Hydrocarbons In Groundwater Research Articles

Microbiota associated with the migration and transformation of chlorinated aliphatic hydrocarbons in groundwater

Pollution of groundwater with chlorinated aliphatic hydrocarbons (CAHs) is a serious environmental problem which is threatening human health. Microorganisms are the major participants in degrading these contaminants. Here, groundwater contaminated for a decade with CAHs was investigated. Numerical simulation and field measurements were used to track and forecast the migration and transformation of the pollutants. The diversity, abundance, and possible activity of groundwater microbial communities at CAH-polluted sites were characterized by molecular approaches. The number of microorganisms was between 5.65E+05 and 1.49E+08 16S rRNA gene clone numbers per liter according to quantitative real-time PCR analysis. In 16S rRNA gene clone libraries constructed from samples along the groundwater flow, eight phyla were detected, and Proteobacteria were dominant (72.8%). The microbial communities varied with the composition and concentration of pollutants. Meanwhile, toluene monooxygenases and methane monooxygenases capable of degradation of PCE and TCE were detected, demonstrating the major mechanism for PCE and TCE degradation and possibility for in situ remediation by addition of oxygen in this study.

Evaluation of petroleum hydrocarbon biodegradation in shallow groundwater by hydrogeochemical indicators and C, S-isotopes

Biodegradation is one of the main natural attenuation processes in groundwater contaminated with petroleum hydrocarbons. In this work, preliminary studies have been carried out by analyzing the concentrations of total petroleum hydrocarbons (TPH), dissolved inorganic carbon (DIC), dominant terminal electron accepters or donors, as well as δ 13CDIC and δ 34SSO4, to reveal the biodegradation mechanism of petroleum hydrocarbons in a contaminated site. The results show that along groundwater flow in the central line of the plume, the concentrations of electron acceptors, pH, and E h increased but TPH and DIC decreased. The δ 13CDIC values of the contaminated groundwater were in the range of −14.02 to −22.28 ‰PDB and −7.71 to 8.36 ‰PDB, which reflected a significant depletion and enrichment of 13C, respectively. The increase of DIC is believed to result from the non-methanogenic and methanogenic biodegradation of petroleum hydrocarbon in groundwater. Meanwhile, from the contaminated source to the downgradient of the plume, the 34S in the contaminated groundwater became more depleted. The Rayleigh model calculation confirmed the occurrence of bacterial sulfate reduction as a biodegradation pathway of the petroleum hydrocarbon in the contaminated aquifers. It was concluded that stable isotope measurements, combined with other biogeochemical measurements, can be a useful tool to prove the occurrence of the biodegradation process and to identify the dominant terminal electron-accepting process in contaminated aquifers.

Denatured ethanol release into gasoline residuals, Part 1: Source behaviour

With the increasing use of ethanol in fuels, it is important to evaluate its fate when released into the environment. While ethanol is less toxic than other organic compounds present in fuels, one of the concerns is the impact ethanol might have on the fate of gasoline hydrocarbons in groundwater. One possible concern is the spill of denatured ethanol (E95: ethanol containing 5% denaturants, usually hydrocarbons) in sites with pre-existing gasoline contamination. In that scenario, ethanol is expected to increase the mobility of the NAPL phase by acting as a cosolvent and decreasing interfacial tension. To evaluate the E95 behaviour and its impacts on pre-existing gasoline, a field test was performed at the CFB-Borden aquifer. Initially gasoline contamination was created releasing 200 L of E10 (gasoline with 10% ethanol) into the unsaturated zone. One year later, 184 L of E95 was released on top of the gasoline contamination. The site was monitored using soil cores, multilevel wells and one glass access tube. At the end of the test, the source zone was excavated and the compounds remaining were quantified. E95 ethanol accumulated and remained within the capillary fringe and unsaturated zone for more than 200 days, despite ~1m oscillations in the water table. The gasoline mobility increased and it was redistributed in the source zone. Gasoline NAPL saturations in the soil increased two fold in the source zone. However, water table oscillations caused a separation between the NAPL and ethanol: NAPL was smeared and remained in deeper positions while ethanol moved upwards following the water table rise. Similarly, the E95 denaturants that initially were within the ethanol-rich phase became separated from ethanol after the water table oscillation, remaining below the ethanol rich zone. The separation between ethanol and hydrocarbons in the source after water table oscillation indicates that ethanol's impact on hydrocarbon residuals is likely limited to early times.

