Philip C. Bennett

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.

Quartz dissolution in organic-rich aqueous systems

Abstract Organic electrolytes are a common component of natural waters and are known to be important in many rock-water interactions. The influence of organic electrolytes on silica mobility, quartz solubility, and quartz dissolution kinetics, however, is less well understood. While there is mounting evidence supporting the presence of an aqueous organic-silica complex in natural waters, the significance of this species is difficult to characterize because of competing interactions in mixed inorganic-organic electrolyte environments. In the experiments reported here, the kinetics of quartz dissolution in dilute aqueous organic-acid solutions between 25 and 70°C were investigated to determine the influence of both organic and inorganic electrolytes. Batch-reactor dissolution experiments in inorganic and organic electrolyte solutions were designed to investigate the hypothesis that organic acids at circum-neutral pH accelerate the dissolution and increase the solubility of quartz in water. Results suggest that multi-functional organic acids such as citrate and oxalate accelerate quartz dissolution by decreasing the activation energy by approximately 20%. The increase in dissolution rate was accompanied by a 100% increase in apparent quartz solubility at 25°C. Experiments using inorganic electrolytes, in contrast, increase the rate of quartz dissolution without decreasing the activation energy, and without increasing solubility. From these data, a model for both a solution complex between dissolved organic acid and monomeric silicic acid, and an activated complex on quartz surfaces is proposed. The model suggests that dissolved organic compounds in natural waters at near-neutral pH and low temperatures are capable of accelerating the dissolution of quartz and increasing its solubility.

Silicates, Silicate Weathering, and Microbial Ecology

Mineralogy, microbial ecology, and mineral weathering in the subsurface are an intimately linked biogeochemical system. Although bacteria have been implicated indirectly in the accelerated weathering of minerals, it is not clear if this interaction is simply the coincidental result of microbial metabolism, or if it represents a specific strategy offering the colonizing bacteria a competitive ecological advantage. Our studies provide evidence that silicate weathering by bacteria is sometimes driven by the nutrient requirements of the microbial consortium, and therefore depends on the trace nutrient content of each aquifer mineral. This occurrence was observed in reducing groundwaters where carbon is abundant but phosphate is scarce; here, even resistant feldspars are weathered rapidly. This suggests that the progression of mineral weathering may be influenced by a minerals nutritional potential, with microorganisms destroying only beneficial minerals. The rock record, therefore, may contain a remnant mine...

Microbial Control of Silicate Weathering in Organic-Rich Ground Water

An in situ microcosm study of the influence of surface-adhering bacteria on silicate diagenesis in a shallow petroleum-contaminated aquifer showed that minerals were colonized by indigenous bacteria and chemically weathered at a rate faster than theoretically predicted. Feldspar and quartz fragments were placed in anoxic, organic-rich ground water, left for 14 months, recovered, and compared to unreacted controls with scanning electron microscopy. Ground-water geochemistry was characterized before and after the experiment. Localized mineral etching probably occurred in a reaction zone at the bacteria-mineral interface where high concentrations of organic acids, formed by bacteria during metabolism of hydrocarbon, selectively mobilized silica and aluminum from the mineral surface.

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.

Microbial precipitation of dolomite in methanogenic groundwater

We report low-temperature microbial precipitation of dolomite in dilute natural waters from both field and laboratory experiments. In a freshwater aquifer, microorganisms colonize basalt and nucleate nonstoichiometric dolomite on cell walls. In the laboratory, ordered dolomite formed at near-equilibrium conditions from groundwater with molar Mg:Ca ratios of <1; dolomite was absent in sterile experiments. Geochemical and microbiological data suggest that methanogens are the dominant metabolic guild in this system and are integral to dolomite precipitation. We hypothesize that the attached microbial consortium reacts with the basalt surface, releasing Mg and Ca into solution, which drives dolomite precipitation via nucleation on the cell wall. These findings provide insight into the long-standing dolomite problem and suggest a fundamental role for microbial processes in the formation of dolomite across a wide range of environmental conditions.

