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  • Brantley, Susan  (153)
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  • 1
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States of America, 19 May 2015, Vol.112(20), pp.6325-30
    Description: High-volume hydraulic fracturing (HVHF) has revolutionized the oil and gas industry worldwide but has been accompanied by highly controversial incidents of reported water contamination. For example, groundwater contamination by stray natural gas and spillage of brine and other gas drilling-related fluids is known to occur. However, contamination of shallow potable aquifers by HVHF at depth has never been fully documented. We investigated a case where Marcellus Shale gas wells in Pennsylvania caused inundation of natural gas and foam in initially potable groundwater used by several households. With comprehensive 2D gas chromatography coupled to time-of-flight mass spectrometry (GCxGC-TOFMS), an unresolved complex mixture of organic compounds was identified in the aquifer. Similar signatures were also observed in flowback from Marcellus Shale gas wells. A compound identified in flowback, 2-n-Butoxyethanol, was also positively identified in one of the foaming drinking water wells at nanogram-per-liter concentrations. The most likely explanation of the incident is that stray natural gas and drilling or HF compounds were driven ∼ 1-3 km along shallow to intermediate depth fractures to the aquifer used as a potable water source. Part of the problem may have been wastewaters from a pit leak reported at the nearest gas well pad-the only nearby pad where wells were hydraulically fractured before the contamination incident. If samples of drilling, pit, and HVHF fluids had been available, GCxGC-TOFMS might have fingerprinted the contamination source. Such evaluations would contribute significantly to better management practices as the shale gas industry expands worldwide.
    Keywords: Marcellus Shale ; High-Volume Hydraulic Fracturing ; Natural Gas ; Shale Gas ; Water Quality ; Water Movements ; Extraction and Processing Industry -- Methods ; Groundwater -- Chemistry ; Natural Gas -- Adverse Effects ; Water Pollutants, Chemical -- Analysis ; Water Supply -- Analysis
    ISSN: 00278424
    E-ISSN: 1091-6490
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  • 2
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States of America, 04 December 2018, Vol.115(49), pp.12349-12358
    Description: Extensive development of shale gas has generated some concerns about environmental impacts such as the migration of natural gas into water resources. We studied high gas concentrations in waters at a site near Marcellus Shale gas wells to determine the geological explanations and geochemical implications. The local geology may explain why methane has discharged for 7 years into groundwater, a stream, and the atmosphere. Gas may migrate easily near the gas wells in this location where the Marcellus Shale dips significantly, is shallow (∼1 km), and is more fractured. Methane and ethane concentrations in local water wells increased after gas development compared with predrilling concentrations reported in the region. Noble gas and isotopic evidence are consistent with the upward migration of gas from the Marcellus Formation in a free-gas phase. This upflow results in microbially mediated oxidation near the surface. Iron concentrations also increased following the increase of natural gas concentrations in domestic water wells. After several months, both iron and SO concentrations dropped. These observations are attributed to iron and SO reduction associated with newly elevated concentrations of methane. These temporal trends, as well as data from other areas with reported leaks, document a way to distinguish newly migrated methane from preexisting sources of gas. This study thus documents both geologically risky areas and geochemical signatures of iron and SO that could distinguish newly leaked methane from older methane sources in aquifers.
