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Barbara A. Bekins

Publications and source records attributed to Barbara A. Bekins.

At least 55 records · Page 3Linked to original sources
USGS2011

Permeability-porosity relationships of subduction zone sediments

Permeability–porosity relationships for sediments from the northern Barbados, Costa Rica, Nankai, and Peru subduction zones were examined based on sediment type, grain size distribution, and general mechanical and chemical compaction history. Greater correlation was observed between permeability and porosity in siliciclastic sediments, diatom oozes, and nannofossil chalks than in nannofossil oozes. For siliciclastic sediments, grouping of sediments by percentage of clay-sized material yields relationships that are generally consistent with results from other marine settings and suggests decreasing permeability as percentage of clay-sized material increases. Correction of measured porosities for smectite content improved the correlation of permeability–porosity relationships for siliciclastic sediments and diatom oozes. The relationship between permeability and porosity for diatom oozes is very similar to the relationship in siliciclastic sediments, and permeabilities of both sediment types are related to the amount of clay-size particles. In contrast, nannofossil oozes have higher permeability values by 1.5 orders of magnitude than siliciclastic sediments of the same porosity and show poor correlation between permeability and porosity. More indurated calcareous sediments, nannofossil chalks, overlap siliciclastic permeabilities at the lower end of their measured permeability range, suggesting similar consolidation patterns at depth. Thus, the lack of correlation between permeability and porosity for nannofossil oozes is likely related to variations in mechanical and chemical compaction at shallow depths. This study provides the foundation for a much-needed global database with fundamental properties that relate to permeability in marine settings. Further progress in delineating controls on permeability requires additional carefully documented permeability measurements on well-characterized samples.

Marine Geology↗
USGS2011

Long-term natural attenuation of crude oil in the subsurface

The time frame for natural attenuation of crude oil contamination in the subsurface has been studied for the last 27 years at a spill site located near Bemidji, Minnesota, USA. Data from the groundwater contaminant plume show that dissolved benzene concentrations adjacent to the oil decreased by 50% between 1993 and 2007. To assess how this decrease is related to benzene concentrations in the crude oil, samples of oil were bailed from floating oil in five wells and analysed for volatile components. Compared to reference oil collected from the pipeline in 1984, benzene concentrations in the well located farthest downgradient in the oil have decreased an average of 50%. Benzene and ethylbenzene depletion are linearly correlated with oil saturation in the pore space suggesting that dissolution is the primary removal mechanism and biodegradation within the oil body is insignificant.

Conference Paper↗
USGS2010

Mixing effects on apparent reaction rates and isotope fractionation during denitrification in a heterogeneous aquifer

Gradients in contaminant concentrations and isotopic compositions commonly are used to derive reaction parameters for natural attenuation in aquifers. Differences between field‐scale (apparent) estimated reaction rates and isotopic fractionations and local‐scale (intrinsic) effects are poorly understood for complex natural systems. For a heterogeneous alluvial fan aquifer, numerical models and field observations were used to study the effects of physical heterogeneity on reaction parameter estimates. Field measurements included major ions, age tracers, stable isotopes, and dissolved gases. Parameters were estimated for the O 2 reduction rate, denitrification rate, O 2 threshold for denitrification, and stable N isotope fractionation during denitrification. For multiple geostatistical realizations of the aquifer, inverse modeling was used to establish reactive transport simulations that were consistent with field observations and served as a basis for numerical experiments to compare sample‐based estimates of “apparent” parameters with “true“ (intrinsic) values. For this aquifer, non‐Gaussian dispersion reduced the magnitudes of apparent reaction rates and isotope fractionations to a greater extent than Gaussian mixing alone. Apparent and true rate constants and fractionation parameters can differ by an order of magnitude or more, especially for samples subject to slow transport, long travel times, or rapid reactions. The effect of mixing on apparent N isotope fractionation potentially explains differences between previous laboratory and field estimates. Similarly, predicted effects on apparent O 2 threshold values for denitrification are consistent with previous reports of higher values in aquifers than in the laboratory. These results show that hydrogeological complexity substantially influences the interpretation and prediction of reactive transport.

Water Resources Research↗
USGS2010

Simulation of aerobic and anaerobic biodegradation processes at a crude oil spill site

A two-dimensional, multispecies reactive solute transport model with sequential aerobic and anaerobic degradation processes was developed and tested. The model was used to study the field-scale solute transport and degradation processes at the Bemidji, Minnesota, crude oil spill site. The simulations included the biodegradation of volatile and nonvolatile fractions of dissolved organic carbon by aerobic processes, manganese and iron reduction, and methanogenesis. Model parameter estimates were constrained by published Monod kinetic parameters, theoretical yield estimates, and field biomass measurements. Despite the considerable uncertainty in the model parameter estimates, results of simulations reproduced the general features of the observed groundwater plume and the measured bacterial concentrations. In the simulation, 46% of the total dissolved organic carbon (TDOC) introduced into the aquifer was degraded. Aerobic degradation accounted for 40% of the TDOC degraded. Anaerobic processes accounted for the remaining 60% of degradation of TDOC: 5% by Mn reduction, 19% by Fe reduction, and 36% by methanogenesis. Thus anaerobic processes account for more than half of the removal of DOC at this site.