Microbial colonization and weathering of silicates in a petroleum-contaminated groundwater

Abstract The influence of native microorganisms on mineral dissolution and precipitation was examined in a petroleum-contaminated aquifer near Bemidji, Minnesota. In-situ microcosms containing clean silicate fragments were established in the contaminated, microbially active groundwater over one-year periods. The recovered minerals were then examined by SEM and ESEM for microbial colonization patterns and weathering features. These experiments reveal distinct patterns of colonization and weathering associated with microbial attachment and growth. Microcline, anorthoclase, and oligoclase were widely colonized, and the colonized surfaces deeply weathered, with secondary clays precipitated on some uncolonized surfaces. Other feldspars, in contrast, were uncolonized and unweathered. Specific mineral removal rate was estimated from etch pit depth, while bulk weathering rate was estimated from the rate of change of silica concentration in the groundwater. In this system the primary control of silicate dissolution is apparently microbial colonization and metabolic activity.

Microbial contributions to cave formation: New insights into sulfuric acid speleogenesis

The sulfuric acid speleogenesis (SAS) model was introduced in the early 1970s from observations of Lower Kane Cave, Wyoming, and was proposed as a cave-enlargement process due to primarily H 2 S autoxidation to sulfuric acid and subaerial replacement of carbonate by gypsum. Here we present a reexamination of the SAS type locality in which we make use of uniquely applied geochemical and microbiological methods. Little H 2 S escapes to the cave atmosphere, or is lost by abiotic autoxidation, and instead the primary H 2 S loss mechanism is by subaqueous sulfur-oxidizing bacterial communities that consume H 2 S. Filamentous “ Epsilonproteobacteria ” and Gammaproteobacteria , characterized by fluorescence in situ hybridization, colonize carbonate surfaces and generate sulfuric acid as a metabolic byproduct. The bacteria focus carbonate dissolution by locally depressing pH, compared to bulk cave waters near equilibrium or slightly supersaturated with calcite. These findings show that SAS occurs in subaqueous environments and potentially at much greater phreatic depths in carbonate aquifers, thereby offering new insights into the microbial roles in subsurface karstification.

Filamentous “Epsilonproteobacteria” Dominate Microbial Mats from Sulfidic Cave Springs

ABSTRACT Hydrogen sulfide-rich groundwater discharges from springs into Lower Kane Cave, Wyoming, where microbial mats dominated by filamentous morphotypes are found. The full-cycle rRNA approach, including 16S rRNA gene retrieval and fluorescence in situ hybridization (FISH), was used to identify these filaments. The majority of the obtained 16S rRNA gene clones from the mats were affiliated with the “Epsilonproteobacteria” and formed two distinct clusters, designated LKC group I and LKC group II, within this class. Group I was closely related to uncultured environmental clones from petroleum-contaminated groundwater, sulfidic springs, and sulfidic caves (97 to 99% sequence similarity), while group II formed a novel clade moderately related to deep-sea hydrothermal vent symbionts (90 to 94% sequence similarity). FISH with newly designed probes for both groups specifically stained filamentous bacteria within the mats. FISH-based quantification of the two filament groups in six different microbial mat samples from Lower Kane Cave showed that LKC group II dominated five of the six mat communities. This study further expands our perceptions of the diversity and geographic distribution of “Epsilonproteobacteria” in extreme environments and demonstrates their biogeochemical importance in subterranean ecosystems.

Process‐based approach to CO2 leakage detection by vadose zone gas monitoring at geologic CO2 storage sites

A critical issue for geologic carbon sequestration is the ability to detect CO2 in the vadose zone. Here we present a new process-based approach to identify CO2 that has leaked from deep geologic storage reservoirs into the shallow subsurface. Whereas current CO2concentration-based methods require years of background measurements to quantify variability of natural vadose zone CO2, this new approach examines chemical relationships between vadose zone N2, O2, CO2, and CH4 to promptly distinguish a leakage signal from natural vadose zone CO2. The method uses sequential inspection of the following gas concentration relationships: 1) O2 versus CO2to distinguish in-situ vadose zone background processes (biologic respiration, methane oxidation, and CO2 dissolution) from exogenous deep leakage input, 2) CO2 versus N2 to further distinguish dissolution of CO2 from exogenous deep leakage input, and 3) CO2 versus N2/O2 to assess the degree of respiration, CH4 oxidation and atmospheric mixing/dilution occurring in the system. The approach was developed at a natural CO2-rich control site and successfully applied at an engineered site where deep gases migrated into the vadose zone. The ability to identify gas leakage into the vadose zone without the need for background measurements could decrease uncertainty in leakage detection and expedite implementation of future geologic CO2 storage projects.