaerobic pathways are not included in this work. Reaeration of oxygen-depleted aquifers by diffusive transport of oxygen through the vadose zone has generally been neglected as a way to introduce oxygen into surficial aquifers. The observed plume lengths and preliminary laboratory results indicate that this source of oxygen should be accounted for in any natural attenuation model of BTEX contamination. This approach to modeling reaeration has been incorporated into the finite-element groundwater flow and contaminant transport code, FLOTRAN. Adding diffusion-driven reaeration to the modeling process produces BTEX plumes consistent with observed plume lengths.     

Definition, Objectives, and Evaluation of Natural Attenuation

Natural attenuation offers large benefits to owners and managers of contaminated sites, but often raises strong objections from those who live and work near a site and are asked to assume most of the long-term risks. Part of the controversy comes about because published definitions of natural attenuation do not identify a realistic end-point objective, and they also are ambiguous about the naturally occurring processes that can achieve the objective. According to guidance from the U.S. National Research Council (NRC 2000), destruction and strong immobilization are the naturally occurring processes that achieve a realistic objective: containing the contaminant relatively nears its source, thereby minimizing exposure risks. The strategy for obtaining solid evidence that the objective is being achieved requires measurements that establish a cause-and-effect relationship between contaminant loss and a destruction or strong-immobilization reaction. The cause-and-effect relationship is best documented with reaction footprints, which typically are concentration changes in reactants or products of the destruction or immobilization reaction. MTBE presents a contemporary example in which footprint evidence for biodegradation is especially crucial, since aerobic biodegradation of MTBE requires special conditions not present at all sites: a high availability of dissolved oxygen and bacteria expressing particular oxygenase enzymes.     

Synthetic Chemicals with Potential for Natural Attenuation

Natural attenuation of petroleum hydrocarbons is predictable and self-sustaining because bacteria able to use the contaminants as growth substrates are widely distributed. In contrast, bacteria able to grow at the expense of chlorinated aliphatic compounds are less common and the natural attenuation of such compounds is, therefore, less predictable. The purpose of this paper is to describe examples of other synthetic organic compounds that are known to be biodegradable and have the potential for natural attenuation in the field.     

Rate of Natural Attenuation of Tert-Butyl Alcohol at a Chemical Plant

Tert-butyl alcohol (TBA) may be present in groundwater as an original component of leaked gasoline, or as a degradation product of methyl tert-butyl ether (MTBE). Evidence for natural attenuation of TBA in groundwater is presented from a chemical plant in Pasadena, Texas. Shallow groundwater in several areas of the plant has been affected by historic leaks and spills of TBA. A decade of regular groundwater monitoring of one groundwater plume, consisting primarily of TBA, shows generally declining concentrations and a plume area that is shrinking. Natural attenuation mechanisms are limiting the advective transport of TBA. The principal mechanism of attenuation in this case is probably biodegradation as the other physical components of natural attenuation (dilution, dispersion, diffusion, adsorption, chemical reactions, and volatilization) cannot explain the behavior of the plume over time. Biodegradation was also indicated by the enrichment of stable carbon isotope composition (¹³C/¹²C) of TBA along the flow path. Preliminary dissolved gas and electron acceptor analyses indicate the groundwater is at least under sulfate reducing condition in the core of the plume and the process responsible for biodegradation of TBA may include fermentation under aerobic (plume fringes) and possible anaerobic conditions. This case history demonstrates that natural attenuation of TBA is important, and can be used as a groundwater management tool at this site.     

Anaerobic Biodegradation and Hydrogeochemical Controls on Natural Attenuation of Trichloroethene in an Inland Forested Wetland

A field and laboratory investigation of natural attenuation, focusing on anaerobic biodegradation, was conducted in a forested wetland where a plume of trichloroethene discharges from a sand aquifer through organic-rich wetland and stream-bottom sediments. The rapid response of the wetland hydrology to precipitation events altered groundwater flow and geochemistry during wet conditions in the spring compared to the drier conditions in the summer and fall. During dry conditions, partial reductive dechlorination of trichloroethene to cis-1,2-dichloroethene occurred in methanogenic wetland porewater. Influx of oxygenated recharge during wet conditions resulted in a change from methanogenic to iron-reducing conditions and a lack of 1,2-dichloroethene production in the wet spring conditions. During these wet conditions, dilution was the primary attenuation mechanism evident for trichloroethene in the wetland porewater. Trichloroethene degradation was insignificant in anaerobic microcosms constructed with the shallow wetland sediment, and microbiological analyses showed a low microbial biomass and absence of known dehalorespiring microorganisms. Despite the typically organic-rich characteristic of wetland sediments, natural attenuation by anaerobic degradation may not be an effective groundwater remediation for chlorinated solvents at all sites.     

