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).     

Experimental Evaluation of Catalyzed Hydrogen Peroxide and Sodium Persulfate for Destruction of BTEX Contaminants

Due to the toxicity and prevalence of BTEX contaminants (benzene, toluene, ethylbenzene, and xylenes) at hazardous waste sites, approaches for their remediation are of interest, especially those that particularly address benzene, which is often the limiting factor for achieving regulatory cleanup at these contaminated sites. In situ chemical oxidation (ISCO) is a viable technology for BTEX destruction, and hydrogen peroxide and sodium persulfate are two oxidants of interest for BTEX treatment.

Laboratory studies were conducted to compare BTEX contaminant destruction and oxidant persistence for these two oxidants and for varied methods of oxidant activation/propagation. Additionally, studies were performed to compare contaminant destruction and oxidant persistence in laboratory contaminant spike systems vs. field site contaminant systems. Finally, contaminant destruction and oxidant persistence in field porous media with varied characteristics were evaluated. Contaminant and oxidant concentrations were measured at multiple time points over a three-week reaction period in each oxidant and oxidant activation/propagation system.

Under the comparable conditions evaluated here, sodium persulfate systems demonstrated greater BTEX contaminant destruction and greater oxidant persistence than hydrogen peroxide systems. FeSO₄ and citric acid activation of sodium persulfate resulted in greater BTEX destruction and greater oxidant persistence than pH adjustment or hydrogen peroxide activation in both laboratory contaminant spike systems and field gas condensate systems. Additionally, results indicate that the response of the contaminant(s) and oxidant (extent and rate of depletion) are both contaminant-and porous media type-dependent.     

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.     

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.     

Legal Aspects of Natural Attenuation in Germany

The legal aspects of Natural Attenuation are multifarious. Therefore, the specific legal situation in Germany shall be described in the following as an example of the classification of NA in an European country. Additionally, a short reference to the international dimension of NA is included at the end. The article is meant to introduce the reader to the ongoing legal discussion in Germany. A comprehensive legal examination will be part of the final report of the so-called 'KORA' project (Natural Attenuation: Retention and Degradation Processes Reducing Contaminations in Groundwater and Soil) funded by the German Federal Ministry of Education and Research. The authors participate in this project as coordinator for legal affairs. The final report is expected in 2006.     

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.     

Challenges in Monitoring the Natural Attenuation of Spatially Variable Plumes

Monitored natural attenuation may be applied as a risk-based remediation strategy if it can be established that contaminants are or will be reduced to some acceptable level at or before a compliance point. Contaminant attenuation is often attributed to intrinsic biodegradation, which in some circumstances may occur only at the plume fringes where electron acceptors from the surrounding uncontaminated zones mix by dispersion and diffusion with the plume. However, due to the common spatial and temporal variability exhibited by many plumes, the centreline monitoring approaches advocated in many natural attenuation protocols may be unable to detect natural attenuation occurring primarily by fringe processes. Snapshot data from a multilevel sampling well transect across an MTBE plume at Vandenberg Air Force Base, CA, USA, illustrate the difficulty of centreline monitoring and the challenge of providing sufficient detail to detect attenuation processes that may be occurring primarily at plume fringes. In a study of a phenols plume in Wolverhampton, UK, high-resolution multilevel wells demonstrated that the key biodegradation processes were restricted spatially to the upper fringe of the plume and were rate-limited by transverse dispersion and diffusion of electron acceptors into the plume. Thus the overall extent of biodegradation was considerably less than suggested by a plume-scale analysis of total electron acceptor and contaminant budgets. These examples indicate that more robust and cost-effective MNA assessments can be obtained using monitoring strategies that focus on the location of key biodegradation processes.     

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.     

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.     

Enhancement of TCE Attenuation in Soils by Natural Amendments

The objective of this study was to identify low-cost natural amendments that could be used as carbon sources and sustain a bioactive zone to promote biodegradation of TCE in contaminated shallow groundwater. The natural amendments were compared based on their geophysical characteristics as well as TCE adsorption capacities. The amendments studied included low-cost natural and agricultural materials such as eucalyptus tree mulch, pine bark mulch, muck from the Florida Everglades, SRS wetland peat, commercial compost, and peat humus. These natural substrates have relatively high organic fractions that can retard the movement of TCE while serving as carbon sources. Batch sorption studies were conducted to determine the sorption and retardation characteristics of the amendments for TCE. The experimental results were analyzed in relation to the geophysical characteristics of the amendments and compared with those of natural soils.     

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.     

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).     

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.     

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.     

Natural Attenuation, Biostimulation, and Bioaugmentation in 4-Chloroaniline-Contaminated Soil

Bioremediation treatments including natural attenuation (NA), biostimulation (BS), and bioaugmentation (BA) were performed and compared regarding the degradation of 4-chloroaniline (4CA) contaminating two types of agricultural soil collected from Nakornnayok (NN) and Chiangmai (CM) provinces, Thailand. Despite the different soil properties, both soil types exhibited intrinsic potential for biodegradation. 4CA degradation by NA in loam soil-NN was fairly effective (ca. 40%), while in sandy-clay loam soil-CM it occurred poorly (<10%). Compared to NA, BS with aniline and BA with 4CA-degrading Klebseilla sp. CA17 were comparatively more effective techniques, although the degradation occurred differently in each soil type. In soil-NN, the biodegradation of 4CA took place at a higher rate, achieving biodegradation of 70–75% within 4 weeks, than in soil-CM, i.e., up to 40–46% within 8 weeks. During each treatment, changes in soil microbial activity, numbers of 4CA-degrading micro-organisms, and dynamic modification of soil microbial community structure were also monitored. The results suggest that both BS and BA are feasible techniques for bioremediation of 4CA accumulated in soil, although the biodegrading efficiency in soil environment depends not only on site characteristics but also on the characteristics of either indigenous microbial population or the survival and stability of bioaugmented cultures.