SEARCH · USGS Science

Results for “Biodegradation”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 217 records · Page 12Linked to original sources

USGS2023

Intramolecular carbon isotope geochemistry of butane isomers from laboratory maturation and Monte-Carlo simulations of kerogen types I, II, and III

Position-specific (PS) carbon isotope compositions of light hydrocarbons such as propane and butane isomers (n-butane and i-butane) can provide a wealth of information on the history of natural gases in the subsurface reservoirs and other environments. For PS carbon isotope analysis of butane isomers, we have established a GC-pyrolysis-GC-isotope ratio mass spectrometry method with demonstrated accuracy. With this method, we analyzed PS δ 13 C values of butane isomers generated from the systematic laboratory pyrolysis experiments of three different kerogen types (I, II, and III) at temperatures of 310–430 °C with corresponding thermal maturity (Easy %R o ) ranging from 0.7 to 3.3. The observed evolution in the abundances of butane isomers can be interpreted and semi-quantitatively modeled based on the abundances of different C C bonds within the kerogens at low maturity and thermal degradation of butane isomers at high maturity. The δ 13 C values at the central sites of both nC 4 and iC 4 were heavier than those at the terminal positions, similar to our previous observations of propane. Their isotopic evolution with the maturity were controlled largely by kinetic isotope effects associated with breaking of different C C bonds during the generation and degradation of butane isomers. Kinetic Monte Carlo (kMC) simulations of n-butane generated from thermal cracking of model kerogens (I, II, and III) and an oil with a series of reactions (homolytic cleavage, β-scission, radical isomerization, H-abstraction, and termination by radical recombination) provided generally consistent results with the experimental observations, although the difference in PS δ 13 C values between the central and terminal positions are somewhat overestimated. On the other hand, the kMC simulation with homolytic cleavage and capping reactions alone produced significant deviations from the experimental results. Re-assessment of very limited data of PS δ 13 C values of natural butanes with our experimental and simulation results show that biodegradation significantly increased δ 13 C values at the central positions, not only of propane, but also of both butane isomers. This study lays a foundation and demonstrates the potential of PS isotope geochemistry of butane isomers to further improve our understanding of the sources, and geochemical and microbial processes of light hydrocarbons in the subsurface and other natural environments.

Geochimica et Cosmochimica Acta↗

USGS2006

Recharge processes drive sulfate reduction in an alluvial aquifer contaminated with landfill leachate

Natural attenuation of contaminants in groundwater depends on an adequate supply of electron acceptors to stimulate biodegradation. In an alluvial aquifer contaminated with leachate from an unlined municipal landfill, the mechanism of recharge infiltration was investigated as a source of electron acceptors. Water samples were collected monthly at closely spaced intervals in the top 2 m of the saturated zone from a leachate-contaminated well and an uncontaminated well, and analyzed for δ 18 O, δ 2 H, non-volatile dissolved organic carbon (NVDOC), SO 4 2− , NO 3 − and Cl − . Monthly recharge amounts were quantified using the offset of the δ 18 O or δ 2 H from the local meteoric water line as a parameter to distinguish water types, as evaporation and methanogenesis caused isotopic enrichment in waters from different sources. Presence of dissolved SO 4 2− in the top 1 to 2 m of the saturated zone was associated with recharge; SO 4 2− averaged 2.2 mM, with maximum concentrations of 15 mM. Nitrate was observed near the water table at the contaminated site at concentrations up to 4.6 mM. Temporal monitoring of δ 2 H and SO 4 2− showed that vertical transport of recharge carried SO 4 2− to depths up to 1.75 m below the water table, supplying an additional electron acceptor to the predominantly methanogenic leachate plume. Measurements of δ 34 S in SO 4 2− indicated both SO 4 2− reduction and sulfide oxidation were occurring in the aquifer. Depth-integrated net SO 4 2− reduction rates, calculated using the natural Cl − gradient as a conservative tracer, ranged from 7.5 × 10 − 3 to 0.61 mM·d − 1 (over various depth intervals from 0.45 to 1.75 m). Sulfate reduction occurred at both the contaminated and uncontaminated sites; however, median SO 4 2− reduction rates were higher at the contaminated site. Although estimated SO 4 2− reduction rates are relatively high, significant decreases in NVDOC were not observed at the contaminated site. Organic compounds more labile than the leachate NVDOC may be present in the root zone, and SO 4 2− reduction may be coupled to methane oxidation. The results show that sulfur (and possibly nitrogen) redox processes within the top 2 m of the aquifer are directly related to recharge timing and seasonal water level changes in the aquifer. The results suggest that SO 4 2− reduction associated with the infiltration of recharge may be a significant factor affecting natural attenuation of contaminants in alluvial aquifers.

