Susan Brantley

While unconventional oil and gas development (UOGD) is changing the world economy, processes that are used during UOGD such as high-volume hydraulic fracturing (“fracking”) have been linked with water contamination. Water quality risks include leaks of gas and salty fluids (brines) that are co-produced at wellpads. Identifying the cause of contamination is difficult, however, because UOG wells are often co-located with other contaminant sources. We investigated the world’s largest shale gas play with publicly accessible groundwater data (~29,000 analyses from the Marcellus Shale in Pennsylvania, U.S.A.) and discovered that concentrations of brine-associated species barium ([Ba]) and strontium ([Sr]) show small regional increases within 1km of UOGD. Higher concentrations in groundwaters are associated with greater proximity to and density of UOG wells. Concentration increases are even larger when considering the locations of i) spill-related violations and ii) some wastewater impoundments. These statistically significant relationships persist even after correcting for other natural and anthropogenic sources of salts. The most likely explanation is that UOGD slightly increases salt concentrations in regional groundwaters not because of fracking but because of the ubiquity of wastewater management issues. The high frequency of spills and leaks across shale gas basins suggests other plays could show similar effects.

Earth’s climate may be stabilized over millennia by solubilization of atmospheric carbon dioxide (CO2) as minerals weather, but the temperature sensitivity of this thermostat is poorly understood. We discovered that the temperature dependence of weathering expressed as an activation energy increases from laboratory to watershed as transport, clay precipitation, disaggregation, and fracturing increasingly couple to dissolution. A simple upscaling to the global system indicates that the temperature dependence decreases to ~22 kilojoules per mole because (i) the lack of runoff limits weathering and retains base metal cations on half the land surface and (ii) other landscapes are regolith-shielded and show little weathering response to temperature. By comparing weathering from laboratory to globe, we reconcile some aspects of kinetic and thermodynamic controls on CO2 drawdown by natural or enhanced weathering.

Although metal redox reactions in soils can strongly affect carbon mineralization and other important soil processes, little is known about temporal variations in this redox cycling. Recently, potentiostatically poised electrodes (fixed-potential electrodes) have shown promise for measuring the rate of oxidation and reduction at a specific reduction potential in situ in riparian soils. Here for the first time, we used these electrodes in unsaturated soils to explore the fine-scale temporal redox fluctuations of both iron and manganese in response to environmental conditions. We used three-electrode systems with working electrodes fixed at 100 mV (vs. SHE) and 400 mV at 50 cm and 70 cm in the valley floor soil of a headwater watershed. Electrodes fixed at 100 mV to mimic iron oxides and at 400 mV to mimic manganese oxides allowed real-time reduction and oxidation rates to be calculated from temporal variations in the …

As drinking‐water scarcity grows worldwide, we need to improve predictions of the quantity and quality of our water resources. An overarching problem for model improvement is that we do not know the geological structure of aquifers in sufficient detail. In this work, we demonstrate that mineral‐water reactions imprint structure in the subsurface that impacts the flow and transport of some chemical species. Specifically, pyrite, a ubiquitous mineral, commonly oxidizes and depletes in the upper layers of the weathering profile in most humid watersheds, only remaining at depths of meters. We hypothesize that variations in concentrations (C) of pyrite‐derived sulfate released into rivers as a function of discharge (q) reflect the rate‐limiting step and depth of pyrite‐oxidizing layers. We found that log C − log q behaviors thus differ in small and large watersheds in the Susquehanna River Basin as well as in selected …

The oxidation of petrogenic organic carbon (OCpetro) is a source of carbon dioxide to the atmosphere over geological timescales. The rates of OCpetro oxidation in locations that experience low rates of denudation remain poorly constrained, despite these landscapes dominating Earth's continental surface area. Here, we track OCpetro oxidation using radiocarbon and the trace element rhenium (Re) in the deep weathering profiles, soils and stream waters of the Susquehanna Shale Hills Critical Zone Observatory (PA, USA). In a ridge‐top borehole, radiocarbon measurements reveal the presence of a broad OCpetro weathering front, with a first‐order assessment of ~40% loss occurring over ~6 m. However, the low OCpetro concentration (< 0.05 wt%) and inputs of radiocarbon throughout the deepest parts of the profile complicate the assessment of OCpetro loss. The OCpetro weathering front coincides with a …