Fate and transport of ethanol-blended dissolved BTEX hydrocarbons: a quantitative tracing study of a sand tank experiment

To estimate the behavior of ethanol-blended dissolved BTEX hydrocarbons in groundwater, a quantitative tracing study instead of qualitative analysis was carried out by using a large sand tank, into which 2-L solution including bromide, ethanol and dissolved BTEX was injected under a controlled hydraulic condition. Mean residence time (MRT), pore volume swept by solute (Vp), retardation coefficient (R) and biodegradation rate constant (k) of injected solutes were estimated. Compared with bromide that was used as a conservative tracer, ethanol and BTEX had shorter MRT and smaller Vp with the sequence of EtOH T > E > m/p-X > o-X > B was just opposite to the sequences of MRT and Vp. The relationship among above sequences implies that MRT and Vp can be used as indicators to assess in situ biodegradability of a solute. Biodegradation of a reactive solute can make its MRT shortened and Vp shrunk. In addition, the sorption of ethanol could be neglected (R = 1.0), whereas BTEX compounds were adsorbed (R = 1.04–1.15). It should be noted that biodegradation of a solute can affect the estimation of its retardation coefficient. To our knowledge, this paper provides an available route to quantitatively estimate biodegradability of a solute in groundwater.

Bacterial community evidence for anaerobic degradation of petroleum hydrocarbons in cold climate groundwater

Abstract There is currently limited scientific data to assess whether groundwater bacterial communities in fractured rock environments can degrade petroleum hydrocarbon plumes in cold regions. The former Colomac Mine is located in the Canadian Shield in the Northwest Territories (mean annual air temperature of − 5 °C) and is currently listed on the Federal Contaminated Sites inventory. Groundwater in fractured rock beneath the former fuel tank-farm at the mine site is contaminated by petroleum hydrocarbons. The objectives of this study were to investigate the bacterial community structure in the groundwater associated with hydrocarbon contamination, and to probe for potential anaerobic microbial processes involved in intrinsic bioremediation. Groundwater monitoring wells previously installed at the site were used to collect samples for chemical, isotopic and microbial analyses. Denaturing gradient gel electrophoresis (DGGE) analysis identified a relatively high bacterial diversity in the least contaminated locations, but as the hydrocarbon contamination increased, bacterial diversity decreased. Sequence analysis of the 16S rRNA gene demonstrated that the bacteria belonged to a wide range of genera, such as Pseudomonas, Thiobacillus, and Geobacter, all of which have been associated with the anaerobic degradation of hydrocarbons. Two putative nitrate-reducing genes were detected in samples with high nitrate reducing activity. Both chemical and microbiological results indicated the presence of microbial anaerobic processes by using nitrate, manganese(IV), ferric iron and sulfate as electron acceptors and suggest that these anaerobic processes play an important role in the biodegradation of dissolved petroleum hydrocarbons in the groundwater at the Colomac site. Our results also revealed that more than one biogeochemical process linked to hydrocarbon degradation could be present in a single borehole and these processes could vary spatially at this site.

Biodegradation of Soybean and Castor Oil Biodiesel: Implications on the Natural Attenuation of Monoaromatic Hydrocarbons in Groundwater

