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Mountainous watersheds are characterized by variability in functional traits, including vegetation, topography, geology, and geomorphology, which together determine nitrogen (N) retention, and release. Coal Creek and East River are two contrasting catchments within the Upper Colorado River Basin that differ markedly in total nitrate (NO3-) export. The East River has a diverse vegetation cover, sinuous floodplains, and is underlain by N-rich marine shale, resulting in a three to twelve times greater total NO3- export relative to the conifer-dominated Coal Creek. While this can partly be explained by the larger size of the East River, the distinct watershed traits of these two catchments imply different mechanisms controlling the aggregate N-export signal. A causality analysis shows biogenic and geogenic processes were critical in determining NO3- export from the East River catchment. Stable isotope ratios of NO3- (δ15NNO3 and δ18ONO3) show the East River catchment is a strong hotspot for biogeochemical processing of NO3- at the soil-saprolite interface and within the floodplain prior to export. By contrast, the conifer-dominated Coal Creek retained nearly all (~97 %) atmospherically-deposited NO3-, and its export was controlled by catchment hydrological traits (i.e., snowmelt periods and water table depth). The conservative N-cycle within Coal Creek is likely due to the abundance of conifer trees, and a smaller riparian region, retaining more NO3- overall and reduced processing prior to export. This study highlights the value of integrating isotope systematics to link watershed functional traits to mechanisms of watershed element retention and release.

Minimizing loss of injected hydraulic fracturing fluids into shale along fracture-matrix boundaries is desired because imbibed water restricts gas production and wastes valuable water resources. This problem has motivated the addition of surfactants into water-based hydraulic fracturing fluids in order to reduce the capillary driving force for imbibition. Here, we show that reduction in interfacial tension and wettability alteration has negligible ability to reduce imbibition in deep gas reservoirs. The effectiveness of altering capillary forces acting at the wetting front also depends on the injection pressure acting at the fracture-matrix boundary. The pressure at the interface between the fracture and the shale matrix is constrained between the reservoir pore pressure and formation pressure (rock fracture pressure, also known as breakdown pressure of the rock) and increases with depth to magnitudes that greatly exceed that of capillary pressures. The analyses presented here show that even maximum alteration of interfacial properties that result in strongly hydrophobic interactions between the fracturing fluid and reservoir rock is incapable of significantly reducing imbibition in deep reservoirs. Instead of using surfactants, this analysis points to decreases in wellbore shut-in pressures and shut-in times as practical options for reducing imbibition losses of water-based fluids.

Atmospheric nitrous oxide contributes directly to global warming, yet models of the nitrogen cycle do not account for bedrock, the largest pool of terrestrial nitrogen, as a source of nitrous oxide. Although it is known that release rates of nitrogen from bedrock are large, there is an incomplete understanding of the connection between bedrock-hosted nitrogen and atmospheric nitrous oxide. Here, we quantify nitrogen fluxes and mass balances at a hillslope underlain by marine shale. We found that, at this site, bedrock weathering contributes 78% of the subsurface reactive nitrogen, while atmospheric sources (commonly regarded as the sole sources of reactive nitrogen in pristine environments) account for only the remaining 22%. About 56% of the total subsurface reactive nitrogen denitrifies, including 14% emitted as nitrous oxide. The remaining reactive nitrogen discharges in porewaters to a floodplain where additional denitrification probably occurs. We also found that the release of bedrock nitrogen occurs primarily within the zone of the seasonally fluctuating water table and suggest that the accumulation of nitrate in the vadose zone, often attributed to fertilization and soil leaching, may also include contributions from weathered nitrogen-rich bedrock. Our hillslope study suggests that, under oxygenated and moisture-rich conditions, weathering of deep, nitrogen-rich bedrock makes an important contribution to the nitrogen cycle. Weathering of deep bedrock releases reactive nitrogen into the subsurface, which contributes to the flux of nitrous oxide to the atmosphere, according to a field study that combines soil, rock and groundwater data within a river catchment.

The mixed-wet nature of reservoir formations imposes a wide range of rock wettability from strong resident-fluid wetting to strong invading-fluid wetting. The characteristics of two-phase flow in porous media composed of mixed-wetting surfaces remain poorly understood. In this study, we investigated the displacement of resident ethylene glycol (EG) by hexane in two mixed-wet micromodels of identical 2.5-D geometry heterogeneity, with uniformly or heterogeneously distributed patches strongly wetting to hexane. These patches are mixed among pores with unaltered EG-wetting surfaces. Along with control tests in the originally EG-wet micromodel, we show the classic fingering and transitions in flow regimes at (Formula presented.) (capillary number) from −7.2 to −3.9. Moreover, pore-scale distributions of wettability and their spatial correlation influence displacement efficiency. In the two mixed-wet micromodels, we found (a) an increase of steady-state hexane saturation at the end of experiments by up to 0.12 in the capillary fingering regime and a decrease of at most by 0.06 in the viscous fingering regime, compared to the EG-wet micromodel, and (b) dispersed and fragmented hexane distribution after displacement. Brine drainage during supercritical CO2 (scCO2) injections in these micromodels occurs with lower wettability contrasts, and under similar viscosity ratios and interfacial tensions resulted in higher displacement efficiency relative to displacement of EG by hexane. While mixed-wettability can enhance displacement efficiency compared to uniform wettability, the dynamics of immiscible fluids in strong mixed-wet reservoirs are expected to be less pronounced in contributing to the efficiency of geological CO2 sequestration, oil recovery, and remediation of hydrocarbon-contaminated aquifers.

The ability to predict multiphase fluid transport in nanoporous rocks such as shales is critical for many geoscience applications, for example unconventional hydrocarbon production, geologic carbon sequestration, and nuclear waste disposal. When the pore sizes approach nanoscales, the impact of the molecular interaction forces between fluids and solids becomes increasingly important. These forces can alter macroscopic fluid phase behavior and control transport. Recent experimental studies have shown that capillary condensation and subsequent imbibition of liquid water can occur in hydrophilic nanoporous media even if the vapor phase is at a critical relative humidity (rhcrit) well below vapor saturation. This study presents a theoretical investigation of the processes controlling adsorption, capillary condensation and imbibition in nanoporous media, using the square-gradient classical density functional theory. The proposed theoretical model explicitly includes the relevant interaction forces among fluids and solids in macroscopic porous media. Application of the model to a relative-humidity-controlled water adsorption experiment is presented to demonstrate the impact of water-pore wall attractive forces on multiphase water behavior in a hydrophilic silicon nanoporous medium. The model represents well the measured time-dependent evolution of the water imbibition front inside the nanoporous medium and also explains the diffusion-like water transport regimes observed at rhrhcrit. The study furthermore gives an insight on hysteresis phenomenon in adsorption and desorption isotherms.