Despite its importance, only a few researchers have incorporated the effects of fracturing into models of reactive transport for rock weathering. Here we explore 2D simulations that describe weathering under conditions of diffusive and advective transport within heterogeneous media consisting of rocky blocks and fractures. In our simulations, the Darcy velocities vary in space and time and depend on weathering processes within the rock matrix. We explore simulations with saturated and unsaturated flow for weathering bedrock that consists of blocks separated by inert or weathered material. The simulations show that a simplified homogenized model can approximate exact solutions for some of the simulated columns and hills and can allow exploration of coupling between flow and reaction in fractured rock. These hillslope simulations document that, even in the presence of 2D water flow, i) an increase in fracture density results in faster weathering advance rates; and ii) the water table locates deeper for a rock system that is weathered and fractured rather than weathered and unfractured. Some of these patterns have also been observed for natural systems. But these simulations also highlight how simplified models that do not use appropriate averaging of heterogeneities can be inaccurate in predicting weathering rate for natural systems. For example, if water flows both vertically and laterally through the vadose zone of a hill, then a prediction of the depth of regolith that is based on modeling strictly unidirectional downward infiltration will be unrealistically large. Likewise, if the fracture density observed near the land surface is used in a model to …

To understand geological systems that are important to our national well-being -- such as nuclear waste repositories or hydraulically fractured rocks -- requires the ability to make quantitative projections of the evolution of rock-water systems forward in time. A fundamental goal of much of our geochemical research was focused on development of numerical models of reactive transport to simulate the evolution of natural water-rock systems. The geochemical community has provided such models and they are rapidly being improved with field-model cross-testing. This project was especially impactful in field-model testing for well-constrained weathering systems to provide new conceptual understanding of how these systems function.

Understanding emissions of methane from legacy and ongoing shale gas development requires both regional studies that assess the frequency of emissions and case studies that assess causation. We present the first direct measurements of emissions in a case study of a putatively leaking gas well in the largest shale gas play in the United States. We quantify atmospheric methane emissions in farmland >2 km from the nearest shale gas well cited for casing and cementing issues. We find that emissions are highly heterogeneous as they travel long distances in the subsurface. Emissions were measured near observed patches of dead vegetation and methane bubbling from a stream. An eddy covariance flux tower, chamber flux measurements, and a survey of enhancements of the near-surface methane mole fraction were used to quantify emissions and evaluate the spatial and temporal variability. We combined …

Analytical and numerical solutions have been proposed to model reaction fronts to study soil formation. With growing access to large geo-datasets and powerful computational capacity, data-driven models are becoming increasingly useful. We therefore explored the use of a neural network (NN) guided by a physics-based model (PBM) to simulate the depth profile of feldspar dissolution in soils. Specifically, we explored this hybrid neural network (HNN) to see if it could predict reaction fronts as a function of important variables known from domain knowledge: site climate characteristics (temperature T; precipitation P), geomorphic parameters (soil residence time t; erosion rate E), and parent material mineralogy (quartz content Q; albitic feldspar content of the feldspar A). We evaluated the mean square error (MSE) for 63 HNNs, each using a different combination of training data (i.e., soil profiles) and environmental …

Oxidative weathering of pyrite plays an important role in the biogeochemical cycling of Fe and S in terrestrial environments. While the mechanism and occurrence of biologically accelerated pyrite oxidation under acidic conditions are well established, much less is known about microbially mediated pyrite oxidation at circumneutral pH. Recent work (Percak‐Dennett et al., 2017, Geobiology, 15, 690) has demonstrated the ability of aerobic chemolithotrophic microorganisms to accelerate pyrite oxidation at circumneutral pH and proposed two mechanistic models by which this phenomenon might occur. Here, we assess the potential relevance of aerobic microbially catalyzed circumneutral pH pyrite oxidation in relation to subsurface shale weathering at Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Pennsylvania, USA. Specimen pyrite mixed with native shale was incubated in groundwater for 3 …

The reaction mechanism of weathering of chlorite, an important rock-forming phyllosilicate, is not well understood in natural settings. In this work we investigated the weathering of Fe-rich chlorite from deep protolith to saprock to soil across a small shale-underlain watershed in the Appalachian Mountains, USA (Shale Hills). We found that oxidation of Fe(II) in chlorite always occurs prior to dissolution of the interlayers of the mineral. The oxidation of pyrite and chlorite commence near the water table across narrow depth intervals under the upper-catchment ridges, but well below the water table across wide depth intervals under the valley. We hypothesize that these patterns can be explained by hydrological and geochemical differences between the ridge and the valley: oxygenated water descends sub-vertically (1D flow) under the ridge, while under the valley, oxygen-depleted water moves upward to the stream and …

Unconventional oil and gas development (UOGD) sometimes impacts water resources, including incidents of methane (CH4) migration from compromised wells and spills that degrade water with salts, organics, and metals. We hypothesized that contamination may be more common where UOGD overlaps with legacy coal, oil, and gas extraction. We tested this hypothesis on ∼7000 groundwater analyses from the largest U.S. shale gas play (Marcellus), using data mining techniques to explore UOGD contamination frequency. Corroborating the hypothesis, we discovered small, statistically significant regional correlations between groundwater chloride concentrations ([Cl]) and UOGD proximity and density where legacy extraction was extremely dense (southwestern Pennsylvania (SWPA)) but no such correlations where it was minimal (northeastern Pennsylvania). On the other hand, legacy extraction of shallow gas …