    Keywords: Hydraulic Fracturing ; Methane ; Noble Gases ; Shale Gas ; Water Quality
    ISSN: 00278424
    E-ISSN: 1091-6490
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(2), pp.337-351
    Description: We have compiled time-series concentration data for the biological reduction of manganese(III/IV) published between 1985 and 2004 and fit these data with a simple hyperbolic rate expression or, when appropriate, one of its limiting forms. The compiled data and rate constants are available in . The zero- and first-order rate constants appear to follow a log–normal distribution that could be used, for example, in predictive modeling of Mn-oxide reduction in a reactive transport scenario. We have also included details of the experimental procedures used to generate each time-series data-set in our compilation. These meta-data—mostly pertaining to the type and concentration of micro-organism, electron donor, and electron acceptor—enable us to examine the rate data for trends. We have computed a number of rudimentary, mono-variate statistics on the compiled data with the hope of stimulating both more detailed statistical analyses of the data and new experiments to fill gaps in the existing data-set. We have also analyzed the data with parametric models based on the log–normal distribution and rate equations that are hyperbolic in the concentration of cells and Mn available for reduction. This parametric analysis allows us to provide best estimates of zero- and first-order rate constants both ignoring and accounting for the meta-data.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 4
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(23), pp.7644-7667
    Description: Saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution. Here we investigate the chemical and physical transformations that occur during weathering by studying small-scale “saprolites” in the form of weathering rinds, which form on rock in soil or saprolite and grow in thickness without physical disturbance with time. We compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in Costa Rica to diffusion-reaction simulations of rind formation using the fully coupled reactive transport model CrunchFlow. The four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include (1) the mineralogy of weathering products, (2) weathering rind thickness, (3) the coincidence of plagioclase and augite reaction fronts, and (4) the thickness of the zones of mineral reaction, i.e. reaction fronts. Four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations. To fit the model to all four criteria, however, it was necessary to (1) treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9%, and (2) treat mineral surface area as a fitting factor. This latter approach was presumably necessary because the mineral-water surface area of the connected (accessible) porosity in the Costa Rica samples is much less than the total porosity ( ). The model-fit surface area, here termed reacting surface area, was much smaller than the BET-measured surface area determined for powdered basaltic material. In the parent basalt, reacting surface area and diffusivity are low due to low pore connectivity, and early weathering is therefore transport controlled. However, as pore connectivity increases as a result of weathering, the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics. The transition point between transport and kinetic control appears to be related to a critical porosity (9%) at which pore connectivity is high enough to allow rapid transport. Based on these simulations, we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface, and that this porosity increases because of mineral dissolution following a rate that is largely surface-reaction controlled.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 5
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(2), pp.401-415
    Description: The dissolution–precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH) solutions with pH 8.2–9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at or silicon sites, respectively. The dissolution mechanism varies with Δ such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by . However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated reaction rate on pH, and a poorly-constrained pH dependence of the postulated rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 February 2014, Vol.126, pp.555-573
    Description: We investigate how mineral spatial distribution in porous media affects their dissolution rates. Specifically, we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration (v/v) was held constant. The largest difference was observed between a “Mixed column” containing uniformly distributed magnesite and a “One-zone column” containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid (flow-parallel One-zone column). The columns were flushed with acid water at a pH of 4.0 at flow velocities of 3.6 or 0.36 m/d. Breakthrough data show that the rate of magnesite dissolution is 1.6–2 times slower in the One-zone column compared to the Mixed column. This extent of rate limitation is much larger than what was observed in our previous work (14%) for a similar One-zone column where the magnesite was packed in a layer aligned perpendicular to flow (flow-transverse One-zone column). Two-dimensional reactive transport modeling with CrunchFlow revealed that ion activity product (IAP) and local dissolution rates at the grid block scale (0.1 cm) vary by orders of magnitude. Much of the central magnesite zone in the One-zone flow-parallel column is characterized by close or equal to equilibrium conditions with IAP/ 〉 0.1. Two important surface areas are defined to understand the observed rates: the effective surface area ( ) reflects the magnesite that effectively dissolves under far from equilibrium conditions (IAP/ 〈 0.1), while the interface surface area ( ) reflects the effective magnesite surface that lies along the quartz–magnesite interface. Modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of and . Under the