Selecting Remediation Goals by Assessing the Natural Attenuation Capacity of Groundwater Systems

Remediation goals for the source areas of a chlorinated ethene-contaminated groundwater plume were identified by assessing the natural attenuation capacity of the aquifer system. The redox chemistry of the site indicates that sulfate-reducing (H₂ ∼ 2 nanomoles [nM]) per liter conditions near the contaminant source grade to Fe(III)-reducing conditions (H₂ ∼ 0.5 nM) downgradient of the source. Sulfate-reducing conditions facilitate the initial reduction of perchloroethene (PCE) to trichloroethene (TCE), cis-dichloroethene (cis-DCE), and vinyl chloride (VC). Subsequently, the Fe(III)-reducing conditions drive the oxidation of cis-DCE and VC to carbon dioxide and chloride. This sequence gives the aquifer a substantial capacity for biodegrading chlorinated ethenes. Natural attenuation capacity (the slope of the steady-state contaminant concentration profile along a groundwater flowpath) is a function of biodegradation rates, aquifer dispersive characteristics, and groundwater flow velocity. The natural attenuation capacity at the Kings Bay, Georgia site was assessed by estimating groundwater flowrates (∼0.23±0.12 m/d) and aquifer dispersivity (∼1 m) from hydrologic and scale considerations. Apparent biodegradation rate constants (PCE and TCE ∼0.01 d^-1; cis-DCE and VC ∼0.025 d^-1) were estimated from observed contaminant concentration changes along aquifer flowpaths. A boundary-value problem approach was used to estimate levels to which contaminant concentrations in the source areas must be lowered (by engineered removal), or groundwater flow velocities lowered (by pumping) for the natural attenuation capacity to achieve maximum concentration limits (MCLs) prior to reaching a predetermined regulatory point of compliance.     

Natural Attenuation of Oily Sludge Used for the Maintenance of an Unpaved Road in Arauca (Colombia)

IN this study the biodegradation and leaching of total petroleum hydrocarbons (TPHs) during the natural attenuation (NA) processes of the oily sludge used for the maintenance of unpaved roads were evaluated. The study was conducted on a road located in Arauca (Colombia) and simultaneously in a laboratory setting where rain conditions were simulated by using columns. In the in situ study, three depths (5, 50, and 80 cm) were assessed. The degradation of TPH was evaluated by monitoring physicochemical and microbiological conditions in situ and in the columns during 538 and 119 days, respectively. The pH levels observed during the study were relatively constant and ranged between the optimum values for biodegradation (6.2 ± 0.9). Water content in situ was low (< 14%) and observed concentrations of nitrogen (as ammonia and nitrate) were < 30 and < 10 mg/kg_dw, respectively, indicating that no mass balance was maintained, both possible factors limiting intrinsic biodegradation. During the in situ study a 95% TPH degradation was observed at 5 cm depth, whereas no degradation was evident at 50 and 80 cm. In the column experiments, TPH concentration in the leachate was < 1 mg/L, indicating that the leaching process did not play a key role in this study.     

Methyl-tertiary Butyl Ether Natural Attenuation Case Studies

The occurrence of methyl-tertiary butyl ether (MtBE), a gasoline additive in ground water and surface water, is causing regulatory agencies, owner/operators, environmental professionals, and researchers to reevaluate remediation strategies at sites where gasoline containing this additive has been released. Over the last 5 to 10 years, monitored natural attenuation has been applied at petroleum hydrocarbon-impacted sites with increasing frequency. The efficacy of this remediation method for releases containing MtBE is now coming under increased scrutiny. The question of natural attenuation efficacy stems from uncertainty about MtBE biodegradability and behavior in the subsurface. Researchers and applied environmental scientists have completed and are continuing studies concerning MtBE biodegradability and behavior. This article briefly summarizes the history of MtBE, its physicochemical properties, its behavior in the environmental, and the applicability of monitored natural attenuation as a remediation tool. Case studies representing past and current research are then presented and followed by a brief discussion. Results from the documented research reviewed show that MtBE does biodegrade in the laboratory and at actual release sites. “Plumathon” studies document MtBE concentration and mass reduction and/or plume stabilization over time. MtBE concentrations in monitoring wells may show declining concentrations indicating natural attenuation. The data reviewed from past and current research suggest that natural attenuation may be the appropriate remediation strategy at some release sites. Yet, the data also indicate that care must be exercised in determining the efficacy of applying monitored natural attenuation at sites impacted with MtBE.     

