Ean Warren

Utah Geological Survey, Salt Lake City, Utah, United States

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Publications (18)14.01 Total impact

  • Journal of Contaminant Hydrology. 01/2014;
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    ABSTRACT: 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. Previous studies at the site showed that benzene and ethylbenzene undergo minimal degradation in the methanogenic zone of the plume while toluene and o-xylene degrade rapidly in this zone. Other studies have shown that degradation of benzene under methanogenic conditions occurs in some cases but is generally unreliable in the field. In this study concentrations of volatile components in the crude oil source were examined to determine if the observed benzene decrease near the oil source zone was due a change in the ability of the methanogenic microbial community to degrade benzene or long-term depletion of the oil source. Oil samples collected in 2008 had benzene concentrations ranging from 7-61% of values measured in archived oil representative of the spill consistent with depletion of the oil source. Several lines of evidence indicate that dissolution and conservative transport control the losses of benzene and ethylbenzene from the crude oil. Laboratory microcosms constructed using sediments from the methanogenic zone near the source and incubated for over 13 months with an anaerobic mineral salt solution spiked with ~2 mg/L benzene exhibited no benzene losses. Concentrations of benzene and ethylbenzene in oil samples collected from five wells were linearly correlated to interpolated maximum pore space oil saturations adjacent to each well (R2 =0.72 and 0.55 respectively), indicating that losses of these compounds from the oil were controlled by the relative permeability of groundwater through the oil body. Moreover benzene loss from the oil was greater than ethylbenzene, consistent with their relative aqueous solubilities. Losses of other oil compounds appear to be more strongly controlled by methanogenic degradation occurring in the source zone. Concentrations of these compounds, which include the n-alkanes, toluene, and o-xylene, correlate better with location in the oil body than with pore space oil saturation. Greater degradation rates occur below a topographic depression where focussing of surface runoff leads to an annual recharge rate of almost twice that of a nearby higher elevation site. The oxygen in the recharge over the source zone never reaches the oil at the water table because it is rapidly consumed in the vadose zone by aerobic methanotrophs oxidizing methane produced from oil degradation in the source zone. Other electron acceptors including nitrate and sulphate are insignificant at this site. The data suggest that transport by recharge of the growth nutrients phosphorus and nitrogen is the explanation for the higher degradation rates of the oil components in the focussed recharge area.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Crude oil contamination from a 1979 oil spill near Bemidji, Minnesota resulted in a subsurface oil body and a dissolved hydrocarbon plume in the groundwater. Benzene concentrations in the plume adjacent to the oil decreased from nearly 5 mg/L in 1993 to less than 3 mg/L in 2007. Benzene depletion within the plume and oil body was investigated with a microcosm study and analyses of the benzene content remaining in the oil. To test if the methanogenic microbial consortium adjacent to the oil is capable of degrading benzene, eight anaerobic microcosms were constructed with sediment from a methanogenic area of the plume that has been exposed to benzene. Microcosms were constructed in 120 mL serum bottles containing 60 g of anaerobic sediment, 20 mL pre-reduced mineral salts solution, and spiked with benzene to attain a target concentration of 2 mg/L. The methanogenic inhibitor, 2-bromoethanesulfonate (BES), was added to four bottles to achieve a concentration of 13.5 g/L. One bottle containing BES and another not containing BES were autoclaved. Over a year the average aqueous benzene concentrations decreased 2.9±0.4 mug/L-day in the inhibited microcosms, 3.1±0.3 mug/L-day in the uninhibited microcosms, and 2.1±0.5 mug/L-day in the autoclaved microcosms. Although the linear regressions slopes differed, the difference in the rates between the BES inhibited, uninhibited, and autoclaved treatments is not statistically significant with the existing data (p=0.19). Headspace analyses for methane conducted after 392 days showed concentrations in the uninhibited microcosms were about 40 times greater than the inhibited microcosms and autoclaved controls. The methane concentrations in the inhibited microcosm were lower than dissolved methane originally present in the pore water of the added sediment. Oil samples bailed from four wells in the oil body were analyzed for benzene concentrations and normalized to the benzene content of an archived sample representative of the original spilled oil. Relative benzene content in the four oil samples ranged from 7-61% of the original oil. The benzene percentage was compared with pore space oil saturations determined from sediment cores. Results show that benzene depletion is linearly correlated (R2 =0.93) with oil saturations over a range of saturations from 8 to 62%. The linear relation suggests that the relative permeability for water flowing past the oil controls the benzene depletion. That is, the more oily sediments had lower permeabilities resulting in less dissolution and higher benzene concentrations relative to less oily sediments. The results of the oil analyses and microcosm experiment suggest dissolution, not methanogenic biodegradation, is the main control on the decrease of benzene concentration in the oil body and the adjacent plume.