Water Resources Research↗
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↗
USGS2010

Sustainability of natural attenuation of nitrate in agricultural aquifers

Increased concentrations of nitrate in groundwater in agricultural areas, coinciding with increased use of chemical and organic fertilizers, have raised concern because of risks to environmental and human health. At some sites, these problems are mitigated by natural attenuation of nitrate as a result of microbially mediated reactions. Results from U.S. Geological Survey (USGS) research under the National Water-Quality Assessment (NAWQA) program show that reactions of dissolved nitrate with solid aquifer minerals and organic carbon help lower nitrate concentrations in groundwater beneath agricultural fields. However, increased fluxes of nitrate cause ongoing depletion of the finite pool of solid reactants. Consumption of the solid reactants diminishes the capacity of the aquifer to remove nitrate, calling into question the long-term sustainability of these natural attenuation processes.

Fact Sheet↗
USGS2010

Fate and transport of petroleum hydrocarbons in the subsurface near Cass Lake, Minnesota

The U.S. Geological Survey (USGS) investigated the natural attenuation of subsurface petroleum hydrocarbons leaked over an unknown number of years from an oil pipeline under the Enbridge Energy Limited Partnership South Cass Lake Pumping Station, in Cass Lake, Minnesota. Three weeks of field work conducted between May 2007 and July 2008 delineated a dissolved plume of aromatic hydrocarbons and characterized the biodegradation processes of the petroleum. Field activities included installing monitoring wells, collecting sediment cores, sampling water from wells, and measuring water-table elevations. Geochemical measurements included concentrations of constituents in both spilled and pipeline oil, dissolved alkylbenzenes and redox constituents, sediment bioavailable iron, and aquifer microbial populations. Groundwater in this area flows east-southeast at approximately 26 meters per year. Results from the oil analyses indicate a high degree of biodegradation, characterized by nearly complete absence of n-alkanes. Cass Lake oil samples were more degraded than two oil samples collected in 2008 from the similarly contaminated USGS Bemidji, Minnesota, research site 40 kilometers away. Based on 19 ratios developed for comparing oil sources, the conclusion is that the oils at the two sites appear to be from the same hydrocarbon source. In the Cass Lake groundwater plume, benzene concentrations decrease by three orders of magnitude within 150 meters (m) downgradient from the oil body floating on the water table (between well MW-10 and USGS-4 well nest). The depths of the highest benzene concentrations increase with distance downgradient from the oil, a condition typical of plumes in shallow, unconfined aquifers. Background groundwater, which is nearly saturated with oxygen, becomes almost entirely anaerobic in the plume. As at the Bemidji site, the most important biodegradation processes are anaerobic and dominated by iron reduction. The similarity between the Cass Lake and Bemidji benzene degradation rates, redox conditions, and aquifer material all support a hypothesis that the Cass Lake plume, like the Bemidji plume, is decades old. As concentrations of alkylbenzenes in the oil decrease over time, the benzene concentrations in the groundwater plume will also decrease and the plume is expected to shrink. The Fox Creek wetland, about 250 m south of the Cass Lake site, is the nearest receptor to the south.

Scientific Investigations Report↗
USGS2009

Crude oil at the Bemidji Site: 25 years of monitoring, modeling, and understanding

The fate of hydrocarbons in the subsurface near Bemidji, Minnesota, has been investigated by a multidisciplinary group of scientists for over a quarter century. Research at Bemidji has involved extensive investigations of multiphase flow and transport, volatilization, dissolution, geochemical interactions, microbial populations, and biodegradation with the goal of providing an improved understanding of the natural processes limiting the extent of hydrocarbon contamination. A considerable volume of oil remains in the subsurface today despite 30 years of natural attenuation and 5 years of pump‐and‐skim remediation. Studies at Bemidji were among the first to document the importance of anaerobic biodegradation processes for hydrocarbon removal and remediation by natural attenuation. Spatial variability of hydraulic properties was observed to influence subsurface oil and water flow, vapor diffusion, and the progression of biodegradation. Pore‐scale capillary pressure‐saturation hysteresis and the presence of fine‐grained sediments impeded oil flow, causing entrapment and relatively large residual oil saturations. Hydrocarbon attenuation and plume extent was a function of groundwater flow, compound‐specific volatilization, dissolution and biodegradation rates, and availability of electron acceptors. Simulation of hydrocarbon fate and transport affirmed concepts developed from field observations, and provided estimates of field‐scale reaction rates and hydrocarbon mass balance. Long‐term field studies at Bemidji have illustrated that the fate of hydrocarbons evolves with time, and a snap‐shot study of a hydrocarbon plume may not provide information that is of relevance to the long‐term behavior of the plume during natural attenuation.