Mineral weathering is a major control on long term atmospheric CO2. The sensitivity of weathering rates to changes in climate and erosion depends on what limits weathering. For example, when reaction kinetics, supply of fresh mineral, or water throughput is limiting, weathering fluxes increase with temperature, erosion, and runoff, respectively. We call these three weathering regimes kinetic limited (KL), erosive transport limited (ETL), and runoff limited (RL). Watersheds can also exhibit complex weathering dependencies when they are partly KL and partly ETL. We refer to these watersheds as transition regimes (TR). Although silicate weathering and CO2 drawdown have been extensively studied for granitic and basaltic watersheds, weathering in shale watersheds has not been fully characterized despite the widespread coverage of shales globally. Here we identified and analyzed 142 shale watersheds across …

Weathering continuously converts rock to regolith at Earth’s surface while regulating the atmospheric concentrations of CO2 and O2. Shale weathering is of particular interest because shale, the most abundant rock type exposed on continents, stores much of the ancient organic carbon (OCpetro) buried in rocks. Using geochemical and mineralogical analysis combined with neutron scattering and imaging, we investigated the weathering profile of OCpetro in saprock in a black shale (Marcellus Formation) in the Ridge and Valley Appalachians in Pennsylvania, U.S.A. Consistent with the low erosion rate of the landscape, we discovered that Marcellus is completely depleted in carbonate, plagioclase, and pyrite in saprock below the soil layer. On the contrary, only ∼60% of OCpetro was depleted in saprock. By comparing the pore structure of saprock to bedrock and samples combusted to remove organic matter (OM …

Development of horizontal drilling and high-pressure high-volume hydraulic fracturing has ushered in a new era of energy development in the USA. At the same time, like older hydrocarbon extraction industries, water quality impacts from shale gas development have been noted. In the mid-2000s, public outcry about" fracking" reached a fever pitch whereas today, the pushback is more muted. In this talk we look at what has been learned over the last decade about water impacts. To understand the nature of contamination with multiple lines of evidence, highly localized field studies are needed. On the other hand, to assess regional impact or to calculate frequency of impact requires statistical analysis, including many different techniques ranging from machine learning to fixed effects analysis and geospatial analysis. We present results from field to statistical studies to illustrate that impacts happen but the impact is …

Meteoric waters move along pathways in the subsurface that differ as a function of lithology because of the effects of chemical and physical weathering. To explore how this affects stream chemistry, we investigated rocks around an igneous intrusion in the Luquillo Mountains (Puerto Rico). We investigated streams on i) unmetamorphosed country rock (volcaniclastic sedimentary strata, VC) surrounding an igneous intrusion, ii) the quartz-diorite intrusion (QD), and iii) the metamorphosed aureole rock (hornfels-facies volcaniclastics, HF). These lithologies differ physically and chemically but weather under the same tropical rain forest conditions. The sedimentary VC lithology is pervasively fractured while the massive QD and HF lithologies are relatively unfractured. However, the QD fractures during weathering to produce spheroidally-weathered corestones surrounded by cm-thick rindlets of increasingly weathered rock. Meteoric waters flow pervasively through the network of already-fractured VC rock and the spheroidally weathered rindlets on the QD, but only access a limited fraction of the HF, explaining why streams draining HF are the most dilute in the mountains. This results in various thicknesses of regolith from thick (VC) to moderate (QD) to thin or nonexistent (HF). The pervasive fractures allow groundwater to flow deeply through the VC and then return to the mainstem river (Río Mameyes) at lower elevations. These “rock waters” drive concentrations of rock-derived solutes (silica, base cations, sulfate, phosphate) higher in the lower reaches of the stream. Water also flows through weathering-induced fractures on the QD at high elevations …

Historical oil and gas extraction has left a high density of poorly maintained, orphaned, or abandoned wells in many hydrocarbon-bearing basins across the world. Aging and abandoned wells are a well-established source of methane (CH4) emissions to the atmosphere, particularly in regions such as the Appalachian Basin where extraction dates back two centuries, but less is understood about their impacts to water resources. Anticipating the impacts of such wells on water quality thus requires a more extensive understanding of the mechanisms through which contamination may occur. In Pennsylvania, the state with the longest history of commercial oil extraction in the US, high-CH4 artesian flows from leaking abandoned wells generally contain aqueous chemistry distinct from CH4-contaminated groundwater seepages. Using field-, lab-, and modeling-based efforts, we investigated how the leakage and …

