Mary Jo Baedecker

Crude oil in a shallow sand and gravel aquifer-III. Biogeochemical reactions and mass balance modeling in anoxic groundwater

Abstract Crude oil floating on the water table in a sand and gravel aquifer provides a constant source of hydrocarbons to the groundwater at a site near Bemidji, Minnesota. The degradation of hydrocarbons affects the concentrations of oxidized and reduced aqueous species in the anoxic part of the contaminant plume that developed downgradient from the oil body. The concentrations of Fe2+, Mn2+ and CH4, Eh measurements, and the δ13C ratios of the total inorganic C indicate that the plume became more reducing ver a 5-a period. However, the size of the contaminant plume remained stable during this time. Field data coupled with laboratory microcosm experiments indicate that benzene and the alkylbenzenes are degraded in an anoxic environment. In anaerobic microcosm experiments conducted under field conditions, almost complete degradation (98%) was observed for benzene in 125 d and for toluene in 45 d. Concentrations of aqueous Fe2+ and Mn2+ increased in these experiments, indicating that the primary reactions were hydrocarbon degradation coupled with Fe and Mn reduction. Mass transfer calculations on a 40-m flowpath in the anoxic zone, downgradient from the oil body, indicated that the primary reactions in the anoxic zone are oxidation of organic compounds, precipitation of siderite and a ferroan calcite, dissolution of iron oxide and outgassing of CH4 and CO2. The major difference in the two models presented is the ratio of CO2 and CH4 that outgasses. Both models indicate quantitatively that large amounts of Fe are dissolved and reprecipitated as ferrous iron in the anoxic zone of the contaminant plume.

Progression of natural attenuation processes at a crude-oil spill site . I. Geochemical evolution of the plume

A 16-year study of a hydrocarbon plume shows that the extent of contaminant migration and compound-specific behavior have changed as redox reactions, most notably iron reduction, have progressed over time. Concentration changes at a small scale, determined from analysis of pore-water samples drained from aquifer cores, are compared with concentration changes at the plume scale, determined from analysis of water samples from an observation well network. The small-scale data show clearly that the hydrocarbon plume is growing slowly as sediment iron oxides are depleted. Contaminants, such as ortho-xylene that appeared not to be moving downgradient from the oil on the basis of observation well data, are migrating in thin layers as the aquifer evolves to methanogenic conditions. However, the plume-scale observation well data show that the downgradient extent of the Fe2+ and BTEX plume did not change between 1992 and 1995. Instead, depletion of the unstable Fe (III) oxides near the subsurface crude-oil source has caused the maximum dissolved iron concentration zone within the plume to spread at a rate of approximately 3 m/year. The zone of maximum concentrations of benzene, toluene, ethylbenzene and xylene (BTEX) has also spread within the anoxic plume. In monitoring the remediation of hydrocarbon-contaminated ground water by natural attenuation, subtle concentration changes in observation well data from the anoxic zone may be diagnostic of depletion of the intrinsic electron-accepting capacity of the aquifer. Recognition of these subtle patterns may allow early prediction of growth of the hydrocarbon plume.

Hydrogeological Processes and Chemical Reactions at a Landfill

Chemical and isotopic analyses were made of water from wells in and downgradient from a landfill to determine chemical and isotopic effects of generation and migration of leachate on ground water. The distribution and wide concentration range of oxygen and methane permit the delineation of an anaerobic zone, a regional oxygenated zone and an intermediate zone. The ratio of reduced nitrogen to nitrate indicates location of reducing fronts as the leachate migrates. The pH of the native ground water is low (≥5.0) primarily because of the low pH of rainfall and the lack of calcareous or other soluble minerals in the aquifer material. The pH is higher (∼6.6) in the leachate because of generation of carbon dioxide, ammonia, and methane. The native ground water has a low TDS (80 mg/l) while the leachate has an average TDS of 2800 mg/l and is primarily a NaHCO3 type water. Sulfate concentrations are extremely low and H2 S was not detected. We suggest that a major source of cations may be their exchange from the clays by the ammonium generated in the leachate. High concentrations of Fe and Mn are attributed to a source in the refuse but more important to reduction of oxide cements and coatings resulting from degradation of organic matter. The main source of bicarbonate is from organic degradation with minimal CO2 from the soil zone. At one landfill site 52% of the total alkalinity is attributed to organic compounds, mainly organic acid anions. The δ13 C of bicarbonate in the leachate is exceedingly heavy (+18.400 /00 ) which results from fractionation during the formation of methane. The 10 per mil deuterium enrichment of water may be due to decomposition of deuterium-enriched compounds and bacterial processes that preferentially consume the lighter hydrogen isotope.

