Search

This study presents experimental copper isotope (65Cu/63Cu) fractionation data between chalcopyrite and dissolved Cu at physiochemical conditions representative of high temperature mid-ocean ridge hydrothermal environments. The experimental data are compared with chemical and Cu isotope data from high temperature fluids and chalcopyrite sampled from the Main Endeavour Field and ASHES hydrothermal fields located along the Juan de Fuca Ridge. Specifically, three long-term (>250–2500 hr) chalcopyrite recrystallization experiments were conducted at 350 °C and 50 MPa in acidic chloride-bearing fluid. One of the experiments contained 57Fe isotope tracer to quantify reaction progress and the extent of exchange between chalcopyrite and dissolved Fe, which serves as a proxy for Cu isotope exchange ( Syverson et al., 2017 ). The dissolved 57Fe tracer demonstrated rapid isotope exchange between chalcopyrite and dissolved metals within the duration of the long term experiments, approximately 2500 hr, where complete exchange was achieved within approximately 1000 hr. The experimentally determined δ65Cu equilibrium fractionation between chalcopyrite and dissolved Cu, Δ 65 C u [ C p y - C u a q ] = −0.22 ± 0.16‰ (1σ), is comparable in sign and magnitude to theoretical predictions. Comparison of the δ65Cu values of chimney-derived chalcopyrite and hydrothermal fluids sampled from the Main Endeavour Field and ASHES hydrothermal systems demonstrate a range in (dis)equilibrium from theoretical and experimental Δ 65 C u [ C p y - C u a q ] constraints, possibly due to temporal fluctuations in the δ65Cu value of hydrothermal fluid associated with changes in the physiochemical processes controlling Cu mass transfer in the subseafloor. Overall, the small Δ 65 C u [ C p y - C u a q ] effectively allows chalcopyrite to be a reliable recorder of the variability in the δ65Cu value of end-member hydrothermal fluids throughout the temporal evolution of MOR hydrothermal systems.

Stresses on water systems can be quantitatively assessed through indices that account for water demand relative to water availability, e.g., the Water Supply Stress Index (WaSSI). However, as a result of adopting deterministic supply-driven approaches, limited attention is paid to the potential impacts of climatic variability on quantifying water stresses. The current study aimed to account for the impacts of inter-annual and intra-annual variability in the WaSSI stress index and to provide insights into potential opportunities for better water management practices. The results from our analysis indicate that looking only at average stresses can substantially mask the important impacts of climate variability. Louisiana, as a typical example of humid regions in the USA, is subjected to high levels of stresses (WaSSI exceeds 1.0) with higher inter-annual variability in watersheds where thermoelectric power plants exist and extensive water is used for cooling process. In addition, intra-annual variability in some watersheds shows periodicity in terms of seasonal stress distributions due to variability in surface water supply and water demand. Our analysis indicated that the stress variability grows as the median WaSSI increases but up to a certain threshold level and then the variability decreases for very high stress levels. For the annual and monthly scales, the peak variability, quantified as the width of the 2.5–97.5 stress percentiles, reached 68% for a median annual WaSSI of 1.00 and 100% for a median monthly WaSSI of 1.15, respectively. Various decisions related to water use and management can be driven by such variability, at both annual and intra-annual scales. Hence, these results have important implications for applied water resource studies aiming to formulate water management policies and improve water system sustainability under climate variability.

Hydraulic fracturing activities have been gaining more attention; therefore, a better understanding of the potential stress that fracturing may cause to freshwater resources is needed. The ...

Many unconventional clay-rich oil and gas reservoirs are characterized by fine-scale heterogeneity that is challenging to characterize and predict at larger scales. This is particularly problematic in emerging shale plays with limited shared knowledge such as the Tuscaloosa Marine Shale (TMS). The TMS is located within the Central U.S. Gulf Coast region and contains an estimated 7 billion barrels of potentially recoverable oil. In this study we synthesized and interpreted newly available mineralogical and organic geochemical data from 11 wells located in the currently producing area of the TMS. The data are extremely heterogeneous within individual wells, but generally suggest that the basal region of the TMS within the producing region is characterized by higher concentrations of calcite and TOC relative to other areas of the TMS. Data from an ∼18 m section near the base of the TMS (including samples from all 11 wells) averaged 1.65 wt% TOC, 22.8 wt% quartz, 17.2 wt% calcite, 16.2 wt% kaolinite, 14.1 wt% illite, 11.8 wt% smecite, 5.7 wt% chlorite, 3.9 wt% plagioclase, and 4.4 wt% pyrite. Kerogen in this region is a mixture of Type II and Type III and Tmax and vitrinite reflectance data suggest that samples from most wells are within the oil and wet gas generation window. Comparisons to previous investigations of the TMS show that most studies are in reasonable agreement only at the largest scales of observation and break down at smaller scales such that the concentrations of minerals and organic matter reported depend largely on the scale and locations of the investigation and the sampling density. Hence, the best approach for quantifying, describing, and ultimately predicting the heterogeneous physical and chemical properties within TMS may be to rely upon statistically-based comparisons of groups of data such that probability distributions of these groups can be generated that vary with the changing stratigraphy and depositional environments.

The over-use of groundwater is an increasing issue even in regions of the U.S. traditionally characterized by abundant rainfall and surface water. Part of the groundwater withdrawals in these regions can potentially be replaced by surface water, and quantitative spatial analysis to demonstrate this potential may help to spur policy changes. However, one challenge for this type of spatial analysis is the lack of groundwater withdrawal data at the watershed scale. For example, estimates of groundwater withdraw are only available at the county-level for most of the U.S., which is not granular enough to be useful for local management decisions. The present study developed a GIS-based framework for spatially disaggregating available groundwater withdrawal information based on ancillary information such as the well’s minimum casing diameter. This information was used to identify potential opportunities where surface water could be used to offset groundwater withdrawals. The application of this framework to irrigation water demand in the state of Louisiana shows that a significant fraction of groundwater withdrawals can potentially be offset by available surface water. This basic framework can be complemented with additional considerations such as the availability of surface water to more fully evaluate water management scenarios.

Teapot Dome oilfield near the town of Midwest, Wyoming, has been the testing site of numerous drilling and production techniques since the early 1900's. The structure is a doubly plunging basement-cored anticline that formed during the Laramide orogeny, with sediments ranging from Cambrian to Quaternary. The Steele, Niobrara, and Mowry formations are potential source rocks in the area. In this study, we combine geochemical analyses with one-dimensional modeling of burial history, thermal maturity, and timing of petroleum generation to evaluate the potential of the field for unconventional exploration. Rock Eval™ pyrolysis and oxidation results show a mixture of Type II and Type III kerogen in the Niobrara and Mowry formations, and no hydrocarbon generation potential for the Steele Formation. These geochemical results, combined with regional tectonics, stratigraphy, and sedimentological conditions, were implemented into burial history and maturation models. Results from the modeling show that the Niobrara and Mowry formations are in the early oil generation window. The Mowry is the most mature of the three formations, but only 10% of the kerogen has been converted to oil. The study area did not reach the thermal maturity required to generate the significant amount of oil and gas produced to date from the structure. The results and method of this study can easily be extrapolated to better constrain the basin evolution of nearby fields. This study is significant for further exploration in other established Laramide basins with similar formations.