Journal of Contaminant Hydrology↗

USGS2010

Vadose zone attenuation of organic compounds at a crude oil spill site - Interactions between biogeochemical reactions and multicomponent gas transport

Contaminant attenuation processes in the vadose zone of a crude oil spill site near Bemidji, MN have been simulated with a reactive transport model that includes multicomponent gas transport, solute transport, and the most relevant biogeochemical reactions. Dissolution and volatilization of oil components, their aerobic and anaerobic degradation coupled with sequential electron acceptor consumption, ingress of atmospheric O 2 , and the release of CH 4 and CO 2 from the smear zone generated by the floating oil were considered. The focus of the simulations was to assess the dynamics between biodegradation and gas transport processes in the vadose zone, to evaluate the rates and contributions of different electron accepting processes towards vadose zone natural attenuation, and to provide an estimate of the historical mass loss. Concentration distributions of reactive (O 2 , CH 4 , and CO 2 ) and non-reactive (Ar and N 2 ) gases served as key constraints for the model calibration. Simulation results confirm that as of 2007, the main degradation pathway can be attributed to methanogenic degradation of organic compounds in the smear zone and the vadose zone resulting in a contaminant plume dominated by high CH 4 concentrations. In accordance with field observations, zones of volatilization and CH 4 generation are correlated to slightly elevated total gas pressures and low partial pressures of N 2 and Ar, while zones of aerobic CH 4 oxidation are characterized by slightly reduced gas pressures and elevated concentrations of N 2 and Ar. Diffusion is the most significant transport mechanism for gases in the vadose zone; however, the simulations also indicate that, despite very small pressure gradients, advection contributes up to 15% towards the net flux of CH 4 , and to a more limited extent to O 2 ingress. Model calibration strongly suggests that transfer of biogenically generated gases from the smear zone provides a major control on vadose zone gas distributions and vadose zone carbon balance. Overall, the model was successful in capturing the complex interactions between biogeochemical reactions and multicomponent gas transport processes. However, despite employing a process-based modeling approach, honoring observed parameter ranges, and generally obtaining good agreement between field observations and model simulations, accurate quantification of natural attenuation rates remains difficult. The modeling results are affected by uncertainties regarding gas phase saturations, tortuosities, and the magnitude of CH 4 and CO 2 flux from the smear zone. These findings highlight the need to better delineate gas fluxes at the model boundaries, which will help constrain contaminant degradation rates, and ultimately source zone longevity.

Minnesota↗

USGS2017

The role of alluvial aquifer sediments in attenuating a dissolved arsenic plume

In a crude-oil-contaminated sandy aquifer at the Bemidji site in northern Minnesota, biodegradation of petroleum hydrocarbons has resulted in release of naturally occurring As to groundwater under Fe-reducing conditions. This study used chemical extractions of aquifer sediments collected in 1993 and 2011–2014 to evaluate the relationship between Fe and As in different redox zones (oxic, methanogenic, Fe-reducing, anoxic-suboxic transition) of the contaminated aquifer over a twenty-year period. Results show that 1) the aquifer has the capacity to naturally attenuate the plume of dissolved As, primarily through sorption; 2) Fe and As are linearly correlated in sediment across all redox zones, and a regression analysis between Fe and As reasonably predicted As concentrations in sediment from 1993 using only Fe concentrations; 3) an As-rich “iron curtain,” associated with the anoxic-suboxic transition zone, migrated 30 m downgradient between 1993 and 2013 as a result of the hydrocarbon plume evolution; and 4) silt lenses in the aquifer preferentially sequester dissolved As, though As is remobilized into groundwater from sediment after reducing conditions are established. Using results of this study coupled with historical data, we develop a conceptual model which summarizes the natural attenuation of As and Fe over time and space that can be applied to other sites that experience As mobilization due to an influx of bioavailable organic matter.