The potential groundwater impacts of biodiesel releases have received limited attention despite the increasing probability of such events. In this work, microcosms were prepared with unacclimated sediment and groundwater from the Ressacada Experimental Site (Florianopolis, Santa Catarina, Brazil) and spiked with 54.8 mg/L of pure soybean or castor oil biodiesel (B100). Oxygen was purged from the microcosms to mimic commonly anoxic and hypoxic conditions at fuel‐impacted sites; low background concentrations of nitrate (1.2 to 2.5 mg/L) and sulfate (2.2 to 3.0 mg/L) were present. Biodegradation was assessed by the removal of fatty acid methyl esters and hydrocarbon components relative to sterile controls. Approximately 80% of soybean biodiesel was biotransformed in 41 d, compared to only 40% of castor oil biodiesel removed in 90 d. The higher persistence of castor biodiesel was attributed to its higher viscosity and lower bioavailability. Additional microcosms were prepared similarly to assess the impact of biodiesel on hydrocarbon degradation. These microcosms were spiked with benzene (2.9 mg/L) and toluene (0.8 mg/L) with or without soybean biodiesel (54.8 mg/L). The biodiesel had an inhibitory effect, increasing the time required to remove toluene from 25 to 34 d. Similarly, 45% of benzene was removed in the presence of biodiesel within 34 d, compared to 90% in the absence of biodiesel. Overall, we postulate that the relatively high viscosity of biodiesel is conducive to limited migration potential and a smaller but longer lasting inhibitory region of influence, compared to that exerted by more soluble, more mobile, and readily degradable biofuels such as ethanol. However, controlled release studies are needed to test this hypothesis and characterize the complex dynamics of such releases.

High-pressure injection of dissolved oxygen for hydrocarbon remediation in a fractured dolostone aquifer

A field experiment was completed at a fractured dolomite aquifer in southwestern Ontario, Canada, to assess the delivery of supersaturated dissolved oxygen (supersaturated with respect to ambient conditions) for enhanced bioremediation of petroleum hydrocarbons in groundwater. The injection lasted for 1.5 h using iTi's gPro® oxygen injection technology at pressures of up to 450 kPa and at concentrations of up to 34 mg O 2/L. A three-dimensional numerical model for advective–dispersive transport of dissolved oxygen within a discretely-fractured porous medium was calibrated to the observed field conditions under a conservative (no-consumption) scenario. The simulation demonstrated that oxygen rapidly filled the local intersecting fractures as well as the porous matrix surrounding the injection well. Following injection, the local fractures were rapidly flushed by the natural groundwater flow system but slow back-diffusion ensured a relatively longer residence time in the matrix. A sensitivity analysis showed significant changes in behaviour with varying fracture apertures and hydraulic gradients. Applying the calibrated model to a 7-day continuous injection scenario showed oxygen residence times (at the 3 mg/L limit), within a radius of 2–4 m from the injection well, of up to 100 days. This study has demonstrated that supersaturated dissolved oxygen can be effectively delivered to this type of a fractured and porous bedrock system at concentrations and residence times potentially sufficient for enhanced aerobic biodegradation.

Synthesis and characterization of hydrophobic zeolite for the treatment of hydrocarbon contaminated ground water

Hydrophobic zeolite was synthesized, modified and characterized for its suitability as a permeable reactive barrier (PRB) material for treatment of hydrocarbons in groundwater. Batch sorption tests were performed along with a number of standard characterization techniques. High and low ionic strength and pH tests were also conducted to determine their impact on hydrocarbon uptake. Further ion exchange tests were conducted to determine the potential for the zeolite to act as both a hydrocarbon capture material and nutrient a delivery system for bioremediation. The zeolite was coated with octadecyltrichlorosilane (C18) to change its surface properties. The results of the surface characterization tests showed that the underlying zeolite structure was largely unaffected by the coating. TGA measurements showed a reactive carbon content of 1–2%. Hydrocarbon (o-xylene and naphthalene) sorption isotherms results compared well with the behaviour of similar materials investigated by other researchers. Ionic strength and pH had little effect on hydrocarbon sorption and the treated zeolite had an ion exchange capacity of 0.3 mequiv./g, indicating it could be utilised as a nutrient source in PRBs. Recycle tests indicated that the zeolite could be used cleaned and reused at least three times without significant reduction in treatment effectiveness.

The Application of Air-sparging, Soil Vapor Extraction and Pump and Treat for Remediation of a Diesel-contaminated Fractured Formation

The present study addresses the efficiency of an integrated air sparging, soil vapor extraction, and pump and treat system in the remediation of a diesel contaminated site in Oman. Cleanup efforts have targeted groundwater and soil in fractured formations. Site hydrogeological characterization was conducted including sampling and analysis of water and soil. Within seven months of the start of the treatment system, benzene gas in the unsaturated zone fell from an initial range of 15–60 ppm to below detection level, while total petroleum hydrocarbon in the groundwater dropped from 25–50 ppm to less than 0.5 ppm. Treatment processes have ceased while groundwater and soil are being monitored. Thus far, benzene gas has been undetected for the past 18 months, but total petroleum hydrocarbon in groundwater has rebounded to 1.2 ppm during the last four months.