Advancing our understanding of the connections among groundwater, food, and climate is critical to meet global food demands while optimizing water resources usage. However, our understanding of the linkages among groundwater, food, and climate is still limited. Here, we offer a Bayesian framework to simulate crop yield at a regional scale and quantify its relationships and associated uncertainty with climate, groundwater, agricultural, and energy-related variables. We implemented the framework in the rice-producing regions of Louisiana from 1960–2015. To build a parsimonious model, we used a probability-based variable selection approach to detect the key drivers of rice yield. Rice yield increased, groundwater declined, and area planted declined or did not change over 56yrs. The number of irrigation wells, groundwater level, air temperature, and area planted were found to be the key drivers of rice yield. The regression coefficients showed that rice yield was positively related to groundwater level, and negatively related to area planted and the number of irrigation wells. The limited influence of N fertilizer was noted on rice yield for the period when fertilizer data were available. The inverse relationship between rice yield and area planted pointed to the adaption of efficient crop management practices that maintained or increased yield, despite the decline in area planted. The farmers' ability to install irrigation wells during droughts sustained the yields over long-term but not short-term. This decline in rice yield in response to drought over the short-term might explain the negative relation between yield and irrigation wells. Overall, this work highlighted the uncertainty in relationships between rice yield and key drivers and quantified the intimate connection between food and groundwater. This work may have implications for managing two highly competing commodities (i.e., groundwater and food) in agricultural regions.

Many alluvial aquifer systems are characterized in part by groundwater rich in sodium (Na) and bicarbonate (HCO3) and elevated concentrations of dissolved iron (Fe) and arsenic (As). However the relationships among Fe and As concentrations and the formation of Na HCO3 waters are unclear. Here we evaluated the formation of Na HCO3 groundwater in the Lower Mississippi River Alluvial Aquifer (LMRAA) in Louisiana, USA, where concentrations of dissolved Fe and As reached values as high as 16 mg/L and 64 μg/L, respectively. Water isotope data showed that groundwater was predominantly recharged via local precipitation outboard of the Mississippi River, while groundwater nearer the river and to the south of the study area was more hydraulically connected to the river. Much of the geochemical data, including water isotopes, calcite saturation index, Na excess, and As correlated to some degree with well depth, highlighting the complex interactions between the shallower recharge from local precipitation and the deeper reaction pathways laterally connected to the Mississippi River and/or vertically connected with regional groundwater. The δ13C(DIC) of groundwater ranged from −17.4 to −11.1‰, likely representing a mixture of microbially- and abiotically-derived alkalinity. Within this context, the production of groundwater rich in Na HCO3 can be explained by reaction pathways involving the weathering of Ca Na silicate phases. This weathering, driven in part by the production of microbial CO2 during Fe and sulfate reduction, may continue past the point of calcite saturation. Once calcite reaches saturation the concentration of Ca is buffered, but there is a possibility of further buildup in the concentrations of Na and HCO3. The silicate weathering model serves as an alternative conceptual model to calcite dissolution and clay exchange for the development of Na HCO3 waters. If globally important, this water-rock reaction pathway could place new constraints on the processes influencing the fate and transport of Fe and As in alluvial aquifer systems.

Growing populations demand more food, putting more pressure on soil productivity and sustainability around the world. In western Texas along the Rio Grande Valley, the low natural rainfall requires frequent irrigations for sustaining agriculture. To investigate the impacts of irrigation on soil quality, we collected and modelled geochemical data (major elements and nutrients) on irrigation water, soil pore water, drainage water, and soil samples, and monitored soil moisture, temperature, and electrical conductivity with sensors from two pecan, one cotton, and one alfalfa fields in western Texas. This study showed that flood irrigation with both surface (Rio Grande river) and ground waters significantly increased the root-zone salinity, soil sodicity, and nutrient leaching from soils to the underlying aquifers and Rio Grande river from agricultural fields of the arid southwest. The water used for irrigation was high in total dissolved solids (>500 ppm generally), dominated by Na+, Cl−, Ca2+ and SO42−. After flood irrigation, infiltrating water dissolved salts such as gypsum that have accumulated in the soils due to previous irrigations, or/and mixed existing concentrated soil waters, and approached saturation with respect to these evaporite minerals. Soil water was supersaturated with respect to carbonates as pedogenic calcite precipitated out and reached concentrations of ∼10 wt% of total soil mass. This suggested that pedogenic carbonate is an important carbon reservoir and precipitation kinetics and controls of such secondary calcite need further investigation for the irrigated agricultural fields in arid regions of the world. Chemistry of agricultural return flow samples collected from drainage ditches was similar to that of irrigation water, suggesting that most of the irrigation water had taken a shallow and short flowpath through the fields to drains. Between irrigation events, soil water became more concentrated as water was lost through evapotranspiration that led to precipitation of evaporite salts. As a result, sodicity and salinity of soils, especially clayey soils, frequently exceeded the tolerance levels of major crops grown in the region. Here in these fine-textured soils, combination of high evapotranspiration rates, intensive irrigation with water of elevated salinity, and limited infiltration stunted crop growth, decreased soil porosity and permeability, led to poor aeration, and accelerated salt buildup via a positive feedback mechanism. During initial irrigation where soils were saturated, soil water also percolated and recharged to underlying aquifers, and thus salts, nutrients, and trace metals from agricultural practices (i.e., application of fertilizers, irrigation, soil amendments, and pesticide) could be mobilized to shallow groundwaters. This implied that chemistry of Rio Grande river, groundwater, and soil was closely linked. Thus the sustainability of agriculture depended on appropriate water, soil and crop management practices.

The generation of H 2 during serpentinization of ultramafic oceanic crust along Earth's mid-ocean ridges provides a key energy source for the deep biosphere and has important implications for metal transport and mineral stability in the subseafloor hydrothermal system. Nonetheless, the driving forces and rates of the coupled processes of Fe partitioning and H 2 generation during serpentinization are complicated by many factors, but especially, the activity of SiO 2 (aq) in the serpentinizing fluid. To examine the role of this key variable in H 2 generation and Fe partitioning between magnetite and Fe-serpentine during serpentinization, San Carlos olivine was reacted with a NaCl-SiO 2 (aq)-bearing fluid at 300 °C and 500 bars for approximately 90 days using flexible gold cell hydrothermal equipment. Time-series changes in solution chemistry and dissolved H 2 are coupled with magnetic susceptibility, Mossbauer spectroscopy, and Fe isotope measurements of reactant olivine and coexisting alteration products to provide a complete description of Fe mass transfer and oxidation as a function of reaction progress. Utilization of talc as reactant not only provided a source of SiO 2 (aq) for serpentinization, but effectively prevented brucite formation, in keeping with constraints imposed by many natural hydrothermal systems, which are inherently open systems. Moreover, this experimental strategy allowed Fe derived from olivine dissolution to transfer completely to serpentine and magnetite, facilitating reaction path modeling. Initial changes in solution chemistry indicate “buffering” of dissolved SiO 2 (aq), owing to the apparently balanced rates of olivine dissolution and serpentine precipitation. During this stage of reaction, a negligible amount of H 2 is generated, suggesting that Fe 2+ -serpentine is stable and the dominant Fe-bearing product. With continued reaction, the rate of SiO 2 (aq) production lessens likely due to reduction in surface area of talc, such that the continued hydration and hydrolysis of olivine titrates the previously buffered activity of SiO 2 (aq), eventually approaching serpentine-brucite coexistence. During this secondary stage of reaction, the rate of H 2 generation abruptly increases, which reflects the formation of magnetite and Fe 3+ -serpentine. Temporal changes in the isotope composition of dissolved Fe, throughout these stages, likely reflect different mineral alteration and formation processes, where initially, 1) preferential mass transfer of isotopically light Fe into solution occurred during olivine alteration, without considerable oxidation, as demonstrated by the low H 2 production rate, and, 2) the preferential removal of isotopically light Fe from solution in connection with enhanced magnetite and H 2 formation during the later stages of reaction. Post experimental analysis of the Fe isotopic composition of Fe-bearing minerals demonstrates statistically negligible enrichment of δ 56 Fe of magnetite relative to olivine and serpentine, consistent with observations from natural altered oceanic crust. Observations from this study demonstrate the utility of using coupled chemical, isotopic, and analytical approaches to ascertain time series changes in rates of mass transfer and mechanisms of oxidation of Fe during serpentinization of olivine. Ultimately, these insights provide important constraints for the chemical and Fe isotopic evolution of fluids and minerals in ultramafic hosted hydrothermal systems at mid-ocean ridges.