Journal of Contaminant Hydrology↗

USGS2021

Arsenic release to the environment from hydrocarbon production, storage, transportation, use and waste management

Arsenic (As) is a toxic trace element with many sources, including hydrocarbons such as oil, natural gas, oil sands, and oil- and gas-bearing shales. Arsenic from these hydrocarbon sources can be released to the environment through human activities of hydrocarbon production, storage, transportation and use. In addition, accidental release of hydrocarbons to aquifers with naturally occurring (geogenic) As can induce mobilization of As to groundwater through biogeochemical reactions triggered by hydrocarbon biodegradation. In this paper, we review the occurrence of As in different hydrocarbons and the release of As from these sources into the environment. We also examine the occurrence of As in wastes from hydrocarbon production, including produced water and sludge. Last, we discuss the potential for As release related to waste management, including accidental or intentional releases, and recycling and reuse of these wastes.

Journal of Hazardous Materials↗

USGS2008

Natural gas geochemistry of sediments drilled on the 2005 Gulf of Mexico JIP cruise

In April and May 2005, cores were acquired and sub-sampled for gases in lease blocks Atwater Valley 13 and 14 and Keathley Canyon 151 during deep subseafloor drilling conducted as part of the JIP study of gas hydrates in the northern Gulf of Mexico. Sample types included sediment headspace gas, free gas derived from sediment gas exsolution, and gas exsolution from controlled degassing of pressurized cores. The gases measured both onboard and in shore-based labs were nitrogen, oxygen, hydrogen sulfide, carbon dioxide, and the hydrocarbons methane through hexane. The presence of seafloor mounds, seismic anomalies, a shallow sulfate-methane interface, and similar gas compositions and isotopic compositions near the seafloor and at depth suggest an upward flux of methane at both sites. Sediment gases at the Atwater Valley sites, where seafloor mounds and adjacent sediments were cored, strongly suggest a microbial source of methane, with very little thermogenic gas input. Sediment gas from all cores contained from about 96 to 99.9% methane, with the balance composed primarily of carbon dioxide. Methane to ethane ratios were greater than 1000, and often over 10,000. Gases from cores at Keathley Canyon were similar to those at Atwater Valley, however, deeper cores from Keathley Canyon contained more ethane, propane, and butane suggesting mixing with minor concentrations thermogenic gas. The isotopic composition of methane, ethane, and carbon dioxide were measured, and ??13C values range from -84.3 to -71.5???, -65.2 to -46.8???, and -23.5 to -3.0???, respectively, all consistent with microbial gas sources, early diagenesis of organic matter and perhaps biodegradation of petroleum. The presence of deep microbial gas at these sites here and elsewhere highlights a potentially significant, predominantly microbial gas source in the northern Gulf of Mexico.

Marine and Petroleum Geology↗

USGS2011

Gas geochemistry of the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope: implications for gas hydrate exploration in the Arctic

Gases were analyzed from well cuttings, core, gas hydrate, and formation tests at the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well, drilled within the Milne Point Unit, Alaska North Slope. The well penetrated a portion of the Eileen gas hydrate deposit, which overlies the more deeply buried Prudhoe Bay, Milne Point, West Sak, and Kuparuk River oil fields. Gas sources in the upper 200 m are predominantly from microbial sources (C 1 isotopic compositions ranging from −86.4 to −80.6‰). The C 1 isotopic composition becomes progressively enriched from 200 m to the top of the gas hydrate-bearing sands at 600 m. The tested gas hydrates occur in two primary intervals, units D and C, between 614.0 m and 664.7 m, containing a total of 29.3 m of gas hydrate-bearing sands. The hydrocarbon gases in cuttings and core samples from 604 to 914 m are composed of methane with very little ethane. The isotopic composition of the methane carbon ranges from −50.1 to −43.9‰ with several outliers, generally decreasing with depth. Gas samples collected by the Modular Formation Dynamics Testing (MDT) tool in the hydrate-bearing units were similarly composed mainly of methane, with up to 284 ppm ethane. The methane isotopic composition ranged from −48.2 to −48.0‰ in the C sand and from −48.4 to −46.6‰ in the D sand. Methane hydrogen isotopic composition ranged from −238 to −230‰, with slightly more depleted values in the deeper C sand. These results are consistent with the concept that the Eileen gas hydrates contain a mixture of deep-sourced, microbially biodegraded thermogenic gas, with lesser amounts of thermogenic oil-associated gas, and coal gas. Thermal gases are likely sourced from existing oil and gas accumulations that have migrated up-dip and/or up-fault and formed gas hydrate in response to climate cooling with permafrost formation.

