Geology topics

Research about Bemidji, Minnesota

Source-linked reports with geographic coverage including Bemidji, Minnesota.

At least 19 recordsLinked to original sources

USGS2023

Attenuation of barium, strontium, cobalt, and nickel plumes formed during microbial iron-reduction in a crude-oil-contaminated aquifer

We assessed the spatial distribution of 35 elements in aquifer sediments and groundwater of a crude-oil-contaminated aquifer and show evidence of the dissolution of barium (Ba), strontium (Sr), cobalt (Co), and nickel (Ni) during hydrocarbon oxidation coupled to historic microbial Fe(III)-reduction near the oil. Trace element plumes occur in the crude-oil-contaminated aquifer, where 50% Co, 47% Ni, 24% Ba, and 15% Sr have been mobilized from the sediment near the oil into groundwater, resulting in dissolved masses >33, 18, three, and two times greater than estimated dissolved masses prior to contamination, respectively. Ba 2+ and Ni 2+ concentrations exceeded the World Health Organization’s drinking-water guidelines of 700 and 20 μg/L, respectively. Sediments attenuate trace element plumes in two geochemically distinct zones, resulting in <0.01% total trace element masses dissolved in groundwater, despite the substantial mobilization near the oil body. Geochemical modeling of the modern Fe(III)-reducing zone suggests trace elements are likely attenuated via coprecipitation with/without sorption on iron carbonate precipitates. In the suboxic transition zone at the leading edge of the plume, Fe(III)-hydroxides sorb Ba 2+ , Sr 2+ , Co 2+ , and Ni 2+ . This study emphasizes that slow but persistent biogeochemical activity can substantially alter aquifer chemistry over decadal timeframes, a phenomenon we term biogeochemical gradualism.

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USGS2023

Photochemical mobilization of dissolved hydrocarbon oxidation products from petroleum contaminated soil into a shallow aquifer activate human nuclear receptors

Elevated non-volatile dissolved organic carbon (NVDOC) concentrations in groundwater monitoring wells under oil-contaminated hydrophobic soils originating from a pipeline rupture at the National Crude Oil Spill & Natural Attenuation Research Site near Bemidji, MN are documented.. We hypothesized the elevated NVDOC is comprised of water-soluble photooxidation products transported from the surface to the aquifer. We use field and laboratory samples in combination with complementary analytical methods to test this hypothesis and determine the biological response to these products. Observations from optical spectroscopy and ultrahigh-resolution mass spectrometry reveal a significant correlation between the chemical composition of NVDOC leached from photochemically weathered soils and groundwater monitoring wells with high NVDOC concentrations measured in the aquifer beneath the contaminated soil. Conversely, the chemical composition from the uncontaminated soil photoleachate, matches the NVDOC observed in the uncontaminated wells. Contaminated groundwater and photodissolution leachates from contaminated soil activated biological targets indicative of xenobiotic metabolism and exhibited potential for adverse effects. Newly formed hydrocarbon oxidation products (HOPs) from fresh oil could be distinguished from those downgradient. This study illustrates another pathway for dissolved HOPs to infiltrate groundwater and potentially affect human health and the environment.

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USGS2022

Understanding the evolution of groundwater-contaminant plume chemistry emanating from legacy contaminant sources: An example from a long-term crude oil spill

Understanding the evolution of plumes emanating from residual hydrocarbon contaminant sources requires evaluating how changes in source compositions over time cause changes in dissolved plume chemistry as residual sources age. This study investigates such changes at the site of a 1979 crude-oil pipeline spill and is the first comprehensive look at groundwater chemistry associated with a residual hydrocarbon source zones in different stages of aging. The data show a direct relationship between concentrations of benzene and naphthalene in the residual oil and those measured in water samples collected below the oil. Groundwater associated with oil near the spill site had different chemical composition compared with water associated with oil that had spread downgradient from the spill zone, indicating a shift in biodegradation reactions. These results emphasize that source zone processes are spatially and temporally heterogeneous and should be accounted for in natural attenuation studies where residual source zones persist.