Equilibrium Fe isotope fractionation between chalcopyrite and dissolved Fe was determined in acidic chloride-bearing fluid at 350 °C and 500 bars. The study utilized deformable gold-cell technology, which allowed time-series sampling of solution during chalcopyrite recrystallization and isotope exchange. A key element of the experimental design involved the addition of anomalous dissolved 57 Fe to an on-going experiment as a means of determining the degree and rate of isotope exchange. Taking explicit account of imposed chemical and isotopic mass balance constraints of Fe in fluid and mineral (chalcopyrite) reservoirs, these data indicate that no more than 1000 h is required for the isotopically anomalous dissolved Fe reservoir to exchange completely with the coexisting chalcopyrite. The experimental calibration of the rate of Fe isotope exchange for the δ 57 Fe-spiked experiment provides critical insight for the time necessary to achieve Fe isotope exchange in two non-spiked, but otherwise identical experiments. The Fe isotope data indicate that the equilibrium fractionation between chalcopyrite and dissolved Fe, Δ 56 Fe Cpy-Fe (aq) , at 350 °C is small, 0.09 ± 0.17‰ (2 σ ), and is in good agreement with recent theoretical equilibrium predictions. Owing to the apparent rate of Fe isotope exchange at 350 °C, it is likely that chalcopyrite formed at high temperature deep-sea vents (black smoker systems) achieves isotopic equilibrium, and effectively records the Fe isotopic composition of the coexisting end-member hydrothermal fluid. Comparison of the experimental mineral– fluid equilibrium fractionation factors with conjugate chalcopyrite and dissolved Fe pairs sampled from high temperature hydrothermal vent systems at Axial Caldera and Main Endeavour Field (Juan de Fuca Ridge) are in agreement with this inference. The experimental data were further used to determine the mineral– mineral equilibrium Fe isotope fractionation between pyrite-chalcopyrite, Δ 56 Fe Pyr-Cpy , at 350 °C by combining previously determined pyrite-Fe 2+ (aq) equilibrium fractionation data with chalcopyrite-Fe 2+ (aq) from this study. The empirically determined Δ 56 Fe Pyr-Cpy value, 0.90 ± 0.34‰ (2 σ ), is consistent with theoretical predictions, and when coupled with mineral– fluid Fe isotope fractionation systematics and experimentally determined exchange rates, helps to delineate processes of sulfide mineralization in hydrothermal systems.

Groundwater depletion in many areas of the world has been broadly attributed to irrigation. However, more formal, data-driven, causal mechanisms of long-term groundwater patterns have not been assessed. Here, we conducted the first Granger causality analysis to identify the “causes” of groundwater patterns using the rice-producing parishes of Louisiana, USA, as an example. Trend analysis showed a decline of up to 6 m in groundwater level over 51 years. We found that no single cause explained groundwater patterns for all parishes. Causal linkages were noted between groundwater and area harvested, number of irrigation wells, summer precipitation totals, and drought. Bi-directional linkages were noted between groundwater and rice yield, suggesting feedback between both time series. Causal linkages were absent between groundwater and many drivers where significant correlations were noted, highlighting the importance of using robust causal relationships over illusive correlations to detect the cause. These results advance our understanding of groundwater dynamics and can reveal a key connection between food and groundwater.

Inorganic geochemical data are becoming increasingly important in the characterization of hydrocarbon source rocks. However, it is often difficult to establish which elements are associated with organic matter versus the various mineral phases using whole rock data alone. In this study we present a new geochemical analysis method designed to target elements that are primarily associated with kerogen. The method involves high-temperature leaching of samples that have had their kerogen destroyed during pyrolysis. To evaluate the impact of kerogen destruction on the leaching results, we compare the leachates of post-pyrolysis samples to leachates from splits of the same sample that did not undergo pyrolysis. Using this approach, in combination with whole rock XRF data, we evaluated the inorganic geochemistry of a core collected from the Eagle Ford Shale. Elements including Al, Fe, K, Ti, and Si were leached in greater amounts using post-pyrolysis samples where kerogen had first been destroyed. This suggests that detrital minerals are intimately associated with the organic matter. This detrital input correlated with substantial increases in TOC in the Eagle Ford and cross plots with Al showed that most trace metals (Mo, V, Ni, Co, Cu, Zn, Cr, etc.) were linked to the detrital fraction. Enrichment factors for Mo and V were the greatest among the trace metals examined and were highest nearest the base of the Eagle Ford and generally decreased toward the top of the unit. Estimated DOP ranged from about 0.4 to 0.8 and was highest at the base, but was also elevated at the top of the Eagle Ford section. This suggests that the lower trace metal content and TOC concentrations in the upper member of the Eagle Ford are not explainable by changes in the degree of anoxia alone. Instead, the metal and organic carbon inventory in the upper Eagle Ford was reduced due to changes in productivity associated with sea level changes and a substantial decrease in detrital influx. This geochemical analysis demonstrates the utility of the post-pyrolysis leaching method developed here and highlights some important concepts regarding the changing paleoenvironmental conditions during the deposition of the Eagle Ford.

The volume of petroleum produced water (PPW) has increased dramatically over the last decade. PPWs are rich in salts and often contain high concentrations of potentially toxic trace elements such as Barium (Ba), Strontium (Sr), Selenium (Se), and Arsenic (As) which, under the right circumstances, may contaminate freshwater supplies. To avoid these issues, PPW is frequently disposed of in subsurface aquifers such as the dolomitic Arbuckle Group in the state of Oklahoma, USA. This may not be a permanent solution if the injected PPW migrates within the disposal sites to ultimately reach conductive fault zones and/or existing wells with casing and/or cementing failures. In these cases the water could pose an environmental risk to potable groundwater. In order to understand the mobility of these elements under conditions similar to produced water injection, we conducted a series of batch sorption experiments. We investigated the effect of brine salinity and temperature on the sorption of Ba, Sr, Se, and As by dolomite. The results revealed that the sorption of the tested elements on dolomite did not substantially change with increasing salinity from 18 to 90 g/L. However, the sorption for all elements did increase with increasing temperature from 22 to 150 °C. This is likely due to a combination of strong surface complexation or ion exchange reactions coupled with precipitation/co-precipitation on the dolomite mineral surfaces.