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.

The geochemical evolution of low-molecular-weight organic acids derived from the degradation of petroleum contaminants in groundwater

Abstract The geochemical evolution of low-molecular-weight organic acids in groundwater downgradient from a crude-oil spill near Bemidji, Minnesota, was studied over a five year period (1986–1990). The organic acids are metabolic intermediates of the degradation of components of the crude oil and are structurally related to hydrocarbon precursors. The concentrations of organic acids, particularly aliphatic acids, increase as the microbial alteration of hydrocarbons progresses. The organic-acid pool changes in composition and concentration over time and in space as the degradation processes shift from Fe(III) reduction to methanogenesis. Over time, the aquifer system evolves into one in which the groundwater contains more oxidized products of hydrocarbon degradation and the reduced forms of iron, manganese, and nitrogen. Laboratory microcosm experiments with aquifer material support the hypothesis that organic acids observed in the groundwater originate from the microbial degradation of aromatic hydrocarbons under anoxic conditions. The geochemistry of two other shallow aquifers in coastal plain sediments, one contaminated with creosote waste and the other with gasoline, were compared to the Bemidji site. The geochemical evolution of the low-molecular-weight organic acid pool in these systems is controlled, in part, by the presence of electron acceptors available for microbially mediated electron-transfer reactions. The depletion of electron acceptors in aquifers leads to the accumulation of aliphatic organic acids in anoxic groundwater.

Crude oil in a shallow sand and gravel aquifer—I. Hydrogeology and inorganic geochemistry

Abstract Changes in the distribution of inorganic solutes in a shallow ground water contaminated by crude oil document a series of geochemical reactions initiated by biodegradation of the oil. Upgradient of an oil body floating on the water table, oxidation of oil to carbonic acid dissolves carbonate minerals in the aquifer matrix. In this oxidized zone pH is depressed ∼1 pH unit, and the concentrations of Ca, Mg and HCO 3 − increase to more than twice that of the native ground water. In the anoxic zone beneath the oil body concentrations of dissolved SiO 2 , Sr, K, Fe and Mn increase significantly. Here, Fe is mobilized by microbial reduction, pH is buffered by the carbonate system, and silicates weather via hydrolysis and organic-acid-enhanced dissolution. Farther down-gradient the ground water is reoxygenated and Fe precipitates from solution, possibly as iron hydroxide or iron carbonates, while SiO 2 precipitates as amorphous silica. Other solutes, such as Mg, are transported more conservatively down-gradient where contaminated and native ground water mix. The observed changes in inorganic aqueous chemistry document changes in water-mineral interactions caused by the presence of an organic contaminant. These organic-initiated interactions are likely present in many contaminated aquifers and may be analogous to interactions occurring in other organic-rich natural waters.

Transformation of Monoaromatic hydrocarbons to organic acids in anoxic groundwater environment

The transformation of benzene and a series of alkylbenzenes was studied in anoxic groundwater of a shallow glacial-outwash aquifer near Bemidji, Minnesota, U.S.A. Monoaromatic hydrocarbons, the most water-soluble components of crude oil, were transported downgradient of an oil spill, forming a plume of contaminated groundwater. Organic acids that were not original components of the oil were identified in the anoxic groundwater. The highest concentrations of these oxidized organic compounds were found in the anoxic plume where a decrease in concentrations of structurally related alkylbenzenes was observed. These results suggest that biological transformation of benzene and alkylbenzenes to organic acid intermediates may be an important attenuation process in anoxic environments. The transformation of a complex mixture of hydrocarbons to a series of corresponding oxidation products in an anoxic subsurface environment provides new insight into in situ anaerobic degradation processes.