conditions examined in this work, the value of varies from 2% to 67% of the total magnesite BET surface area. Column-scale bulk rates (in units of mol/s) vary linearly with and . Using to normalize rates, we calculate a rate constant (10 mol/m /s) that is very close to the value of 10 mol/m /s under well-mixed conditions at the grid block scale. This implies that the laboratory-field rate discrepancy can potentially be caused by differences in the effective surface area. If we know the effective surface area of dissolution, we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems. In this work, approximately 60–70% of the is at the magnesite–quartz interface. This implies that in some field systems where the detailed information that we have for our columns is not available, the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones. Although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport, our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution (chemical heterogeneity) on dissolution.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 December 2012, Vol.99, pp.159-178
    Description: This paper demonstrates a method for systematic analysis of published mineral dissolution rate data using forsterite dissolution as an example. The steps of the method are: (1) identify the data sources, (2) select the data, (3) tabulate the data, (4) analyze the data to produce a model, and (5) report the results. This method allows for a combination of of data, based on expert knowledge of theoretical expectations and experimental pitfalls, and of the data using statistical methods. Application of this method to all currently available forsterite dissolution rates (0 〈 pH 〈 14, and 0 〈 〈 150 °C) normalized to geometric surface area produced the following rate equations: For pH 〈 5.6 and 0° 〈 〈 150 °C, based on 519 data For pH 〉 5.6 and 0° 〈 〈 150 °C, based on 125 data The values show that ∼10% of the variance in is not explained by variation in 1/ and pH. Although the experimental error for rate measurements should be ± ∼30%, the observed error associated with the log values is ∼0.5 log units (±300% relative error). The unexplained variance and the large error associated with the reported rates likely arises from the assumption that the rates are directly proportional to the mineral surface area (geometric or BET) when the rate is actually controlled by the concentration and relative reactivity of surface sites, which may be a function of duration of reaction. Related to these surface area terms are other likely sources of error that include composition and preparation of mineral starting material. Similar rate equations were produced from BET surface area normalized rates. Comparison of rate models based on geometric and BET normalized rates offers no support for choosing one normalization method over the other. However, practical considerations support the use of geometric surface area normalization. Comparison of Mg and Si release rates showed that they produced statistically indistinguishable dissolution rates because dissolution was stoichiometric in the experiments over the entire pH range even though the surface concentrations of Mg and Si are known to change with pH. Comparison of rates from experiments with added carbonate, either from CO partial pressures greater than atmospheric or added carbonate salts, showed that the existing data set is not sufficient to quantify any effect of dissolved carbonate species on forsterite dissolution rates.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 8
    Language: English
    In: Earth and Planetary Science Letters, 2010, Vol.297(1), pp.211-225
    Description: In the Critical Zone where rocks and life interact, bedrock equilibrates to Earth surface conditions, transforming to regolith. The factors that control the rates and mechanisms of formation of regolith, defined here as material that can be augered, are still not fully understood. To quantify regolith formation rates on shale lithology, we measured uranium-series (U-series) isotopes ( U, U, and Th) in three weathering profiles along a planar hillslope at the Susquehanna/Shale Hills Observatory (SSHO) in central Pennsylvania. All regolith samples show significant U-series disequilibrium: ( U/ U) and ( Th/ U) activity ratios range from 0.934 to 1.072 and from 0.903 to 1.096, respectively. These values display depth trends that are consistent with fractionation of U-series isotopes during chemical weathering and element transport, i.e., the relative mobility decreases in the order U 〉 U 〉 Th. The activity ratios observed in the regolith samples are explained by i) loss of U-series isotopes during water–rock interactions and ii) re-deposition of U-series isotopes downslope. Loss of U and Th initiates in the meter-thick zone of “bedrock” that cannot be augered but that nonetheless consists of up to 40% clay/silt/sand inferred to have lost K, Mg, Al, and Fe. Apparent equivalent regolith production rates calculated with these isotopes for these profiles decrease exponentially from 45 m/Myr to 17 m/Myr, with increasing regolith thickness from the ridge top to the valley floor. With increasing distance from the ridge top toward the valley, apparent equivalent regolith residence times increase from 7 kyr to 40 kyr. Given that the SSHO experienced peri-glacial climate ∼ 15 kyr ago and has a catchment-wide averaged erosion rate of ∼ 15 m/Myr as inferred from cosmogenic Be, we conclude that the hillslope retains regolith formed before the peri-glacial period and is not at geomorphologic steady state. Both chemical weathering reactions of clay minerals and translocation of fine particles/colloids are shown to contribute to mass loss of U and Th from the regolith, consistent with major element data at SSHO. This research documents a case study where U-series isotopes are used to constrain the time scales of chemical weathering and regolith production rates. Regolith production rates at the SSHO should be useful as a reference value for future work at other weathering localities.