Produced water is a byproduct of oil and gas production. The chemistry of produced water may provide information about the source of the fluid and its evolution, leading to an improved understanding of the hydrology of petroleum systems. In this study, samples from 19 wells from the Tuscaloosa Marine Shale (TMS) in Mississippi and Louisiana, USA were analyzed for their major and trace element compositions. Data obtained from produced waters from the TMS were compared to existing chemical data from produced waters collected from nearby hydrocarbon reservoir rocks within the Gulf Coast Basin. The results show that produced waters from the TMS are highly saline, with a mean concentration of 15.9 g/L of total dissolved solids. Comparison of the chemistry of produced water from the TMS to early flowback waters demonstrated a rapid shift from the more dilute fracturing fluid to the formation water endmember composition. Most of the trace metals showed a moderate to strong correlation with the overall salinity of the waters. Concentrations of Cu and V showed a moderate correlation with the amount of oil produced from the TMS wells, suggesting that these elements are strongly affiliated with the kerogen and subsequent dissolved (

In order to better understand the factors influencing zinc (Zn) isotope composition in hydrological systems, we analyzed the δ 66 Zn of dissolved Zn in the streams and groundwater of the Upper and Middle Rio Grande watershed in Colorado and New Mexico, United States. The stream water samples have a wider variation of δ 66 Zn (−0.57 to + 0.41 ‰ relative to the JMC 3-0749-Lyon standard) than groundwater samples (−0.13 to + 0.12 ‰ ) and than samples from streams that are in close proximity to abandoned mining sites (+0.24 to + 0.40 ‰ ). Regional changes of bedrock geology, from primarily igneous rocks to primarily sedimentary rocks, have no resolvable effect on the δ 66 Zn of aqueous samples. Instead, an increase in water pH from 7.5 to 8.5 corresponds to a considerable decrease in the δ 66 Zn of dissolved Zn ( R 2 = − 0.37 , p = 0.003 , n = 22 ). Consequently, we link the observed Zn isotope variations to the process of adsorption of Zn onto suspended sediment and bedrock minerals (average Δ 66 Zn adsorbed-dissolved = + 0.31 ‰ ). Our results are in good agreement with previous experimental and empirical studies suggesting that Zn adsorption leads to a residual dissolved pool enriched in light Zn isotopes. Given that anthropogenic Zn sources can also be responsible for lowering of δ 66 Zn, and may overlap with the pH/adsorption effect on δ 66 Zn, the latter needs to be carefully considered in future studies to differentiate between natural and anthropogenic factors influencing Zn isotopes in this and other aquatic systems.

In order to characterize the sulfide-derived SO 4 fluxes in the Rio Grande, we collected seasonally (from 2009 to 2011) riverine, agricultural drain and groundwater samples and analyzed them for their major element chemistries and the δ 34 S and δ 18 O of dissolved SO 4 . The observed variation of δ 34 S (− 4 to + 8‰) and δ 18 O (− 2 to + 7‰) in the Rio Grande mainly resulted from mixing between sulfide- and sulfate-derived SO 4 of volcanic and sedimentary origin. Our S isotope mass balance suggests that the average sulfide-derived SO 4 flux usually accounted for 83–94% (± 10–20%) of the sulfate source in the upstream Rio Grande and decreased downstream to 45–51% because of increasing contributions of sulfate-derived SO 4 . The sulfide-derived SO 4 was related to snow melt in the high elevation watersheds and recycling of surficial sulfate-rich salts by episodic water activity in dry areas at lower elevations. Additionally, elevated bedrock sulfide contents in volcanic and some sedimentary terrains of the studied area have been recognized as important factors contributing to sulfide-derived SO 4 in the Rio Grande.

Ignoring the presence of pyrite can lead to errors in the estimation of Total Organic Carbon (TOC) since pyrite has significantly higher density and conductivity compared to other minerals in shale formations. This study aims to improve the accuracy of estimating TOC from well log data by accounting for the pyrite effect in Eagle Ford shale. To this end, more than 50 feet of preserved cores samples from the Eagle Ford were analyzed using laboratory pyrolysis, X-ray fluorescence (XRF), X-Ray Diffraction (XRD), and spectral core gamma system. Since there is significant vertical heterogeneity in the Eagle Ford shale, parameters such as TOC, pyrite content, Gamma ray intensity, content of Fe and S, and concentrations of U, Th, and K were analyzed on a fine scale in the Upper and Lower Eagle Ford respectively. Analysis of laboratory TOC data were applied to calibrate TOC data using geophysical well logs methodologies. Pyrite data from XRD analysis were used to find the relationship between pyrite and organic matter and to determine the effect of pyrite on well logs. Well-log-based TOC calculation methods were improved by considering pyrite as an adjustable parameter in equations. Schmoker's (1983) four-component system rock model and Alfred and Vernik's (2013) two-pore system model are two representatives of density-log-based TOC calculation methods. Based on these two models, a new petrophysical model considering pyrite and organic porosity was developed. In this research, empirical correlation between TOC and pyrite was explored. Changes of Fe and S concentrations with depth and Gamma ray intensity was determined. The trends of Fe sand S contents matched Gamma ray intensity very well in the depth range from 13790 ft to 13825 ft. Empirical relationships were found between TOC and Gamma ray intensity, TOC and Uranium, respectively. Furthermore, a new petrophysical model considering pyrite and organic porosity was validated with TOC and density data from shale formations. The proposed model improves the estimation of TOC calculation in Eagle Ford formation by the incorporation of pyrite effect.

Dryland river systems are becoming saltier, severely degrading the quality of water used for agriculture and limiting its domestic use. In this context, it is becoming important to understand how the sources and abundances of salts in these river systems will respond to increasingly arid climates. Using the Rio Grande in the Southwest U.S. as an example dryland river system, we show that changes in climate over the last century are closely linked to variations in salt chemistry and therefore salt sources. Starting ∼25 years ago there has been a shift toward more Cl-rich surface waters in the Rio Grande. This shift may reflect a tipping point in the relative influence of anthropogenic activities on the overall surface water budget. Climate change is accelerating this transition through the loss of rain and snow in the headwaters region, resulting in a loss of connectivity of surface flow in the upper and lower sections of the river. The implication of this relationship for dryland rivers is that salt chemistry and sources are likely to become more heterogeneous in the future, reflecting more localized natural (inflow of groundwater) and anthropogenic (waste and industrial effluents, irrigation returns) influences.