Monitored Natural Attenuation of Manufactured Gas Plant Tar Mono‐ and Polycyclic Aromatic Hydrocarbons in Ground Water: A 14‐Year Field Study

AbstractSite 24 was the subject of a 14‐year (5110‐day) study of a ground water plume created by the disposal of manufactured gas plant (MGP) tar into a shallow sandy aquifer approximately 25 years prior to the study. The ground water plume in 1988 extended from a well‐defined source area to a distance of approximately 400 m down gradient. A system of monitoring wells was installed along six transects that ran perpendicular to the longitudinal axis of the plume centerline. The MGP tar source was removed from the site in 1991 and a 14‐year ground water monitored natural attenuation (MNA) study commenced. The program measured the dissolved mono‐ and polycyclic aromatic hydrocarbons (MAHs and PAHs) periodically over time, which decreased significantly over the 14‐year period. Naphthalene decreased to less than 99% of the original dissolved mass, with mass degradation rates of 0.30 per year (half‐life 2.3 years). Bulk attenuation rate constants for plume centerline concentrations over time ranged from 0.33 ± 0.09 per year (half‐life 2.3 ± 0.8 years) for toluene and 0.45 ± 0.06 per year (half‐life 1.6 ± 0.2 years) for naphthalene. The hydrogeologic setting at Site 24, having a sandy aquifer, shallow water table, clay confining layer, and aerobic conditions, was ideal for demonstrating MNA. However, these results demonstrate that MNA is a viable remedial strategy for ground water at sites impacted by MAHs and PAHs after the original source is removed, stabilized, or contained.

Feasibility of using microbial fuel cell technology for bioremediation of hydrocarbons in groundwater

A single-cell microbial fuel cell (MFC) design was used to study anaerobic microbes that utilize petroleum contaminants as a sole substrate to produce power during remediation. Additionally, we tested various proton bridge designs to physically separate the anode and cathode chambers of a two-cell MFC by ∼ 9 m (∼ 30 ft.). This separation enables the potential use of MFC technology for in situ bioremediation of petroleum hydrocarbons in the groundwater, in which oxygen is usually depleted and oxygen availability only exists at or near the surface. Sustained power generation (as high as 120 mW/m2 cathode) was recorded for ∼ 6 d in a single-cell MFC utilizing a mixture of refinery waste (containing various concentrations of hydrocarbon contaminants) and cell growth media. MFC cell potential (1KΩ external resistance) decreased by ∼ 55% over the length of the 9 m proton bridge with a 6.9% decrease in potential per m of bridge. This preliminary data indicates that using MFC technology (with our modifications) may enhance bioremediation of petroleum contaminants in groundwater under anaerobic conditions. Because oxygen is eventually used as the terminal electron acceptor for anaerobic biodegradation inside an MFC, this technology may be a cost-effective innovation to enhanced biodegradation in groundwater, by substituting or eliminating conventional in situ aeration. To our knowledge, this is the first report on power generation from MFCs utilizing mixed hydrocarbon substrates. In addition, this study is the first to show the applicability of using extended proton bridges for the physical separation of anode and cathode chambers over extended distances that may be encountered in the field.

Microbial signature lipid profiling and exopolysaccharides: Experiences initiated with Professor David C White and transported to Tasmania, Australia

Developments and applications with signature lipid and exopolysaccharide (EPS) methodologies covering a thirty year period in the DC White laboratories at Florida State University and the University of Tennessee at Knoxville are illustrated. These powerful techniques were used to gain new insight into microbial communities, not obtainable by classical approaches. Selected case examples are highlighted and include: use of a specific dimethyl disulphide (DMDS) derivitization procedure with monounsaturated fatty acids (MUFA) to precisely determine double bond position and geometry; application of the DMDS procedure in taxonomic and environmental studies including the degradation of pollutant halogenated hydrocarbons in groundwater and subsurface aquifers; exploiting the ubiquitous nature of uronic acids in microbial EPS to quantify these exopolymers in complex environmental samples; development of rapid and non-destructive approaches including FT-IR to follow biofilm formation in a unique manner not possible with other approaches. The foundations laid in the DC White laboratories have seen a wide suite of applications in modern microbial ecology and associated fields. The training of young scientists by DC White will also ensure that his unique approach and quest for new and or novel methodologies for use in environmental microbiology will continue.