Alaska↗

USGS2007

Characterization of the origin of coalbed gases in southeastern Illinois Basin by compound-specific carbon and hydrogen stable isotope ratios

Coalbed gases and waters from exploratory and production gas wells in the southeastern Illinois Basin were sampled to assess geochemically the origin of coalbed gases, with an emphasis on the Springfield and Seelyville Coal Members that are commercially targeted for coalbed methane production in Indiana. On-line analyses of hydrocarbon gases methane to butanes (C1, C2, C3, n-C4, i-C4) and CO2 yielded gas concentrations, plus ??D and ??13C values. The low thermal maturity of Indiana coals with vitrinite reflectance R0 ??? 0.6% is in agreement with an overwhelmingly biogenic isotopic signature of coalbed gases containing ???96% methane generated via bacterial CO2-reduction. In contrast, thermogenic gas was generated in the stratigraphically equivalent coal beds in western Kentucky's Rough Creek Graben zone where higher maturities of up to R0 ??? 0.8% were reached owing to tectonic and hydrothermal activity. No secondary biogenic methane was observed in more mature western Kentucky coal beds where greater burial depth limits the recharge of meteoric water. Biogenic and thermogenic coalbed gases represent two end-members that are compositionally and isotopically distinct. Microbial biodegradation of thermogenic C2+ hydrocarbon gases in Indiana coal beds preferentially targets C3 and introduces isotope fractionation whereby remaining C3 is enriched in deuterium and 13C.

Organic Geochemistry↗

USGS2008

Distinguishing solid bitumens formed by thermochemical sulfate reduction and thermal chemical alteration

Insoluble solid bitumens are organic residues that can form by the thermal chemical alteration (TCA) or thermochemical sulfate reduction (TSR) of migrated petroleum. TCA may actually encompass several low temperature processes, such as biodegradation and asphaltene precipitation, followed by thermal alteration. TSR is an abiotic redox reaction where petroleum is oxidized by sulfate. It is difficult to distinguish solid bitumens associated with TCA of petroleum from those associated with TSR when both processes occur at relatively high temperature. The focus of the present work was to characterize solid bitumen samples associated with TCA or TSR using X-ray photoelectron spectroscopy (XPS). XPS is a surface analysis conducted on either isolated or in situ (>25 ??m diameter) solid bitumen that can provide the relative abundance and chemical speciation of carbon, organic and inorganic heteroatoms (NSO). In this study, naturally occurring solid bitumens from three locations, Nisku Fm. Brazeau River area (TSR-related), LaBarge Field Madison Fm. (TSR-related), and the Alaskan Brooks range (TCA-related), are compared to organic solids generated during laboratory simulation of the TSR and TCA processes. The abundance and chemical nature of organic nitrogen and sulfur in solid bitumens can be understood in terms of the nature of (1) petroleum precursor molecules, (2) the concentration of nitrogen by way of thermal stress and (3) the mode of sulfur incorporation. TCA solid bitumens originate from polar materials that are initially rich in sulfur and nitrogen. Aromaticity and nitrogen increase as thermal stress cleaves aliphatic moieties and condensation reactions take place. Organic sulfur in TCA organic solids remains fairly constant with increasing maturation (<3.4 sulfurs per 100 carbons) due to offsetting preservation and H2S elimination reactions. In contrast, TSR solid bitumens are sulfur rich and nitrogen poor solids. These heteroatom distributions are attributed to the ability of TSR to incorporate copious amounts of inorganic sulfur (>3.5 to ???17 sulfur per 100 carbons) into aromatic structures and to the low levels of nitrogen in their hydrocarbon precursors. Hence, XPS results provide organic chemical composition information that helps to distinguish whether solid bitumen, either in situ or removed and concentrated from the rock matrix, was formed via the TCA or TRS process. ?? 2008 Elsevier Ltd.