    Keywords: U-Series Isotopes ; Regolith Formation ; Chemical Weathering ; Erosion ; Critical Zone ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 9
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 September 2012, Vol.93, pp.235-261
    Description: The reactive transport model FLOTRAN was used to forward-model weathering profiles developed on granitic outwash alluvium over 40–3000 ka from the Merced, California (USA) chronosequence as well as deep granitic regolith developed over 800 ka near Davis Run, Virginia (USA). Baseline model predictions that used laboratory rate constants ( ), measured fluid flow velocities ( ), and BET volumetric surface areas for the parent material ( ) were not consistent with measured profiles of plagioclase, potassium feldspar, and quartz. Reaction fronts predicted by the baseline model are deeper and thinner than the observed, consistent with faster rates of reaction in the model. Reaction front depth in the model depended mostly upon saturated versus unsaturated hydrologic flow conditions, rate constants controlling precipitation of secondary minerals, and the average fluid flow velocity ( ). Unsaturated hydrologic flow conditions (relatively open with respect to CO ) resulted in the prediction of deeper reaction fronts and significant differences in the separation between plagioclase and potassium feldspar reaction fronts compared to saturated hydrologic flow (relatively closed with respect to CO ). Under saturated or unsaturated flow conditions, the rate constant that controls precipitation rates of secondary minerals must be reduced relative to laboratory rate constants to match observed reaction front depths and measured pore water chemistry. Additionally, to match the observed reaction front depths, was set lower than the measured value, , for three of the four profiles. The reaction front gradients in mineralogy and pore fluid chemistry could only be modeled accurately by adjusting values of the product . By assuming values were constrained by laboratory data, field observations were modeled successfully with TST-like rate equations by dividing measured values of by factors from 50 to 1700. Alternately, with sigmoidal or Al-inhibition rate models, this adjustment factor ranges from 5 to 170. Best-fit models of the wetter, hydrologically saturated Davis Run profile required a smaller adjustment to than the drier hydrologically unsaturated Merced profiles. We attributed the need for large adjustments in and necessary for the Merced models to more complex hydrologic flow that decreased the reactive surface area in contact with bulk flow water, e.g., dead-end pore spaces containing fluids that are near or at chemical equilibrium. Thus, rate models from the laboratory can successfully predict weathering over millions of years, but work is needed to understand how to incorporate changes in what controls the relationship between reactive surface area and hydrologic flow.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 10
    Language: English
    In: Science of the Total Environment, 15 May 2015, Vol.515-516, pp.49-59
    Description: Atmospheric emissions of metals from anthropogenic activities have led to deposition and contamination of soils worldwide. We quantified addition of manganese (Mn) to soils around the largest emitter of Mn in the United States (U.S.) using chemical analyses and atmospheric dispersion modeling (Second-Order Closure Integrated Puff (SCIPUFF)). Concentrations of soil-surface Mn were enriched by 9-fold relative to that of the parent material within 1 km of the facility. Elevated concentrations of Mn and chromium (Cr), another potentially toxic element that was emitted, document contamination only within 1 m of the soil surface. Total mass of Mn added per unit land area integrated over 1 m, , equals ~ 80 mg Mn cm near the facility. Values of remained above background up to tens of kilometers from the source. Air concentrations of Mn particles of 7.5-micron diameter simulated with SCIPUFF using available data for the emission rate and local meteorological conditions for 2006 were consistent with measured air concentrations. However, the Mn deposition calculated for 2006 with SCIPUFF yielded a cumulative value over the lifetime of the refinery (60 years) that is a factor of 15 lower than the Mn observed to have been added to the soils. This discrepancy can be easily explained if Mn deposition rates before 1988 were more than an order of magnitude greater than today. Such higher emissions are probable, given the changes in metal production with time and the installation of emission controls after the Clean Air Act (1970). This work shows that atmospheric dispersion models can be used with soil profiles to understand the changes in metal emissions over decadal timescales. In addition, the calculations are consistent with the Clean Air Act accounting for a 15-fold decrease in the Mn deposition to soils around the refinery per metric ton of Mn alloy produced.
    Keywords: Scipuff ; Metal Deposition ; Industrial Emissions ; Environmental Sciences ; Biology ; Public Health
    ISSN: 0048-9697
    E-ISSN: 1879-1026
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