Natural Attenuation of Trichloroethylene in Rhizosphere Soils at the Savannah River Site

Extensive trichloroethylene (TCE) groundwater contamination has resulted from discharges to a former seepage basin in the A/M Area at the Department of Energy's Savannah River Site. The direction of groundwater flow has been determined and a seep line where the contaminated groundwater is estimated to emerge as surface water has been identified in a region of the Southern Sector of the A/M Area. This study was undertaken to estimate the potential of four rhizosphere soils along the seep line to naturally attenuate TCE. Microcosms were setup to evaluate both biotic and abiotic attenuation of TCE. Results demonstrated that sorption to soil was the dominant mechanism during the first week of incubation, with as much as 90% of the TCE removed from the aqueous phase. Linear partitioning coefficients (K_d) ranged from 0.83 to 7.4?mL/g, while organic carbon partition coefficients (K_oc) ranged from 72 to 180?mL/gC. Diffu-sional losses from the microcosms appeared to be a dominant fate mechanism during the remainder of the experiment, as indicated by results from the water controls. A limited amount of TCE biodegradation was observed, and attempts to stimulate TCE biodegradation by either methanotrophic or methanogenic activity through amendments with methane, oxygen, and methanol were unsuccessful. The appearance of cis-1,2-dichloroethylene (c-DCE), and trans-1,2-dichloroethylene (t-DCE) confirmed the potential for anaerobic reductive dechlorination. However, these daughter products represented less than 5% of the initial TCE added. The sorption results indicate that natural attenuation may represent a viable remediation option for the TCE plume as it passes through the rhizosphere.     

A Site-Specific Approach for the Evaluation of Natural Attenuation at Metals-Impacted Sites

Consideration of monitored natural attenuation (MNA) as a remedy component for metals-contaminated sites can be achieved using a site-specific screening approach, followed by application of one or a series of sequential extraction measurements. Hazardous waste sites contaminated with metals can be screened for the implementation of monitored natural attenuation on the basis of contaminant-specific soil chemical characteristics (i.e., K_d's, solubilities, and nonexchangeable sorbed fraction). Field cases are used to demonstrate the screening approach and to outline the primary considerations involved in accurately applying sequential extraction procedures to support the of MNA for site remediation. The results of these case studies provide strong evidence that site-specific screening and the use of sequential extraction procedures are effective methods for evaluating natural attenuation for metals impacted sites.     

Evaluation of Natural Attenuation of Benzene and Dichloroethanes at the KL Landfill

Natural attenuation of benzene and dichloroethanes in groundwater contaminated by leachate from the West KL Avenue landfill in Kalamazoo, Michigan, was evaluated in three phases. Existing data from the previous site investigations were used to locate a series of high-resolution vertical profile samples. By analyzing data from the discrete vertical profile samples, the rates of attenuation of benzene and dichloroethanes in the plume were forecasted. Permanent monitoring wells were installed over the depth intervals associated with high concentrations in the vertical profile sampling. These wells were monitored over time to extract independent estimates of the rates of degradation of benzene and dichloroethanes. Estimates of first-order attenuation rate constants were obtained using two methods: a method due to Buscheck and Alcantar (1995), which is based on a one-dimensional steady-state analytical solution, and the tracer correction method of Wiedemeier et al. (1996). The rates of attenuation predicted from the vertical profile sampling were found to be in good agreement with the rates obtained from the permanent monitoring well data, indicating that the long-term behavior of the contaminant plumes is consistent with the initial forecast. The results also indicated that the natural attenuation of benzene, 1,1-dichloroethane (DCA), and 1,2-DCA was statistically significant (at the 0.05 level).     

