Alexis K. Navarre-Sitchler

Basalt weathering rates on Earth and the duration of liquid water on the plains of Gusev Crater, Mars

Where Martian rocks have been exposed to liquid water, chemistry versus depth profi les could elucidate both Martian climate history and potential for life. The persistence of primary minerals in weathered profi les constrains the exposure time to liquid water: on Earth, mineral persistence times range from ~10 k.y. (olivine) to ~250 k.y. (glass) to ~1 m.y. ( pyroxene) to ~5 m.y. (plagioclase). Such persistence times suggest mineral persistence minima on Mars. However, Martian solutions may have been more acidic than on Earth. Relative mineral weathering rates observed for basalt in Svalbard (Norway) and Costa Rica demonstrate that laboratory pH trends can be used to estimate exposure to liquid water both qualitatively (mineral absence or presence) and quantitatively (using reactive transport models). Qualitatively, if the Martian solution pH >~2, glass should persist longer than olivine; therefore, persistence of glass may be a pH indicator. With evidence for the pH of weathering, the reactive transport code CrunchFlow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profi le. For the profi le measured on the surface of the exposed Martian rock known as Humphrey in Gusev Crater, the calculated exposure time is 22 k.y., which is a minimum due to physical erosion. If correct, this estimate is consistent with short-term, episodic alteration accompanied by ongoing surface erosion. More of these depth profi les should be measured to illuminate the weathering history of Mars.

Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast

Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast Alexis Navarre-Sitchler 1,3* Carl Steefel 2,3 , Li Yang 2,3 , Liviu Tomutsa 2 , Susan Brantley 3,4 Corresponding Author: alexis.sitchler@uwyo.edu Department of Geology and Geophysics, The University of Wyoming, Laramie WY 82071 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley CA Center for Environmental Kinetics Analysis, The Pennsylvania State University, University Park PA 16802 Earth and Environmental Systems Institute, The Pennsylvania State University, University Park PA 16802

Metal release from sandstones under experimentally and numerically simulated CO2 leakage conditions.

Leakage of CO2 from a deep storage formation into an overlying potable aquifer may adversely impact water quality and human health. Understanding CO2-water-rock interactions is therefore an important step toward the safe implementation of geologic carbon sequestration. This study targeted the geochemical response of siliclastic rock, specifically three sandstones of the Mesaverde Group in northwestern Colorado. To test the hypothesis that carbonate minerals, even when present in very low levels, would be the primary source of metals released into a CO2-impacted aquifer, two batch experiments were conducted. Samples were reacted for 27 days with water and CO2 at partial pressures of 0.01 and 1 bar, representing natural background levels and levels expected in an aquifer impacted by a small leakage, respectively. Concentrations of major (e.g., Ca, Mg) and trace (e.g., As, Ba, Cd, Fe, Mn, Pb, Sr, U) elements increased rapidly after CO2 was introduced into the system, but did not exceed primary Maximum Contaminant Levels set by the U.S. Environmental Protection Agency. Results of sequential extraction suggest that carbonate minerals, although volumetrically insignificant in the sandstone samples, are the dominant source of mobile metals. This interpretation is supported by a simple geochemical model, which could simulate observed changes in fluid composition through CO2-induced calcite and dolomite dissolution.

Human health risk assessment of CO2 leakage into overlying aquifers using a stochastic, geochemical reactive transport approach.