How does hillslope structure (e.g., hillslope shape and permeability variation) regulate its hydro‐geochemical functioning (flow paths, solute export, chemical weathering)? Numerical reactive transport experiments and particle tracking were used to answer this question. Results underscore the first‐order control of permeability variations (with depth) on vertical connectivity (VC), defined as the fraction of water flowing into streams from below the soil zone. Where permeability decreases sharply and VC is low, >95% of water flows through the top 6 m of the subsurface, barely interacting with reactive rock at depth. High VC also elongates mean transit times (MTTs) and weathering rates. VC however is less of an influence under arid climates where long transit times drive weathering to equilibrium. The results lead to three working hypotheses that can be further tested. H1: The permeability variations with depth …

Despite a multitude of small catchment studies, we lack a deep understanding of how variations in critical zone architecture lead to variations in hydrologic states and fluxes. This study characterizes hydrologic dynamics of 15 catchments of the U.S. Critical Zone Observatory (CZO) network where we hypothesized that our understanding of subsurface structure would illuminate patterns of hydrologic partitioning. The CZOs collect data sets that characterize the physical, chemical, and biological architecture of the subsurface, while also monitoring hydrologic fluxes such as streamflow, precipitation, and evapotranspiration. For the first time, we collate time series of hydrologic variables across the CZO network and begin the process of examining hydrologic signatures across sites. We find that catchments with low baseflow indices and high runoff sensitivity to storage receive most of their precipitation as rain and contain …

Data sharing benefits the researcher, the scientific community, and the public by allowing the impact of data to be generalized beyond one project and by making science more transparent. However, many scientific communities have not developed protocols or standards for publishing, citing, and versioning datasets. One community that lags in data management is that of low-temperature geochemistry (LTG). This paper resulted from an initiative from 2018 through 2020 to convene LTG and data scientists in the U.S. to strategize future management of LTG data. Through webinars, a workshop, a preprint, a townhall, and a community survey, the group of U.S. scientists discussed the landscape of data management for LTG – the data-scape. Currently this data-scape includes a “street bazaar” of data repositories. This was deemed appropriate in the same way that LTG scientists publish articles in many journals. The …

We report volatile contents (H2O, CO2, F, Cl, S) of 102 fresh submarine glasses from across the Bransfield Strait (BS) back-arc extensional feature and the nearby Phoenix Ridge (PR) spreading center in the Antarctic Peninsula (AP). New major element, trace element, and isotope (Sr, Nd, Pb, Hf) data have been used to identify geochemical end-members in the BS and PR upper mantles and estimate their volatile contents and CO2-H2O melt saturation pressures. Phoenix samples form a spectrum from N-MORB to EMORB, with volatile/refractory element of similar incompatibility values that correlate with enrichment. The most enriched PR lavas have the highest F/Nd, F/Cl and H2O/Ce and lowest CO2/Ba. This difference with CO2/Ba may reflect a relationship between degassing and melt enrichment. Phoenix N-MORB and E-MORB span similar ranges in Cl/K and S/Dy, and only the most enriched alkali basalts extend to lower S/Dy. Our data support a model of slowdown at the PR resulting in increasingly tapping more enriched, volatile-rich portions of upper mantle over time. Bransfield samples form three groups: 1) Weak arc lavas with LILE/HFSE ratios higher than PR MORB and slightly more enriched isotope ratios, 2) Strong arc lavas with very high LILE/HFES ratios and a clear isotopic enrichment towards a subduction end-member, 3) Enriched alkali lavas with high concentrations of incompatible trace elements and distinct isotope ratios. The strongest arc lavas in the BS have the highest F/Nd and H2O/Ce and lowest F/Cl and S/Dy of all samples. The low S/Dy may be explained by higher oxidation driven by subduction and the effect of …

Numerous geochemical approaches have been proposed to ascertain if methane concentrations in groundwater, [CH4], are anomalous, i.e., migrated from hydrocarbon production wells, rather than derived from natural sources. We propose a machine-learning model to consider alkalinity, Ca, Mg, Na, Ba, Fe, Mn, Cl, sulfate, TDS, specific conductance, pH, temperature, and turbidity holistically together. The model, an ensemble of sub-models targeting one parameter pair per sub-model, was trained with groundwater chemistry from Pennsylvania (n=19,086) and a set of 16 analyses from putatively contaminated groundwater. For cases where [CH4] ≥ 10 mg/L, salinity- and redox-related parameters sometimes show that CH4 may have moved into the aquifer recently and separately from natural brine migration, i.e., anomalous CH4. We applied the model to validation and hold-out data for Pennsylvania (n=4,786) and …

