R. Ryan Dupont

Enhancing biodegradation of petroleum hydrocarbons through soil venting

Abstract Aerobic bioremediation of jet fuel in contaminated soil in an unsaturated vadose zone at Hill Air Force Base, Utah, was stimulated by soil venting. In situ respiration studies conducted at the contaminated site revealed that microbial respiration was occurring as a result of oxygen introduced by the venting process. Stable isotopic ratios ( 13 C/ 12 C) in soil gas were measured to confirm a biogenic source of carbon during venting of the contaminated soil. Although volatilization was the primary mechanism of removal, from 15 to 25% of the jet fuel was biodegraded in situ , making soil venting a promising approach for enhancing the aerobic remediation of contaminated vadose zones.

Arsenic(V) Reduction in Relation to Iron(III) Transformation and Molecular Characterization of the Structural and Functional Microbial Community in Sediments of a Basin-Fill Aquifer in Northern Utah

ABSTRACT Basin-fill aquifers of the Southwestern United States are associated with elevated concentrations of arsenic (As) in groundwater. Many private domestic wells in the Cache Valley Basin, UT, have As concentrations in excess of the U.S. EPA drinking water limit. Thirteen sediment cores were collected from the center of the valley at the depth of the shallow groundwater and were sectioned into layers based on redoxmorphic features. Three of the layers, two from redox transition zones and one from a depletion zone, were used to establish microcosms. Microcosms were treated with groundwater (GW) or groundwater plus glucose (GW+G) to investigate the extent of As reduction in relation to iron (Fe) transformation and characterize the microbial community structure and function by sequencing 16S rRNA and arsenate dissimilatory reductase (arrA) genes. Under the carbon-limited conditions of the GW treatment, As reduction was independent of Fe reduction, despite the abundance of sequences related to Geobacter and Shewanella, genera that include a variety of dissimilatory iron-reducing bacteria. The addition of glucose, an electron donor and carbon source, caused substantial shifts toward domination of the bacterial community by Clostridium-related organisms, and As reduction was correlated with Fe reduction for the sediments from the redox transition zone. The arrA gene sequencing from microcosms at day 54 of incubation showed the presence of 14 unique phylotypes, none of which were related to any previously described arrA gene sequence, suggesting a unique community of dissimilatory arsenate-respiring bacteria in the Cache Valley Basin.

Arsenic(V) Reduction in Relation to Iron(III) Transformation and Molecular Characterization of the Structural and Functional Microbial Community in Sediments of a Northern Utah, Basin-Fill Aquifer

ABSTRACT Basin-fill aquifers of the Southwestern United States are associated with elevated concentrations of arsenic (As) in groundwater. Many private domestic wells in the Cache Valley Basin, UT, have As concentrations in excess of the U.S. EPA drinking water limit. Thirteen sediment cores were collected from the center of the valley at the depth of the shallow groundwater and were sectioned into layers based on redoxmorphic features. Three of the layers, two from redox transition zones and one from a depletion zone, were used to establish microcosms. Microcosms were treated with groundwater (GW) or groundwater plus glucose (GW+G) to investigate the extent of As reduction in relation to iron (Fe) transformation and characterize the microbial community structure and function by sequencing 16S rRNA and arsenate dissimilatory reductase (arrA) genes. Under the carbon-limited conditions of the GW treatment, As reduction was independent of Fe reduction, despite the abundance of sequences related to Geobacter and Shewanella, genera that include a variety of dissimilatory iron-reducing bacteria. The addition of glucose, an electron donor and carbon source, caused substantial shifts toward domination of the bacterial community by Clostridium-related organisms, and As reduction was correlated with Fe reduction for the sediments from the redox transition zone. The arrA gene sequencing from microcosms at day 54 of incubation showed the presence of 14 unique phylotypes, none of which were related to any previously described arrA gene sequence, suggesting a unique community of dissimilatory arsenate-respiring bacteria in the Cache Valley Basin.

iSAW: Integrating Structure, Actors, and Water to Study Socio-Hydro-Ecological Systems

Urbanization, climate, and ecosystem change represent major challenges for managing water resources. Although water systems are complex, a need exists for a generalized representation of these systems to identify important components and linkages to guide scientific inquiry and aid water management. We developed an integrated Structure-Actor-Water framework (iSAW) to facilitate the understanding of and transitions to sustainable water systems. Our goal was to produce an interdisciplinary framework for water resources research that could address management challenges across scales (e.g., plot to region) and domains (e.g., water supply and quality, transitioning, and urban landscapes). The framework was designed to be generalizable across all human–environment systems, yet with sufficient detail and flexibility to be customized to specific cases. iSAW includes three major components: structure (natural, built, and social), actors (individual and organizational), and water (quality and quantity). Key linkages among these components include: (1) ecological/hydrologic processes, (2) ecosystem/geomorphic feedbacks, (3) planning, design, and policy, (4) perceptions, information, and experience, (5) resource access and risk, and (6) operational water use and management. We illustrate the flexibility and utility of the iSAW framework by applying it to two research and management problems: understanding urban water supply and demand in a changing climate and expanding use of green storm water infrastructure in a semi-arid environment. The applications demonstrate that a generalized conceptual model can identify important components and linkages in complex and diverse water systems and facilitate communication about those systems among researchers from diverse disciplines.