Increased human health risk associated with groundwater contamination from potential carbon dioxide (CO2) leakage into a potable aquifer is predicted by conducting a joint uncertainty and variability (JUV) risk assessment. The approach presented here explicitly incorporates heterogeneous flow and geochemical reactive transport in an efficient manner and is used to evaluate how differences in representation of subsurface physical heterogeneity and geochemical reactions change the calculated risk for the same hypothetical aquifer scenario where a CO2 leak induces increased lead (Pb(2+)) concentrations through dissolution of galena (PbS). A nested Monte Carlo approach was used to take Pb(2+) concentrations at a well from an ensemble of numerical reactive transport simulations (uncertainty) and sample within a population of potentially exposed individuals (variability) to calculate risk as a function of both uncertainty and variability. Pb(2+) concentrations at the well were determined with numerical reactive transport simulation ensembles using a streamline technique in a heterogeneous 3D aquifer. Three ensembles with variances of log hydraulic conductivity (σ(2)lnK) of 1, 3.61, and 16 were simulated. Under the conditions simulated, calculated risk is shown to be a function of the strength of subsurface heterogeneity, σ(2)lnK and the choice between calculating Pb(2+) concentrations in groundwater using equilibrium with galena and kinetic mineral reaction rates. Calculated risk increased with an increase in σ(2)lnK of 1 to 3.61, but decreased when σ(2)lnK was increased from 3.61 to 16 for all but the highest percentiles of uncertainty. Using a Pb(2+) concentration in equilibrium with galena under CO2 leakage conditions (PCO2 = 30 bar) resulted in lower estimated risk than the simulations where Pb(2+) concentrations were calculated using kinetic mass transfer reaction rates for galena dissolution and precipitation. This study highlights the importance of understanding both hydrologic and geochemical conditions when numerical simulations are used to perform quantitative risk calculations.

Kinetic metal release from competing processes in aquifers.

Understanding groundwater time scales wherein kinetic metal-desorption and mineral-dissolution are important mechanisms is essential for realistic modeling of metal release. In this study, release rate constants were compiled and the Damköhler number was applied to calculate residence times where kinetic formulations are relevant. Desorption rate constants were compiled for arsenic, barium, cadmium, copper, lead, mercury, nickel, and zinc, and span 6 orders of magnitude, while mineral-dissolution rate constants compiled for calcite, kaolinite, smectite, anorthite, albite, K-feldspar, muscovite, quartz, goethite, and galena ranged over 13 orders of magnitude. This Damköhler analysis demonstrated that metal-desorption kinetics are potentially influential at residence times up to about two years, depending on the metal and groundwater conditions. Kinetic mineral-dissolution should be considered for nearly all residence times relevant to groundwater modeling, provided the rate, solubility, and availability of the mineral generates a non-negligible concentration. Geochemical models of competitive desorption and dissolution for an illustrative metal demonstrate total metal concentrations may be sensitive to dissolution rate variations despite the predominance of release from desorption. Ultimately, this analysis provides constraints on relevant processes for incorporation into transport models.

The effects of physical and geochemical heterogeneities on hydro-geochemical transport and effective reaction rates

The role of coupled physical and geochemical heterogeneities in hydro-geochemical transport is investigated by simulating three-dimensional transport in a heterogeneous system with kinetic mineral reactions. Ensembles of 100 physically heterogeneous realizations were simulated for three geochemical conditions: 1) spatially homogeneous reactive mineral surface area, 2) reactive surface area positively correlated to hydraulic heterogeneity, and 3) reactive surface area negatively correlated to hydraulic heterogeneity. Groundwater chemistry and the corresponding effective reaction rates were calculated at three transverse planes to quantify differences in plume evolution due to heterogeneity in mineral reaction rates and solute residence time (τ). The model is based on a hypothetical CO2 intrusion into groundwater from a carbon capture utilization and storage (CCUS) operation where CO2 dissolution and formation of carbonic acid created geochemical dis-equilibrium between fluids and the mineral galena that resulted in increased aqueous lead (Pb(2+)) concentrations. Calcite dissolution buffered the pH change and created conditions of galena oversaturation, which then reduced lead concentrations along the flow path. Near the leak kinetic geochemical reactions control the release of solutes into the fluid, but further along the flow path mineral solubility controls solute concentrations. Simulation results demonstrate the impact of heterogeneous distribution of geochemical reactive surface area in coordination with physical heterogeneity on the effective reaction rate (Krxn,eff) and Pb(2+) concentrations within the plume. Dissimilarities between ensemble Pb(2+) concentration and Krxn,eff are attributed to how geochemical heterogeneity affects the time (τeq) and therefore advection distance (Leq) required for the system to re-establish geochemical equilibrium. Only after geochemical equilibrium is re-established, Krxn,eff and Pb(2+) concentrations are the same for all three geochemical conditions. Correlation between reactive surface area and hydraulic conductivity, either positive or negative, results in variation in τeq and Leq.