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USGS2021

Arsenic in petroleum-contaminated groundwater near Bemidji, Minnesota is predicted to persist for centuries

We used a reactive transport model to investigate the cycling of geogenic arsenic (As) in a petroleum-contaminated aquifer. We simulated As mobilization and sequestration using surface complexation reactions with Fe(OH) 3 during petroleum biodegradation coupled with Fe-reduction. Model results predict that dissolved As in the plume will exceed the U.S. and EU 10 µg/L drinking water standard for ~400 years. Non-volatile dissolved organic carbon (NVDOC) in the model promotes As mobilization by exerting oxygen demand, which maintains anoxic conditions in the aquifer. After NVDOC degrades, As re-associates with Fe(OH) 3 as oxygenated conditions are re-established. Over the 400-year simulation, As transport resembles a “roll front” in which: (1) arsenic sorbed to Fe(OH) 3 is released during Fe-reduction coupled to petroleum biodegradation; (2) dissolved As resorbs to Fe(OH) 3 at the plume’s leading edge; and (3) over time, the plume expands, and resorbed As is re-released into groundwater. This “roll front” behavior underscores the transience of sorption as an As attenuation mechanism. Over the plume’s lifespan, simulations suggest that As will contaminate more groundwater than benzene from the oil spill. At its maximum, the model simulates that ~5.7× more groundwater will be contaminated by As than benzene, suggesting that As could pose a greater long-term water quality threat than benzene in this petroleum-contaminated aquifer.

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USGS2021

Hydrocarbons to carboxyl-rich alicyclic molecules: A continuum model to describe biodegradation of petroleum-derived dissolved organic matter in contaminated groundwater plumes

Relationships between dissolved organic matter (DOM) reactivity and chemical composition in a groundwater plume containing petroleum-derived DOM (DOM HC ) were examined by quantitative and qualitative measurements to determine the source and chemical composition of the compounds that persist downgradient. Samples were collected from a transect down the core of the plume in the direction of groundwater flow. An exponential decrease in dissolved organic carbon concentration resulting from biodegradation along the transect correlated with a continuous shift in fluorescent DOM HC from shorter to longer wavelengths. Moreover, ultrahigh resolution mass spectrometry showed a shift from low molecular weight (MW) aliphatic, reduced compounds to high MW, unsaturated (alicyclic/aromatic), high oxygen compounds that are consistent with carboxyl-rich alicyclic molecules. The degree of condensed aromaticity increased downgradient, indicating that compounds with larger, conjugated aromatic core structures were less susceptible to biodegradation. Nuclear magnetic resonance spectroscopy showed a decrease in alkyl (particularly methyl) and an increase in aromatic/olefinic structural motifs. Collectively, data obtained from the combination of these complementary analytical techniques indicated that changes in the DOM HC composition of a groundwater plume are gradual, as relatively low molecular weight (MW), reduced, aliphatic compounds from the oil source were selectively degraded and high MW, alicyclic/aromatic, oxidized compounds persisted.

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USGS2020

Effects of a crude-oil recovery remediation system operated 1999–2003 on groundwater plumes and unsaturated-zone vapor concentrations at a crude-oil spill site near Bemidji, Minnesota