Clay weathering in shales is an important component of the global Li budget because Li is mobilized from Li-rich clay minerals and shale represents about one quarter of the exposed rocks on Earth. We investigate Li isotopes and concentrations to explore implications and mechanisms of Li isotopic fractionation in Shale Hills, a first-order catchment developed entirely on shale in a temperate climate in the Appalachian Mountains, northeastern USA. The Li isotopic compositions (δ7Li) of aqueous Li in stream water and groundwater vary between 14.5 and 40.0‰. This range is more than half that observed in rivers globally. The δ7Li of aqueous Li increases with increasing Li retention in secondary minerals, which is simulated using a box model that considers pore fluid advection to be the dominant transport process, silicate dissolution to be the source of Li to the pore fluid, and uptake of Li by kaolinite, Fe-oxides …

More above‐ground biomass (kg m−2) grows in the northern Appalachian Mountains (USA) in forests on shale than on sandstone at all landscape positions other than ridgetops. This has been tentatively attributed to physical (rather than chemical) attributes of the substrates, such as elevation, particle size, and water capacity. However, shales have generally similar phosphorus (P) concentrations to sandstones and, in the Valley and Ridge province, they erode more quickly. This led us to hypothesize that faster replenishment of the lithogenic nutrient P in shale soils through erosion + soil production could instead control the differences in biomass. To test this, soils and foliage from 10 sites on shales and sandstones in the northern Appalachians from roughly the same elevation and aspect were analysed. We discovered that, when controlling for location, concentrations of bioavailable P in soils and P in foliage …

Headwater catchments are the fundamental units that connect the land to the ocean. Hydrological flow and biogeochemical processes are intricately coupled, yet their respective sciences have progressed without much integration. Reaction kinetic theories that prescribe rate dependence on environmental variables (e.g., temperature and water content) have advanced substantially, mostly in well‐mixed reactors, columns, and warming experiments without considering the characteristics of hydrological flow at the catchment scale. These theories have shown significant divergence from observations in natural systems. On the other hand, hydrological theories, including transit time theory, have progressed substantially yet have not been incorporated into understanding reactions at the catchment scale. Here we advocate for the development of integrated hydro‐biogeochemical theories across gradients of climate …

Endmember mixing analysis (EMMA) is often used by hydrogeochemists to interpret the sources of stream solutes, but variations in stream concentrations and discharges remain difficult to explain. We discovered that machine learning can be used to highlight patterns in stream chemistry that reveal information about sources of solutes and subsurface groundwater flowpaths. The investigation has implications, in turn, for the balance of CO2 in the atmosphere. For example, CO2-driven weathering of silicate minerals removes carbon from the atmosphere over ∼106-year timescales. Weathering of another common mineral, pyrite, releases sulfuric acid that in turn causes dissolution of carbonates. In that process, however, CO2 is released instead of sequestered from the atmosphere. Thus, understanding long-term global CO2 sequestration by weathering requires quantification of CO2- versus H2SO4-driven reactions. Most researchers estimate such weathering fluxes from stream chemistry, but interpreting the reactant minerals and acids dissolved in streams has been fraught with difficulty. We apply a machine-learning technique to EMMA in three watersheds to determine the extent of mineral dissolution by each acid, without pre-defining the endmembers. The results show that the watersheds continuously or intermittently sequester CO2, but the extent of CO2 drawdown is diminished in areas heavily affected by acid rain. Prior to applying the new algorithm, CO2 drawdown was overestimated. The new technique, which elucidates the importance of different subsurface flowpaths and long-timescale changes in the watersheds, should have utility as …

The critical zone sustains terrestrial life, but we have few tools to explore it efficiently beyond the first few meters of the subsurface. Using analyses of high‐frequency ambient seismic noise from densely spaced seismometers deployed in the forested Shale Hills subcatchment of the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), we show that temporal changes in seismic velocities at depths from ∼1 m to tens of m can be detected. These changes are driven by variations at the land surface. The Moving‐Window Cross‐Spectral (MWCS) method was employed to measure seismic‐velocity changes in coda waves at hourly resolution in 10 different frequency bands. We observed a diurnal signal, a seasonal signal, and a meteorological‐event‐based signal. These signals were compared to time‐series measurements of precipitation, well water levels, soil moisture, soil temperature, air temperature …

Predicting the partitioning between aqueous and gaseous C across landscapes is difficult because many factors interact to control carbon dioxide (CO2) concentrations and removal as dissolved inorganic carbon (DIC). For example, carbonate minerals buffer soil pH and allow CO2 dissolution in porewaters, but nitrification of fertilizers may decrease pH so that carbonate weathering results in a gaseous CO2 efflux. Here, we investigate CO2 partitioning in an agricultural, first‐order, mixed‐lithology humid, temperate watershed. We quantified soil mineralogy and measured porewater chemistry, soil moisture, and soil pCO2 and pO2 as a function of depth at three hillslope positions. Variation of soil moisture along the hillslope was the dominant control on the concentration of soil CO2, but mineralogy acted as a secondary control on the partitioning of CO2 between gaseous and aqueous phases. Regression slopes of …