ParCrunchFlow: an efficient, parallel reactive transport simulation tool for physically and chemically heterogeneous saturated subsurface environments

Understanding the interactions between physical, geochemical, and biological processes in the shallow subsurface is integral to the development of effective contamination remediation techniques, or the accurate quantification of nutrient fluxes and biogeochemical cycling. Hydrology is a primary control on the behavior of shallow subsurface environments and must be realistically represented if we hope to accurately model these systems. ParCrunchFlow is a new parallel reactive transport model that was created by coupling a multicomponent geochemical code (CrunchFlow) with a parallel hydrologic model (ParFlow). These models are coupled in an explicit operator-splitting manner. ParCrunchFlow can simulate three-dimensional multicomponent reactive transport in highly resolved, field-scale systems by taking advantage of ParFlow’s efficient parallelism and robust hydrologic abilities, and CrunchFlow’s extensive geochemical abilities. Here, the development of ParCrunchFlow is described and two simple verification simulations are presented. The parallel performance is evaluated and shows that ParCrunchFlow has the ability to simulate very large problems. A series of simulations involving the biologically mediated reduction of nitrate in a floodplain aquifer were conducted. These floodplain simulations show that this code enables us to represent more realistically the variability in chemical concentrations observed in many field-scale systems. The numerical formulation implemented in ParCrunchFlow minimizes numerical dispersion and allows the use of higher-order explicit advection schemes. The effects that numerical dispersion can have on finely resolved, field-scale reactive transport simulations have been evaluated. The smooth gradients produced by a first-order advection scheme create an artificial mixing effect, which decreases the spatial variance in solute concentrations and leads to an increase in overall reaction rates. The work presented here is the first step in a larger effort to couple these models in a transient, variably saturated surface-subsurface framework, with additional geochemical abilities.

Metal fate and partitioning in soils under bark beetle-killed trees

Recent mountain pine beetle infestation in the Rocky Mountains of North America has killed an unprecedented acreage of pine forest, creating an opportunity to observe an active re-equilibration in response to widespread land cover perturbation. This work investigates metal mobility in beetle-impacted forests using parallel rainwater and acid leaches to estimate solid-liquid partitioning coefficients and a complete sequential extraction procedure to determine how metals are fractionated in soils under trees experiencing different phases of mortality. Geochemical model simulations analyzed in consideration with experimental data provide additional insight into the mechanisms controlling metal complexation. Metal and base-cation mobility consistently increased in soils under beetle-attacked trees relative to soil under healthy trees. Mobility increases were more pronounced on south facing slopes and more strongly correlated to pH under attacked trees than under healthy trees. Similarly, soil moisture was significantly higher under dead trees, related to the loss of transpiration and interception. Zinc and cadmium content increased in soils under dead trees relative to living trees. Cadmium increases occurred predominantly in the exchangeable fraction, indicating increased mobilization potential. Relative increases of zinc were greatest in the organic fraction, the only fraction where increases in copper were observed. Model results reveal that increased organic complexation, not changes in pH or base cation concentrations, can explain the observed differences in metal partitioning for zinc, nickel, cadmium, and copper. Predicted concentrations would be unlikely to impair human health or plant growth at these sites; however, higher exchangeable metals under beetle-killed trees relative to healthy trees suggest a possible decline in riverine ecosystem health and water quality in areas already approaching criteria limits and drinking water standards. Impairment of water quality in important headwater streams from the increased potential for metal mobilization and storage will continue to change as beetle-killed trees decompose and forests begin to recover.