Potential for bioremediation of petroleum hydrocarbons in groundwater under cold climate conditions: A review

Globally, bioremediation is a common choice for remediation of petroleum hydrocarbon-contaminated sites. For application at cold climate sites, bioremediation approaches are appealing because they have potential to be more efficient and cost-effective than alternative, more energy intensive approaches. Several bioremediation approaches have been reported to be successful for petroleum hydrocarbon-contaminated soils at cold climate sites. In contrast, there are relatively few publications on applications of bioremediation for petroleum-contaminated groundwater at cold climate sites. Most of the existing relevant groundwater studies were conducted at sites with either no permafrost, or with sporadic to discontinuous permafrost. To date, the majority of cold climate groundwater investigations were at fuel spill sites; few studies on bioremediation of dissolved hydrocarbon plumes derived from crude oil or gas condensate have been published. Some studies reported that extents of hydrocarbon plumes in groundwater were limited by natural attenuation, including intrinsic bioremediation. At other sites, oxygenation of groundwater or amendments with nitrate were reported to be successful techniques for enhancing biodegradation of petroleum hydrocarbons. Both aerobic and anaerobic processes appear to be important at these sites. Based on three case studies, bioremediation (in situ or ex situ) may be feasible for sites with extensive permafrost. Further research and field demonstrations are required to establish or confirm the applicability of bioremediation technologies to clean up hydrocarbons in groundwater in various hydrogeological settings at cold climate sites.

Concentrations of heavy metals and hydrocarbons in groundwater near petrol stations and mechanic workshops in Calabar metropolis, southeastern Nigeria

Groundwater and soil samples from 16 locations near petrol stations (PS) and mechanic workshops (MW) around Calabar, Nigeria, were analyzed for heavy metals and hydrocarbons to determine their concentrations and assess the impact of the PS and MW on groundwater in the area. Results show that mean concentrations of arsenic, cadmium, chromium, copper, iron, mercury, manganese, nickel, lead, and zinc in groundwater are higher than the reference data (maximum admissible concentration, baseline value, and control site value). The mean concentration of total organic content in soil is low (4.03%), but the mean concentrations of total petroleum hydrocarbons (46.87 mg kg1) and naphthalene (340.00 g g1) in the soil are high in comparison to the reference data. The mean concentrations of parameters in soil and groundwater are higher where PS and MW are located near each other (PS/MW). The mean total hydrocarbon content (0.66 mg L1) and total polycyclic aromatic hydrocarbon (PAH) (66.64 g L1) in groundwater are higher than the maximum admissible concentration of 0.1 mg L1 and 0.2 g L1, respectively. The concentration of PAH compounds with mean concentrations greater than 1 g L1 are of the order phenanthrene anthracene fluorene benzo(b)fluoranthene-benzo(k)fluoranthene pyrene naphthalene. The ratios of phenanthrene/anthracene, fluoranthene/pyrene, and benzo(a)anthracene/chrysene and factor analysis indicate several sources for the PAH. The most important sources include anthropogenic (petrogenic, pyrolytic) and natural.

Review of Fundamentals of Environmental Engineering by James R. MihelcicFundamentals of Environmental EngineeringWiley0-471-24313-2$78.95