A distinctive sulfur cycle dominates many geological processes on Mars and hydrated sulfate minerals are found in numerous topographic settings with widespread occurrences on the Martian surface. However, many of the key processes controlling the hydrological transport of sulfur, including sulfur sources, climate and the depositional history that led to precipitation of these minerals, remain unclear. In this paper, we use a model for the formation of sulfate efflorescent salts (Mg–Ca–Na sulfates) in the Rio Puerco watershed of New Mexico, a terrestrial analog site from the semiarid Southwest U.S., to assess the origin and environmental conditions that may have controlled deposition of hydrated sulfates in Valles Marineris on Mars. Our terrestrial geochemical results ( δ S 34 of −36.0 to + 11.1 ‰ ) show that an ephemeral arid hydrological cycle that mobilizes sulfur present in the bedrock as sulfides, sulfate minerals, and dry/wet atmospheric deposition can lead to widespread surface accumulations of hydrated sulfate efflorescences. Repeating cycles of salt dissolution and reprecipitation appear to be major processes that migrate sulfate efflorescences to sites of surface deposition and ultimately increase the aqueous SO 4 2 − flux along the watershed (average 41,273 metric tons/yr). We suggest that similar shallow processes may explain the occurrence of hydrated sulfates detected on the scarps and valley floors of Valles Marineris on Mars. Our estimates of salt mass and distribution are in accord with studies that suggest a rather short-lived process of sulfate formation (minimum rough estimate ∼100 to 1000 years) and restriction by prevailing arid conditions on Mars.

Seepage beneath artificial levees is a common concern during flooding events. Risk of levee failure is elevated when piping erodes channels beneath the levee, evidenced by the formation of sand boils where transported sediments discharge. Along the lower Mississippi River, pathways of floodwater beneath the levee vary with surface geology, following deeper paths where the levee overlies fine-grained channel-fill deposits, and shallower (higher risk) paths where it overlies sand-bar deposits. Shallow, organic-rich alluvial aquifers are often geochemically stratified into upper oxic and lower anoxic zones, raising the possibility of using the geochemical signatures of discharging water from sand boils to differentiate flow pathways. A preliminary investigation north of Vicksburg, MS, (USA) during the 2011 Mississippi River flood demonstrated the potential of using geochemistry to identify deep and shallow pathways, though the study was limited to cation and trace element analyses. Sampling during the 2015 and 2016 floods for temperature, conductivity, redox potential (Eh), dissolved oxygen (DO), major ions, trace elements, tritium, and stable isotope ratios of oxygen, hydrogen, and strontium, facilitated a greater understanding of the nature of flow and geochemical evolution of groundwater in this environment. Characteristics of deeper flow pathways (relative to shallow) included (1) lower Eh and higher Fe and As, reflecting anoxic conditions and high-Fe sediments, (2) proportional increases in Fe and HCO3, indicating reductive dissolution of Fe-oxyhydroxides, and (3) higher ratios of Ba/Ca and Sr/Ca, reflecting differences in the elemental composition of minerals with depth. Tritium results indicate that subsurface flow pathways are dynamic, shifting spatially with the rapid changes in hydraulic gradients during and between flooding events. Estimated residence times of groundwater discharging from sand boils and relief wells ranged from essentially zero (discharge of concurrent floodwater) up to a quarter century. Lower strontium isotope ratios (87Sr/86Sr) were observed in the aquifer relative to river water, though with no clear variation with flow depth. Oxygen and hydrogen isotope ratios (δ18O and δ2H) show evidence of partial evaporation prior to recharge, also with no apparent variation with subsequent flow depth.

Fe isotope fractionation between pyrite (FeS 2 ) and dissolved Fe in NaCl- and sulfur-bearing aqueous fluids was determined under hydrothermal conditions (300–350 °C, 500 bars). The data were collected using two different, but complementary, experimental approaches, one involving classical Fe isotope exchange between Fe in pyrite and dissolved Fe in coexisting fluid, while the other involved homogenous precipitation of pyrite in a redox and pH buffered chemical system. Results from these experiments indicate equilibrium Fe isotopic fractionation between pyrite and fluid, Δ 56 Fe Pyr–Fe(aq) , of 0.99 ± 0.29‰ (2 σ ), in 56 Fe/ 54 Fe. The experimentally determined equilibrium pyrite–fluid Fe isotopic fractionation agrees with theoretical and spectrally-based predictions. The second series of experiments were conducted to better constrain the effect of precipitation rate on the temporal evolution of the Fe isotopic composition of pyrite and Fe bearing fluids in dynamic mixing environments, such as hydrothermal vent sites at mid-ocean ridges. Rapid homogenous precipitation of pyrite at 300 and 350 °C indicates that δ 56 Fe of dissolved Fe is significantly greater than pyrite that formed during the earliest stage of the experiment, possibly facilitated by either equilibrium or kinetic isotope effects involving FeS as a reactant during pyrite formation. Subsequent recrystallization of pyrite results in a Fe isotopic fractionation with dissolved Fe that moves towards the experimentally determined equilibrium Fe isotopic fractionation with reaction progress. The experimental data reported here may help to decipher the complex kinetic and thermodynamic processes involved in pyrite formation at deep sea vents, while also providing constraints for the rapidly developing theoretical models used to estimate equilibrium Fe isotope fractionation between pyrite and fluid at elevated temperatures and pressures.

The Najd aquifers in Oman, are located in one of the most arid zones in the world. In such regions, there is a shortage in the water resources where groundwater is a very critical component for human life. The main aim of this contribution is to use the satellite remote sensing data of the Gravity Recovery and Climate Experiment (GRACE) along with the Global Land Data Assimilation System (GLDAS), to estimate the groundwater storage changes at the Najd aquifers. Groundwater storage changes were calculated from both GRACE/GLDAS data and from the groundwater level measurements. It was found that the estimated groundwater storage changes from GRACE and water levels coincide in their trends showing a noticeable depletion within the time period from Oct. 2002 to Sept. 2014. The spatial distribution maps of the groundwater storage changes show slightly changes from Oct. 2003 to Sept. 2010, but a significant decreasing were observed from 2010 to 2014. The groundwater storage over Najd aquifers was decreased by about 0.44 and 0.46 km3/year as calculated from GRACE data and groundwater levels, respectively. We also found that groundwater storage was affected by the strong storm events as observed in 2007 and 2010. This contribution could be helpful for the long term sustainable groundwater management in the study area and other arid regions.

Bioleaching has been suggested as an alternative to traditional mining techniques in extraterrestrial environments because it does not require extensive infrastructure and bulky hardware. In situ bioleaching of silicate minerals, such as those found on the moon or Mars, has been proposed as a feasible alternative to traditional extraction techniques that require either extreme heat and/or substantial chemical treatment. In this study, we investigated the biotic and abiotic leaching of basaltic rocks (analogues to those found on the moon and Mars) and the mineral ilmenite (FeTiO3) in aqueous environments under acidic (pH ∼ 2.5) and circumneutral pH conditions. The biological leaching experiments were conducted using Acidithiobacillus ferrooxidans, an iron (Fe)-oxidizing bacteria, and Pseudomonas mendocina, an Fe-scavenging bacteria. We found that both strains were able to grow using the Fe(II) derived from the tested basaltic rocks and ilmenite. Although silica leaching rates were the same or slightly less in the bacterial systems with A. ferrooxidans than in the abiotic control systems, the extent of Fe, Al and Ti released (and re-precipitated in new solid phases) was actually greater in the biotic systems. This is likely because the Fe(II) leached from the basalt was immediately oxidized by A. ferrooxidans, and precipitated into Fe(III) phases which causes a change in the equilibrium of the system, i.e. Le Chatelier's principle. Iron(II) in the abiotic experiment was allowed to build up in solution which led to a decrease in its overall release rate. For example, the percentage of Fe, Al and Ti leached (dissolved + reactive mineral precipitates) from the Mars simulant in the A. ferrooxidans experimental system was 34, 41 and 13% of the total Fe, Al and Ti in the basalt, respectively, while the abiotic experimental system released totals of only 11, 25 and 2%. There was, however, no measurable difference in the amounts of Fe and Ti released from ilmenite in the experiments with A. ferrooxidans versus the abiotic controls. P. mendocina scavenged some Fe from the rock/mineral substrates, but the overall amount of leaching was small (A. ferrooxidans does lead to enhanced release of Fe, Al and Ti and subsequent sequestration of Fe (and other metals) in Fe(III)-precipitates.