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: Surface hydrologic properties controlling groundwater recharge can have a large effect on biodegradation rates in the subsurface. Two studies of crude oil contamination show that degradation rates are dramatically increased where recharge rates are enhanced. The first site, located near Bemidji, Minnesota, was contaminated in August, 1979 when oil from a pipeline rupture infiltrated into a surficial glacial outwash aquifer. Discrete oil phases form three separate pools at the water table, the largest of which is 25x75 m at a depth of 6-8 m. Gas and water concentrations and microbial community data show that methanogenic conditions prevail in this oil pool. There is extreme spatial dependence in the degradation rates such that most of the n-alkanes have been degraded in the upgradient end, but in the downgradient end n-alkane concentrations are nearly unaltered from the original spill. Recharge rates through the two ends of the oil body were estimated using a water table fluctuation method. In 2002, the more degraded end received 15.2 cm of recharge contrasted to 10.7 cm at the less degraded end. The enhanced recharge is caused by topographic focusing of runoff toward a local depression. Microbial data using the Most Probable Number method show that the methanogen concentrations are 10-100 times greater in the more degraded end of the oil body suggesting that a growth nutrient is supplied by recharge. A decrease in partial pressure of N2 compared to Ar in the soil gas indicates nitrogen fixation probably meets N requirements (Amos et al., 2005, WRR, doi:10.1029/2004WR003433). Organic phosphorus is the main form of P in infiltrating pore water and concentration decreases with depth. The second site is located 40 km southeast of the Bemidji site at an oil pipeline pumping station near Cass Lake, Minnesota. This site was contaminated by oil leaking from a pipe coupling for an unknown duration of time between 1971 and 2002. The oil body at this site lies under a fenced area of the pumping station and is comparable in size to the largest Bemidji site oil pool. The oil is heavily degraded with complete loss of the n-alkane fraction suggesting that degradation is accelerated at this site. The pumping station is flat, gravel-covered, devoid of vegetation, and surrounded by a berm. Thus, the combined effects of no runoff, rapid infiltration, and zero transpiration all enhance recharge to the oil body. Recharge rates through the gravel yard and the adjacent forested area were estimated using a water table fluctuation method. Data for the first six months of 2010 showed that recharge below the gravel yard was 40% greater than below the forested area. Groundwater ammonia concentrations increase from 0.02 to 0.5 mmol/L under the oil body, while background NO3 is only 0.01 mmol/L and there is negligible N in the oil, again suggesting that N fixation meets N requirements. Combined, these studies suggest that enhanced transport of a limiting nutrient other than N from the surface may accelerate degradation of subsurface contamination.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: Benzene and alkylbenzene biodegradation rates and patterns were measured using an in situ microcosm in a crude-oil contaminated aquifer near Bemidji, Minnesota. Benzene-D6, toluene, ethylbenzene, o-, m- and p-xylenes and four pairs of C(3)- and C(4)-benzenes were added to an in situ microcosm and studied over a 3-year period. The microcosm allowed for a mass-balance approach and quantification of hydrocarbon biodegradation rates within a well-defined iron-reducing zone of the anoxic plume. Among the BTEX compounds, the apparent order of persistence is ethylbenzene > benzene > m,p-xylenes > o-xylene >or= toluene. Threshold concentrations were observed for several compounds in the in situ microcosm, below which degradation was not observed, even after hundreds of days. In addition, long lag times were observed before the onset of degradation of benzene or ethylbenzene. The isomer-specific degradation patterns were compared to observations from a multi-year study conducted using data collected from monitoring wells along a flowpath in the contaminant plume. The data were fit with both first-order and Michaelis-Menten models. First-order kinetics provided a good fit for hydrocarbons with starting concentrations below 1mg/L and Michaelis-Menten kinetics were a better fit when starting concentrations were above 1mg/L, as was the case for benzene. The biodegradation rate data from this study were also compared to rates from other investigations reported in the literature.