A crude-oil spill occurred in 1979 when a pipeline burst near Bemidji, Minnesota. More than 70 percent of the 1.7 million liters of spilled crude oil was removed shortly thereafter. In response to a requirement by the State regulatory agency to remove the remaining crude to a sheen in all wells, in 1998, the pipeline company installed a dual-pump recovery system at the site. This additional remediation from 1999 to 2003 resulted in removal of about 115,000 liters of crude oil, representing between 36 and 41 percent of the volume of oil (281,000–317,000 liters) estimated to be present in 1998. Effects of the 1999–2003 remediation on groundwater plumes and unsaturated-zone vapor concentrations were evaluated by the U.S. Geological Survey using several methods including measurements of oil thicknesses in wells; field water-quality properties of dissolved oxygen, specific conductance, temperature, and pH in groundwater; and vapor concentrations of methane, carbon dioxide, nitrogen, and oxygen in the unsaturated zone. Although the recovery system decreased oil thicknesses near the remediation wells, average oil thicknesses measured in all wells at the site were not reduced substantially. Dissolved oxygen and specific conductance measurements indicate that a secondary plume was created during the remediation, caused by the disposal of pumped water from the remediation wells in an upgradient infiltration gallery. This plume expanded rapidly immediately after the start of the remediation in 1999, resulting in expansion of the anoxic zone of groundwater upgradient and beneath the existing natural attenuation plume. Beginning in 2000–1, for example, specific conductance concentrations noticeably increased in many wells at the north oil pool from about 400 to more than 700 microsiemens per centimeter. The rapid expansion of the anoxic and elevated specific conductance plume indicates that the remediation contributed substantial amounts of biodegradable dissolved organic carbon to groundwater through the infiltration gallery. The trends in vapor data collected before, during, and after the remediation generally support the research hypothesis that crude-oil removal would have an insignificant effect on vapor concentrations in the unsaturated zone. Although there were some small changes in the concentration of methane, carbon dioxide, nitrogen, and oxygen in the unsaturated zone, these changes were not coincident with the beginning or cessation of the remediation and are therefore thought to be the result of other factors affecting biodegradation rates. A decrease in methane concentrations in one representative well, for example, is thought to be the result of reduced rates of biodegradation and methane production from the increasingly more weathered crude oil. Oil-phase recovery at this site was determined to be challenging and resulted in considerable volumes of mobile and entrapped oil remaining in the subsurface despite remediation efforts.

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USGS2020

Biological effects of hydrocarbon degradation intermediates: Is the total petroleum hydrocarbon analytical method adequate for risk assessment?

In crude oil contaminant plumes, the dissolved organic carbon (DOC) is mainly hydrocarbon degradation intermediates only partly quantified by the diesel range total petroleum hydrocarbon (TPHd) method. To understand potential biological effects of degradation intermediates, we tested three fractions of DOC: (1) solid-phase extract (HLB); (2) dichloromethane (DCM-total) extract used in TPHd; and (3) DCM extract with hydrocarbons isolated by silica gel cleanup (DCM-SGC). Bioactivity of extracts from five wells spanning a range of DOC was tested using an in vitro multiplex reporter system that evaluates modulation of the activity of 46 transcription factors; extracts were evaluated at concentrations equivalent to the well water samples. The aryl hydrocarbon receptor (AhR) and pregnane X receptor (PXR) transcription factors showed the greatest upregulation, with HLB exceeding DCM-total, and no upregulation in the hydrocarbon fraction (DCM-SGC). The HLB extracts were further studied with HepG2 chemically activated luciferase expression (CALUX) in vitro assays at nine concentrations ranging from 40 to 0.01 times the well water concentrations. Responses decreased with distance from the source but were still present at two wells without detectable hydrocarbons. Thus, our in vitro assay results indicate that risks associated with degradation intermediates of hydrocarbons in groundwater will be underestimated when protocols that remove these chemicals are employed.