Geochemical Implications of Brine Leakage into Freshwater Aquifers

CO(2) injection into deep saline formations as a way to mitigate climate change raises concerns that leakage of saline waters from the injection formations will impact water quality of overlying aquifers, especially underground sources of drinking water (USDWs). This paper aims to characterize the geochemical composition of deep brines, with a focus on constituents that pose a human health risk and are regulated by the U.S. Environmental Protection Agency (USEPA). A statistical analysis of the NATCARB brine database, combined with simple mixing model calculations, show total dissolved solids and concentrations of chloride, boron, arsenic, sulfate, nitrate, iron and manganese may exceed plant tolerance or regulatory levels. Twelve agricultural crops evaluated for decreased productivity in the event of brine leakage would experience some yield reduction due to increased TDS at brine-USDW ratios of < 0.1, and a 50% yield reduction at < 0.2 brine-USDW ratio. A brine-USDW ratio as low as 0.004 may result in yield reduction in the most sensitive crops. The USEPA TDS secondary standard is exceeded at a brine fraction of approximately 0.002. To our knowledge, this is the first study to consider agricultural impacts of brine leakage, even though agricultural withdrawals of groundwater in the United States are almost three times higher than public and domestic withdrawals.

Exploration of Diffuse and Discrete Sources of Acid Mine Drainage to a Headwater Mountain Stream in Colorado, USA