    Journal of contaminant hydrology 01/2010; 111(1-4):48-64. · 2.01 Impact Factor
  • E. Warren, B. A. Bekins
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    ABSTRACT: Crude oil near the water table at spill sites near Bemidji and Cass Lake, Minnesota, has been undergoing aerobic and anaerobic biodegradation for decades. Because the reactions are exothermic, biodegradation of oil compounds will produce measurable temperature increases if heat is generated faster than it is transported away from the oil body. Subsurface temperatures at the two spill sites were measured with thermistors at multiple depths in groundwater monitoring wells and water-filled tubes in the vadose zone. Temperatures in selected wells were measured in the summer of 2007, 2008, and 2009. At the Bemidji site, temperatures measured in the summer ranged from a low of 6.3 oC in the background well to a high of 9.2 oC within wells in the oil-contaminated zone. From year to year, background minimum temperatures were constant within +/- 0.05 oC while maximum temperatures within the oil-contaminated zone remained within +/- 0.25 oC. Seasonal changes in temperature in the plume as measured by data loggers exceeded 4 oC, which was far greater than the year to year change in the summer measurements. Seasonal variability was greater near the water table than at depth. It is unclear whether this variability is due to subsurface hydrology or microbial activity. Temperatures in the vadose zone were warmer near and down-gradient from the oil body compared to the background indicating the heat from the oil and plume propagates up and outward into the vadose zone. At the Cass Lake site, summer temperatures in 2009 were 6.4 oC in the background and 11.5 oC in wells near the oil. Reaction rates inferred from chemical data were compared to heating required in a 3-dimension energy transport model of the subsurface. The increased temperature compared well to the expected heat production from biodegradation reactions occurring in the oil and plume. Results indicate that microbial activity in sediments contaminated with crude oil undergoing biodegradation can be detected using temperature measurements.
    AGU Fall Meeting Abstracts. 12/2009;
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    Environmental Geosciences 01/2005; 12(2):139-152.
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    ABSTRACT: A study of the fate of crude oil in the subsurface at a 1979-spill site near Bemidji, Minnesota, shows the effect of subsurface heterogeneity on oil and methane gas degradation. Separate-phase oil is present in the surficial glacial outwash sediments at residual concentrations in the vadose zone and as an elongated oil body floating on the 6-8-m-deep water table. Gas and water concentrations and microbial data show that methanogenic conditions prevail in the area with separate-phase oil. Within the separate-phase oil, substantial degradation of the n-alkane fraction of the oil has occurred under methanogenic conditions. There is extreme spatial variability in the degradation rates such that the n-alkanes are mostly degraded in the upgradient oil-body limb, but in the downgradient limb n-alkane concentrations are comparable to oil archived from the original spill. Most Probable Number (MPN) microbial data show that numbers of methanogens are ˜10 times greater in the more degraded limb. These differences appear to be related to groundwater recharge. Data from two vertical arrays of moisture probes show that in 2002 the more degraded oil limb received over twice the recharge as the less degraded limb. Typically, samples located near the top of the 1-m-thick floating oil body, are more degraded than those located 10-30 cm lower. Consistent with the degradation state, numbers of methanogens and fermenters are generally greater near the top of the oil body compared to the lower locations. The lateral and vertical variation in degradation state and microbial data together suggest that a nutrient supplied from land surface enhances microbial growth. The resulting heterogeneity of microbial populations leads to greater degradation rates in some locations. There is also evidence of extreme heterogeneity in microbial activity between land surface and the oil body floating on the water table. A narrow methanotrophic horizon exists in the vadose zone where both oxygen from the land surface and ˜20 % (by volume) methane from methanogenic oil degradation are present. MPN data for methanotrophs show similar patterns to those for aerobes indicating that these are the same population. The largest methanotrophic activity (as indicated by aerobe MPN data) occurs just above a silt layer that restricts upward migration of methane and downward migration of oxygen. Changes in physical properties between this silt layer and an overlying unit of coarse sand result in greater than a 1000-fold increase in aerobe numbers over a vertical distance of less than 10 cm.