Detection and Quantification of Dehalococcoides-Related Bacteria in a Chlorinated Ethene-Contaminated Aquifer Undergoing Natural Attenuation

Detection and quantification of bacteria related to Dehalococcoides is essential for the development of effective remediation strategies for tetrachloroethene (PCE)-contaminated sites. In this study, the authors applied three methods for quantifying Dehalococcoides-like bacteria in a PCE-contaminated aquifer undergoing natural attenuation in Grenchen, Switzerland: a catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) protocol, a competitive nested polymerase chain reaction (PCR) approach, and a direct PCR end point quantification with external standards. For the investigated aquifer, multiple lines of evidence indicated that reductive dechlorination (and likely dehalorespiration) was an active process. Both PCR-based quantification methods indicated that low numbers of mostly sediment-bound Dehalococcoides were present in the contaminated zone of the Grenchen aquifer. Estimates based on the quantitative PCR methods ranged from 2.1 × 10⁷ to 1.5 × 10⁸ sediment-bound Dehalococcoides 16S rRNA gene copies per liter of aquifer volume. In contrast, the liquid phase only contained between 8 and 80 copies per liter aquifer volume. CARD-FISH was not sensitive enough for the quantification of Dehalococcoides cell numbers in this aquifer. Cloning and sequencing of the PCR products revealed the presence of sequences closely related to Dehalococcoides isolates such as D. ethenogenes and Dehalococcoides sp. BAV1. An apparently abundant group (termed “Grenchen Cluster”) of sequences more distantly related to Dehalococcoides was also identified, so far without cultured representatives.     

Field Scale Variations in Soil Properties for Spatially Variable Control: A Review

This article reviews the widely scattered information on spatial variability of field-scale properties. It mainly addresses the variations in properties that influence nitrate accumulation and movement resulting from the agricultural activities in the root zone. The drive for this research comes from the perception and evidence of widespread contamination arising from production agriculture. The information is useful in the spatial management of soil for prevention of non-point source groundwater contamination from production practices. The review includes most of the static soil properties, one water transport property, some soil fertility variables, and crop yield. It examines various investigations and parameters identifying the magnitude of variability, and spatial correlation values for a given property are provided in a tabular form. Various sources of spatial variability, as well as methods of evaluating soil variability, are also discussed. For static soil properties, the range of coefficient of variation (CV) value was found to be small given the diversity of soils and field sizes on which the observations were conducted. The selected soil fertility variables and crop yield exhibited intermediate variations. However, CVs were considerably higher for the investigated transport property than those of static soil properties. The static soil properties and fertility variables were found to be correlated over a much larger distance than the transport property. If there was no variability, production through traditional practice with proper adjustment for field conditions would be the most appropriate choice. However, variations in soil and crop growth properties have led to attempts to understand these variations and application of spatially variable control in production agriculture.     

Formulation of the CBC-Model for Modelling the Contaminants and Footprints in Natural Attenuation of BTEX

This paper provides the details of the Coupled Biological and Chemical (CBC) model for representing in situ bioremediation of BTEX. The CBC model contains novel features that allow it to comprehensively track the footprints of BTEX bioremediation, even when the fate of those footprints is confounded by abiotic reactions and complex interactions among different kinds of microorganisms. To achieve this comprehensive tracking of all the footprints, the CBC model contains important new biological features and key abiotic reactions. The biological module of the CBC-model includes these important new aspects: (1) it separates BTEX fermentation from methanogenesis, (2) it explicitly includes biomass as a sink for electrons and carbon, (3) it has different growth rates for each biomass type, and (4) it includes inhibition of the different reactions by other electron acceptors and by sulfide toxicants. The chemical module of the CBC-model includes abiotic reactions that affect the footprints of the biological reactions. In particular, the chemical module describes the precipitation/dissolution of CaCO3, Fe2O3, FeS, FeS2, and S degrees. The kinetics for the precipitation/dissolution reactions follow the critical review in Maurer & Rittmann (2004).     

Iron and Sulfur Mineral Analysis Methods for Natural Attenuation Assessments

Based on electron acceptor abundance, Fe³⁺ and SO₄^2- reduction by bacteria may play a dominant role in intrinsic bioremediation of some organic contaminants in the subsurface. Both Fe³⁺ and SO₄^2- reduction processes involve mineral phases and may not be properly understood by evaluating only groundwater concentrations. Fe and S mineral analyses should be incorporated in natural attenuation studies; however, inherent problems with sample collection and analysis have discouraged such efforts. Methods are presented here for (1) sediment collection and anoxic preservation, (2) evaluation of biologically available Fe³⁺ and biogenically produced Fe²⁺ minerals, and (3) a simplified extended mineral sulfide analysis for ∼FeS and S°+FeS₂. These techniques are demonstrated to evaluate Fe³⁺ and SO₄^2- reduction at three sites where the soil or aquifer matrix had been contaminated with gasoline fuel, methane gas, or landfill leachate. It is expected that these techniques will permit Fe and S mineral analyses to become a routine part of natural attenuation assessments.     