We investigated the impact of acid mine drainage (AMD) contamination from the Minnesota Mine, an inactive gold and silver mine, on Lion Creek, a headwater mountain stream near Empire, Colorado. The objective was to map the sources of AMD contamination, including discrete sources visible at the surface and diffuse inputs that were not readily apparent. This was achieved using geochemical sampling, in-stream and in-seep fluid electrical conductivity (EC) logging, and electrical resistivity imaging (ERI) of the subsurface. The low pH of the AMD-impacted water correlated to high fluid EC values that served as a target for the ERI. From ERI, we identified two likely sources of diffuse contamination entering the stream: (1) the subsurface extent of two seepage faces visible on the surface, and (2) rainfall runoff washing salts deposited on the streambank and in a tailings pile on the east bank of Lion Creek. Additionally, rainfall leaching through the tailings pile is a potential diffuse source of contamination if the subsurface beneath the tailings pile is hydraulically connected with the stream. In-stream fluid EC was lowest when stream discharge was highest in early summer and then increased throughout the summer as stream discharge decreased, indicating that the concentration of dissolved solids in the stream is largely controlled by mixing of groundwater and snowmelt. Total dissolved solids (TDS) load is greatest in early summer and displays a large diel signal. Identification of diffuse sources and variability in TDS load through time should allow for more targeted remediation options.ZusammenfassungDer Einfluss von saurem Grubenabwasser (AMD) aus der Minnesota Mine, einem stillgelegten Gold und Silberbergbau, auf den Lion Creek, einem Gebirgsquellfluss in der Nähe von Empire (Colorado) wurde untersucht. Zielstellung war, die Quellen der AMD-Kontaminationen zu identifizieren und zu kartieren. Diese AMD-Einträge in den Vorfluter stammen sowohl aus diskreten Punktzuflüssen als auch aus diffusen, nicht objektiv sichtbaren Einträgen. Als Untersuchungsmethoden wurden neben geochemischen Beprobungen auch direkte Aufzeichnungen von elektrischen Leitfähigkeiten im Oberflächen- und Sickerwasser sowie elektrische Widerstandsabbildungen des Untergrundes (Elektrical Resistivity Imaging, ERI) verwendet. Die niedrigen pH-Werte im AMD-beeinflussten Wasser korrelieren mit hohen elektrischen Leitfähigkeiten und dienen als Grundlage für die Auswertung der ERI-Analysen. Aus den Widerstandwerten des Untergrundes können zwei diffuse Eintragsquellen in den Vorfluter identifiziert werden: (1) die Ausbreitung von zwei auch oberflächennah sichtbaren Sickerflächen im Untergrund und (2) Niederschlagsabflüsse, welche Salzablagerungen von den Flussbänken und aus den Tailing-Halden der Ostseite des Lion Creeks auswaschen. Zusätzlich bildet der durch die Tailing-Halden versickernde Niederschlag bei hydraulischer Anbindung des Untergrundes an den Vorfluter einen weiteren potenziellen diffusen Eintragspfad. Elektrische Leitfähigkeitswerte im Vorfluter sind am niedrigsten, wenn der Abfluss im Frühsommer hoch ist und stiegen über den Sommer infolge verminderter Abflussmengen an. Dieses Verhalten zeigt, dass die Konzentration der gelösten Stofffracht durch die Mischung von Grundwasserzuflüssen und Schneeschmelzwasser kontrolliert wird. Die gelöste Stofffracht ist im Frühsommer am höchsten und zeigt ein größeres Trübungssignal. Die Identifizierung von diffusen Eintragsquellen und von zeitlichen Veränderungen der gelösten Stofffracht erlaubt die Anwendung zielgerichteter Sanierungsoptionen.ResumenHemos investigado el impacto de la contaminación del drenaje ácido de mina (AMD) de la mina Minnesota, una mina de oro y plata inactiva, sobre Lion Creek, una corriente de montaña cerca de Empire, Colorado. El objetivo fue mapear las fuentes de contaminación con AMD, incluyendo fuentes discretas visibles en la superficie y otras difusas no evidentes fácilmente. Esto fue conseguida usando muestreo geoquímico, la conductividad eléctrica (EC) de la corriente y del fluido de filtración y la resistividad eléctrica (ERI) de la subsuperficie. El bajo pH del agua impactada por AMD correlacionó con los altos valores de EC del fluido que servió para el ERI. A partir del ERI, se identificaron dos fuentes de contaminación difusa que entraban a la corriente: (1) la extensión subsuperficial de dos caras de infiltración visibles en la superficie, y (2) el lavado provocado por la lluvia de las sales depositadas en la ribera y en la pila de relaves de la orilla oriental de Lion Creek. Adicionalmente, la lixiviación provocada por la lluvia en la pila de relaves es una potencial fuente difusa de contaminacion si la pila está conectada hidráulica y subterráneamente con la corriente. En la corriente EC fue mínimo cuando la descarga de la corriente fue máxima en el comienzo del verano y luego se incremento a través del verano a medida que la descarga de la corriente decreció, indicando que la concentración de los sólidos disueltos es controlada principalmente por la mezcla de agua subterránea y agua de deshielo. Los sólidos totales disueltos (TDS) son máximos en los principios del verano y muestra una larga señal diaria. La identificación de las fuentes difusas y la variabilidad en TDS a través del tiempo debería permitir opciones de remediación más específicas.探查汇入美国科罗拉多州山间河流的弥散和分散型酸性矿山废水污染源研究了已停产的Minnesota金银矿酸性矿山废水(AMD)对科罗拉多州Empire附近Lion山间河流的影响。研究旨在测绘酸性废水(AMD)污染源,包括地表可见分散污染源和不易分辨的弥散输入型污染源。研究方法包括地球化学取样、河流和渗流原位电导率(EC)测井和地层视电阻率成像(ERI)。流体受酸性废水(AMD)污染后pH值降低且电导率升高,该相关变化规律是视电阻率成像(ERI)分析的主要原理。依据ERI分析结果,识别出两种汇入山间河流的弥散型污染源:(1) 两个地表可见渗流污染源;(2) 冲刷河岸盐渍沉积的地表径流和Lion河东岸尾矿堆内径流。如果尾矿堆下伏地层与河水具有水力联系,淋滤尾矿堆的雨水也是潜在弥散污染源。夏初,河水流量最大,电导率(EC)最低;其后,河水流量减小,电导率(EC)逐渐增大;该现象表明山间河水溶解固体(TDS) 主要受地下水和雪融水混合水控制。TDS荷载初夏最大,表现出昼夜变化。弥散源辨识和TDS荷载变化规律研究有助于提高废水处理的针对性和有效性。