    AGU Fall Meeting Abstracts. 12/2004;
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    ABSTRACT: 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.
    Bioremediation Journal 01/2004; 8(1-2):1-11. · 0.40 Impact Factor
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    ABSTRACT: In April 1996, a phytoremediation field demonstration site at the Naval Air Station, Fort Worth, Texas, was developed to remediate shallow oxic ground water (< 3.7 m deep) contaminated with chlorinated ethenes. Microbial populations were sampled in February and June 1998. The populations under the newly planted cottonwood trees had not yet matured to an anaerobic community that could dechlorinate trichloroethene (TCE) to cis-1,2-dichloroethene (DCE); however, the microbial population under a mature (approximately 22-year-old) cottonwood tree about 30 m southwest of the plantings had a mature anaerobic population capable of dechlorinating TCE to DCE, and DCE to vinyl chloride (VC). Oxygen-free sediment incubations with contaminated groundwater also demonstrated that resident microorganisms were capable of the dechlorination of TCE to DCE. This suggests that a sufficient amount of organic material is present for microbial dechlorination in aquifer microniches where dissolved O2 concentrations are low. Phenol, benzoic acid, acetic acid, and a cyclic hydrocarbon, compounds consistent with the degradation of root exudates and complex aromatic compounds, were identified by gas chromatography/mass spectrometry (GC/MS) in sediment samples under the mature cottonwood tree. Elsewhere at the site, transpiration and degradation by the cottonwood trees appears to be responsible for loss of chlorinated ethenes.
    International Journal of Phytoremediation 01/2003; 5(1):73-87. · 1.18 Impact Factor
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    ABSTRACT: A multidisciplinary study of a crude-oil contaminated aquifer shows that the distribution of microbial physiologic types is strongly controlled by the aquifer properties and crude oil location. The microbial populations of four physiologic types were analyzed together with permeability, pore-water chemistry, nonaqueous oil content, and extractable sediment iron. Microbial data from three vertical profiles through the anaerobic portion of the contaminated aquifer clearly show areas that have progressed from iron-reduction to methanogenesis. These locations contain lower numbers of iron reducers, and increased numbers of fermenters with detectable methanogens. Methanogenic conditions exist both in the area contaminated by nonaqueous oil and also below the oil where high hydrocarbon concentrations correspond to local increases in aquifer permeability. The results indicate that high contaminant flux either from local dissolution or by advective transport plays a key role in determining which areas first become methanogenic. Other factors besides flux that are important include the sediment Fe(II) content and proximity to the water table. In locations near a seasonally oscillating water table, methanogenic conditions exist only below the lowest typical water table elevation. During 20 years since the oil spill occurred, a laterally continuous methanogenic zone has developed along a narrow horizon extending from the source area to 50-60 m downgradient. A companion paper [J. Contam. Hydrol. 53, 369-386] documents how the growth of the methanogenic zone results in expansion of the aquifer volume contaminated with the highest concentrations of benzene, toluene, ethylbenzene, and xylenes.