At the scale of atomic modeling, the rate of crack propagation or geochemical weathering is the rate of chemical bond breaking. As the spatial scale increases, new processes become rate-limiting. We observed this when we studied pyrite oxidation in a shale-underlain watershed (Shale Hills, Pennsylvania). At the atomic scale, the rate of oxidation is limited by the interfacial reaction but at the scale of pyrite grains, we saw evidence that the rate is limited by oxygen diffusion. At the scale of pedons, we inferred that the rate of diffusion is limited by the rate of fracture formation. Finally, at the scale of the borehole, the rate of physical erosion of soil limits the rate that oxygenated water reaches unoxidized pyrite at depth. Thus, with increasing scale of observation, the rate-limiting step for pyrite oxidation changes from bond breaking to diffusion to fracturing to erosion. This paradigm was developed by inspecting 1D profiles …

Weathering plays a crucial role in a number of environmental processes, and the microstructure and evolution of multi-scale pore space is a critically important factor in weathering. In igneous rocks the infiltration of meteoric water into initially relatively dry material can initiate disaggregation, increasing porosity and surface area, and allowing further disaggregation and weathering. These processes, in turn, allow biota to colonize the rock, further enhancing the weathering rate. In some rocks this may be driven by primary mineral oxidation. One such mineral, biotite, has been repeatedly mentioned as a cause of cracking during oxidation. However, the scale-dependence of the processes by which this occurs are poorly understood. We cannot, therefore, accurately extrapolate laboratory reaction rates to the field in predictive numerical models.In order to better understand the effects of oxidation and test the hypothesis …

Large‐scale models often use a single grid to represent an entire catchment assuming homogeneity; the impacts of such an assumption on simulating evapotranspiration (ET) and streamflow remain poorly understood. Here, we compare hydrological dynamics at Shale Hills (PA, USA) using a complex model (spatially explicit, >500 grids) and a simple model (spatially implicit, two grids using “effective” parameters). We asked two questions: What hydrological dynamics can a simple model reproduce at the catchment scale? What processes does it miss by ignoring spatial details? Results show the simple model can reproduce annual runoff ratios and ET, daily discharge peaks (e.g., storms, floods) but not discharge minima (e.g., droughts) under dry conditions. Neither can it reproduce different streamflow from the two sides of the catchment with distinct land surface characteristics. The similar annual runoff ratios …

Knowing little about how porosity and permeability are distributed at depth, we commonly develop models of groundwater by treating the subsurface as a homogeneous black box even though porosity and permeability vary with depth. One reason for this depth variation is that infiltrating meteoric water reacts with minerals to affect porosity in localized zones called reaction fronts. We are beginning to learn to map and model these fronts beneath headwater catchments and show how they are distributed. The subsurface landscapes defined by these fronts lie subparallel to the soil‐air interface but with lower relief. They can be situated above, below, or at the water table. These subsurface landscapes of reaction are important because porosity developed from weathering can control subsurface water storage. In addition, porosity often changes at the weathering fronts, and when this affects permeability significantly, the …

We used seismic refraction to image the P‐wave velocity structure of a shale watershed experiencing regional compression in the Valley and Ridge Province (USA). From estimates showing strong compressional stress, we expected the depth to unweathered bedrock to mirror the hill‐valley‐hill topography (“bowtie pattern”) by analogy to seismic velocity patterns in crystalline bedrock in the North American Piedmont that also experience compression. Previous researchers used failure potentials calculated for strong compression in the Piedmont to suggest fractures are open deeper under hills than valleys to explain the “bowtie” pattern. Seismic images of the shale watershed, however, show little evidence of such a “bowtie.” Instead, they are consistent with weak (not strong) compression. This contradiction could be explained by the greater importance of infiltration‐driven weathering than fracturing in determining …

The migration of methane into shallow groundwater via leaking oil and gas wells is the most frequently cited water quality issue surrounding unconventional shale gas extraction in Pennsylvania, USA, and has been documented in other shale gas basins across the country. Recent studies have documented geochemical evidence for anaerobic oxidation of methane (AOM) coupled to iron and/or sulfate reduction in groundwaters contaminated by methane migration from oil and gas activities. While AOM has the potential to naturally attenuate methane contamination, harmful species associated with iron oxides (eg, arsenic) may be mobilized by iron reduction coupled with AOM. However, sulfide produced via sulfate reduction coupled AOM may draw metals out of solution via metal sulfide precipitation. As such, the respective rates of these reactions and availability of iron oxides/sulfate may have profound effects on …