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USGS2019

Geophysical mapping of plume discharge to surface water at a crude oil spill site: Inversion versus machine learning

The interpretation of geophysical survey results to answer hydrologic, engineering, and geologic questions is critical to diverse problems for management of water, energy, and mineral resources. Although geophysical images provide valuable qualitative insight into subsurface architecture and conditions, translating geophysical images into quantitative information (e.g., saturation, concentration, and hydraulic properties) often involves substantial nonuniqueness and uncertainty owing to the limited resolution of geophysical imaging and uncertainty in petrophysical relations. We have developed a machine-learning approach to address these challenges in the context of a field-based investigation to map zones where a hydrocarbon plume was discharging to surface water at the National Crude Oil Spill Fate and Natural Attenuation Research Site in Bemidji, Minnesota, USA. The two-step approach combines multiple types of geophysical and direct information and effectively bypasses inversion and its associated assumptions. Integrating multifrequency electromagnetic induction, ground-penetrating radar, and fluid-sampling data, we first identify discharge zones and second estimate specific conductance versus depth. Compared with conventional inversion results, the machine-learning results (1) directly address the study objectives (delineating the discharge zones); (2) better extract depth-dependent information from the data, for which sensitivity diminishes rapidly with depth; and (3) quantify the uncertainty of the predictions (i.e., discharge versus nondischarge zones), rather than the uncertainty of the geophysical estimates (i.e., the standard error of estimation for the logarithm of electrical conductivity).

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USGS2019

Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill

A long-term study of a 40-year-old crude oil spill provides insights about petroleum hydrocarbon natural attenuation processes and rates. In the source zone, fermentation coupled to methanogenesis is the dominant natural source zone depletion (NSZD) process, and most of the carbon mass exits the surface as CO2 efflux. Monitored natural attenuation (MNA) of the groundwater plume shows that benzene degradation is coupled to iron reduction and that the benzene plume is stable. A plume of hydrocarbon oxidation products measured as nonvolatile dissolved organic carbon (NVDOC) expanded ~20 m in 20 years. Most of the NVDOC is biodegraded by 200 m from the source, but optical data suggest there are components that persists for 300 m. Biological effects screening indicates decreasing biological effects with distance from the source.

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USGS2018

Relative contributions of microbial and infrastructure heat at a crude oil-contaminated site

Biodegradation of contaminants can increase the temperature in the subsurface due to heat generated from exothermic reactions, making temperature observations a potentially low-cost approach for determining microbial activity. For this technique to gain more widespread acceptance, it is necessary to better understand all the factors affecting the measured temperatures. Biodegradation has been occurring at a crude oil-contaminated site near Bemidji, Minnesota for 39 years, creating a quasi-steady-state plume of contaminants and degradation products. A model of subsurface heat generation and transport helps elucidate the contribution of microbial and infrastructure heating to observed temperature increases at this site. We created a steady-state, two-dimensional, heat transport model using previous-published parameter values for physical, chemical and biodegradation properties. Simulated temperature distributions closely match the observed average annual temperatures measured in the contaminated area at the site within less than 0.2 °C in the unsaturated zone and 0.4 °C in the saturated zone. The model results confirm that the observed subsurface heat from microbial activity is due primarily to methane oxidation in the unsaturated zone resulting in a 3.6 °C increase in average annual temperature. Another important source of subsurface heat is from the active, crude-oil pipelines crossing the site. The pipelines impact temperatures for a distance of 200 m and contribute half the heat. Model results show that not accounting for the heat from the pipelines leads to overestimating the degradation rates by a factor of 1.7, demonstrating the importance of identifying and quantifying all heat sources. The model results also highlighted a zone where previously unknown microbial activity is occurring at the site.