Climate change in the SW USA is likely to involve drier conditions and higher surface temperatures. In order to better understand the evolution of water chemistry and the sources of aqueous SO4 in these semi-arid settings, chemical and S isotope compositions were determined of springs, groundwater, and bedrock associated with a Permian fractured carbonate aquifer located in the southern Sacramento Mountains, New Mexico, USA. The results suggest that the evolution of water chemistry in the semi-arid carbonate aquifer is mainly controlled by dedolomitization of bedrock, which was magnified by increasing temperature and increasing dissolution of gypsum/anhydrite along the groundwater flow path. The δ34S of dissolved SO4 in spring and groundwater samples varied from +9.0‰ to +12.8‰, reflecting the mixing of SO4 from the dissolution of Permian gypsum/anhydrite (+12.3‰ to +13.4‰) and oxidation of sulfide minerals (−24.5‰ to −4.2‰). According to S isotope mass balance constraints, the contribution of sulfide-derived SO4 was considerable in the High Mountain recharge areas, accounting for up to ∼10% of the total SO4 load. However, sulfide weathering decreased in importance in the lower reaches of the watershed. A smaller SO4 input of ∼2–4% was contributed by atmospheric wet deposition. This study implies that the δ34S variation of SO4 in semi-arid environments can be complex, but that S isotopes can be used to distinguish among the different sources of weathering. Here it was found that H2SO4 dissolution due to sulfide oxidation contributes up to 5% of the total carbonate weathering budget, while most of the SO4 is released from bedrock sources during dedolomitization.

The sources of SO 4 along a ~ 550 km stretch of the Rio Grande in New Mexico and western Texas were investigated using stable S isotopes. During 2007 and 2008, the δ 34 S of dissolved SO 4 in the Rio Grande surface water varied over a narrow range from − 1.6 to + 0.9‰, which was consistent with the δ 34 S of local fertilizers (− 2.1 to + 1.6‰) and was not consistent with Paleozoic evaporite sources of SO 4 in regional bedrock (+ 7.6 to + 12.9‰). This is likely due the fact that SO 4 is the major component of N–P–K fertilizers used in the Rio Grande Valley, constituting about half of the total fertilizers by mass. The SO 4 /Cl ratios of the groundwater system are relatively low (0.06 to 3) compared to the fertilizer source, suggesting that more Cl is added to the Rio Grande from geological sources as compared to SO 4 . In the Mesilla Basin in southern New Mexico, we identified zones of mixing between recharging irrigation water with groundwater within the depth range of ~ 50–200 m below the ground surface. For this aquifer, Principal Component Analysis (PCA) indicated that Na–K–Cl concentrations were largely attributable to geological sources and SO 4 –Mg–Ca concentrations to anthropogenic sources. Here, an additional anthropogenic source of SO 4 (with a δ 34 S of − 2.7‰) was linked to anaerobic decomposition of manure on a horse farm. In this case SO 4 concentrations (800 mg/L) increased by about three times compared to background SO 4 concentrations in groundwater ( 2 SO 4 in fertilizer manufacturing, anthropogenic SO 4 fluxes to rivers and shallow aquifers from irrigation waters can be significant worldwide.

Copper isotopes may prove to be a useful tool for investigating bacteria–metal interactions recorded in natural waters, soils, and rocks. However, experimental data which attempt to constrain Cu isotope fractionation in biologic systems are limited and unclear. In this study, we utilized Cu isotopes (δ 65 Cu) to investigate Cu–bacteria interactions, including surface adsorption and intracellular incorporation. Experiments were conducted with individual representative species of Gram-positive ( Bacillus subtilis ) and Gram-negative ( Escherichia coli ) bacteria, as well as with wild-type consortia of microorganisms from several natural environments. Ph-dependent adsorption experiments were conducted with live and dead cells over the pH range 2.5–6. Surface adsorption experiments of Cu onto live bacterial cells resulted in apparent separation factors (Δ 65 Cu solution–solid = δ 65 Cu solution − δ 65 Cu solid ) ranging from +0.3‰ to +1.4‰ for B. subtilis and +0.2‰ to +2.6‰ for E. coli . However, because heat-killed bacterial cells did not exhibit this behavior, the preference of the lighter Cu isotope by the cells is probably not related to reversible surface adsorption, but instead is a metabolically-driven phenomenon. Adsorption experiments with heat-killed cells yielded apparent separation factors ranging from +0.3‰ to −0.69‰ which likely reflects fractionation from complexation with organic acid surface functional group sites. For intracellular incorporation experiments the lab strains and natural consortia preferentially incorporated the lighter Cu isotope with an apparent Δ 65 Cu solution–solid ranging from ∼+1.0‰ to +4.4‰. Our results indicate that live bacterial cells preferentially sequester the lighter Cu isotope regardless of the experimental conditions. The fractionation mechanisms involved are likely related to active cellular transport and regulation, including the reduction of Cu(II) to Cu(I). Because similar intracellular Cu machinery is shared by fungi, plants, and higher organisms, the influence of biological processes on the δ 65 Cu of natural waters and soils is probably considerable.

The service life of mechanically stabilized earth walls depends on the rate of corrosion of the metallic reinforcements used in their construction. The assessment of corrosion potential requires an accurate evaluation of pH, resistivity, and sulfate and chloride concentrations of aqueous solutions in contact with the surrounding aggregate. Highway agencies tend to use larger aggregates that contain only a small amount of fine material (passing the Number 40 sieve) in the backfill. Evaluation of the electrochemical parameters of coarse aggregates is challenging because traditional evaluation methods call for the use of fine material. In this study, the suitability of traditional soil characterization techniques for use with coarse aggregates was assessed through leaching experiments performed on coarse limestone and dolomite aggregates from six quarries in Texas. Chemical differences were isolated from size-related kinetic leaching effects by comparing the results from same-sized material collected in the field with material derived from the crushing of larger (≥⅜ in.) aggregates in the laboratory. The testing demonstrated that the fines collected from the field were enriched in chemicals that, when exposed to water, decreased pH and resistivity and increased sulfate concentrations compared with the bulk rock. This was likely the result of sulfur compounds in the atmosphere reacting with carbonate rocks to produce reactive surface layers that were mechanically abraded into the fines. This phenomenon could bias traditional soil testing results and, therefore, the assessment of corrosion potential. This study demonstrated that a more accurate assessment of the electrochemical parameters can be obtained by crushing the coarse material to meet testing size specifications.