ABATEMENT OF GAS-CONDENSATE HYDROCARBONS IN A NATURAL WETLAND

ABSTRACT Results of a five-year research study on natural attenuation processes in a wetland, located downgradient of a sour gas processing plant in central Alberta, Canada, show that natural attenuation may present a favourable remedial solution. Both free-phase and dissolved phase condensate have been discharging to the wetland since 1984. This condensate is primarily composed of C5 to C12 hydrocarbons, including BTEX compounds. The condensate enters the base of the wetland at 1 m below ground surface, resulting in contamination of the wetland peat and underlying clay till. The lateral extent of contamination in the wetland has remained stable, and apparent free product thickness and BTEX concentrations have decreased over time. Sorption, aerobic biodegradation, volatilization, and anaerobic biodegradation were identified as active attenuation processes at this site. Sorption and desorption processes were evaluated by laboratory testing of site soils using 14C-benzene. Linear sorption coefficients (K d) for the surface and subsurface peat were similar (4.48–4.62 l/kg), while the K d for the underlying silt was 0.096 l/kg. The significantly higher K d values for the peat are attributed to the peats higher organic content (40%), relative to the clayey silt (1%). No significant resistance to desorption was observed, however, indicating that benzene would remain mobile and bioavailable over time. Aerobic biodegradation and volatilization appear to be the main removal processes. They are enhanced by a seasonal drop in the water level from surface down to 1 m depth, resulting in an aerobic unsaturated zone. Respiration testing in the peat indicates a significant aerobic biodegradation rate of 27 mg/kg/day, equating to an estimated hydrocarbon removal rate of 5 kg/day across the 3600 m2 plume area. Surface vapour measurements indicate hydrocarbon volatilization is occurring at a rate of 3 × 10−4 kg/m2/day, equating to a mass removal of 1 kg/day across the plume. Anaerobic biodegradation is occurring primarily in the clayey silt, based on geochemical indicator parameters, microbial analyses, and soil vapour sampling. Overall, natural attenuation appears to be a feasible remedial solution for this wetland, by providing continued removal and degradation of condensate components before they reach the downgradient surface water receptor.

Field monitoring and performance evaluation of an in situ air sparging system at a gasoline-contaminated site

In situ air sparging (IAS) has been used since the mid-1980s, but few carefully designed field studies have been performed to evaluate its effectiveness. In this study, 27 discrete monitoring points (MPs) were installed at a gasoline-contaminated site to investigate the efficacy of IAS. Each MP was instrumented with a pressure transducer and a Technalithics dissolved oxygen (DO) probe, and located so they could be used to characterize subsurface changes in total head and DO with depth, distance and orientation around a central injection well. Because the blower over-heated and automatically shut down after approximately 30 min and short-circuiting of air into two MPs occurred within 2 min, the study was designed as three sets of three 30-min trials. Longer trials would not have yielded different nor more insightful results. A volume of soil was not oxygenated during any injection. Instead, air traveled directly to at least four of seven different MPs during eight of the nine trials, probably as a result of an air bubble forming beneath a confining layer. The order of air arrival at the MPs varied during the first few trials, but once a preferential pathway was established, it did not collapse between trials and provided the shortest distance to the vadose zone during subsequent trials. Oxygen uptake rates estimated for MPs that received air during any trial exceeded the consumption rates of the Technalithics DO probes, and indicate that the probes could be used for estimating oxygen transfer during system operation or for oxygen uptake measurements during shut-down tests. The data from the monitoring system indicate that IAS is infeasible for remediation of soil and groundwater at this site due to its low horizontal hydraulic conductivity. Similar behavior is anticipated when IAS is applied at other sites with low hydraulic conductivity materials.

Tracer studies for evaluation of in situ air sparging and in-well aeration system performance at a gasoline-contaminated site

Field-scale tracer studies were conducted at a gasoline-contaminated site in order to evaluate the effectiveness of in situ air sparging (IAS) and in-well aeration (IWA) in controlling the movement of soil gas and groundwater in the subsurface. The field site was comprised of silty sand (SM) and silty clay (CL), underlain by a clay layer at approximately 7.6 m. Depth to groundwater ranged from 2.4 to 3 m. Soil permeability and the natural hydraulic gradient were both low. Helium was used to trace the movement of soil gas in the unsaturated zone during the IAS field study, and successfully confirmed short-circuit pathways for injected air and demonstrated the limited distribution of injected gases at this site. Fluorescein, bromide, and rhodamine were used to trace the movement of groundwater during the IWA system field study, and successfully documented the inability of the IWA system to recirculate enough groundwater to enhance subsurface dissolved oxygen levels or to remediate groundwater by air stripping at this site. The inability of the systems to remediate the site was likely due to site conditions which consist of low-permeability soils and decreasing permeability with depth. As a result, relatively impermeable layers exist at the depth of the IAS screen and the lower IWA screen. These site conditions are not conducive to successful performance of either remediation system.