To further develop boron isotopes as a tool for understanding shale weathering, we explored patterns of boron concentrations and isotopes across the forested Susquehanna Shale Hills Critical Zone Observatory (CZO). We present boron measurements for all watershed components that provided a foundation for examining water-rock interactions in a shale dominated watershed, including water compartments (e.g., precipitation, stream water, groundwater) and solid compartments (e.g., soil, bedrock, stream sediments, suspended load, and leaf litter). Results show boron isotopes (δ11B) in the bedrock (− 4.6‰) and soil (− 5.9 to - 4.2‰) were very similar. All waters were enriched in 11B by comparison: precipitation (7.2 to 22.6‰), stream (10.3 to 15.5‰), and groundwater (2.2 to 17.4‰). Modeling revealed that isotopic fractionation observed in the surface water and groundwater could mainly be explained by water …

Soil CO2 and O2 are coupled in some processes (eg respiration) but uncoupled in others (eg mineral weathering), such that simultaneous measurement of these two gases can yield insight into an array of soil chemical reactions and biogeochemical processes. Because soil CO2 production and O2 consumption are tightly coupled when aerobic respiration and diffusion persist in the soil system, the deviations from that coupling can be interpreted to signify various biotic and abiotic reactions. Here, we used such measurements as a function of depth to understand mineral, hillslope, and seasonal controls on soil pCO2 relative to pO2 in three watersheds of different bedrock lithology. We made our measurements over a growing season in three neighboring humid, temperate watersheds underlain by three different sedimentary bedrocks-acidic shale, calcareous shale, and acidic sandstone. Across these three …

Since the early 2000s, an increasing number of power plants in the U.S. have switched from burning coal to burning gas and thus have released less SO2 emissions into the atmosphere. We investigated whether stream chemistry (i.e., SO42−) also benefits from this transition. Using publicly available data from Pennsylvania (PA), a U.S. state with heavy usage of coal as fuel, we found that the impact of SO2 emissions on stream SO42− can be observed as far as 63 km from power plants. We developed a novel model that incorporates an emission-control technology trend for coal-fired power plants to quantify potentially avoided SO2 emissions and stream SO42− as power plants switched from coal to gas. The results show that, if 30% of the electricity generated by coal in PA in 2017 had been replaced by that from natural gas, a total of 20.3 thousand tons of SO2 emissions could have been avoided and stream SO42 …

To investigate how bedrock transforms to soil, we mapped the topography of the interface demarcating onset of weathering under an east‐west trending shale watershed in the Valley and Ridge province in the USA Using wave equation travel‐time tomography from a seismic array of >4,000 geophones, we obtained a 3D P‐wave velocity (Vp) model that resolves structures ∼20 m below land surface (mbls). The depth of mobile soil and the onset of dissolution of chlorite roughly match Vp = 600 m/s and Vp = 2,700 m/s, respectively. Chlorite dissolution initiates porosity growth in the shale matrix. Depth to the 2,700 m/s contour is greater under the N‐ as compared to S‐facing hillslopes and under sub‐planar as compared to concave‐up land surfaces. Broadly, the geometries of the ‘soil’ and ‘chlorite’ Vp contours are consistent with the calculated potential for shear fracture opening under weak regional compression …

The Río Icacos watershed is located in the Luquillo Mountains of northeastern Puerto Rico. It’s hot and humid climate (with mean annual air temperature of 22 C and mean annual precipitation of 4200 mm, White et al., 1998) results in dense, tropical vegetation. The steep relief varies from 620 to 832 m in elevation. The Río Icacos, a tributary of the Río Blanco, flows north to south in its watershed with an average channel gradient of~ 0.9%(Orlando et al., 2016). The Río Icacos watershed is almost exclusively developed on Río Blanco Quartz Diorite, an igneous pluton that contains predominantly plagioclase (50–60%) and quartz (20–30%) with lesser amounts of biotite and hornblende, and accessory magnetite, K-feldspar, sphene, ilmenite, apatite, and zircon (White et al., 1998; Turner et al., 2003; Buss et al., 2008). The bedrock is now classified as a tonalite according to the current IUGS classification scheme (Buss et al., 2017) but has been referred to as a quartz diorite repeatedly in the literature. In the (unweathered) corestones, plagioclase grains are 0.2–2 mm in size, and typically contain calcic cores with more sodic rims (Fig. S-2a, b). Biotite grains are similar in size as the plagioclase, and the edges are replaced by chlorite in some cases (Fig. S-2d). Magnetite is present as exsolved inclusions within silicates, and as coarse grains (Fig. S-2a, c, d).