Author(s): Tokunaga, TK; Wan, J; Williams, KH; Brown, W; Henderson, A; Kim, Y; Tran, AP; Conrad, ME; Bill, M; Carroll, RWH; Dong, W; Xu, Z; Lavy, A; Gilbert, B; Carrero, S; Christensen, JN; Faybishenko, B; Arora, B; Siirila-Woodburn, ER; Versteeg, R; Raberg, JH; Peterson, JE; Hubbard, SS | Abstract: Major components of hydrologic and elemental cycles reside underground, where their complex dynamics and linkages to surface waters are obscure. We delineated seasonal subsurface flow and transport dynamics along a hillslope in the Rocky Mountains (USA), where precipitation occurs primarily as winter snow and drainage discharges into the East River, a tributary of the Gunnison River. Hydraulic and geochemical measurements down to 10 m below ground surface supported application of transmissivity feedback of snowmelt to describe subsurface flow and transport through three zones: soil, weathering shale, and saturated fractured shale. Groundwater flow is predicted to depths of at least 176 m, although a shallower limit exists if hillslope-scale hydraulic conductivities are higher than our local measurements. Snowmelt during the high snowpack water year 2017 sustained flow along the weathering zone and downslope within the soil, while negligible downslope flow occurred along the soil during the low snowpack water year 2018. We introduce subsurface concentration-discharge (C-Q) relations for explaining hillslope contributions to C-Q observed in rivers and demonstrate their calculations based on transmissivity fluxes and measured pore water specific conductance and dissolved organic carbon. The specific conductance data show that major ions in the hillslope pore waters, primarily from the weathering and fractured shale, are about six times more concentrated than in the river, indicating hillslope solute loads are disproportionately high, while flow from this site and similar regions are relatively smaller. This methodology is applicable in different representative environments within snow-dominated watersheds for linking their subsurface exports to surface waters.

Multiphase flow phenomena in nanoporous media are encountered in many science and engineering applications. Shales, for example, possessing complex nanopore networks, have considerable importance as source rocks for unconventional oil and gas production and as low-permeability seals for geologic carbon sequestration or nuclear waste disposal. This study presents a theoretical investigation of the processes controlling adsorption, capillary condensation, and imbibition in such nanoporous media, with a particular focus on understanding the effects of fluid-fluid and fluid-pore wall interaction forces in the interconnected nanopore space. Building on a new theoretical framework, we developed a numerical model for the multiphase nanoporous flow and tested it against water vapor uptake measurements conducted on a shale core sample. The model, which is based on the density functional approach, explicitly includes the relevant interaction forces among fluids and solids while allowing for a continuum representation of the porous medium. The experimental data include gravimetrically measured mass changes in an initially dry core sample exposed to varying levels of relative humidity, starting with a low relative humidity (rh = 0.31) followed by a period of a higher relative humidity (rh = 0.81). During this process, water vapor uptake in the dry core is recorded as a function of time. Our model suggests that, under low rh conditions, the flow within the shale sample is controlled by adsorption- and diffusion-type processes. After increasing the rh to 0.81, the uptake of water vapor becomes more significant, and according to our model, this can be explained by capillary condensation followed by immiscible displacement in the core sample. It appears that strong fluid-pore wall attractive forces cause condensation near the inlet, which then induces water imbibition further into sample.

Author(s): Chang, Chun; Kneafsey, Timothy J; Wan, Jiamin; Tokunaga, Tetsu K; Nakagawa, Seiji

Author(s): Tokunaga, TK | Abstract: Predicting water imbibition into porous materials is important in a wide variety of fields, and hydraulic fracturing of low permeability hydrocarbon reservoirs has emerged as an application that is imposing a large water footprint. Reliable predictions of imbibition are needed to better manage water use, yet are challenging because of uncertainties in both the permeability and capillary pressure driving force. Here, this uncertainty is reduced through evaluating correlations between the permeability and the effective capillary pressure associated with the wetting front, Pc,f. These correlations allow elimination of Pc,f from the Green and Ampt equation and concentrate all uncertainties in fluxes on the effective permeability k. Over a wide range of k and porosities n, imbibition scales approximately with k1/3. Although Leverett k1/4 scaling for predicting Pc,f is shown to be inferior when tested with data spanning a wide range of n, it nevertheless predicted imbibition fairly well. From simple imbibition measurements, both the empirical and Leverett scaling approaches allow estimates of k that have root-mean-square deviations of about one order of magnitude relative to measurements that ranged over 10 orders of magnitude in k.

Author(s): Wan, J; Tokunaga, TK; Dong, W; Williams, KH; Kim, Y; Conrad, ME; Bill, M; Riley, WJ; Hubbard, SS | Abstract: Understanding terrestrial carbon cycling has relied primarily on studies of topsoils that are typically characterized to depths shallower than 0.5nm. At a semiarid site instrumented down to 7nm, we measured seasonal- and depth-resolved carbon inventories and fluxes and groundwater and unsaturated zone flow rates. Measurements showed that ~30% of the CO2 efflux to the atmosphere (60% in winter) originates from below 1nm, contrary to predictions of less than 1% by Earth System Model land modules. Respiration from deeper roots and deeper microbial communities is supported by favorable subsurface temperatures, moisture, and oxygen availability. Below 1nm, dissolved organic carbon fluxes from the overlying soil and C from deep roots and exudates are expected to be important in sustaining microbial respiration. Because these conditions are characteristic of semiarid climate regions, we contend that Earth System Model land modules should incorporate such deeper soil processes to improve CO2 flux predictions.

Author(s): Wang, L; Wan, J; Tokunaga, TK; Kim, Y; Yu, Q | Abstract: Shale gas equilibrates through gas-liquid-solid interactions in reservoirs, but the role of moisture is rarely investigated. To determine how adsorbed water influences methane behavior, three carboniferous shale samples from the Qaidam Basin, China, were humidified at five levels up to a relative humidity of 89%, and their methane capacities at pressures up to 12nMPa were studied. The experimental results indicate that two water-related mechanisms, “water blocking for methane transport” and “surface competition for gas-solid interaction,” are primarily responsible for the methane capacity variations. A compositional comparison suggests that a high abundance of clay minerals plays a favorable role in methane migration by retaining water in interlayer pores. Based on the experimental data, an optimized method for calculating the adsorption amount based on an approximation of density distribution is proposed. The model predicts the average thickness of the adsorption layer and the adsorbed methane density distribution on the surface at a given pressure. The methane adsorption layer “thins” in a stepped pattern by up to 45% in the presence of water, with little further change observed at relative humidities greater than 75% in the studied samples.

Interactions between supercritical (sc) CO2 and minerals are important when CO2 is injected into geologic formations for storage and as working fluids for enhanced oil recovery, hydraulic fracturing, and geothermal energy extraction. It has previously been shown that at the elevated pressures and temperatures of the deep subsurface, scCO2 alters smectites (typical swelling phyllosilicates). However, less is known about the effects of scCO2 on nonswelling phyllosilicates (illite and muscovite), despite the fact that the latter are the dominant clay minerals in deep subsurface shales and mudstones. Our studies conducted by using single crystals, combining reaction (incubation with scCO2), visualization [atomic force microscopy (AFM)], and quantifications (AFM, X-ray photoelectron spectroscopy, X-ray diffraction, and off-gassing measurements) revealed unexpectedly high CO2 uptake that far exceeded its macroscopic surface area. Results from different methods collectively suggest that CO2 partially entered the muscovite interlayers, although the pathways remain to be determined. We hypothesize that preferential dissolution at weaker surface defects and frayed edges allows CO2 to enter the interlayers under elevated pressure and temperature, rather than by diffusing solely from edges deeply into interlayers. This unexpected uptake of CO2, can increase CO2 storage capacity by up to ∼30% relative to the capacity associated with residual trapping in a 0.2-porosity sandstone reservoir containing up to 18 mass % of illite/muscovite. This excess CO2 uptake constitutes a previously unrecognized potential trapping mechanism.