Field monitoring and performance evaluation of a field-scale in-well aeration system at a gasoline-contaminated site.

Several in-well aeration (IWA) technologies have been used since the early 1990s, but few field studies have been performed to evaluate the extent of water circulation around IWA systems. In this study, 27 discrete monitoring points (MPs) were installed at a gasoline-contaminated site to assess the efficacy of IWA. Pressure transducers and dissolved oxygen (DO) probes were sealed into the MPs, allowing them to be used to characterize subsurface changes in total head and DO with depth, distance and orientation from a central injection well. No change in DO or in hydrocarbon total mass or distribution occurred across the site during two trials (41 and 20 days) of the system. Water level fluctuations during the trials were similar in all MPs, and were due to seasonal water table changes and rainfall events. No circulation cell was established around the IWA well after 41 days of operation, and the impact of the well extended less than 90cm from it. Groundwater only circulated through the sand pack around the well. Little, if any, recharge occurred through the lower screen. Silt accumulated in the well, limiting its operation time, even with a fabric filter sock over the lower screen. Obviously, IWA was ineffective at this site, probably because the horizontal hydraulic conductivity (K(h)) of the soil opposite the lower screen was low (0.09cm per day) and because the distance between the two screens was short relative to the borehole radius. Long remediation times would likely make IWA unattractive at this or other sites where the K(h) of the soil is so low that the air injection rate would have to be low to prevent blowing the well dry.

Volatilization of Trichloroethylene from Trees and Soil: Measurement and Scaling Approaches

Trichloroethylene (TCE) volatilization from leaves, trunk, and soil was measured to assess the significance of these pathways from phytoremediation sites at Travis and Fairchild Air Force Bases. Measurements were scaled temporally and spatially to estimate the annual volatilization of TCE at the Travis (0.82 ± 0.51 kg/yr) and Fairchild sites (0.014 ± 0.008 kg/yr). Volatilization was primarily through the leaf (0.34 ± 0.16 kg/yr at Travis and 0.01 ± 0.06 kg/yr at Fairchild) and soil (0.48 ± 0.36 kg/yr at Travis, 0.003 ± 0.002 kg/yr at Fairchild) pathways. The larger volatilization estimate at Travis was expected because of the sites higher TCE groundwater concentrations. Using groundwater data collected in 2004 and 2009, calculations show that over the 5 year period, 1.7 and 0.015 kg of TCE were removed each year at the Travis and Fairchild sites, respectively. On the basis of the scaled field measurements, volatilization from the leaves and soil may play a significant role in TCE removal at both sites. Daily and seasonal variations were not addressed during the limited daytime sampling events, but the methods described here provide a novel and practical framework for evaluating the potential importance of volatilization of TCE and similar compounds at phytoremediation sites.

Aerobic Biotransformation of N-Nitrosodimethylamine and N-Nitrodimethylamine by Benzene-, Butane-, Methane-, Propane-, and Toluene-Fed Cultures

ABSTRACT N-Nitrosodimethylamine (NDMA) is an emerging contaminant of concern. N-nitrodimethylamine (DMNA) is a structural analog to NDMA. NDMA and DMNA have been found in drinking water, groundwater, and other media and are of concern due their toxicity. The authors evaluated biotransformation of NDMA and DMNA by cultures enriched from contaminated groundwater growing on benzene, butane, methane, propane, or toluene. Maximum specific growth rates of enriched cultures on butane (μmax = 1.1 h−1) and propane (μmax = 0.65 h−1) were 1 to 2 orders of magnitude higher than those presented in the literature. Growth rates of mixed cultures grown on benzene (μmax = 1.3 h−1), methane (μmax = 0.09 h−1), and toluene (μmax = 0.99 h−1) in these studies were similar to those presented in the literature. NDMA biotransformation rates for methane oxidizers (υmax = 1.4 ng min−1 mg−1) and toluene oxidizers (υmax = 2.3 ng min−1 mg−1) were comparable to those presented in the literature, whereas the biotransformation rate for propane oxidizers (υmax = 0.37 ng min−1 mg−1) was lower. NDMA biotransformation rates for benzene oxidizers (υmax = 1.02 ng min−1 mg−1) and butane oxidizers (υmax = 1.2 ng min−1 mg−1) were comparable to those reported for other primary substrates. These studies showed that DMNA biotransformation rates for benzene (υmax = 0.79 ng min−1 mg−1), butane (υmax = 1.0 ng min−1 mg−1), methane (υmax = 2.1 ng min−1 mg−1), propane (υmax = 1.46 ng min−1 mg−1), and toluene (υmax = 0.52 ng min−1 mg−1) oxidizers were all comparable. These studies highlight potential bioremediation methods for NDMA and DMNA in contaminated groundwater.