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USGS2017

Weathering of oil in a surficial aquifer

The composition of crude oil in a surficial aquifer was determined in two locations at the Bemidji, MN, spill site. The abundances of 71 individual hydrocarbons varied within 16 locations sampled. Little depletion of these hydrocarbons (relative to the pipeline oil) occurred in the first 10 years after the spill, whereas losses of 25% to 85% of the total measured hydrocarbons occurred after 30 years. The C 6‐30 n ‐alkanes, toluene, and o ‐xylene were the most depleted hydrocarbons. Some hydrocarbons, such as the n‐ C 10–24 cyclohexanes, tri‐ and tetra‐ methylbenzenes, acyclic isoprenoids, and naphthalenes were the least depleted. Benzene was detected at every sampling location 30 years after the spill. Degradation of the oil led to increases in the percent organic carbon and in the δ 13 C of the oil. Another method of determining hydrocarbon loss was by normalizing the total measured hydrocarbon concentrations to that of the most conservative analytes. This method indicated that the total measured hydrocarbons were depleted by 47% to 77% and loss of the oil mass over 30 years was 18% to 31%. Differences in hydrocarbon depletion were related to the depth of the oil in the aquifer, local topography, amount of recharge reaching the oil, availability of electron acceptors, and the presence of less permeable soils above the oil. The results from this study indicate that once crude oil has been in the subsurface for a number of years there is no longer a “starting oil concentration” that can be used to understand processes that affect its fate and the transport of hydrocarbons in groundwater.

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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.

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USGS2016

Molecular-level evidence provided by ultrahigh resolution mass spectrometry for oil-derived doc in groundwater at Bemidji, Minnesota

Dissolved organic matter samples extracted from ground water at the USGS Bemidji oil spill site in Minnesota were investigated by ultrahigh resolution mass spectrometry. Principle component analysis (PCA) of the elemental composition assignments of the samples showed that the score plots for the contaminated sites were well separated from those for the uncontaminated sites. Additionally, spectra obtained from the same sampling site 7 and 19 years after the spill were grouped together in the score plot, strongly suggesting a steady state of contamination within the 12 year interval. The double bond equivalence (DBE) of O x class compounds was broader for the samples from the contaminated sites, because of the complex nature of oil and the consequent formation of compounds with saturated and/or aromatic structures from the oxygenated products of oil. In addition, O x class compounds with a relatively smaller number of x (x < 8; x = number of oxygen) and O x S 1 class compounds were more abundant in the samples from the contaminated sites, because of the lower oxygen and higher sulfur contents of the oil compared to humic substances. The molecular-level signatures presented here can be a fundamental basis for in-depth analysis of oil contamination.

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USGS2015

Probing the carbonyl functionality of a petroleum resin and asphaltene through oximation and schiff base formation in conjunction with N-15 NMR

Despite recent advances in spectroscopic techniques, there is uncertainty regarding the nature of the carbonyl groups in the asphaltene and resin fractions of crude oil, information necessary for an understanding of the physical properties and environmental fate of these materials. Carbonyl and hydroxyl group functionalities are not observed in natural abundance 13 C nuclear magnetic resonance (NMR) spectra of asphaltenes and resins and therefore require spin labeling techniques for detection. In this study, the carbonyl functionalities of the resin and asphaltene fractions from a light aliphatic crude oil that is the source of groundwater contamination at the long term USGS study site near Bemidji, Minnesota, have been examined through reaction with 15 N-labeled hydroxylamine and aniline in conjunction with analysis by solid and liquid state 15 N NMR. Ketone groups were revealed through 15 N NMR detection of their oxime and Schiff base derivatives, and esters through their hydroxamic acid derivatives. Anilinohydroquinone adducts provided evidence for quinones. Some possible configurations of the ketone groups in the resin and asphaltene fractions can be inferred from a consideration of the likely reactions that lead to heterocyclic condensation products with aniline and to the Beckmann reaction products from the initially formed oximes. These include aromatic ketones and ketones adjacent to quaternary carbon centers, β-hydroxyketones, β-diketones, and β-ketoesters. In a solid state cross polarization/magic angle spinning (CP/MAS) 15 N NMR spectrum recorded on the underivatized asphaltene as a control, carbazole and pyrrole-like nitrogens were the major naturally abundant nitrogens detected.