Gas production from shale gas reservoirs plays a significant role in satisfying increasing energy demands. Compared with conventional sandstone and carbonate reservoirs, shale gas reservoirs are characterized by extremely low porosity, ultra-low permeability and high clay content. Slip flow, diffusion, adsorption and desorption are the primary gas transport processes in shale matrix, while Darcy flow is restricted to fractures. Understanding methane diffusion and adsorption, and gas flow and equilibrium in the low-permeability matrix of shale is crucial for shale formation evaluation and for predicting gas production. Modeling of diffusion in low-permeability shale rocks requires use of the Dusty gas model (DGM) rather than Fick’s law. The DGM is incorporated in the TOUGH2 module EOS7C-ECBM, a modified version of EOS7C that simulates multicomponent gas mixture transport in porous media. Also included in EOS7C-ECBM is the extended Langmuir model for adsorption and desorption of gases. In this study, a column shale model was constructed to simulate methane diffusion and adsorption through shale rocks. The process of binary $$\hbox {CH}_{4}{-}\hbox {N}_{2}$$ diffusion and adsorption was analyzed. A sensitivity study was performed to investigate the effects of pressure, temperature and permeability on diffusion and adsorption in shale rocks. The results show that methane gas diffusion and adsorption in shale is a slow process of dynamic equilibrium, which can be illustrated by the slope of a curve in $$\hbox {CH}_{4}$$ mass variation. The amount of adsorption increases with the pressure increase at the low pressure, and the mass change by gas diffusion will decrease due to the decrease in the compressibility factor of the gas. With the elevated temperature, the gas molecules move faster and then the greater gas diffusion rates make the process duration shorter. The gas diffusion rate decreases with the permeability decrease, and there is a limit of gas diffusion if the permeability is less than $$1.0\,\times \,10^{-15}\, \hbox { m}^{2}$$ . The results can provide insights for a better understanding of methane diffusion and adsorption in the shale rocks so as to optimize gas production performance of shale gas reservoirs.

Author(s): Hubbard, SS; Williams, KH; Agarwal, D; Banfield, J; Beller, H; Bouskill, N; Brodie, E; Carroll, R; Dafflon, B; Dwivedi, D; Falco, N; Faybishenko, B; Maxwell, R; Nico, P; Steefel, C; Steltzer, H; Tokunaga, T; Tran, PA; Wainwright, H; Varadharajan, C | Abstract: Extreme weather, fires, and land use and climate change are significantly reshaping interactions within watersheds throughout the world. Although hydrological–biogeochemical interactions within watersheds can impact many services valued by society, uncertainty associated with predicting hydrologydriven biogeochemical watershed dynamics remains high. With an aim to reduce this uncertainty, an approximately 300-km2 mountainous headwater observatory has been developed at the East River, CO, watershed of the Upper Colorado River Basin. The site is being used as a testbed for the Department of Energy supported Watershed Function Project and collaborative efforts. Building on insights gained from research at the “sister” Rifle, CO, site, coordinated studies are underway at the East River site to gain a predictive understanding of how the mountainous watershed retains and releases water, nutrients, carbon, and metals. In particular, the project is exploring how early snowmelt, drought, and other disturbances influence hydrological–biogeochemical watershed dynamics at seasonal to decadal timescales. A system-of-systems perspective and a scale-adaptive simulation approach, involving the combined use of archetypal watershed subsystem “intensive sites” are being tested at the site to inform aggregated watershed predictions of downgradient exports. Complementing intensive site hydrological, geochemical, geophysical, microbiological, geological, and vegetation datasets are long-term, distributed measurement stations and specialized experimental and observational campaigns. Several recent research advances provide insights about the intensive sites as well as aggregated watershed behavior. The East River “community testbed” is currently hosting scientists from more than 30 institutions to advance mountainous watershed methods and understanding.

How the wettability of pore surfaces affects supercritical (sc) CO2 capillary trapping in geologic carbon sequestration (GCS) is not well understood, and available evidence appears inconsistent. Using a high-pressure micromodel-microscopy system with image analysis, we studied the impact of wettability on scCO2 capillary trapping during short-term brine flooding (80 seconds, 8 to 667 pore volumes). Experiments on brine displacing scCO2 were conducted at 8.5 MPa and 45°C in water-wet (static contact angle θ = 20° ± 8°) and intermediate-wet (θ = 94° ± 13°) homogeneous micromodels under four different flow rates (capillary number Ca ranging from 9 × 10−6 to 8 × 10−4) with a total of eight conditions (four replicates for each). Brine invasion processes were recorded and statistical analysis was performed for over two thousand images of scCO2 saturations, and scCO2 cluster characteristics. The trapped scCO2 saturation under intermediate-wet conditions is 15% higher than under water-wet conditions under the slowest flow rate (Ca ∼ 9 × 10−6). Based on the visualization and scCO2 cluster analyses, we show that the scCO2 trapping process in our micromodels is governed by bypass trapping that is enhanced by the larger contact angle. Smaller contact angles enhance cooperative pore filling and widen brine fingers (or channels), leading to smaller volumes of scCO2 being bypassed. Increased flow rates suppress this wettability effect.

Wettability is an important factor controlling the displacement of immiscible fluids in porous media and therefore affects the flow and transport of supercritical (sc) CO2 in geologic carbon sequestration. Because few studies have focused on the wettability effect in scCO2-brine flow systems, we experimentally and numerically tested influences of wettability on drainage at the pore network scale. Using a high-pressure micromodel-microscopy system, we performed experiments of scCO2 invasion into brine-saturated water-wet and intermediate-wet micromodels, and recorded the scCO2 invasion morphology under reservoir relevant conditions. We also performed pore-scale numerical simulations to infer 3D details of fluid–fluid displacement processes. During drainage under intermediate-wet conditions, we found higher scCO2 saturation, wider scCO2 fingering, and more compact displacement patterns. The simulation results are qualitatively consistent with the experiments. Through quantitative analyses of the experiments, we found that the reduced wettability decreases the displacement front velocity, promotes non-wetting phase pore-filling events in the longitudinal direction, delays the breakthrough time of invading fluid, and increases the displacement efficiency. Simulated results also show that the fluid–fluid interface area follows a unified power-law relation with scCO2 saturation, and show smaller interface area in intermediate-wet case which suppresses the mass transfer between the phases. These pore-scale results provide insights for the wettability effects on CO2–brine immiscible displacement in geologic carbon sequestration.

