Oxygen-Containing Compounds Identified in Groundwater from Fuel Release Sites Using GCxGC-TOF-MS

This research continues a 7-year study of oxygen-containing organic compounds present in groundwater at gasoline and diesel fuel release sites that are quantified as diesel-range “total petroleum hydrocarbons” when measured by methods utilizing solvent extraction and gas chromatography. Two-dimensional gas chromatography with time-of-flight mass spectrometry was used to tentatively identify 1162 compounds (TICs) in 113 groundwater samples from 22 sites. Samples were collected from wells either upgradient of the release, within the source zone, or downgradient of the source but still within the plume of dissolved organics associated with release. The names and formulas of all TICs found in samples from each well type are presented and the results from upgradient and downgradient locations are compared in detail. About 60% of the most frequently detected TICs in downgradient wells were also detected in upgradient wells. A majority of these were saturated straight chain alkyl acids, commonly called fatty acids, or fatty acid esters. Of TICs frequently detected in downgradient wells but not upgradient wells, over half were branched alkyl alcohols. Hierarchical cluster analysis results suggest about 80% of the chemical composition of downgradient samples is more similar to upgradient samples than to source area samples. This similarity is due to the presence of the same types of fatty acids and esters. Principal component analysis indicates a continuum of biodegradation between the source area and downgradient samples with the latter becoming more consistent with upgradient samples. Results suggest some TICs may not be petroleum degradation intermediates but compounds synthesized by microorganisms through secondary production and carbon cycling.     

Comparing Natural Source Zone Depletion Pathways at a Fuel Release Site

Natural source zone depletion (NSZD) refers to processes within chemically impacted vadose and saturated zones that reduce the mass of contaminants remaining in a defined source control volume. Studies of large petroleum hydrocarbon release sites have shown that the depletion rate by vapor phase migration of degradation products from the source control volume through the vadose zone (V‐NSZD) is often considerably higher than the rate of depletion from the source control volume by groundwater flow carrying dissolved petroleum hydrocarbons arising from dissolution, desorption, or back diffusion, and degradation products arising from biodegradation (GW‐NSZD). In this study, we quantified vadose zone and GW‐NSZD at a small unpaved fuel release site in California typical of those in settings with predominantly low permeability media. We estimated vadose zone using a dense network of efflux monitoring locations at four sampling events over 2 years, and GW‐NSZD using groundwater monitoring data downgradient of the source control volume in three depth intervals spanning up to 9 years. On average, vadose zone was 17 times greater than GW‐NSZD during the time interval of comparison, and vadose zone was in the range of rates quantified at other sites with petroleum hydrocarbon releases. Estimating vadose zone and GW‐NSZD rates is challenging but the vadose zone rate is the best indicator of overall source mass depletion, whereas GW‐NSZD rates may be useful as baselines to quantify progress of natural or engineered remediation in portions of the saturated zone in which there are impediments to loss of methane and other gases to the vadose zone.     

Arsenic Cycling in Hydrocarbon Plumes: Secondary Effects of Natural Attenuation

Monitored natural attenuation is widely applied as a remediation strategy at hydrocarbon spill sites. Natural attenuation relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe‐hydroxides, a potential secondary effect of natural attenuation of hydrocarbons coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. At a crude‐oil‐contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction has been well documented. We collected groundwater samples at the site annually from 2009 to 2013 to examine if As is released to groundwater and, if so, to document relationships between As and Fe inside and outside of the dissolved hydrocarbon plume. Arsenic concentrations in groundwater in the plume reached 230 µg/L, whereas groundwater outside the plume contained less than 5 µg/L As. Combined with previous data from the Bemidji site, our results suggest that (1) naturally occurring As is associated with Fe‐hydroxides present in the glacially derived aquifer sediments; (2) introduction of hydrocarbons results in reduction of Fe‐hydroxides, releasing As and Fe to groundwater; (3) at the leading edge of the plume, As and Fe are removed from groundwater and retained on sediments; and (4) downgradient from the plume, patterns of As and Fe in groundwater are similar to background. We develop a conceptual model of secondary As release due to natural attenuation of hydrocarbons that can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction.     