Minnesota↗
USGS2007

Forensic fingerprinting of oil-spill hydrocarbons in a methanogenic environment-Mandan, ND and Bemidji, MN

In recent decades forensic fingerprinting of oil-spill hydrocarbons has emerged as an important tool for correlating oils and for evaluating their source and character. Two long-term hydrocarbon spills, an off-road diesel spill (Mandan, ND) and a crude oil spill (Bemidji, MN) experiencing methanogenic biodegradation were previously shown to be undergoing an unexpected progression of homologous n-alkane and n-alkylated cyclohexane loss. Both exhibited degradative losses proceeding from the high-molecular-weight end of the distributions, along with transitory concentration increases of lower-molecular-weight homologs. Particularly in the case of the diesel fuel spill, these methanogenic degradative patterns can result in series distributions that mimic lower cut refinery fuels or admixture with lower cut fuels. Forensic fingerprinting in this long-term spill must therefore rely on more recalcitrant series, such as polycyclic aromatic hydrocarbon or drimane sesquiterpane profiles, to prove if the spilled oil is single-sourced or whether there is verifiable admixture with other extraneous refinery fuels. Degradation processes impacting n-alkanes and n-alkylated ring compounds, which make these compounds unsuitable for fingerprinting, nevertheless are of interest in understanding methanogenic biodegradation.

Environmental Forensics↗
USGS2006

Natural attenuation strategy for groundwater cleanup focuses on demonstrating cause and effect

In the 20 years since cleanup of contaminated groundwater has been a high priority in the United States, recognition of both the scope of the problem and the technical difficulties involved has grown steadily. Estimates of the number of hazardous waste sites where groundwater may be contaminated vary between 300,000 and 400,000 nationwide [NRC, 1994]. Legislation passed in the 1980s by Congress and the states generally required that groundwater in contaminated aquifers be restored to background or drinking water standards. Unfortunately, attempts to meet these goals using conventional methods, such as pump and treat systems, frequently have been unsuccessful [NRC, 1994].

Eos, Transactions, American Geophysical Union↗
USGS2005

Percolation and transport in a sandy soil under a natural hydraulic gradient

Unsaturated flow and transport under a natural hydraulic gradient in a Mediterranean climate were investigated with a field tracer experiment combined with laboratory analyses and numerical modeling. Bromide was applied to the surface of a sandy soil during the dry season. During the subsequent rainy season, repeated sediment sampling tracked the movement of bromide through the profile. Analysis of data on moisture content, matric pressure, unsaturated hydraulic conductivity, bulk density, and soil texture and structure provides insights into parameterization and use of the advective‐dispersive modeling approach. Capturing the gross features of tracer and moisture movement with model simulations required an order‐of‐magnitude increase in laboratory‐measured hydraulic conductivity. Wetting curve characteristics better represented field results, calling into question the routine estimation of hydraulic characteristics based only on drying conditions. Measured increases in profile moisture exceeded cumulative precipitation in early winter, indicating that gains from dew drip can exceed losses from evapotranspiration during periods of heavy (“Tule”) fog. A single‐continuum advective‐dispersive modeling approach could not reproduce a peak of bromide that was retained near the soil surface for over 3 years. Modeling of this feature required slow exchange of solute at a transfer rate of 0.5–1 × 10 −4 d −1 with an immobile volume approaching the residual moisture content.

Water Resources Research↗
USGS2005

Data report: Permeabilities of eastern equatorial Pacific and Peru margin sediments

Constant-flow permeability tests were conducted on core samples from Ocean Drilling Program Leg 201 from the eastern equatorial Pacific and the Peru margin. Eighteen whole-round core samples from Sites 1225, 1226, 1227, 1230, and 1231 were tested for vertical permeabilities. Sites 1225, 1226, and 1231 represent sediments of the open ocean, whereas Sites 1227 and 1230 represent sediments of the ocean margin. Measured vertical permeabilities vary from ~8 x 10 –19 m 2 to ~1 x 10 –16 m 2 for a porosity range of 45%–90%.