The book is not new but was recently brought to my attention. As the field of environmental hydraulics becomes increasingly popular, how can we educate our hydraulic engineers to gain basic knowledge in environmental engineering? Can we combine knowledge from various sciences like chemistry and biology with the knowledge of physics and mathematics that is also required in the traditional civil engineering curriculum? This book provides a reasonable answer to these questions. It is the outgrowth of team teaching for a course required of all civil and environmental engineering students at Michigan Technological University. Several coauthors share partial credit for this text besides the primary author. The faculty members from MTU include M. T. Auer, D. W. Hand, R. E. Honrath, J. A. Perlinger, and N. R. Urban. M. R. Penn at the University of WisconsinPlatteville also contributed to the biology chapter. The course is intended to provide a bridge between basic sciences and math courses and upper-level engineering courses. The material is divided into four main chapters: concentrations in air, water, and soils; chemistry; physical processes; and biology. The brief preface uses practical examples to illustrate the need for a strong command of the fundamentals in chemistry, biology, and physical processes. The examples of municipal wastewater treatment, acid rain, and fate of chlorinated aliphatic hydrocarbons in groundwater clearly state the need for an integrated multidisciplinary approach to environmental problems. Chapter 2 delves into the nitty-gritty details of unit conversions and sets the tone for the quantitative approach of the authors. The reader is exposed to all possible units used in measuring contaminant levels in soils, water, and the atmosphere. Chapter 3 covers about 100 pages and explains the chemistry of pollutants in a variety of engineered and natural systems. This chapter includes a nice and detailed presentation of chemical kinetics, thermodynamics, equilibrium processes, oxidation/reduction processes, and photochemistry. Chapter 4 provides a 60-page discussion of mass and energy balances. There is also a brief presentation of advection and dispersion, along with a review of the movement of particles in fluids

Preliminary observations on the release of arsenic to groundwater in the presence of hydrocarbon contaminants in UK aquifers

AbstractDegradation of contaminant hydrocarbons in groundwater by microbially mediated oxidation, linked to the reduction of electron acceptors, is fundamental to the strategy of ‘monitored natural attenuation’ (MNA) for oxidizable hydrocarbons, which is increasingly being adopted at polluted aquifer sites throughout Europe and North America. Commonly, oxygen is depleted and following the reduction of nitrate, solid-phase Fe oxides become the dominant electron acceptors. Arsenic, associated with Fe and Mn oxides in soils and sediments, may therefore be mobilized to groundwater and pose an additional threat to environmental receptors. In a pilot study of three aquifers in England, we have examined the extent to which arsenic is released to groundwater under Fe(III)-reducing conditions imposed by contaminant hydrocarbons. Results show that arsenic is locally mobilized in the Chalk to <10 μg/1, in Quaternary gravels to 70 μg/1 and in the Triassic sandstones to 160 μg/1. At the Chalk and Quaternary gravels sites arsenic mobilization is demonstrably linked to reduction of Fe- and Mn-oxides. This is not so at the Triassic sandstone site, where release of arsenic is related to elevated bicarbonate alkalinity. Redox-driven arsenic mobilization at other Triassic sandstone locations is possible. Further work is required on the solid-phase sources of arsenic in the aquifers, and to relate the hydrochemical observations to groundwater hydraulic conditions.

Assessment of Degradation Pathways in an Aquifer with Mixed Chlorinated Hydrocarbon Contamination Using Stable Isotope Analysis

The demonstration of monitored natural attenuation (MNA) of chlorinated hydrocarbons in groundwater is typically conducted through the evaluation of concentration trends and parent-daughter product relationships along prevailing groundwater flow paths. Unfortunately, at sites contaminated by mixtures of chlorinated ethenes, ethanes, and methanes, the evaluation of MNA by using solely concentration data and parent-daughter relationships can result in erroneous conclusions regarding the degradation mechanisms that are truly active at the site, since many of the daughter products can be derived from multiple parent compounds. Stable carbon isotope analysis was used, in conjunction with concentration data, to clarify and confirm the active degradation pathways at a former waste solvent disposal site where at least 14 different chlorinated hydrocarbons have been detected in the groundwater. The isotope data indicate that TCE, initially believed to be present as a disposed product and/or a PCE dechlorination intermediate, is attributable to dehydrochlorination of 1,1,2,2-PCA. The isotope data further support that vinyl chloride and ethene in the site groundwater result from dichloroelimination of 1,1,2-trichlorethane and 1,2-dichloroethane, respectively, rather than from reductive dechlorination of the chlorinated ethenes PCE, TCE, or 1,2-DCE. The isotope data confirm that the chlorinated ethanes and chlorinated methanes are undergoing significant intrinsic degradation, whereas degradation of the chlorinated ethenes may be limited. In addition to the classical trend of enriched isotope values of the parent compounds with increasing distance associated to biodegradation, shifts of isotope ratios of degradation byproduct in the opposite direction due to mixing of isotopically light byproducts of biodegradation with compounds from the source are shown to be of high diagnostic value. These data underline the value of stable isotope analysis in confirming transformation processes at sites with complex mixtures of chlorinated compounds.