Organic Geochemistry↗

USGS2021

Tandem field and laboratory approaches to quantify attenuation mechanisms of pharmaceutical and pharmaceutical transformation products in a wastewater effluent-dominated stream

Evolving complex mixtures of pharmaceuticals and transformation products in effluent-dominated streams pose potential impacts to aquatic species; thus, understanding the attenuation dynamics in the field and characterizing the prominent attenuation mechanisms of pharmaceuticals and their transformation products (TPs) is critical for hazard assessments. Herein, we determined the attenuation dynamics and the associated prominent mechanisms of pharmaceuticals and their corresponding TPs via a combined long-term field study and controlled laboratory experiments. For the field study, we quantified spatiotemporal exposure concentrations of five pharmaceuticals and six associated TPs in a small, temperate-region effluent-dominated stream during baseflow conditions where the wastewater plant was the main source of pharmaceuticals. We selected four sites (upstream, at, and two progressively downstream from effluent discharge) and collected water samples at 16 time points (64 samples in total, approximately twice monthly, depending on flows) for 1 year. Concurrently, we conducted photolysis, sorption, and biodegradation batch tests under controlled conditions to determine the major attenuation mechanisms. We observed 10-fold greater attenuation rates in the field compared to batch tests, demonstrating that connecting laboratory batch tests with field measurements to enhance predictive power is a critical need. Batch systems alone, often used for assessment, are useful for determining fate processes but poorly approximate in-stream attenuation kinetics. Sorption was the dominant attenuation process (t 1/2 <7.7 d) for 5 of 11 compounds in the batch tests, while the other compounds ( n = 6) persisted in the batch tests and along the 5.1 km stream reach. In-stream parent-to-product transformation was minimal. Differential attenuation contributed to the evolving pharmaceutical mixture and created changing exposure conditions with concomitant implications for aquatic and terrestrial biota. Tandem field and laboratory characterization can better inform modeling efforts for transport and risk assessments.

Water Research↗

USGS2003

Comparison of approaches for simulating reactive solute transport involving organic degradation reactions by multiple terminal electron acceptors

Reactive solute transport models are useful tools for analyzing complex geochemical behavior resulting from biodegradation of organic compounds by multiple terminal electron acceptors (TEAPs). The usual approach of simulating the reactions of multiple TEAPs by an irreversible Monod rate law was compared with simulations that assumed a partial local equilibrium or kinetically controlled reactions subject to the requirement that the Gibbs free energy of reaction (Δ G) was either less than zero or less than a threshold value. Simulations were performed using a single organic substrate and O 2 , FeOOH, SO 4 -2 and CO 2 as the terminal electron acceptors. It was assumed that the organic substrate was slowly and completely fermented to CO 2 and H 2 and the H 2 was oxidized by the TEAPs. Simulations using the Monod approach showed that this irreversible rate law forced the reduction of both FeOOH and CO 2 to proceed even when Δ G was positive. This resulted in an over prediction in amount of FeOOH reduced to Fe(II) in parts of the domain and it resulted in large errors in pH. Simulations using mass action kinetics agreed with equilibrium simulations for the case of large rate constants. The extent of reductive dissolution of FeOOH was strongly dependent on the thermodynamic stability of the FeOOH phase. Transport simulations performed assuming that the reactions of the TEAPs stopped when Δ G exceeded a threshold value showed that only simulated H 2 concentrations were affected if the threshold value was the same for each TEAP. Simulated H2 concentrations were controlled by the fastest reaction of the TEAP, but it was common for reactions to occur concomitantly rather than sequentially.

Computers & Geosciences↗

USGS1997

Lignin phenols in sediments of Lake Baikal, Siberia: Application to paleoenvironmental studies