© 2017 American Chemical Society. Three-dimensional variably saturated flow and multicomponent biogeochemical reactive transport modeling, based on published and newly generated data, is used to better understand the interplay of hydrology, geochemistry, and biology controlling the cycling of carbon, nitrogen, oxygen, iron, sulfur, and uranium in a shallow floodplain. In this system, aerobic respiration generally maintains anoxic groundwater below an oxic vadose zone until seasonal snowmelt-driven water table peaking transports dissolved oxygen (DO) and nitrate from the vadose zone into the alluvial aquifer. The response to this perturbation is localized due to distinct physico-biogeochemical environments and relatively long time scales for transport through the floodplain aquifer and vadose zone. Naturally reduced zones (NRZs) containing sediments higher in organic matter, iron sulfides, and non-crystalline U(IV) rapidly consume DO and nitrate to maintain anoxic conditions, yielding Fe(II) from FeS oxidative dissolution, nitrite from denitrification, and U(VI) from nitrite-promoted U(IV) oxidation. Redox cycling is a key factor for sustaining the observed aquifer behaviors despite continuous oxygen influx and the annual hydrologically induced oxidation event. Depth-dependent activity of fermenters, aerobes, nitrate reducers, sulfate reducers, and chemolithoautotrophs (e.g., oxidizing Fe(II), S compounds, and ammonium) is linked to the presence of DO, which has higher concentrations near the water table. (Figure Presented).

Although bedrock weathering strongly influences water quality and global carbon and nitrogen budgets, the weathering depths and rates within subsurface are not well understood nor predictable. Determination of both porewater chemistry and subsurface water flow are needed in order to develop more complete understanding and obtain weathering rates. In a long-term field study, we applied a multiphase approach along a mountainous watershed hillslope transect underlain by marine shale. Here we report three findings. First, the deepest extent of the water table determines the weathering front, and the range of annually water table oscillations determines the thickness of the weathering zone. Below the lowest water table, permanently water-saturated bedrock remains reducing, preventing deeper pyrite oxidation. Secondly, carbonate minerals and potentially rock organic matter share the same weathering front depth with pyrite, contrary to models where weathering fronts are stratified. Thirdly, the measurements-based weathering rates from subsurface shale are high, amounting to base cation exports of about 70 kmolc ha−1 y−1, yet consistent with weathering of marine shale. Finally, by integrating geochemical and hydrological data we present a new conceptual model that can be applied in other settings to predict weathering and water quality responses to climate change.

SummaryWatersheds are important for supplying fresh water, the quality of which depends on complex interplay involving physical, chemical and biological processes. As water percolates through the soil and underlying weathering rock en route to the river corridor, microorganisms mediate key geochemical transformations, yet the distribution and functional capacities of subsurface microbial communities remain little understood. We have studied metabolic capacities of microbial communities along a meadow to floodplain hillslope transect within the East-River watershed, Colorado, using genome resolved metagenomics and carbon and hydrogen stable isotopes. Very limited strain/species overlap was found at different depths below the ground surface and at different distances along the hillslope, possibly due to restricted hydraulic connectivity after early stages of snowmelt. Functions such as carbon fixation and selenate reduction were prevalent at multiple sites, although the lineages of organisms responsible tend to be location-specific. Based on its abundance, sulfur is significantly more important for microbial metabolism at the floodplain compared to on the hillslope. Nitrification and methylamine oxidation are likely only occurring within the floodplain, with nitrification capacity in shallow soil, and methylamine oxidation in deeper unsaturated sediment. Biogenic methane was detected in deep surface samples, but methanogenic organisms were not identified.Originality-Significance StatementIn a previous study within a hillslope to riparian zone transect of a sub-alpine watershed, the community structure was explored using ribosomal protein S3 genes, and the metabolic potential was hypothesized based on the presence of metabolism related genes. However, tying specific strains and species to metabolic functioning was not discussed as resolved genomes were not available.In the current study, we use genome-resolved metagenomics along with carbon and hydrogen stable isotopes to explore the spatial distribution of biogeochemical processes. By linking taxonomy and function, using multiple functional genes indicative of full metabolic pathways, we detect heterogeneity in the distribution of metabolic potential and the organisms involved with depth and landscape position. Thus, we infer how microbiome genomic variation impacts biogeochemical cycling across the watershed.We found very limited strain/species overlap at different depths below the surface and along the hillslope, possibly due to the restricted site to site hydraulic connectivity, and show that communities are largely distinct in their metabolic capacities. Both proximity to the river and the underlying Mancos shale apparently control species distribution and metabolic potential.Functions such as carbon fixation and selenate reduction were prevalent at multiple sites, although the lineages of organisms responsible tend to be location-specific. Arsenate detoxification was found to be prevalent in the riparian zone whereas selenate reduction was detected within weathered Mancos shale. We conclude that important ecosystem functions are strongly associated with the riparian zone, some of which may have crucial implications as to water quality and human health.

Capillary pressure (Pc) – saturation (Sw) relations are essential for predicting equilibrium and flow of immiscible fluid pairs in soils and deeper geologic formations. In systems that are difficult to measure, behavior is often estimated based on capillary scaling of easily measured Pc–Sw relations (e.g., air-water, and oil-water), yet the reliability of such approximations needs to be examined. In this study, seventeen sets of brine drainage and imbibition curves were measured with air-brine, decane-brine, and supercritical (sc) CO2-brine in homogeneous quartz and carbonate sands, using porous plate systems under ambient (0.1 MPa, 23 ˚C) and reservoir (12.0 MPa, 45 ˚C) conditions. Comparisons between these measurements showed significant differences in residual nonwetting phase saturation, Snw,r. Through applying capillary scaling, changes in interfacial properties were indicated, particularly wettability. With respect to the residual trapping of the nonwetting phases, Snwr, CO2 > Snwr, decane > Snwr, air. Decane-brine and scCO2-brine Pc–Sw curves deviated significantly from predictions assuming hydrophilic interactions. Moreover, neither the scaled capillary behavior nor Snw,r for scCO2-brine were well represented by decane-brine, apparently because of differences in wettability and viscosities, indicating limitations for using decane (and other organic liquids) as a surrogate fluid in studies intended to apply to geological carbon sequestration. Thus, challenges remain in applying scaling for predicting capillary trapping and multiphase displacement processes across such diverse fields as vadose zone hydrology, enhanced oil recovery, and geologic carbon sequestration. This article is protected by copyright. All rights reserved.

CO 2 flooding is a proven enhanced oil recovery (EOR) technique and is also considered as a potential method for CO 2 sequestration. Despite having successful field trials on CO 2 EOR, the effects of reservoir wettability and permeability heterogeneity on the efficiency of miscible CO 2 flooding are not well understood. In this work, laboratory investigations have been carried out to evaluate the influence of these properties on the miscible CO 2 EOR performance. The wettability of hydrophilic Berea core samples was altered to be oil-wet by vacuum saturation of the clean and dry core samples with n-hexadecane. The permeability heterogeneity was obtained by combining two half pieces of axially split water-wet core samples of different permeabilities. Core flooding experiments were conducted for n-hexadecane – synthetic brine – CO 2 systems at 1400 psig backpressure to achieve minimum miscibility pressure (MMP) of CO 2 in n-hexadecane at the test temperature (24 ± 1 °C). It was found that wettability strongly influences CO 2 EOR. For the alternate cases of previously brine flooded (to remaining oil saturation) oil-wet and water-wet core samples, five pore volumes (PVs) of CO 2 recovered 100% and only 43% of remaining oil in place (ROIP) respectively. Three PVs of CO 2 could recover only about 0–5% ROIP from the split core samples. The mechanisms underlying these results are discussed. This study sheds light on the significant influence of reservoir wettability and permeability heterogeneity on the performance of miscible CO 2 EOR.