An increased interest in high precision Cu isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has developed recently for various natural geologic systems and environmental applications, these typically contain high concentrations of sulfur, particularly in the form of sulfate (SO 4 2− ) and sulfide (S). For example, Cu, Fe, and Zn concentrations in acid mine drainage (AMD) can range from 100 µg/L to greater than 50 mg/L with sulfur species concentrations reaching greater than 1000 mg/L. Routine separation of Cu, Fe and Zn from AMD, Cu–sulfide minerals and other geological matrices usually incorporates single anion exchange resin column chromatography for metal separation. During chromatographic separation, variable breakthrough of SO 4 2− during anion exchange resin column chromatography into the Cu fractions was observed as a function of the initial sulfur to Cu ratio, column properties, and the sample matrix. SO 4 2− present in the Cu fraction can form a polyatomic 32 S– 14 N– 16 O– 1 H species causing a direct mass interference with 63 Cu and producing artificially light δ 65 Cu values. Here we report the extent of the mass interference caused by SO 4 2− breakthrough when measuring δ 65 Cu on natural samples and NIST SRM 976 Cu isotope spiked with SO 4 2− after both single anion column chromatography and double anion column chromatography. A set of five 100 µg/L Cu SRM 976 samples spiked with 500 mg/L SO 4 2− resulted in an average δ 65 Cu of − 3.50‰ ± 5.42‰ following single anion column separation with variable SO 4 2− breakthrough but an average concentration of 770 µg/L. Following double anion column separation, the average SO 4 2− concentration of 13 µg/L resulted in better precision and accuracy for the measured δ 65 Cu value of 0.01‰ ± 0.02‰ relative to the expected 0‰ for SRM 976. We conclude that attention to SO 4 2− breakthrough on sulfur-rich samples is necessary for accurate and precise measurements of δ 65 Cu and may require the use of a double ion exchange column procedure.

Here the hydrogeochemical constraints of a tracer dilution study are combined with Fe and Zn isotopic measurements to pinpoint metal loading sources and attenuation mechanisms in an alpine watershed impacted by acid mine drainage. In the tested mountain catchment, δ56Fe and δ66Zn isotopic signatures of filtered stream water samples varied by ∼3.5‰ and 0.4‰, respectively. The inherent differences in the aqueous geochemistry of Fe and Zn provided complimentary isotopic information. For example, variations in δ56Fe were linked to redox and precipitation reactions occurring in the stream, while changes in δ66Zn were indicative of conservative mixing of different Zn sources. Fen environments contributed distinctively light dissolved Fe ( +0.4‰). Acidic drainage from mine wastes contributed heavier dissolved Fe (∼+0.5‰) and lighter Zn (∼+0.2‰) isotopes relative to the fen. Upwelling of Fe-rich groundwater near the mouth of the catchment was the major source of Fe (δ56Fe ∼ 0‰) leaving the watershed in surface flow, while runoff from mining wastes was the major source of Zn. The results suggest that given a strong framework for interpretation, Fe and Zn isotopes are useful tools for identifying and tracking metal sources and attenuation mechanisms in mountain watersheds.

We measured the Fe, Cu, and Zn isotopic compositions of the fluids generated during leaching experiments with pyrite-, chalcopyrite-, and sphalerite-rich rocks and with a sphalerite mineral separate. Our study demonstrates that the oxidative weathering of sulfide-rich rocks can produce substantial variations in Fe (− 1.75 to + 1.0‰ Δ 56 Fe solution-pyrite rock ) and Cu (0.0 to + 2.0‰ Δ 65 Cu solution-chalcopyrite rock ) isotopes and small variations in Zn isotopes (0.0 to + 0.2‰ Δ 66 Zn solution-sphalerite ) in the fluid phase relative to the rock. For the Fe and Cu systems we suggest that isotopic fractionation is caused by electron-exchange-driven (e.g., Fe(II)/Fe(III) and Cu(I)/Cu(II) redox) reactions at the surfaces of the sulfide minerals that occur during air and aqueous chemical reactions. Under acidic conditions, these reactions tend to enrich the fluid phase in the heavier Fe and Cu isotopes. However, under circumneutral pH conditions, the Fe isotopic composition in solution was controlled by the precipitation of Fe(III)-oxide phases, which enriched the solution in the lighter Fe isotopes. This investigation provides a preliminary framework for interpreting the impact of sulfide oxidation reactions on the distributions of stable Fe, Cu, and Zn isotopes in natural waters.

Zinc and Cu play important roles in the biogeochemistry of natural systems, and it is likely that these interactions result in mass-dependent fractionations of their stable isotopes. In this study, we examine the relative abundances of dissolved Zn and Cu isotopes in a variety of stream waters draining six historical mining districts located in the United States and Europe. Our goals were to (1) determine whether streams from different geologic settings have unique or similar Zn and Cu isotopic signatures and (2) to determine whether Zn and Cu isotopic signatures change in response to changes in dissolved metal concentrations over well-defined diel (24-h) cycles. Average δ66Zn and δ65Cu values for streams varied from +0.02‰ to +0.46‰ and −0.7‰ to +1.4‰, respectively, demonstrating that Zn and Cu isotopes are heterogeneous among the measured streams. Zinc or Cu isotopic changes were not detected within the resolution of our measurements over diel cycles for most streams. However, diel changes in Zn isotopes were recorded in one stream where the fluctuations of dissolved Zn were the largest. We calculate an apparent separation factor of ∼0.3‰ (66/64Zn) between the dissolved and solid Zn reservoirs in this stream with the solid taking up the lighter Zn isotope. The preference of the lighter isotope in the solid reservoir may reflect metabolic uptake of Zn by microorganisms. Additional field investigations must evaluate the contributions of soils, rocks, minerals, and anthropogenic components to Cu and Zn isotopic fluxes in natural waters. Moreover, rigorous experimental work is necessary to quantify fractionation factors for the biogeochemical reactions that are likely to impact Cu and Zn isotopes in hydrologic systems. This initial investigation of Cu and Zn isotopes in stream waters suggests that these isotopes may be powerful tools for probing biogeochemical processes in surface waters on a variety of temporal and spatial scales.

Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxyhydroxide is examined in experimental mixtures of metal-rich acid rock drainage and relatively pure river water and during batch adsorption experiments using synthetic ferrihydrite. A diverse set of Cu- and Zn-bearing solutions was examined, including natural waters, complex synthetic acid rock drainage, and simple NaNO3 electrolyte. Metal adsorption data are combined with isotopic measurements of dissolved Cu (65Cu/63Cu) and Zn (66Zn/64Zn) in each of the experiments. Fractionation of Cu and Zn isotopes occurs during adsorption of the metal onto amorphous ferric oxyhydroxide. The adsorption data are modeled successfully using the diffuse double layer model in PHREEQC. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (αsoln–solid) are 0.99927 ± 0.00008 for Cu and 0.99948 ± 0.00004 for Zn or, alternately, the separation factors (Δsoln–solid) are −0.73 ± 0.08‰ for Cu and −0.52 ± 0.04‰ for Zn. These factors indicate that the heavier isotope preferentially adsorbs onto the oxyhydroxide surface, which is consistent with shorter metal–oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous ferric oxyhydroxide suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define aqueous metal interactions with oxyhydroxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by the oxyhydroxide with Cu > Zn > Fe(II).