Sediments from three cores obtained from distinct depositional environments in Lake Baikal, Siberia were analyzed for organic carbon, total nitrogen and lignin phenol concentration and composition. Results were used to examine changes in paleoenvironmental conditions during climatic cycles of the late Quaternary (< 125 ka). Average organic carbon, and total nitrogen concentrations, atomic C/N ratios and organic carbon accumulation rates were significantly higher in the Holocene compared with the late Pleistocene, reflecting overall warmer temperatures and increased runoff during the Holocene. A Holocene maximum in organic carbon was observed at about 6 ka, and may represent the warmest wettest period of the Holocene. At one site (Academician Ridge) pronounced late Pleistocene maxima in organic carbon and biogenic silica were observed at about 80-85 ka, probably indicative of an interstadial period with enhanced aquatic productivity. Total sedimentary lignin phenol contents were generally lower in the late Pleistocene compared to the Holocene, but with several peaks in concentration during the late Pleistocene. These late Pleistocene peaks in total sedimentary lignin content (dated at about 80, 50 and 30 ka) directly precede or occur during peaks in sedimentary biogenic silica contents. These periods likely represent relatively warm interstadial times, with increased precipitation producing the observed increase in terrestrial runoff and aquatic productivity. Lignin phenol ratios (S/V, C/V and P/V) were used to examine changes in terrestrial vegetation type resulting from changes in paleoenvironmental conditions during the late Pleistocene. A degree of caution must be used in the interpretation of these ratios with regard to vegetation sources and paleoenvironmental conditions, because of potential compositional changes in lignin resulting from biodegradation. Nevertheless, results show that long glacial periods were characterized by terrestrial vegetation composed of a mix of non-woody angiosperm vegetation and minor gymnosperm forest. Shorter interstadial periods are defined by a change to dominant gymnosperm forest and were observed at about 80, 75, 63, 50 and 30 ka, ranging from about 2-6 kyr in duration. These interstadial periods of the late Pleistocene defined by lignin phenol ratios generally occur during longer periods of enhanced sedimentary biogenic silica content (about 10-15 ka in duration), providing corroborative evidence of these warm interstadial periods.Sediments obtained in Lake Baikal were analyzed for organic carbon, total nitrogen and lignin phenol composition and used to study changes in paleoenvironmental conditions during climatic cycles of the late Quaternary. The organic carbon, total nitrogen concentrations, atomic C/N ratios and organic carbon accumulation rates were higher in the Holocene showing overall warmer temperatures and increased runoff. Total lignin phenol contents were lower in the Pleistocene representing relatively warm interstadial times with increased precipitation, runoff and aquatic productivity. Lignin phenol was used to examine vegetation changes due to paleoenvironmental conditions and showed that long glacial periods were characterized by terrestrial vegetation.

Siberia↗

USGS1997

Protistan communities in aquifers: A review

Eukaryotic microorganisms (protists) are a very important component of microbial communities inhabiting groundwater aquifers This is not unexpected when one considers that many protists feed heterotrophically, by means of either phagotrophy (bacterivory) or osmotrophy. Protistan numbers are usually low (<102 per g dw of aquifer material) in pristine, uncontaminated aquifers but may increase by several orders of magnitude in aquifers subject to organic pout on Stoa flagellates (typically 2-3(5) ??m in size in situ) are by far the dominant protists in aquifers although amoebae and occasionally ciliates may also be present much lower numbers. A though a wealth of new taxonomic information is waiting to be brought to light, interest in the identity of aquifer protists is not exclusively academic If verified, the following hypotheses may prove to be important towards our understanding of the functioning of microbial communities in aquifers: (1) Differences in swimming behavior between species of flagellates lead to feeding heterogeneity and niche differentiation, implying that bacterivorous flagellates graze on different subsets of the bacterial community, and therefore play different roles in controlling bacterial densities. (2) Bacterivorous flagellates grazing on bacteria capable of degrading Organic compounds have an indirect effect on the overall rates of biodegradation.