Recharge of alluvial aquifers is a key component in understanding the interaction between floodplain vadose zone biogeochemistry and groundwater quality. The Rifle Site (a former U-mill tailings site) adjacent to the Colorado River is a well-established field laboratory that has been used for over a decade for the study of biogeochemical processes in the vadose zone and aquifer. This site is considered an exemplar of both a riparian floodplain in a semiarid region and a post-remediation U-tailings site. In this paper we present Sr isotopic data for groundwater and vadose zone porewater samples collected in May and July 2013 to build a mixing model for the fractional contribution of vadose zone porewater (i.e. recharge) to the aquifer and its variation across the site. The vadose zone porewater contribution to the aquifer ranged systematically from 0% to 38% and appears to be controlled largely by the microtopography of the site. The area-weighted average contribution across the site was 8% corresponding to a net recharge of 7.5 cm. Given a groundwater transport time across the site of ~1.5 to 3 years, this translates to a recharge rate between 5 and 2.5 cm/yr, and with the average precipitation to the site implies a loss from the vadose zone due to evapotranspiration of 83% to 92%, both ranges are in good agreement with previously published results by independent methods. A uranium isotopic (234U/238U activity ratios) mixing model for groundwater and surface water samples indicates that a ditch across the site is hydraulically connected to the aquifer and locally significantly affects groundwater. Groundwater samples with high U concentrations attributed to natural bio-reduced zones have 234U/238U activity ratios near 1, suggesting that the U currently being released to the aquifer originated from the former U-mill tailings.

Author(s): Tokunaga, TK; Kim, Y; Wan, J; Bill, M; Conrad, M; Dong, W | Abstract: Transport from the soil surface to groundwater is commonly mediated through deeper portions of the vadose zone and capillary fringe, where variations in temperature and water saturation strongly influence biogeochemical processes. This technical note describes a sediment column design that allows laboratory simulation of thermal and hydrologic conditions found in many field settings. Temperature control is particularly important because room temperature is not representative of most subsurface environments. A 2.0-m-tall column was capable of simulating profiles with temperatures ranging from 3 to 22°C, encompassing the full range of seasonal temperature variation observed in the deep vadose zone and capillary fringe of a semiarid floodplain in western Colorado. The water table was varied within the lower 0.8-m section of the column, and profiles of water content and matric potential were measured. Vadose zone CO2 collected from depth-distributed gas samplers under representative seasonal conditions reflected the influences of temperature and water table depth on microbial respiration. Thus, realistic subsurface biogeochemical dynamics can be simulated in the laboratory through establishing column profiles that represent seasonal thermal and hydrologic conditions.

Author(s): Cihan, A; Wang, S; Tokunaga, TK; Birkholzer, JT | Abstract: Understanding the main mechanisms affecting long-term migration and redistribution of injected CO2 in geological carbon storage is needed for developing predictive models to assess environmental risks and designing monitoring schemes. Preparation of a postinjection site care plan is required for CO2 injection wells, including monitoring of pressure changes and injected CO2 plume. Knowledge gaps exist regarding assessment of postinjection monitoring timeframes for the CO2 plume because the processes driving long-term CO2 plume migration and trapping are not fully understood. In the postinjection stage of geological carbon storage, redistribution of CO2 occurs mainly due to buoyancy and capillary forces. This work presents experimental and modeling studies to investigate processes contributing to postinjection plume distribution and stabilization. We conducted a flow cell experiments (0.5nmn×n0.05nmn×n0.01nm) with two immiscible fluid phases in a glass bead porous medium to study postinjection plume behavior. We employed a hysteretic macroscopic two-phase flow model to interpret the experimental results and to understand main processes leading to plume stabilization. Our findings show that capillary pressure hysteresis explains the experimentally observed plume shape and redistribution at early postinjection stages; however, the long-term plume migration and eventual plume stabilization can only be represented when in addition microscale heterogeneity is accounted for. Results also show that plume stabilization can be extremely slow and that the migration of the plume front can occur through multiple intermittent bursts over long times. Further studies are needed to understand implications of the results for more realistic porous media and large-scale storage reservoirs.

Author(s): Hu, R; Wan, J; Yang, Z; Chen, YF; Tokunaga, T | Abstract: When a more viscous fluid displaces a less viscous one in porous media, viscous pressure drop stabilizes the displacement front against capillary pressure fluctuation. For this favorable viscous ratio conditions, previous studies focused on the front instability under slow flow conditions but did not address competing effects of wettability and flow rate. Here we study how this competition controls displacement patterns. We propose a theoretical model that describes the crossover from fingering to stable flow as a function of invading fluid contact angle θ and capillary number Ca. The phase diagram predicted by the model shows that decreasing θ stabilizes the displacement for θ≥45° and the critical contact angle θc increases with Ca. The boundary between corner flow and cooperative filling for θ l 45° is also described. This work extends the classic phase diagram and has potential applications in predicting CO2 capillary trapping and manipulating wettability to enhance gas/oil displacement efficiency.

Significance Reliable estimates of geologic carbon storage capacities (needed for policymaking) in both saline aquifers and unconventional gas/oil shales rely on understanding trapping mechanisms. We found that CO2 uptake by muscovite (a common mineral and a conservative proxy for illite) far exceeds the maximum adsorption capacity of its external surface area. Our measurements using different methods collectively suggest that CO2 enters muscovite interlayers without bulk interlayer expansion, contrary to the conventional wisdom that only swelling clays take up CO2 into interlayers. Because the nonswelling illitic clay is the major clay mineral in deep subsurface tight rocks, their excess uptake of CO2 may significantly contribute to CO2 storage capacity and warrants further in-depth studies., Interactions between supercritical (sc) CO2 and minerals are important when CO2 is injected into geologic formations for storage and as working fluids for enhanced oil recovery, hydraulic fracturing, and geothermal energy extraction. It has previously been shown that at the elevated pressures and temperatures of the deep subsurface, scCO2 alters smectites (typical swelling phyllosilicates). However, less is known about the effects of scCO2 on nonswelling phyllosilicates (illite and muscovite), despite the fact that the latter are the dominant clay minerals in deep subsurface shales and mudstones. Our studies conducted by using single crystals, combining reaction (incubation with scCO2), visualization [atomic force microscopy (AFM)], and quantifications (AFM, X-ray photoelectron spectroscopy, X-ray diffraction, and off-gassing measurements) revealed unexpectedly high CO2 uptake that far exceeded its macroscopic surface area. Results from different methods collectively suggest that CO2 partially entered the muscovite interlayers, although the pathways remain to be determined. We hypothesize that preferential dissolution at weaker surface defects and frayed edges allows CO2 to enter the interlayers under elevated pressure and temperature, rather than by diffusing solely from edges deeply into interlayers. This unexpected uptake of CO2, can increase CO2 storage capacity by up to ∼30% relative to the capacity associated with residual trapping in a 0.2-porosity sandstone reservoir containing up to 18 mass % of illite/muscovite. This excess CO2 uptake constitutes a previously unrecognized potential trapping mechanism.