    Journal of Contaminant Hydrology 01/2002; 53(3-4):387-406. · 2.89 Impact Factor
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    ABSTRACT: We have identified several subsurface habitats for microorganisms in a crude oil contaminated located near Bemidji, Minnesota. These aquifer habitats include: 1) the unsaturated zone contaminated by hydrocarbon vapors, 2) the zones containing separate-phase crude oil, and 3) the aqueous-phase contaminant plume. The surficial glacial outwash aquifer was contaminated when a crude oil pipeline burst in 1979. We analyzed sediment samples from the contaminated aquifer for the most probable numbers of aerobes, iron reducers, fermenters, and three types of methanogens. The microbial data were then related to gas, water, and oil chemistry, sediment extractable iron, and permeability. The microbial populations in the various contaminated subsurface habitats each have special characteristics and these affect the aquifer and contaminant chemistry. In the eight-meter-thick, vapor-contaminated vadose zone, a substantial aerobic population has developed that is supported by hydrocarbon vapors and methane. Microbial numbers peak in locations where access to both hydrocarbons and nutrients infiltrating from the surface is maximized. The activity of this population prevents hydrocarbon vapors from reaching the land surface. In the zone where separate-phase crude oil is present, a consortium of methanogens and fermenters dominates the populations both above and below the water table. Moreover, gas concentration data indicate that methane production has been active in the oily zone since at least 1986. Analyses of the extracted separate-phase oil show that substantial degradation of C15 -C35 n-alkanes has occurred since 1983, raising the possibility that significant degradation of C15 and higher n-alkanes has occurred under methanogenic conditions. However, lab and field data suggest that toxic inhibition by crude oil results in fewer acetate-utilizing methanogens within and adjacent to the separate-phase oil. Data from this and other sites indicate that toxic inhibition of acetoclastic methanogenesis in the proximity of separate phase contaminant sources may result in build-up of acetate in contaminant plumes. Within the aqueous-phase contaminant plume steep vertical hydrocarbon concentration gradients are associated with sharp transitions in the dominant microbial population. In the 20 years since the aquifer became contaminated, sediment iron oxides have been depleted and the dominant physiologic type has changed in areas of high contaminant flux from iron reducing to methanogenic. Thus, methanogens are found in high permeability horizons down gradient from the oil while iron reducers persist in low permeability zones. Expansion of the methanogenic zone over time has resulted in a concomitant increase in the aquifer volume contaminated with the highest concentrations of benzene and ethylbenzene.
    AGU Fall Meeting Abstracts. 12/2001;
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    ABSTRACT: We characterized the microbial population in an 8-meter-thick, hydrocarbon-contaminated vadose zone using Most Probable Number (MPN) estimates for four physiologic types: aerobes, heterotrophic fermenters, iron-reducers and methanogens. The site is a surficial sand and gravel aquifer near Bemidji, MN, that was contaminated in 1979 when crude oil infiltrated the subsurface from a broken pipeline. Substantial liquid and vapor-phase petroleum hydrocarbons remain in the vadose zone. We examined three vadose-zone profiles located in: 1) the residual oil, 2) a vapor-contaminated area, and 3) the capillary fringe above the contaminated aquifer. In the residual oil ~100 methanogens per gram dry weight of sediment (g-1) are present throughout the profile, and fermenter numbers g-1 are 10,000 times those of iron-reducers, suggesting that methanogenesis is now the dominant degradation process. Analyses of extracted oil from these sediments show that substantial degradation of C15 -C35 n-alkanes has occurred since 1983. Moreover, gas concentration measurements indicate that methane production in this location has been active since at least 1986, raising the possibility that significant degradation of C15 and higher n-alkanes has occurred under methanogenic conditions. In the vapor-contaminated profile, aerobe numbers g-1 are 10,000 times higher than uncontaminated background values. Methanotrophic activity also was detected in laboratory incubations of these sediments. Apparently, a substantial microbial population has developed that is supported by the hydrocarbon vapors and methane. Downgradient from the oil, where groundwater is contaminated but no hydrocarbon vapors are detected, fermenter and aerobe numbers g-1 above the capillary fringe match those of uncontaminated sediments (100-1,000 g-1). Within the capillary fringe, numbers increase rapidly with depth to values typically found in the contaminated saturated zone. In the vadose zone profiles with significant hydrocarbon sources from residual oil and vapors, microbial populations are typically 10-100 times higher than in the underlying contaminated saturated zone. Moreover, in areas with residual oil, numbers g-1 increase significantly upward toward the land surface. This pattern suggests that supply of an unknown, essential nutrient from the land surface may be facilitating growth in the hydrocarbon-contaminated vadose zone. In contrast, at the location with no significant vadose zone hydrocarbons, numbers g-1 in the capillary fringe are less than or equal to those in the contaminated saturated zone below.