This paper provides previously unpublished drill core and mapping evidence of the spatial relationship between regional dolomitization and Mississippi Valley-type (MVT) mineralization in the midcontinent of the United States. Dolomitization is discussed for several key carbonate units and their contained MVT mineralization. This more complete picture of the relationship between regional dolomitization and mineralization provides constraints on models for MVT mineralization and serves as an exploration guide for new deposits.

The measurement of Cu, Fe, and Zn isotopes in natural samples may provide valuable information about biogeochemical processes in the environment. However, the widespread application of stable Cu, Fe, and Zn isotope chemistry to natural water systems remains limited by our ability to efficiently separate these trace elements from the greater concentrations of matrix elements. In this study, we present a new method for the isolation of Cu, Fe, and Zn from complex aqueous solutions using a single anion-exchange column with hydrochloric acid media. Using this method we are able to quantitatively separate Cu, Fe, and Zn from each other and from matrix elements in a single column elution. Elution of the elements of interest, as well as all other elements, through the anion-exchange column is a function of the speciation of each element in the various concentrations of HCl. We highlight the column chemistry by comparing our observations with published studies that have investigated the speciation of Cu, Fe, and Zn in chloride solutions. The functionality of the column procedure was tested by measuring Cu, Fe, and Zn isotopes in a variety of stream water samples impacted by acid mine drainage. The accuracy and precision of Zn isotopic measurements was tested by doping Zn-free stream water with the Zn isotopic standard. The reproducibility of the entire column separation process and the overall precision of the isotopic measurements were also evaluated. The isotopic results demonstrate that the Cu, Fe, and Zn column separates from the tested stream waters are of sufficient purity to be analyzed directly using a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS), and that the measurements are fully-reproducible, accurate, and precise. Although limited in scope, these isotopic measurements reveal significant variations in δ 65 Cu (− 1.41 to + 0.30‰), δ 56 Fe (− 0.56 to + 0.34‰), and δ 66 Zn (0.31 to 0.49‰) among samples collected from different abandoned mines within a single watershed. Hence, Cu, Fe, and Zn isotopic measurements may be a powerful tool for fingerprinting specific metal sources and/or examining biogeochemical reactions within fresh water systems.

In this study, we conducted experiments that tested for the existence of ternary surface complexes involving bacteria, metal cations and natural organic matter (NOM). We performed a variety of batch complexation experiments in single (NOM only), binary ( Bacillus subtilis –metal, NOM–metal, B. subtilis –NOM), and ternary ( B. subtilis –metal–NOM) systems in order to determine the significance of ternary complexation. We conducted experiments as a function of 1) pH, 2) component concentrations, 3) fraction of NOM (humic, fulvic, or bulk), and 4) metal of interest (Pb, Cu, Cd, and Ni). Our investigative approach was to quantify the differences between the binary and ternary experimental systems under these different conditions. The concentration of NOM bound in ternary form was calculated directly from experimental data by comparing binary and ternary systems. The concentration of metal bound in ternary form was calculated using chemical equilibrium modeling. Experimental results demonstrate that bacteria–metal–NOM complex formation is a rapid, fully-reversible chemical process. The pH of the aqueous system is the biggest factor in the stability, and therefore significance of ternary complexes. As pH decreases, bacteria–metal–NOM complexes become more stable. The fraction of NOM involved and the identity of the aqueous metal cation have a comparatively modest impact on the stability of ternary complexes. All NOM fractions form ternary complexes to similar extents at circumneutral pH, but humic acid becomes the dominant NOM fraction in ternary complexes at low pH. The abundance of humic acid in ternary form is greatest in experimental systems with Ni or Cd and less in systems with Pb and Cu. These observations suggest that bacteria–metal–NOM complexes are abundant under the conditions present in most natural waters. Hence, ternary complexes may impact the mobility of aqueous metal cations in natural systems by changing dissolved NOM–metal complexes to colloidal bacteria–metal–NOM complexes.

Adsorption onto bacterial cell walls can significantly affect the specia- tion and mobility of aqueous metal cations in many geologic settings. However, a unified thermodynamic framework for describing bacterial adsorption reactions does not exist. This problem originates from the numerous approaches that have been chosen for modeling bacterial surface protonation reactions. In this study, we compile all currently available potentiometric titration datasets for individual bacterial species, bacterial consortia, and bacterial cell wall components. Using a consistent, four discrete site, non-electrostatic surface complexation model, we determine total func- tional group site densities for all suitable datasets, and present an averaged set of 'universal' thermodynamic proton binding and site density parameters for modeling bacterial adsorption reactions in geologic systems. Modeling results demonstrate that the total concentrations of proton-active functional group sites for the 36 bacterial species and consortia tested are remarkably similar, averaging 3.2 1.0 (1) 10 4 moles/wet gram. Examination of the uncertainties involved in the development of proton-binding modeling parameters suggests that ignoring factors such as bacterial species, ionic strength, temperature, and growth conditions introduces relatively small error compared to the unavoidable uncertainty associated with the determination of cell abundances in realistic geologic systems. Hence, we propose that reasonable estimates of the extent of bacterial cell wall deprotonation can be made using averaged thermodynamic modeling parameters from all of the experiments that are considered in this study, regardless of bacterial species used, ionic strength, temperature, or growth condition of the experiment. The average site densities for the four discrete sites are 1.1 0.7 10 4 , 9.1 3.8 10 5 , 5.3 2.1 10 5 , and 6.6 3.0 10 5 moles/wet gram bacteria for the sites with pKa values of 3.1, 4.7, 6.6, and 9.0, respectively. It is our hope that this thermodynamic framework for modeling bacteria-proton binding reactions will also provide the basis for the development of an internally consistent set of bacteria-metal binding constants. 'Universal' constants for bacteria-metal binding reactions can then be used in conjunc- tion with equilibrium constants for other important metal adsorption and complex- ation reactions to calculate the overall distribution of metals in realistic geologic systems.

To model the effects of bacterial metal adsorption in contaminated environments, results from metal adsorption experiments involving individual pure stains of bacteria must be extrapolated to systems in which potentially dozens of bacterial species are present. This extrapolation may be made easier because bacterial consortia from natural environments appear to exhibit similar metal binding properties. However, bacteria that thrive in highly perturbed contaminated environments may exhibit significantly different adsorptive behavior. Here we measure proton and Cd adsorption onto a range of bacterial consortia grown from heavily contaminated industrial wastes, groundwater, and soils. We model the results using a discrete site surface complexation approach to determine binding constants and site densities for each consortium. The results demonstrate that bacterial consortia from different contaminated environments exhibit a range of total site densities (approximately a 3-fold difference) and Cd-binding constants (approximately a 10-fold difference). These ranges for Cd binding constants may be small enough to suggest that bacteria-metal adsorption in contaminated environments can be described using relatively few "averaged" bacteria-metal binding constants (in conjunction with the necessary binding constants for competing surfaces and ligands). However, if additional precision is necessary, modeling parameters must be developed separately for each contaminated environment of interest.