It is commonly true that a diluted colloidal suspension is more stable over time than a concentrated one because dilution reduces collision rates of the particles and therefore delays the formation of aggregates. However, this generalization does not apply for some engineered ligand-coated nanoparticles (NPs). We observed the opposite relationship between stability and concentration of NPs. We tested 4 different types of NPs: CdSe-11-mercaptoundecanoic acid, CdTe-polyelectrolytes, Ag-citrate, and Ag-polyvinylpyrrolidone. The results showed that dilution alone induced aggregation and subsequent sedimentation of the NPs that were originally monodispersed at very high concentrations. Increased dilution caused NPs to progressively become unstable in the suspensions. The extent of the dilution impact on the stability of NPs is different for different types of NPs. We hypothesize that the unavoidable decrease in free ligand concentration in the aqueous phase following dilution causes detachment of ligands from the suspended NP cores. The ligands attached to NP core surfaces must generally approach exchange equilibrium with free ligands in the aqueous phase; therefore, ligand detachment and destabilization are expected consequences of dilution. More studies are necessary to test this hypothesis. Because the stability of NPs determines their physicochemical and kinetic behavior including toxicity, dilution-induced instability needs to be understood to realistically predict the behavior of engineered ligand-coated NPs in aqueous systems. Environ Toxicol Chem 2018;37:1301-1308. © 2018 SETAC.

© 2017 Elsevier Inc. When a wetting liquid is displaced by air in a capillary tube, a wetting film develops between the tube wall and the air that is responsible for the snap-off mechanism of the gas phase. By dissolving a dye in the wetting phase it is possible to relate a measure of the absorbance in the capillary to the thickness of liquid films. These data could be used to compare with cutting edge numerical simulations of the dynamics of snap-off for which experimental and numerical data are lacking. Drainage experiments in constricted capillary tubes were performed where a dyed wetting liquid is displaced by air for varying flow rates. We developed an optical method to measure liquid film thicknesses that range from 3 to 1000 μm. The optical measures are validated by comparison with both theory and direct numerical simulations. In a constricted capillary tube we observed, both experimentally and numerically, a phenomenon of snap-off coalescence events in the vicinity of the constriction that bring new insights into our understanding and modeling of two-phase flows. In addition, the good agreement between experiments and numerical simulations gives confidence to use the numerical method for more complex geometries in the future.

In geologic carbon sequestration, capillary pressure (Pc)-saturation (Sw) relations are needed to predict reservoir processes. Capillarity and its hysteresis have been extensively studied in oil-water and gas-water systems, but few measurements have been reported for supercritical (sc) CO2-water. Here, Pc-Sw relations of scCO2 displacing brine (drainage), and brine rewetting (imbibition) were studied to understand CO2 transport and trapping behavior under reservoir conditions. Hysteretic drainage and imbibition Pc-Sw curves were measured in limestone sands at 45 °C under elevated pressures (8.5 and 12.0 MPa) for scCO2-brine, and in limestone and dolomite sands at 23 °C (0.1 MPa) for air-brine using a new computer programmed porous plate apparatus. scCO2-brine drainage and imbibition curves shifted to lower Pc relative to predictions based on interfacial tension, and therefore deviated from capillary scaling predictions for hydrophilic interactions. Fitting universal scaled drainage and imbibition curves show that wettability alteration resulted from scCO2 exposure over the course of months-long experiments. Residual trapping of the nonwetting phases was determined at Pc = 0 during imbibition. Amounts of trapped scCO2 were significantly larger than for those for air, and increased with pressure (depth), initial scCO2 saturation, and time. These results have important implications for scCO2 distribution, trapping, and leakage potential.

Renewed interest in managing C balance in soils is motivated by increasing atmospheric concentrations of CO2 and consequent climate change. Here, experiments were conducted in soil columns to determine C mass balances with and without addition of CaSO4-minerals (anhydrite and gypsum), which were hypothesized to promote soil organic carbon (SOC) retention and soil inorganic carbon (SIC) precipitation as calcite under slightly alkaline conditions. Changes in C contents in three phases (gas, liquid and solid) were measured in unsaturated soil columns tested for one year and comprehensive C mass balances were determined. The tested soil columns had no C inputs, and only C utilization by microbial activity and C transformations were assumed in the C chemistry. The measurements showed that changes in C inventories occurred through two processes, SOC loss and SIC gain. However, the measured SOC losses in the treated columns were lower than their corresponding control columns, indicating that the amendments promoted SOC retention. The SOC losses resulted mostly from microbial respiration and loss of CO2 to the atmosphere rather than from chemical leaching. Microbial oxidation of SOC appears to have been suppressed by increased Ca(2+) and SO4(2)(-) from dissolution of CaSO4 minerals. For the conditions tested, SIC accumulation per m(2) soil area under CaSO4-treatment ranged from 130 to 260 g C m(-1) infiltrated water (20-120 g C m(-1) infiltrated water as net C benefit). These results demonstrate the potential for increasing C sequestration in slightly alkaline soils via CaSO4-treatment.

The current ambiguity on wettability of minerals in CO2–brine systems under the geological CO2 sequestration (GCS) reservoir conditions imparts the greatest uncertainty in predicting capillary behavior controlling safe-storage of CO2. To address this issue we conducted a series of experiments using muscovite as a representative of common aluminosilicate minerals. Based on new experimental results we identified several possible causes of the ambiguity problem in contact angle (CA) measurements. We also found that reaction with water-saturated supercritical (sc) CO2 (but not with scN2) phase severely roughened the muscovite surfaces, largely increased CA hysteresis and CO2 adhesion. Although some methodological influences on contact angle uncertainty can be reduced, the high surface-energy of clean and pristine aluminosilicate minerals have strong tendency to adsorb oppositely changed molecules and particles to reduce their surface energy, resulting in less reproducible CA values. Giving the fact that such clean and pristine mineral surfaces do not exist in real reservoirs, our future investigations shall focus on improving understanding of the effect of long-term CO2–mineral–brine reactions on reservoir wettability under realistic reservoir geochemical conditions.

© 2017 American Chemical Society. Environmentally benign, economical, and effective surfactants and additives are needed in engineering subsurface energy recovery processes. Biosurfactants have some advantages over chemically synthesized surfactants, but the high costs of microbial-biosynthesis limit their applications in subsurface engineering. Here we propose to use naturally occurring biosurfactants contained within Earth's readily available and inexpensive humus deposits for subsurface engineering applications. We collected humus samples of different types from four different regions, and developed a simple method for extracting natural biosurfactant (NBS) using only four common chemicals. The average NBS extraction yields are 16 ± 3% of the raw humus. No significant differences in elemental composition and functional group chemistry were found among the NBS extracted from humus of different origins, suggesting that any humus deposit can be used for NBS extraction. Measurements of interfacial tensions between air-water and supercritical (sc) CO2-water interfaces indicate that the NBS is a highly effective surfactant. NBS has good foaming ability. Preliminary tests with only 0.5 mass % NBS in the aqueous phase (no other additives) yielded scCO2-in-water foams of 83% foam quality. The apparent viscosity of 13 cP measured at a shear rate of 2320 s-1indicates that much higher viscosities are achievable at lower shear rates. These results suggest that NBS merits further research and development for potential applications in subsurface energy production.