FEMS Microbiology Reviews↗

USGS1999

Geochemical heterogeneity of a gasoline-contaminated aquifer

The scale of biogeochemical reactions was studied in a physically and chemically heterogeneous surficial Coastal Plain aquifer contaminated by a gasoline spill. The physical heterogeneity of the aquifer is manifested in two hydrologic units, a shallow local aquifer of perched water and a regional sandy aquifer. Over the studied vertical interval of 21.3 ft (6.5 m), concentrations of reactive species varied by orders of magnitude, and the impact of biodegradation was expressed to widely varying degrees. A thin (3 ft thick) section of the perched-water zone was the most contaminated; total aromatic hydrocarbons were as high as 19.4 mg/l. Hydrocarbons were degraded by microbially mediated reactions that varied over short vertical distances and time. Anaerobic processes dominated within the low-permeability clay unit, whereas in the more permeable sandy layers nitrate reduction and aerobic degradation occurred. Hydrocarbons were more persistent over time in the low-permeability layer due to the limited availability of electron acceptors for degradation. The microbial degradation of hydrocarbons was linked to sulfate and iron reduction in the clay unit and led to alterations in the aquifer solids; electron microscopy revealed the presence of FeS minerals encrusting primary aquifer grains. High concentrations of Fe 2+ in groundwater, up to 34.5 mg/l, persist in kinetic disequilibrium in the presence of elevated H 2 S levels of 1.0 mg/l. Assessment of aquifer heterogeneities and groundwater contamination was possible due to sample discrimination at a scale of approximately 2 ft (∼0.6 m), a much finer resolution than is attempted in many remedial investigations of polluted aquifers. The information obtained in this type of study is essential to the development of models capable of estimating the fate of hydrocarbons at a site scale.

Journal of Contaminant Hydrology↗

USGS2000

Diagenetic fate of organic contaminants on the Palos Verdes Shelf, California

Municipal wastes discharged through deepwater submarine outfalls since 1937 have contaminated sediments of the Palos Verdes Shelf. A site approximately 6–8 km downcurrent from the outfall system was chosen for a study of the diagenetic fate of organic contaminants in the waste-impacted sediments. Concentrations of three classes of hydrophobic organic contaminants (DDT+metabolites, polychlorinated biphenyls (PCBs), and the long-chain alkylbenzenes) were determined in sediment cores collected at the study site in 1981 and 1992. Differences between the composition of effluent from the major source of DDT (Montrose Chemical) and that found in sediments suggests that parent DDT was transformed by hydrolytic dehydrochlorination during the earliest stages of diagenesis. As a result, p , p ′-DDE is the dominant DDT metabolite found in shelf sediments, comprising 60–70% of ΣDDT. The p , p -DDE/ p , p ′-DDMU concentration ratio decreases with increasing sub-bottom depth in sediment cores, indicating that reductive dechlorination of p , p ′-DDE is occurring. Approximately 9–23% of the DDE inventory in the sediments may have been converted to DDMU since DDT discharges began ca. 1953. At most, this is less than half of the decline in p , p ′-DDE inventory that has been observed at the study site for the period 1981–1995. Most of the observed decrease is attributable to remobilization by processes such as sediment mixing coupled to resuspension, contaminant desorption, and current advection. Existing field data suggest that the in situ rate of DDE transformation is 10 2 –10 3 times slower than rates determined in recent laboratory microcosm experiments (Quensen, J.F., Mueller, S.A., Jain, M.K., Tiedje, J.M., 1998. Reductive dechlorination of DDE to DDMU in marine sediment microcosms. Science, 280, 722–724.). This explains why the DDT composition (i.e. o , p ′-, p , p ′-isomers of DDE, DDD, DDT) of sediments from this site have not changed significantly since at least 1972. Congener-specific PCB compositions in shelf sediments are highly uniform and show no evidence of diagenetic transformation. Apparently, the agents/factors responsible for reductive dechlorination of DDE are not also effecting alteration of the PCBs. Two types of long-chain alkylbenzenes were found in the contaminated sediments. Comparison of chain length and isomer distributions of the linear alkylbenzenes in wastewater effluent and surficial sediment samples indicate that these compounds undergo biodegradation during sedimentation. Further degradation of the linear alkylbenzenes occurs after burial despite relatively invariant isomer compositions. The branched alkylbenzenes are much more persistent than the linear alkylbenzenes, presumably due to extensive branching of the alkyl side chain. Based on these results, p , p ′-DDE, PCBs, and selected branched alkylbenzenes are sufficiently persistent for use in molecular stratigraphy. The linear alkylbenzenes may also provide information on depositional processes. However, their application as quantitative molecular tracers should be approached with caution.