Thermal DNAPL Source Zone Treatment Impact on a CVOC Plume

The tetrachloroethene (PCE) source zone at a site in Endicott, New York had caused a dissolved PCE plume. This plume was commingled with a petroleum hydrocarbon plume from an upgradient source of fuel oil. The plume required a system for hydraulic containment, using extraction wells located about 360 m downgradient of the source. The source area was remediated using in situ thermal desorption (ISTD). Approximately 1406 kilograms (kg) of PCE was removed in addition to 4082 kg of commingled petroleum‐related compounds. The ISTD treatment reduced the PCE mass discharge into the plume from an estimated 57 kg/year to 0.07 kg/year, essentially removing the source term. In the 5 years following the completion of the thermal treatment in early 2010, the PCE plume has collapsed, and the concentration of degradation products in the PCE‐series plume area has declined by two to three orders of magnitude. Anaerobic dechlorination is the suspected dominant mechanism, assisted by the presence of a fuel oil smear zone and a petroleum hydrocarbon plume from a separate source area upgradient of the PCE source. Based on the post‐thermal treatment groundwater monitoring data, the hydraulic containment system was reduced in 2014 and discontinued in early 2015.     

Nature and Estimated Human Toxicity of Polar Metabolite Mixtures in Groundwater Quantified as TPHd/DRO at Biodegrading Fuel Release Sites

Polar metabolites resulting from petroleum biodegradation are measured in groundwater samples as TPHd unless a silica gel cleanup (SGC) is used on the sample extract to isolate hydrocarbons. Even though the metabolites can be the vast majority of the dissolved organics present in groundwater, SGC has been inconsistently applied because of regulatory concern about the nature and toxicity of the metabolites. A two‐step approach was used to identify polar compounds that were measured as TPHd in groundwater extracts at five sites with biodegrading fuel sources. First, gas chromatography with mass spectrometry (GC‐MS) was used to identify and quantify 57 individual target polar metabolites. Only one of these compounds—dodecanoic acid, which has low potential human toxicity—was detected. Second, nontargeted analysis was used to identify as many polar metabolites as possible using both GC‐MS and GC×GC‐MS. The nontargeted analysis revealed that the mixture of polar metabolites identified in groundwater source areas at these five sites is composed of approximately equal average percentages of organic acids, alcohols and ketones, with few phenols and aldehydes. The mixture identified in downgradient areas at these five sites is dominated by acids, with fewer alcohols, far fewer ketones, and very few aldehydes and phenols. A ranking system consistent with systems used by USEPA and the United Nations was developed for evaluating the potential chronic oral toxicity to humans of the different classes of identified polar metabolites. The vast majority of the identified polar metabolites have a “Low” toxicity profile, and the mixture of identified polar metabolites present in groundwater extracts at these five sites is unlikely to present a significant risk to human health.     

Carbon Budget and Molecular Structure of Natural Organic Matter in Bank Infiltrated Groundwater

Managed aquifer recharge (MAR) provides means to remove natural organic matter (NOM) from surface waters. Recent studies have explored the degree of NOM removal in groundwater. In this study, we further elaborate the NOM removal at a lakeside natural bank infiltration site that functions as a surrogate for MAR. Our objective was to quantify the carbon budget in the aquifer based on concentration measurements of dissolved (in)organic carbon, and the molecular changes in NOM using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). According to the carbon budget, only 25% of the dissolved carbon entering the aquifer was organic, and it predominantly originated from lake water. Of the inorganic majority, on average 40% was produced in the vadose zone above the groundwater table, 31% in the lake bank, 22% in the aquifer as a result of degrading organic matter of lake water, and 7% in the lake. Seasonal concentration variations suggested that the lake bank was the main carbon source in the summer, increasing the carbon concentration of infiltrating lake water, that is, 3.0 mg/L to 7.9 mg/L. FT-ICR MS results showed 4960 to 5330 individual compounds in lake and groundwater. NOM removal in the aquifer was selective: the relative abundance of oxygen-containing species decreased from 75 to 31%, while the relative abundance of sulfur-containing species increased from 15 to 57%. The average molecular weights of both species remained unchanged. The study highlighted the role of lake bank processes and sulfur-containing species in groundwater quality.     