© 2017 American Chemical Society. Diffusion is important in controlling local solute transport and reactions in unsaturated soils and geologic formations. Although it is commonly assumed that thinning of water films controls solute diffusion at low water contents, transport under these conditions is not well understood. We conducted experiments in quartz sands at low volumetric water contents (θ) to quantify ion diffusion within adsorbed films. At the lowest water contents, we employed fixed relative humidities to control water films at nm thicknesses. Diffusion profiles for Rb+ and Br- in unsaturated sand packs were measured with a synchrotron X-ray microprobe, and inverse modeling was used to determine effective diffusion coefficients, De, as low as ∼9 × 10-15 m2 s-1 at θ = 1.0 × 10-4 m3 m-3, where the film thickness = 0.9 nm. Given that the diffusion coefficients (Do) of Rb+ and Br- in bulk water (30 °C) are both ∼2.4 × 10-9 m2 s-1, we found the impedance factor f = De/(θDo) is equal to 0.03 ± 0.02 at this very low saturation, in agreement with the predicted influence of interface tortuosity (τa) for diffusion along grain surfaces. Thus, reduced cross-sectional area (θ) and tortuosity largely accounted for the more than 5 orders of magnitude decrease in De relative to Do as desaturation progressed down to nanoscale films.

Understanding the fate and transport of engineered nanoparticles (ENPs) in subsurface environments is required for developing the best strategy for waste management and disposal of these materials. In this study, the deposition and release of quantum dot (QD) nanoparticles were studied in saturated sand columns. The QDs were first deposited in columns using 100 mM NaCl or 2 mM CaC12 solutions. Deposited QDs were then contacted with deionized (DI) water and/or varying Na(+) concentrations to induce release. QDs deposited in 100 mM Na(+) were easily reversible when the column was rinsed with DI water. Conversely, QDs deposited in the presence of Ca(2+) exhibited resistance to release with DI water. However, significant release occurred when the columns were flushed with NaCl solutions. This release behavior was explained by cation exchange (Ca(2+) in exchange sites were replaced by Na(+)) which resulted in the breakdown of calcium bridging. We also studied the effect of aging time on the QD release. As the aging time increased, smaller amounts of QDs were released following cation exchange. However, deposited QDs were subsequently released when the column was flushed with DI water. The release behavior was modeled using a single first-order kinetic release process and changes in the maximum solid phase concentration of deposited QDs with transition in solution chemistry. The results of this study demonstrate that the presence of carboxyl groups on ENPs and divalent ions in the solution plays a key role in controlling ENP mobility in the subsurface environment.

[1] Brine films remaining on mineral surfaces in deep reservoirs during CO2 sequestration are expected to influence multiphase flow, diffusion, and reactions, but little is known about their behavior. Using synchrotron X-ray fluorescence (XRF), we measured thicknesses of KCsI2 brine films on two difference roughness mica surfaces under conditions representative of geological CO2 sequestration (7.8 MPa and 40°C) to understand the influences of mineral surface roughness and capillary potential. Brine thicknesses measured on the Mica 1 (smooth) and Mica 2 (rough) mica surfaces ranged from 23 to 8 nm and 491 to 412 nm, respectively, over the small range of tested capillary potentials (0.18–3.7 kPa). Within these potentials, brine film thicknesses on mica were governed by surface roughness and only weakly influenced by capillary potentials. In comparing drainage and rewetting isotherms, some film thickness hysteresis was observed, possibly indicative of changes in mica wettability.

Capillary pressure and saturation relations for supercritical CO 2 and brine in sand: High-pressure P c (S w ) controller/meter measurements and capillary scaling predictions Tetsu K. Tokunaga, 1 Jiamin Wan, 1 Jong-Won Jung, 1,2 Tae Wook Kim, 1,3 Yongman Kim, 1 and Wenming Dong 1 [ 1 ] In geologic carbon sequestration, reliable predictions of CO 2 storage require understanding the capillary behavior of supercritical (sc) CO 2 . Given the limited availability of measurements of the capillary pressure (P c ) dependence on water saturation (S w ) with scCO 2 as the displacing fluid, simulations of CO 2 sequestration commonly rely on modifying more familiar air/H 2 O and oil/H 2 O P c (S w ) relations, adjusted to account for differences in interfacial tensions. In order to test such capillary scaling-based predictions, we developed a high-pressure P c (S w ) controller/meter, allowing accurate P c and S w measurements. Drainage and imbibition processes were measured on quartz sand with scCO 2 -brine at pressures of 8.5 and 12.0 MPa (45 o C), and air-brine at 21 o C and 0.1 MPa. Drainage and rewetting at intermediate S w levels shifted to P c values that were from 30% to 90% lower than predicted based on interfacial tension changes. Augmenting interfacial tension-based predictions with differences in independently measured contact angles from different sources led to more similar scaled P c (S w ) relations but still did not converge onto universal drainage and imbibition curves. Equilibrium capillary trapping of the nonwetting phases was determined for P c ¼ 0 during rewetting. The capillary-trapped volumes for scCO 2 were significantly greater than for air. Given that the experiments were all conducted on a system with well-defined pore geometry (homogeneous sand), and that scCO 2 -brine interfacial tensions are fairly well constrained, we conclude that the observed deviations from scaling predictions resulted from scCO 2 -induced decreased wettability. Wettability alteration by scCO 2 makes predicting hydraulic behavior more challenging than for less reactive fluids. and Bachu, 2008 ; Benson and Cole, 2008]. The depend- ence of capillary pressure (P c ) on water saturation (S w ) under reservoir conditions is a basic constitutive relation needed to predict CO 2 flow and capillary trapping during sequestration. Indeed, with only knowledge of how (rela- tive) permeability depends on S w , neither fluid flow nor equilibrium saturation conditions are predictable. At later stages of geologic carbon sequestration, rewetting of reser- voirs with native brine results in capillary trapping (resid- ual trapping) of CO 2 , a main storage mechanism [Bachu et al., 2007; IPCC, 2005]. The capillary trapping capacity of reservoir materials has complex dependence on porosity, pore-size distribution, nonwetting phase displacement extent, and the rewetting process [Iglauer et al., 2011b ; Tanino and Blunt, 2012]. Although capillary trapping of CO 2 at its residual saturation depends on P c (S w ) relations, few investigations to date have directly measured these ba- sic relations with CO 2 /H 2 O at reservoir pressures and tem- peratures [Krevor et al., 2011 ; Pentland et al., 2011 ; Pini et al., 2012 ; Plug and Bruining, 2007]. The study by Plug and Bruining [2007] appears to be the only one in which capillary-trapped scCO 2 saturation was determined while 1. Introduction [ 2 ] Geologic carbon sequestration may become an im- portant technology for mitigating CO 2 buildup in the atmosphere from fossil fuel combustion and for moderating climate change [Intergovernmental Panel on Climate Change (IPCC), 2005]. However, predicting the perform- ance of geologic CO 2 sequestration requires reliable under- standing of CO 2 mobility, and its saturation and pressure distribution within reservoirs [Bachu et al., 2007 ; Bennion Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. Now at Civil and Environmental Engineering Department, Louisiana State University, Baton Rouge, Louisiana, USA. Now at Department of Energy Resources Engineering, Stanford Uni- versity, Stanford, California, USA. Corresponding author : T. K. Tokunaga, Earth Sciences Division, Law- rence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, USA. (tktokunaga@lbl.gov)