    01/2001;
  • BA Bekins, EM Godsy, E Warren
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    ABSTRACT: > Abstract We conducted a plume-scale study of the microbial ecology in the anaerobic portion of an aquifer contaminated by crude-oil compounds. The data provide insight into the patterns of ecological succession, microbial nutrient demands, and the relative importance of free-living versus attached microbial populations. The most probable number (MPN) method was used to characterize the spatial distribution of six physiologic types: aerobes, denitrifiers, iron-reducers, heterotrophic fermenters, sulfate-reducers, and methanogens. Both free-living and attached numbers were determined over a broad cross-section of the aquifer extending horizontally from the source of the plume at a nonaqueous oil body to 66 m downgradient, and vertically from above the water table to the base of the plume below the water table. Point samples from widely spaced locations were combined with three closely spaced vertical profiles to create a map of physiologic zones for a cross-section of the plume. Although some estimates suggest that less than 1% of the subsurface microbial population can be grown in laboratory cultures, the MPN results presented here provide a comprehensive qualitative picture of the microbial ecology at the plume scale. Areas in the plume that are evolving from iron-reducing to methanogenic conditions are clearly delineated and generally occupy 25-50% of the plume thickness. Lower microbial numbers below the water table compared to the unsaturated zone suggest that nutrient limitations may be important in limiting growth in the saturated zone. Finally, the data indicate that an average of 15% of the total population is suspended.http://link.springer-ny.com/link/service/journals/00248/bibs/37n4p263.html
    Microbial Ecology 05/1999; 37(4):263-275. · 3.28 Impact Factor
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    ABSTRACT: In situ microcosms (ISMs) were installed in an aquifer contaminated by crude oil to study the in situ biodegradation of monoaromatic hydrocarbons. One was placed in an area where iron reduction predominates (ISM-Fe) and the other in an area where methanogenesis predominates (ISM-CH 4). Numerous problems were encountered during installation and operation of the ISM-CH 4 microcosm and therefore monitoring of it has been temporarily discontinued. Biodegradation rates determined from field concentrations suggested that degradation for the ISM-Fe should proceed in the following order: (1) toluene and o-xylene – ~12 days; (2) m-, p-xylene – ~400 days; (3) benzene – 700 days; (4) ethylbenzene – ~1600 days. Samples taken from the ISM-Fe at days 36 and 68 indicated that toluene biodegradation was occurring. Iron, manganese, CO 2 , alkalinity, and low molecular weight volatile fatty acids increased while toluene concentration decreased by approximately 20 percent. All other organic and inorganic compounds remained at initial levels including the tracer with the exception of sodium, chloride, and sulfate. These species increased sharply starting at approximately day 8 and have remained elevated since then for an unknown reason.