Proceedings of the Ocean Drilling Program: Scienti↗
USGS2005

Use of dissolved and vapor‐phase gases to investigate methanogenic degradation of petroleum hydrocarbon contamination in the subsurface

At many sites contaminated with petroleum hydrocarbons, methanogenesis is a significant degradation pathway. Techniques to estimate CH 4 production, consumption, and transport processes are needed to understand the geochemical system, provide a complete carbon mass balance, and quantify the hydrocarbon degradation rate. Dissolved and vapor‐phase gas data collected at a petroleum hydrocarbon contaminated site near Bemidji, Minnesota, demonstrate that naturally occurring nonreactive or relatively inert gases such as Ar and N 2 can be effectively used to better understand and quantify physical and chemical processes related to methanogenic activity in the subsurface. In the vadose zone, regions of Ar and N 2 depletion and enrichment are indicative of methanogenic and methanotrophic zones, and concentration gradients between the regions suggest that reaction‐induced advection can be an important gas transport process. In the saturated zone, dissolved Ar and N 2 concentrations are used to quantify degassing driven by methanogenesis and also suggest that attenuation of methane along the flow path, into the downgradient aquifer, is largely controlled by physical processes. Slight but discernable preferential depletion of N 2 over Ar, in both the saturated and unsaturated zones near the free‐phase oil, suggests reactivity of N 2 and is consistent with other evidence indicating that nitrogen fixation by microbial activity is taking place at this site.

Water Resources Research↗
USGS2005

Evaluation of unsaturated-zone solute-transport models for studies of agricultural chemicals

Seven unsaturated-zone solute-transport models were tested with two data sets to select models for use by the Agricultural Chemical Team of the U.S. Geological Survey's National Water-Quality Assessment Program. The data sets were from a bromide tracer test near Merced, California, and an atrazine study in the White River Basin, Indiana. In this study the models are designated either as complex or simple based on the water flux algorithm. The complex models, HYDRUS2D, LEACHP, RZWQM, and VS2DT, use Richards' equation to simulate water flux and are well suited to process understanding. The simple models, CALF, GLEAMS, and PRZM, use a tipping-bucket algorithm and are more amenable to extrapolation because they require fewer input parameters. The purpose of this report is not to endorse a particular model, but to describe useful features, potential capabilities, and possible limitations that emerged from working with the model input data sets. More rigorous assessment of model applicability involves proper calibration, which was beyond the scope of this study. Uncalibrated ("cold") simulations were run using all seven models to predict the transport of bromide (Merced) and the transport and fate of atrazine and three of its transformation products (White River Basin). Among the complex models, HYDRUS2D successfully predicted both the surface retention and accumulation of bromide at depth at the Merced site, whereas RZWQM and VS2DT predicted only the latter. RZWQM predictions of atrazine were closest to observed values at the White River Basin site, where preferential flow has been observed. LEACHP predicted smaller solute concentrations than observed at both the Merced and White River Basin sites. Among the simple models, CALF predicted the highest values of atrazine and deethylatrazine at the measurement depth of 1.5 meters. CALF includes the Addiscott flow option for preferential flow, and also accepts user-specified dispersivity. PRZM underpredicted solute concentrations, probably because control of dispersion is a problem with this model. GLEAMS has a maximum simulation depth of 1.5 meters, which is limiting for mass-balance purposes because it creates a potential disconnect between unsaturated-zone transport and the water table. Of the models tested, RZWQM, HYDRUS2D, VS2DT, GLEAMS and PRZM had graphical user interfaces. Extensive documentation was available for RZWQM, HYDRUS2D, and VS2DT. RZWQM can explicitly simulate water and solute flux in macropores, and both HYDRUS2D and VS2DT can simulate water and solute flux in two dimensions. The version of RZWQM tested had a maximum simulation depth of 3 meters. The complex models simulate the formation, transport, and fate of degradates of up to three to five compounds including the parent, with the exception of VS2DT, which simulates the transport and fate of a single compound.

California, Indiana↗
USGS2004

Inhibition of acetoclastic methanogenesis in crude oil- and creosote-contaminated groundwater

Results from a series of studies of methanogenic processes in crude oil- and creosote-contaminated aquifers indicate that acetoclastic methanogenesis is inhibited near non-aqueous sources. At a crude oil-contaminated site, numbers of acetoclastic methanogens found close to crude oil were one hundred times fewer than those of hydrogen- and formate-utilizing methanogens. In laboratory toxicity assays, crude oil collected from the site inhibited methane production from acetate but not from formate or hydrogen. Toxicity assays with aqueous creosote extract completely inhibited acetate utilization over the range of tested dilutions but only mildly affected formate and hydrogen utilization. The combined results from the laboratory and field studies suggest that in methanogenic contaminated aquifers, inhibition of acetoclastic methanogenesis may lead to a buildup of acetate relative to dissolved organic carbon.

Florida, Minnesota↗