California↗

USGS2004

Molecular markers and their use in environmental organic geochemistry

Molecular markers are organic substances that carry information about sources of organic matter or contamination. The source/marker relation can be used to indicate the presence of a given source material (qualitative), or, under appropriate conditions, to estimate the amount of a source material (quantitative source apportionment) in the environment. Assemblages of markers can also be used as process probes. In this instance, systematic differences and/or similarities in the physical-chemical properties of markers are coupled with compositional changes in marker composition to infer the operation of natural processes. This paper provides an overview of what molecular markers are, what types of markers are present in the environment, the requirements for the use of markers, and some common applications. To illustrate how molecular markers can answer specific environmental questions, three case studies are presented. The first case study examines the impact of municipal waste on a large urban harbor (Boston Harbor). Linear alkylbenzenes (unreacted residues of linear alkylbenzenesulfonate surfactants) and coprostanol (a fecal indicator) provide information on the sources and likely transport pathways of municipal wastes in a complex hydrologic system. The marker data are also used to estimate the proportion of sewage-derived poly chlorinated biphenyls (PCBs) in polluted harbor sediments. The second case study concerns a portion of the continental shelf off southern California (Palos Verdes) where discharge of municipal wastewaters has led to extensive contamination of sediments and biota. Long-chain alkylbenzenes (surfactant residues), PCBs and the pesticide, DDT (dichlorodiphenyltrichloroethane), are used to develop sedimentation rate estimates for several time periods by molecular stratigraphy. These results, when combined with other information, allow conclusions to be drawn about the most likely transport pathway of sediments at the study site and to predict the fate of historically deposited contaminants. Finally, an investigation of a crude-oil spill in Bemidji, MN illustrates how monoaromatic hydrocarbons can be exploited as process probes, providing insights into the relative importance of different attenuation processes in a contaminated aquifer. The results show that natural attenuation of the monoaromatic hydrocarbons is occurring at this site and is dominated, not by physical and/or chemical processes, but by biodegradation.

Book chapter↗

USGS2025

Examining the compositional selectivity of hydrocarbon oxidation products using liquid–liquid extraction and solid-phase extraction techniques

The effect of extraction methods on detecting hydrocarbon oxidation products (HOPs) in groundwater remains unclear. HOPs are polar, water-soluble byproducts of petroleum biodegradation. Our previous work showed that liquid–liquid extraction (LLE), a method commonly used in regulatory monitoring, has a significantly lower extraction efficiency for HOPs compared to solid-phase extraction (SPE). In this study, we evaluate the analytical limitations and compositional selectivity of LLE and SPE using groundwater samples from the Bemidji, MN, crude oil spill site. Optical properties were characterized using excitation–emission matrix spectroscopy (EEMs), and a three-component PARAFAC model was validated, showing consistent trends across both extracts and whole water samples. Ultrahigh-resolution mass spectrometry (UHR-MS) revealed that LLE selectively recovered aliphatic-like compounds but underrepresented more polar oxygenated HOPs. In contrast, SPE methods were more effective at isolating highly oxidized compound classes. These differences were consistent across a gradient of contamination. Overall, the LLE was less precise and less representative of polar HOPs, introducing bias in the characterization of HOPs. This study is the first to quantitatively demonstrate the compositional selectivity and analytical bias of LLE versus SPE for HOPs using combined EEM-PARAFAC and UHR-MS techniques, with implications for long-term monitoring and site assessment protocols.

Environmental Science and Technology↗

USGS2017

Evidence of coupled carbon and iron cycling at a hydrocarbon-contaminated site from time lapse magnetic susceptibility

Conventional characterization and monitoring of hydrocarbon (HC) pollution is often expensive and time-consuming. Magnetic susceptibility (MS) has been proposed as an inexpensive, long-term monitoring proxy of the degradation of HC. We acquired repeated down hole MS logging data in boreholes at a HC-contaminated field research site in Bemidji, MN, USA. The MS data were analyzed in conjunction with redox conditions and iron availability within the source zone to better assess whether MS can serve as a proxy for monitoring HC contamination in unconsolidated sediments. The MS response at the site diminished during the sampling period, which was found to coincide with depletion of solid phase iron in the source zone. Previous geochemical observations and modeling at the site suggest that the most likely cause of the decrease in MS is the transformation of magnetite to siderite, coupled with the exhaustion of ferrihydrite. Although the temporal MS response at this site gives valuable field-scale evidence for changing conditions of iron cycling and stability of iron minerals it does not provide a simple proxy for long-term monitoring of biodegradation of hydrocarbons in the smear zone.

Minnesota↗