Groundwater potential zones using a combination of geospatial technology and geophysical approach: case study in Dehradun,India

ABSTRACT

A combination of geospatial, geophysical and statistical models using satellite data, the weighted index overlay (WIO) method and two-dimensional electrical resistivity tomography (2D-ERT) is applied to generate the highest potential groundwater area and to further explore the groundwater in Dehradun, India. The results show that of 19.7 km² total basin area, 0.26% falls under the “poor” category as a prospect zone for groundwater, 4.3% is “moderate”, 10.10% “moderately good”, 4.9% “good” and 0.17% “very good”. In addition, the demonstration of the geophysical survey is presented, in which Purkal Youth Society Division (PYSD) site is categorized as a shallow aquifer zone and the Guru Nanak Fifth Centenary School (GNFCS) site is a deeper aquifer zone. Our study emphasizes remote sensing and geographic information system integrated with a geophysical survey to support prospecting the most probable area and confirm the existence of groundwater.     

三峡澎溪河高阳平湖碳通量初步分析及不确定性

为明确三峡水库支流澎溪河回水区的碳收支特征,以澎溪河高阳平湖水域为研究对象,建立了河道型水库主要路径碳收支估算方法,对高阳平湖从2011年9月至2012年8月一个完整水文周年内主要路径的碳通量进行了收支动态分析.结果表明,2011年9月至2012年8月,澎溪河高阳平湖水域河流输入的碳通量为133548.55 t C,输出的碳通量为125651.82 t C,水-气界面的扩散碳通量为762.56 t C,消落带土-气界面的扩散碳通量为123.74 t C,水中气泡的释放碳通量为0.38 t C,降水输入的碳通量为104.58 t C,全年高阳平湖水域碳的净积累量为7114.63 t C,宏观上呈现碳积累特征;澎溪河高阳平湖水域水体碳素总体上呈现出河道型水库特有的纵向输移特征.高阳平湖水域上游大量碳素的输入及其在高阳平湖水域的滞留可能会是该水域水-气界面温室气体释放的主要来源.尽管总体上高阳平湖全年呈现出碳积累的特点,但一些方法依然存在不确定性(水-气界面扩散碳通量和气泡释放碳通量的时空异质性等),需要更系统、更长期的工作予以验证或改进.     

Effect of Biofuels on Biodegradation of Benzene and Toluene at Gasoline Spill Sites

The risk that benzene and toluene from spills of gasoline will impact drinking water wells is largely controlled by the natural anaerobic biodegradation of benzene and toluene. Benzene and toluene, as well as ethanol and other biofuels, are degraded under anaerobic conditions to the same pool of degradation products. Biodegradation of biofuels may produce concentrations of degradation products that make the thermodynamics for degradation of benzene and toluene infeasible under methanogenic conditions and produce larger plumes of benzene and toluene. This study evaluated the concentrations of fuel alcohols that are necessary to inhibit the anaerobic degradation of benzene and toluene under methanogenic conditions. At two ethanol spill sites, concentrations of ethanol greater ≥42 mg/L inhibited the anaerobic degradation of toluene. The pH and concentrations of acetate, dissolved inorganic carbon, and molecular hydrogen were used to calculate the Gibbs free energy for the biodegradation of toluene. In general, the anaerobic biodegradation of toluene was not thermodynamically feasible in water with ≥42 mg/L ethanol. In a microcosm study, when the concentrations of ethanol were ≥14 mg/L or the concentrations of n‐butanol were ≥16 mg/L, the biodegradation of the alcohols consistently produced concentrations of hydrogen, dissolved inorganic carbon, and acetate that would preclude natural anaerobic biodegradation of benzene and toluene by syntrophic organisms. In contrast, iso‐butanol and n‐propanol only occasionally produced conditions that would preclude the biodegradation of benzene and toluene.     