MTBE and Gasoline Hydrocarbons in Ground Water of the United States

The occurrence of methyl tert-butyl ether (MTBE) and gasoline hydrocarbons was examined in three types of studies of ground water conducted by the U.S. Geological Survey: major aquifer surveys, urban land-use studies, and agricultural land-use studies. The detection frequency of MTBE was dependent on the study type, with the highest detection frequency in urban land-use studies. Only 13 ground water samples from all study types, or 0.3%, had concentrations of MTBE that exceeded the lower limit of the U.S. EPA's Drinking-Water Advisory. The detection frequency of MTBE was highest in monitoring wells located in urban areas and in public supply wells. The detection frequency of any gasoline hydrocarbon also was dependent on study type and generally was less than the detection frequency of MTBE. The probability of detecting MTBE in ground water was strongly associated with population density, use of MTBE in gasoline, and recharge. Ground water in areas with high population density, in areas where MTBE is used as a gasoline oxygenate, and in areas with high recharge rates had a greater probability of MTBE occurrence. Also, ground water from public supply wells and shallow ground water underlying urban land-use areas had a greater probability of MTBE occurrence compared to ground water from domestic wells and ground water underlying rural land-use areas. The probability of detecting MTBE in ground water was weakly associated with the density of leaking underground storage tanks, soil permeability, and aquifer consolidation, and only concentrations of MTBE >0.5 microg/L were associated with dissolved oxygen.

Interaction between shallow groundwater, saline surface water and contaminant discharge at a seasonally and tidally forced estuarine boundary

This paper presents findings from a 2-year field investigation of a dissolved hydrocarbon groundwater plume flowing towards a tidally and seasonally forced estuarine river system in Perth, Western Australia. Samples collected from transects of multiport wells along the riverbank and into the river, enabled mapping of the fine scale (0.5 m) vertical definition of the hydrocarbon plume and its longitudinal extent. Spear probing beneath the river sediments and water table, and transient monitoring of multiport wells (electrical conductivity) was also carried out to define the zone of mixing between river water and groundwater (the hyporheic zone) and its variability. The results showed that groundwater seepage into the estuarine surface sediments occurred in a zone less than 10 m from the high tide mark, and that this distance and the hyporheic transition zone were influenced by tidal fluctuations and infiltration of river water into the sediments. The dissolved BTEXN (benzene, toluene, ethylbenzene, the xylene isomers and naphthalene) distributions indicated the behaviour of the hydrocarbon plume at the groundwater/surface water transition zone to be strongly influenced by edge-focussed discharge. Monitoring programs and risk assessment studies at similar contaminated sites should therefore focus efforts within the intertidal zone where contaminants are likely to impact the surface water and shallow sediment environments.

Total Petroleum Hydrocarbons in Groundwater—Evaluation of Nondissolved and Nonhydrocarbon Fractions

Total petroleum hydrocarbons (TPH) in groundwater samples are commonly extracted and analyzed for comparison to regulatory standards and for forensic evaluation of the source of spilled petroleum. While this procedure is normally intended to provide information on hydrocarbons dissolved in water, several complicating factors exist. In this study, the significance of reported TPH values is evaluated for a suite of surface water, interstitial, and groundwater samples by comparing analytical data obtained on unfiltered water, filtered water, and water from which nonhydrocarbons were removed by treatment with silica gel. Results indicate that standard measurements of TPH in groundwater not only include dissolved petroleum hydrocarbons, but commonly also include contributions from: (1) hydrocarbons sorbed on particulate matter; (2) droplets or micellular forms of liquid hydrocarbon contamination; (3) biogenic hydrocarbons; (4) contamination by field or laboratory equipment; and (5) dissolved polar organics (nonhydrocarbons), which may be naturally occurring or derived from spilled petroleum products. These factors complicate the regulatory and forensic interpretation of standard TPH data for groundwater samples.