Monitored Natural Attenuation of Explosives

Explosives are subject to several attenuation processes that potentially reduce concentrations in groundwater over time. Some of these processes are well defined, while others are poorly understood. The objective of the project was to optimize data collection and processing procedures for evaluation and implementation of monitored natural attenuation of explosives. After conducting experiments to optimize data quality, a protocol was established for quarterly monitoring of thirty wells over a 2-year period at a former waste disposal site. Microbial biomarkers and stable isotopes of nitrogen and carbon were explored as additional approaches to tracking attenuation processes. The project included a cone penetrometry sampling event to characterize site lithology and to obtain sample material for biomarker studies. A three-dimensional groundwater model was applied to conceptualize and predict future behavior of the contaminant plume. The groundwater monitoring data demonstrated declining concentrations of explosives over the 2 years. Biomarker data showed the potential for microbial degradation and provided an estimate of the degradation rate. Measuring stable isotopic fractions of nitrogen in TNT was a promising method of monitoring TNT attenuation. Overall, results of the demonstration suggest that monitored natural attenuation is a viable option that should be among the options considered for remediation of explosives-contaminated sites.     

Distribution in the environment of degradative capacitiesfor gasoline attenuation

A methodology allowing the detailed assessment ofthe capacities of microflorae to degrade gasoline inaerobic conditions has been developed. It consisted inthe determination of the degradation of a gasolinemodel mixture in liquid cultures in optimalconditions. The gasoline model mixture contained 23representative hydrocarbons of gasoline (GM23). Thekinetics and extent of biodegradation were evaluatedby continuous overall monitoring of CO₂production and final chromatographic analysis (usuallyafter about 30 days) of the consumption of eachhydrocarbon. The methodology was used with soil andwater samples from polluted and non polluted sites.The experimentation aimed at assessing thedistribution of the degradative capacities in theenvironment and the prospects for natural attenuationof gasoline. Nine microflorae were tested. The intrinsicbiodegradability (existence of mechanisms ofbiodegradation) appeared total for GM23 as shown bythe results obtained with several microflorae. Thedegradative capacities of microflorae from nonpolluted samples were high (total degradation rates atleast 85%). Incomplete degradation was observedessentially for trimethylalkanes(2,2,4-trimethylpentane and 2,3,4-trimethylpentane)and for cyclohexane. In several cases, samples frompolluted sites exhibited more extensive degradativecapacities, with total degradation of all hydrocarbonsbeing observed for three out of the six samples.     

Biodegradation of a PAH Mixture by Native Subsurface Microbiota

Laboratory microcosm studies were conducted to estimate biodegradation rates for a mixture of five polycyclic aromatic hydrocarbon compounds (PAHs). Static microcosms were assembled using soil samples from two locations collected at a No. 2 fuel oil-contaminated site in the Atlantic Coastal Plain of Virginia. In microcosms from one location, five PAHs (acenaphthene, fluorene, phenanthrene, pyrene, and benzo(b)fluoranthene) biodegraded at net first-order rates of 1.08, 1.45, 1.13, 1.11, and 1.12 yr^?1, respectively. No observable lag period was noted and degradation in live microcosms ceased with the depletion of oxygen and sulfate after 125 days. In microcosms from a second location, net first-order biodegradation rates after an approximately 2-month lag period were 2.41, 3.28, and 2.98 yr^?1 for fluorene, phenanthrene, and pyrene, respectively. Acenaphthene and benzo(b)fluoranthene mass loss rates in the live microcosms were not statistically different from mass loss rates in control microcosms. Stoichiometric mass balance calculations indicate that the dominant PAH mass loss mechanism was aerobic biodegradation, while abiotic losses (attributed to micropore diffusion and oxidative coupling) ranged from 15 to 33% and biotic losses from sulfate-reduction accounted for 7 to 10% of PAH mass loss. Stoichiometric equations that include biomass yield are presented for PAH oxidation under aerobic and sulfate-reducing conditions.