Three-Dimensional Simulation of Volatile Organic Compound Mass Flux from the Vadose Zone to Groundwater

Low-permeability layers of the vadose zone containing volatile organic compounds (VOCs) may persist as source zones for long time periods and may provide contamination to groundwater. At sites with low recharge rates, where vapor migration is the dominant transport process, the impact of vadose zone sources on groundwater may be difficult to assess. Typical assessment methods include one-dimensional numerical and analytical techniques. The one-dimensional approaches only consider groundwater coupling options through boundary conditions at the water table and may yield artificially high mass flux results when transport is assumed to occur by gas-phase diffusion between a source and an interface with a zero concentration boundary condition. Improvements in mass flux assessments for VOCs originating from vadose zone sources may be obtained by coupling vadose zone gas transport and dissolved contaminant transport in the saturated zone and by incorporating the inherent three-dimensional nature of gas-phase transport, including the potential of density-driven advection. This paper describes a series of three-dimensional simulations using data from the U.S. Department of Energy's Hanford site, where carbon tetrachloride is present in a low-permeability zone about 30 m above the groundwater. Results show that, for most cases, only a relatively small amount of the contaminant emanating from the source zone partitions into the groundwater and that density-driven advection is only important when relatively high source concentrations are considered.     

A Practical Approach to the Design, Monitoring, and Optimization of In Situ MTBE Aerobic Biobarriers

A paradigm for the design, monitoring, and optimization of in situ methyl tert -butyl ether (MTBE) aerobic biobarriers is presented. In this technology, an oxygen-rich biologically reactive treatment zone (the "biobarrier") is established in situ and downgradient of the source of dissolved MTBE contamination in groundwater, typically gasoline-impacted soils resulting from leaks and spills at service station sites or other fuel storage and distribution facilities. The system is designed so that groundwater containing dissolved MTBE flows to, and through, the biobarrier treatment zone, ideally under natural gradient conditions so that no pumping is necessary. As the groundwater passes through the biobarrier, the MTBE is converted by microorganisms to innocuous by-products. The system also reduces concentrations of other aerobically degradable chemicals dissolved in the groundwater, such as benzene, toluene, xylenes, and tert -butyl alcohol. This design paradigm is based on experience gained while designing, monitoring, and optimizing pilot-scale and full-scale MTBE biobarrier systems. It is largely empirically based, although the design approach does rely on simple engineering calculations. The paradigm emphasizes gas injection–based oxygen delivery schemes, although many of the steps would be common to other methods of delivering oxygen to aquifers.     

Carbon fluxes from eroding peatlands – the carbon benefit of revegetation following wildfire

Peatlands are among the largest long‐term soil carbon stores, but their degradation can lead to significant carbon losses. This study considers the carbon budget of peat‐covered sites after restoration, following degradation by past wildfires. The study measured the carbon budget of eight sites: four restored‐revegetated sites, two unrestored bare soil control sites, and two intact vegetated controls over two years (2006–2008). The study considered the following flux pathways: dissolved organic carbon (DOC); particulate organic carbon (POC); dissolved carbon dioxide (CO₂); primary productivity; net ecosystem respiration, and methane (CH₄). The study shows that unrestored, bare peat sites can have significant carbon losses as high as 522 ± 3 tonnes C/km²/yr. Most sites showed improved carbon budgets (decreased source and/or increased sink of carbon) after restoration; this improvement was mainly in the form of a reduction in the size of the net carbon source, but for one restored site the measured carbon budget after four years of restoration was greater than observed for vegetated controls. The carbon sequestration benefit of peatland restoration would range between 122 and 833 tonnes C/km²/yr. Copyright © 2011 John Wiley & Sons, Ltd.     

Hydrogeochemical Evolution Along Groundwater Flow Paths in the Manas River Basin,Northwest China

The impacts of long-term pumping on groundwater chemistry remain unclear in the Manas River Basin, Northwest China. In this study, major ions within five surface water and 105 groundwater samples were analyzed to identify hydrogeochemical processes affecting groundwater composition and evolution along the regional-scale groundwater flow paths using the multivariate techniques of hierarchical cluster analysis (HCA) and principal components analysis (PCA) and traditional graphical methods for analyzing groundwater geochemistry. HCA classified the groundwater samples into four clusters (C1 to C4). PCA reduced the dimensionality of geochemical data into three PCs, which explained 86% of the total variance. The results of HCA and PCA were used to identify three zones: “recharge,” “transition,” and “discharge.” In the recharge zone the groundwater type is Ca-HCO₃-SO₄ and is primarily impacted by the dissolution of calcite and silicate weathering. In the transition zone the groundwater type is Ca-HCO₃-SO₄-Cl and is impacted by rock dissolution and reverse ion exchange. In the discharge zone the groundwater type is Na-Cl and is impacted by evaporation and reverse ion exchange. In addition, anthropogenic activities impact the groundwater chemistry in the study area. The groundwater type generally changes from Ca-HCO₃-SO₄ in the recharge area to Na-Cl in the discharge area along the regional-scale groundwater flow paths. This study provides a process-based knowledge for understanding the interaction of groundwater flow patterns and geochemical evolution within the Manas River Basin.     