    03/1999;
  • B.A. Bekins, E. Warren, E.M. Godsy
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    ABSTRACT: Under some conditions, a first-order kinetic model is a poor representation of biodegradation in contaminated aquifers. Although it is well known that the assumption of first-order kinetics is valid only when substrate concentration, S, is much less than the half-saturation constant, K{sub S}, this assumption is often made without verification of this condition. The authors present a formal error analysis showing that the relative error in the first-order approximation is S/K{sub S} and in the zero-order approximation the error is K{sub S}/S. They then examine the problems that arise when the first-order approximation is used outside the range for which it is valid. A series of numerical simulations comparing results of first- and zero-order rate approximations to Monod kinetics for a real data set illustrates that if concentrations observed in the field are higher than K{sub S}, it may be better to model degradation using a zero-order rate expression. Compared with Monod kinetics, extrapolation of a first-order rate to lower concentrations under-predicts the biotransformation potential, while extrapolation to higher concentrations may grossly over-predict the transformation rate. A summary of solubilities and Monod parameters for aerobic benzene, toluene, and xylene (BTX) degradation shows that the a priori assumption of first-order degradation kinetics at sites contaminated with these compounds is not valid. In particular, out of six published values of K{sub S} for toluene, only one is greater than 2 mg/L, indicating that when toluene is present in concentrations greater than about a part per million, the assumption of first-order kinetics may be invalid. Finally, the authors apply an existing analytical solution for steady-state one-dimensional advective transport with Monod degradation kinetics to a field data set.
    Ground Water 03/1998; 36(2). · 2.13 Impact Factor
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    ABSTRACT: Under some conditions, a first-order kinetic model is a poor representation of biodegradation in contaminated aquifers. Although it is well known that the assumption of first-order kinetics is valid only when substrate concentration, S, is much less than the half-saturation constant, K s, this assumption is often made without verification of this condition. We present a formal error analysis showing that the relative error in the first-order approximation is S/Ks and in the zero-order approximation the error is Ks/S. We then examine the problems that arise when the first-order approximation is used outside the range for which it is valid. A series of numerical simulations comparing results of first- and zero-order rate approximations to Monod kinetics for a real data set illustrates that if concentrations observed in the field are higher than Ks, it may be better to model degradation using a zero-order rate expression. Compared with Monod kinetics, extrapolation of a first-order rate to lower concentrations under-predicts the biotransformation potential, while extrapolation to higher concentrations may grossly over-predict the transformation rate. A summary of solubilities and Monod parameters for aerobic benzene, toluene, and xylene (BTX) degradation shows that the a priori assumption of first-order degradation kinetics at sites contaminated with these compounds is not valid. In particular, out of six published values of Ks for toluene, only one is greater than 2 mg/L, indicating that when toluene is present in concentrations greater than about a part per million, the assumption of first-order kinetics may be invalid. Finally, we apply an existing analytical solution for steady-state one-dimensional advective transport with Monod degradation kinetics to a field data set.
    Ground Water 02/1998; 36(2):261 - 268. · 2.13 Impact Factor
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    ABSTRACT: The shallow ground water at a site near Bemidji, Minnesota is contaminated with crude oil spilled from a broken pipeline in 1979. With a continued source of dissolved crude oil components, the geochemical conditions in the aquifer have evolved into aerobic, iron-reducing, and methanogenic redox zones. The methanogenic zone starts within the crude oil-contaminated region and extends more than 60 meters downgradient. The methanogenic numbers in the aquifer are low, but variable, depending on the subpopulation of methanogen. In areas close to the crude-oil source, hydrogen-and formate-utilizing methanogens are found in numbers more than one hundred times higher than acetate-utilizers. The acetate-utilizers are found only well below the non-aqueous phase oil and further downgradient. This pattern of methanogen distribution suggests that growth of acetate-utilizers is limited near the source. Laboratory results suggest that toxicity of the dissolved crude-oil is an explanation. Serum bottle assays were conducted using crude oil in a mineral salts solution inoculated with an enriched methanogenic consortia from a creosote-contaminated site in Pensacola, Florida. Acetate, hydrogen, and formate were added and gas volume change was monitored. Hydrogen-and formate-utilization were unaffected by the crude oil whereas acetate utilization was significantly inhibited. The distribution of aquifer methanogens together with the toxicity assays form a consistent picture with the hypothesis that acetoclastic methanogenesis is inhibited in the vicinity of the oil at the Bemidji site. Because acetate degradation has been widely documented as the rate-limiting step in anaerobic waste treatment processes, it is likely that the inhibition of acetoclastic methanogenesis by the crude oil affects the overall methanogenic degradation rates of the petroleum hydrocarbon contaminants.