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USGS2015

Reactive transport modeling of geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN

Anaerobic biodegradation of organic amendments and contaminants in aquifers can trigger secondary water quality impacts that impair groundwater resources. Reactive transport models help elucidate how diverse geochemical reactions control the spatiotemporal evolution of these impacts. Using extensive monitoring data from a crude oil spill site near Bemidji, Minnesota (USA), we implemented a comprehensive model that simulates secondary plumes of depleted dissolved O 2 and elevated concentrations of Mn 2+ , Fe 2+ , CH 4 , and Ca 2+ over a two-dimensional cross section for 30 years following the spill. The model produces observed changes by representing multiple oil constituents and coupled carbonate and hydroxide chemistry. The model includes reactions with carbonates and Fe and Mn mineral phases, outgassing of CH 4 and CO 2 gas phases, and sorption of Fe, Mn, and H + . Model results demonstrate that most of the carbon loss from the oil (70%) occurs through direct outgassing from the oil source zone, greatly limiting the amount of CH 4 cycled down-gradient. The vast majority of reduced Fe is strongly attenuated on sediments, with most (91%) in the sorbed form in the model. Ferrous carbonates constitute a small fraction of the reduced Fe in simulations, but may be important for furthering the reduction of ferric oxides. The combined effect of concomitant redox reactions, sorption, and dissolved CO 2 inputs from source-zone degradation successfully reproduced observed pH. The model demonstrates that secondary water quality impacts may depend strongly on organic carbon properties, and impacts may decrease due to sorption and direct outgassing from the source zone.

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USGS2015

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.

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USGS2015

Relating subsurface temperature changes to microbial activity at a crude oil-contaminated site

Crude oil at a spill site near Bemidji, Minnesota has been undergoing aerobic and anaerobic biodegradation for over 30 years, creating a 150–200 m plume of primary and secondary contaminants. Microbial degradation generates heat that should be measurable under the right conditions. To measure this heat, thermistors were installed in wells in the saturated zone and in water-filled monitoring tubes in the unsaturated zone. In the saturated zone, a thermal groundwater plume originates near the residual oil body with temperatures ranging from 2.9 °C above background near the oil to 1.2 °C down gradient. Temperatures in the unsaturated zone above the oil body were up to 2.7 °C more than background temperatures. Previous work at this site has shown that methane produced from biodegradation of the oil migrates upward and is oxidized in a methanotrophic zone midway between the water table and the surface. Enthalpy calculations and observations demonstrate that the temperature increases primarily result from aerobic methane oxidation in the unsaturated zone above the oil. Methane oxidation rates at the site independently estimated from surface CO 2 efflux data are comparable to rates estimated from the observed temperature increases. The results indicate that temperature may be useful as a low-cost measure of activity but care is required to account for the correct heat-generating reactions, other heat sources and the effects of focused recharge.

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USGS2014

Insights into biodegradation through depth-resolved microbial community functional and structural profiling of a crude-oil contaminant plume

Small-scale geochemical gradients are a key feature of aquifer contaminant plumes, highlighting the need for functional and structural profiling of corresponding microbial communities on a similar scale. The purpose of this study was to characterize the microbial functional and structural diversity with depth across representative redox zones of a hydrocarbon plume and an adjacent wetland, at the Bemidji Oil Spill site. A combination of quantitative PCR, denaturing gradient gel electrophoresis, and pyrosequencing were applied to vertically sampled sediment cores. Levels of the methanogenic marker gene, methyl coenzyme-M reductase A (mcrA), increased with depth near the oil body center, but were variable with depth further downgradient. Benzoate degradation N (bzdN) hydrocarbon-degradation gene, common to facultatively anaerobic Azoarcus spp., was found at all locations, but was highest near the oil body center. Microbial community structural differences were observed across sediment cores, and bacterial classes containing known hydrocarbon degraders were found to be low in relative abundance. Depth-resolved functional and structural profiling revealed the strongest gradients in the iron-reducing zone, displaying the greatest variability with depth. This study provides important insight into biogeochemical characteristics in different regions of contaminant plumes, which will aid in improving models of contaminant fate and natural attenuation rates.

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