Land Application of Sulfate Salts for Enhanced Natural Attenuation of Benzene in Groundwater: A Case Study

Sulfate reducing conditions are widely observed in groundwater plumes associated with petroleum hydrocarbon releases. This leads to sulfate depletion in groundwater which can limit biodegradation of hydrocarbons (usually benzene, toluene, ethylbenzene, xylenes [BTEX] compounds) and can therefore result in extended timeframes to achieve groundwater cleanup objectives by monitored natural attenuation. Under these conditions, sulfate addition to the subsurface can potentially enhance BTEX biodegradation and facilitate enhanced natural attenuation. However, a delivery approach that enables effective contact with the hydrocarbons and is able to sustain elevated and uniform sulfate concentrations in groundwater remains a key challenge. In this case study, sulfate addition to a groundwater plume containing predominantly benzene by land application of agricultural gypsum and Epsom salt is described. Over 4 years of groundwater monitoring data from key wells subjected to pilot‐scale and site‐wide land application events are presented. These are compared to data from pilot testing employing liquid Epsom salt injections as an alternate sulfate delivery approach. Sulfate land application, sulfate retention within the vadose zone, and periodic infiltration following ongoing precipitation events resulted in elevated sulfate concentrations (>150 mg/L) in groundwater that were sustained over 12 months between application events and stimulated benzene biodegradation as indicated by declines in dissolved benzene concentration, and compound‐specific isotope analysis data for carbon in benzene. Long‐term groundwater benzene concentration reductions were achieved in spite of periodic rebounds resulting from water table fluctuations across the smear zone. Land application of gypsum is a potentially cost‐effective sulfate delivery approach at sites with open, unpaved surfaces, relatively permeable geology, and shallow hydrocarbon impacts. However, more research is needed to understand the fate and persistence of sulfate and to improve the likelihood of success and effectiveness of this delivery approach.     

Assessing TCE Source Bioremediation by Geostatistical Analysis of a Flux Fence

Mass discharge across transect planes is increasingly used as a metric for performance assessment of in situ groundwater remediation systems. Mass discharge estimates using concentrations measured in multilevel transects are often made by assuming a uniform flow field, and uncertainty contributions from spatial concentration and flow field variability are often overlooked. We extend our recently developed geostatistical approach to estimate mass discharge using transect data of concentration and hydraulic conductivity, so accounting for the spatial variability of both datasets. The magnitude and uncertainty of mass discharge were quantified by conditional simulation. An important benefit of the approach is that uncertainty is quantified as an integral part of the mass discharge estimate. We use this approach for performance assessment of a bioremediation experiment of a trichloroethene (TCE) source zone. Analyses of dissolved parent and daughter compounds demonstrated that the engineered bioremediation has elevated the degradation rate of TCE, resulting in a two‐thirds reduction in the TCE mass discharge from the source zone. The biologically enhanced dissolution of TCE was not significant (～5%), and was less than expected. However, the discharges of the daughter products cis‐1,2, dichloroethene (cDCE) and vinyl chloride (VC) increased, probably because of the rapid transformation of TCE from the source zone to the measurement transect. This suggests that enhancing the biodegradation of cDCE and VC will be crucial to successful engineered bioremediation of TCE source zones.     