The movement of injected CO2 through permeable pore networks determines its distribution and stability within reservoirs used for carbon sequestration (Fig. 1), and this process is dependent on capillary interactions with the displaced brine (IPCC 2005; Benson and Cole 2008). Capillary phenomena controlling the distribution of CO2 and reservoir brine depend on pore size (from mm to nm), wetting and interfacial properties. The pressure of the usually nonwetting CO2 phase relative to that of the native brine is the capillary pressure, P c , which in combination with pore size, wettability, and interfacial properties determines the saturation of each fluid phase. In typically reservoirs used for geologic carbon sequestration, CO2 exists as a supercritical (sc) fluid because pressures and temperatures associated with storage depths exceed the critical point values for CO2 (7.38 MPa, 31.1 °C). Thus, predicting carbon sequestration in reservoirs require understanding of interfacial tension, mineral surface wettability, and the capillary pressure dependence on saturation, for interactions between scCO2, brine, and reservoir mineral surfaces. Investigations into these aspects of scCO2 behavior, especially at elevated pressures and temperatures characteristic of geologic carbon sequestration, have largely only recently begun. Given the fairly early stage of research, many models for CO2 transport in reservoirs rely heavily on the better understood capillary characteristics of other immiscible fluid pairs such as air-water, oil-water, and oil-gas, supplemented with properties of scCO2. However, a growing number of experimental studies are being published on the behavior of scCO2 under reservoir conditions, leading to better understanding of C sequestration while also giving rise to new questions. In this chapter, we review aspects of the current (mostly laboratory-based) understanding of equilibrium capillary interactions between scCO2 and brines, and their implications for scCO2 behavior in …

Improving our ability to estimate the parameters that control water and heat fluxes in the shallow subsurface is particularly important due to their strong control on recharge, evaporation and biogeochemical processes. The objectives of this study are to develop and test a new inversion scheme to simultaneously estimate subsurface hydrological, thermal and petrophysical parameters using hydrological, thermal and electrical resistivity tomography (ERT) data. The inversion scheme – which is based on a nonisothermal, multiphase hydrological model – provides the desired subsurface property estimates in high spatiotemporal resolution. A particularly novel aspect of the inversion scheme is the explicit incorporation of the dependence of the subsurface electrical resistivity on both moisture and temperature. The scheme was applied to synthetic case studies, as well as to real datasets that were autonomously collected at a biogeochemical field study site in Rifle, Colorado. At the Rifle site, the coupled hydrological-thermal-geophysical inversion approach well predicted the matric potential, temperature and apparent resistivity with the Nash–Sutcliffe efficiency criterion greater than 0.92. Synthetic studies found that neglecting the subsurface temperature variability, and its effect on the electrical resistivity in the hydrogeophysical inversion, may lead to an incorrect estimation of the hydrological parameters. The approach is expected to be especially useful for the increasing number of studies that are taking advantage of autonomously collected ERT and soil measurements to explore complex terrestrial system dynamics.

© 2016 In order to understand the transport and humification processes of dissolved organic matter (DOM) within sediments of a semi-arid floodplain at Rifle, Colorado, fluorescence excitation–emission matrix (EEM) spectroscopy, humification index (HIX) and specific UV absorbance (SUVA) at 254 nm were applied for characterizing depth and seasonal variations of DOM composition. Results revealed that late spring snowmelt leached relatively fresh DOM from plant residue and soil organic matter down into the deeper vadose zone (VZ). More humified DOM is preferentially adsorbed by upper VZ sediments, while non- or less-humified DOM was transported into the deeper VZ. Interestingly, DOM at all depths undergoes rapid biological humification process evidenced by the products of microbial by-product-like (i.e., tyrosine-like and tryptophan-like) matter in late spring and early summer, particularly in the deeper VZ, resulting in more humified DOM (e.g., fulvic-acid-like and humic-acid-like substances) at the end of year. This indicates that DOM transport is dominated by spring snowmelt, and DOM humification is controlled by microbial degradation, with seasonal variations. It is expected that these relatively simple spectroscopic measurements (e.g., EEM spectroscopy, HIX and SUVA) applied to depth- and temporally-distributed pore-water samples can provide useful insights into transport and humification of DOM in other subsurface environments as well.

© 2016, Springer-Verlag Berlin Heidelberg. Technologies for increasing carbon storage in soils are gathering attention as a means for mitigating atmospheric CO2emissions. Carbon sequestration can be achieved by controlling the organic carbon stock in soil and by accelerating mineral carbonation. In this study, carbon sequestration capacity was measured in soil columns treated with flue gas desulfurization gypsum (FGDG), a by-product of electric power generation. The feasibility of using FGDG as an environmentally benign alternative to gypsum or anhydrite was examined using a toxicity characteristic leaching procedure and Microtox bioassay. While no toxic leachate was generated from the FGDG treatment, some toxic elements in the soil were removed through absorption reactions. Test results for carbon sequestration based on unsaturated soil column experiments suggest that the application of FGDG for soil treatment holds promise of less microbial CO2emission from soil. The net benefits of carbon sequestration from the FGDG treatment were calculated as 87 and 621 g C/m2/m of infiltrated water, for the 1 % calcite-added column and 3 % calcite-added columns, respectively. The presented test results show that the FGDG treatment for soil carbon sequestration holds a promise when it is applied to slightly alkaline soils.

© Soil Science Society of America. Although CO2fluxes from soils are often assumed to originate within shallow soil horizons (

© 2016, US Government. Flood plains play a potentially important role in the global carbon cycle. The accumulation of organic matter in flood plains often induces the formation of chemically reduced groundwater and sediments along riverbanks. In this study, our objective is to evaluate the cumulative impact of such reduced zones, water table fluctuations, and temperature gradients on subsurface carbon fluxes in a flood plain at Rifle, Colorado located along the Colorado River. 2-D coupled variably-saturated, non-isothermal flow and biogeochemical reactive transport modeling was applied to improve our understanding of the abiotic and microbially mediated reactions controlling carbon dynamics at the Rifle site. Model simulations considering only abiotic reactions (thus ignoring microbial reactions) underestimated CO2partial pressures observed in the unsaturated zone and severely underestimated inorganic (and overestimated organic) carbon fluxes to the river compared to simulations with biotic pathways. Both model simulations and field observations highlighted the need to include microbial contributions from chemolithoautotrophic processes (e.g., Fe+2and S−2oxidation) to match locally-observed high CO2concentrations above reduced zones. Observed seasonal variations in CO2concentrations in the unsaturated zone could not be reproduced without incorporating temperature gradients in the simulations. Incorporating temperature fluctuations resulted in an increase in the annual groundwater carbon fluxes to the river by 170 % to 3.3 g m−2d−1, while including water table variations resulted in an overall decrease in the simulated fluxes. We conclude that spatial microbial and redox zonation as well as temporal fluctuations of temperature and water table depth contribute significantly to subsurface carbon fluxes in flood plains and need to be represented appropriately in model simulations.

Because of its warm water and high salinity, the Red Sea is well known as a challenging water source for seawater reverse osmosis (SWRO) processes. In particular, the warm water provides excellent conditions for growth of bacteriological organisms, and Red Sea desalination plants have a history of biofouling. The ability to apply chlorine to SWRO membranes provides a significant advantage. This article discusses combining cellulose triacetate membranes in the first pass with boron removal technology in the second pass and is based on experiences at the Shuqaiq IWPP project, which included tight specifications for chloride (18 mg/L) and boron (0.5 mg/L), in the final product water.