Crude oil in a shallow sand and gravel aquifer-II. Organic geochemistry

Abstract Crude oil spilled from a pipeline break in a remote area of north-central Minnesota has contaminated a shallow glacial outwash aquifer. Part of the oil was sprayed over a large area to the west of the pipeline and part of it accumulated in an oil body that floats at the water table to the east of the point of discharge. Total dissolved organic carbon (TDOC) concentrations in shallow groundwater collected in the oil spray area reach 16 mg/l. This is nearly an order of magnitude higher than the TDOC concentrations of native groundwater (∼2–3mg/l). The additional TDOC derives from the partial degradation of petroleum residues deposited at the land surface and transported to the aquifer by vertical recharge. In the vicinity of the oil body, TDOC concentrations in groundwater are 48 mg/l, 58% of the TDOC being composed of non-volatile organic C. The majority of the volatile DOC (63%) is a mixture of low-molecular-weight saturated, aromatic and alicyclic hydrocarbons derived from the oil. Downgradient from the oil body along the direction of groundwater flow, concentrations of all measured constituents of the TDOC pool decrease. Concentrations begin to decline most rapidly, however, in the zone where dissolved O 2 concentrations begin to increase, ∼50m downgradient from the leading edge of the oil. Within the anoxic zone near the oil body, removal rates of isometric monoaromatic hydrocarbons vary widely. This indicates that the removal processes are mediated mainly by microbiological activity. Molecular and spectroscopic characterization of the TDOC and its spatial and temporal variation provide evidence of the importance of biogeochemical processes in attenuating petroleum contaminants in this perturbed subsurface environment.

Methane production and consumption monitored by stable H and C isotope ratios at a crude oil spill site, Bemidji, Minnesota

Abstract Stable isotopic ratios of C and H in dissolved CH4 and C in dissolved inorganic C in the ground water of a crude-oil spill near Bemidji, Minnesota, support the concept of CH4 production by acetate fermentation with a contemporaneous increase in HCO3− concentration. Methane concentrations in the saturated zone decrease from 20.6 mg L−1 to less than 0.001 mg L−1 along the investigated flow path. Dissolved N2 and Ar concentrations in the ground water below the oil plume are 25 times lower than background; this suggests that gas exsolution is removing dissolved CH4 (along with other dissolved gases) from the ground water. Oxidation of dissolved CH4 along the flow path seems to be minimal because no measurable change in isotopic composition of CH4 occurs with distance from the oil body. However, CH4 is partly oxidized to CO2 as it diffuses upward from the ground water through a 5- to 7-m thick unsaturated zone; theδ13C of the remaining CH4 increases, theδ13C of the CO2 decreases, and the partial pressure of CO2 increases. Calculations of C fluxes in the saturated and unsaturated zones originating from the degradation of the oil plume lead to a minimum estimated life expectancy of 110 years. This is a minimum estimate because the degradation of the oil body should slow down with time as its more volatile and reactive components are leached out and preferentially oxidized. The calculated life expectancy is an order of magnitude estimate because of the uncertainty in the average linear ground-water velocities and because of the factor of 2 uncertainty in the calculation of the effective CO2 diffusion coefficient.

Modern marine sediments as a natural analog to the chemically stressed environment of a landfill

Abstract Baedecker, M.J. and Back, W., 1979. Modern marine sediments as a natural analog to the chemically stressed environment of a landfill. In: W. Back and D.A. Stephenson (Guest-Editors), Contemporary Hydrogeology — The George Burke Maxey Memorial Volume. J. Hydrol., 43: 393-414. Chemical reactions that occur in landfills are analogous to those reactions that occur in marine sediments. Lateral zonation of C, N, S, O, H, Fe and Mn species in landfills is similar to the vertical zonation of these species in marine sediments and results from the following reaction sequence: (1) oxidation of C, N and S species in the presence of dissolved free oxygen to HCO 3 - , NO 3 - and SO 4 2 ; (2) after consumption of molecular oxygen, then NO 3 - is reduced, and Fe and Mn are solubilized; (3) SO 4 2- is reduced to sulfide; and (4) organic compounds become the source of oxygen, and CH 4 and NH 4 + are formed as fermentation products. In a landfill in Delaware the oxidation potential increases down-gradient and the redox zones in the reducing plume are characterized by: CH 4 , NH 4 + ,Fe 2+ . Mn 2+ , HCO 3 - and NO 3 - . Lack of SO 4 2- at that landfill eliminates the sulfide zone. Although it has not been observed at landfills, mineral alteration should result in precipitation of pyrite and/or siderite downgradient. Controls on the pH of leachate are the relative rates of production of HCO 3 - , NH 4 + and CH 4 . Production of methane by fermentation at landfills results in 13 C isotope fractionation and the accumulation of isotopically heavy σ CO 2 (+10 to +18 0 / 00 PDB). Isotope measurements may be useful to determine the extent of CO 2 reduction in landfills and extent of dilution downgradient. The boundaries of reaction zones in stressed aquifers are determined by head distribution and flow velocity. Thus, if the groundwater flow is rapid relative to reaction rates, redox zones will develop downgradient. Where groundwater flow velocities are low the zones will overlap to the extent that they may be indeterminate.