Denitrifying bacteria and hydrogeochemistry in a natural wetland adjacent to farmlands in Chiba,Japan

This study combined bacterial incubation and hydrogeochemical methods to investigate denitrification in the surface water, top soil (0–20 cm), and shallow groundwater of the Ochi wetland in Japan. Surface water was rich in nitrate (40 mg/l) and denitrifying bacteria (700 per ml). Three functional zones in the wetland were identified in the top soil and shallow groundwater. In the upstream portion of the wetland (Zone I), the counts of denitrifying bacteria in top soil increased from 5200 to 14 970 per ml and nitrate decreased from 25·4 to 1·8 mg/l. Organic carbon concentrations decreased as sulfate increased from 4·0 to 9·6 mg/l in this zone. In the middle‐stream of the wetland (Zone II), all concentrations of major anions, iron, organic carbon, and total nitrogen content in top soil were relatively constant, but the counts of denitrifying bacteria increased up to 70 200 per ml. In the downstream portion of the wetland (Zone III), complete removal of nitrate resulted in sharp reduction of counts of denitrifying bacteria. Correspondingly, dissolved oxygen (DO), organic carbon, and total nitrogen increased in this zone. Counts of denitrifying bacteria were lower in shallow groundwater than in top soil; nitrate concentrations in shallow groundwater were also very low in this zone. DO and the oxidation/reduction potential data suggest that groundwater flows to the surface along an extended flow path, thus providing nitrate for the denitrifying community. Copyright © 2011 John Wiley & Sons, Ltd.     

A kinetic approach for simulating redox‐controlled fringe and core biodegradation processes in groundwater: model development and application to a landfill site in Piedmont,Italy

A three‐dimensional model for predicting redox controlled, multi‐species reactive transport processes in groundwater systems is presented. The model equations were fully integrated within a MODFLOW‐family reactive transport code, RT3D. The model can simulate organic compound biodegradation coupled to different terminal electron acceptor processes. A computational approach, which uses the spatial and temporal distribution of the rates of different redox reactions, is proposed to map redox zones. The method allows one to quantify and visualize the biological degradation reactions occurring in three distinct patterns involving fringe, pseudo‐core and core processes. The capabilities of the numerical model are demonstrated using two hypothetical examples: a batch problem and a simplified two‐dimensional reactive transport problem. The model is then applied to an unconfined aquifer underlying a leaking landfill located near the city of Turin, in Piedmont (Italy). At this site, high organic load from the landfill leachate activates different biogeochemical processes, including aerobic degradation, denitrification, manganese reduction, iron reduction, sulfate reduction and methanogenesis. The model was able to describe and quantify these complex biogeochemical processes. The proposed model offers a rational framework for simulating coupled reactive transport processes occurring beneath a landfill site. Copyright © 2008 John Wiley & Sons, Ltd.     

Push‐Pull Tests for Estimating RDX and TNT Degradation Rates in Groundwater

In situ bioremediation is being considered to optimize an existing pump‐and‐treat remedy for treatment of explosives‐contaminated groundwater at the Umatilla Chemical Depot. Push‐pull tests were conducted using a phased approach to measure in situ hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) and 2,4,6‐trinitrotoluene (TNT) degradation rates associated with various carbon substrates. Phase I included short‐duration transport tests conducted in each well to determine dilution rates and retardation factors for RDX and TNT. Phase II included aquifer “feedings” conducted by injecting 150 gallons of treated site groundwater amended with ethanol, corn syrup, lactose or emulsified oil (concentrations 10, 25 and 27 mM, respectively; 12% by volume for emulsified oil). Wells received up to 6 substrate “feedings” over the course of 3 months followed by monitoring dissolved oxygen, nitrate, Fe(II), and sulfate to gauge in situ redox conditions as indicators of anaerobic microbial activity. Phase III included push‐pull tests conducted by injecting 150 gallons of site groundwater amended with approximately 1000 µg/L RDX, 350 µg/L TNT, carbon substrate and a conservative tracer, followed by sampling over 8 d. Corn syrup resulted in the best RDX removal (82% on average) and the largest RDX degradation rate coefficient (1.4 ± 1.1 d^?1). Emulsified oil resulted in the best TNT removal (99%) and largest TNT degradation rate coefficient (5.7 × 10^?2 d^?1). These results will be used to simulate full‐scale in situ bioremediation scenarios at Umatilla and will support a go/no‐go decision to initiate full‐scale bioremediation remedy optimization.