More than 1 million wells may have been completed using hydraulic fracturing techniques in the USA alone; however, there have been few case studies exploring the impacts on water resources due to methane migration. This study evaluated the results of three investigations initiated by the US Environmental Protection Agency, that were subsequently described in published studies at Dimock in Pennsylvania, Parker-Hood County in Texas, and Pavillion in Wyoming, as well as another study completed at Sugar Run in northeast Pennsylvania. In addition, earlier investigations at Shaws Corner in Pennsylvania, Jackson County in West Virginia, Garfield County in Colorado, and Bainbridge in Ohio are summarized. The most common cause of incidents was the presence of uncemented sections of production casings in wells that allowed gas migration from intermediate depths to shallow freshwater aquifers. In three …

INTRODUCTION Oxidative weathering of pyrite, the most abundant sulfide mineral in Earth’s crust, is coupled to the biogeochemical cycles of sulfur, oxygen, carbon, and iron. Pyrite oxidation is key to these cycles because of its high reactivity with oxygen. Before the Great Oxidation Event (GOE), atmospheric oxygen concentrations were low on early Earth and pyrite was exposed at Earth’s surface, allowing erosion into sediments that were preserved in river deposits. Today, it oxidizes at depth in most rocks and is often not exposed at the land surface. To understand pyrite weathering through geologic time, researchers extrapolate the reaction kinetics based on studies from the laboratory or in acid mine drainage. Such work has emphasized the important role of microorganisms in catalyzing pyrite oxidation. But to interpret the oxidation rates of pyrite on early Earth requires knowledge of the rate-limiting step of the …

Since 2013 Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) has been monitoring using a Ground Hydrological Observation System (GroundHOG) design consisting of various sensor types in the Shavers Creek watershed in central Pennsylvania. The GroundHOG design was established to study interactions between hydrological systems (surface and groundwater), soils, and ecosystems along catenas. We currently have three GroundHOG sites with differing land uses and geology: one located in a pristine shale watershed, another in a pristine sandstone watershed, and the third in an agricultural setting with mixed lithology. Each catena has three pits set up to compare hill slope position and one additional pit to compare north versus south aspect. Each pit is equipped with both automated and manual sensors that measure soil moisture and soil gas at varying depths. The GroundHOG deployment is …

The agricultural use of fertilizer often results in loss of excess nitrate to streams and groundwater via overland flow and infiltration respectively. Flowpaths in the shallow subsurface facilitate transport of high loads of nitrate to surface water. Likewise, groundwater flowpaths move nitrate to streams through aquifers. In this study we investigate the transport of nitrate at two scales: 1) the scale of a cultivated field within a hillslope farm next to a stream, and 2) on a HUC-10 watershed scale. A series of shallow wells (2.5 m-4.5 m) were drilled throughout the farm hillslope to investigate nitrate loads and evidence for denitrification. A synoptic sampling event for the watershed utilized 50 community members, landowners, and students to collect 52 surface water samples and 19 discharge measurements at the same time throughout the watershed. This data was used to create a high-resolution chemical" snapshot" of the …

The relationship between oxidation and dissolution in vermiculitization of chlorite has never been completely understood, especially under different hydrological and geochemical conditions. In this work, weathering of chlorite is studied in several boreholes and soils across a small watershed (Shale Hills) from grain to watershed scale. At watershed scale, the oxidation, dissolution and vermiculitization degree of chlorite are quantified by parameters from different measurements and correlated with each other. Chlorite dissolution is found to lag behind chlorite oxidation under the valley. Both occur simultaneously under the ridge. This is likely caused by the differences in water table position, flow rates, concentrations of dissolved oxygen (DO), and pH between ridge and valley. The oxidation of chlorite and pyrite commence together near the water table under the ridge but commence below the water table under the …

More than 1 million wells may have been completed using hydraulic fracturing techniques in the USA alone; however, there have been few case studies exploring the impacts on water resources due to methane migration. This study evaluated the results of three investigations initiated by the US Environmental Protection Agency, that were subsequently described in published studies at Dimock in Pennsylvania, Parker-Hood County in Texas, and Pavillion in Wyoming, as well as another study completed at Sugar Run in northeast Pennsylvania. In addition, earlier investigations at Shaws Corner in Pennsylvania, Jackson County in West Virginia, Garfield County in Colorado, and Bainbridge in Ohio are summarized. The most common cause of incidents was the presence of uncemented sections of production casings in wells that allowed gas migration from intermediate depths to shallow freshwater aquifers. In three cases, an inadequate depth of the primary top of cement (TOC) also contributed to impacts. Sources of methane were best identified through analyses of isotopes on samples from production casings, annular spaces, and water wells. In Dimock, some isotope signatures changed with time, after the completion of remedial actions. In Parker-Hood County, where impacts were not related to gas well activity, noble gas analyses were also needed to determine the source of gas. At Pavillion, where maximum methane concentrations in water wells were< 1 mg/L, no significant impacts were documented. For all the sites, most or all of the fugitive gas incidents may have been prevented by fully cementing production casings to the land surface.

The rate of chemical weathering has been observed to increase with the rate of physical erosion in published comparisons of many catchments, but the mechanisms that couple these processes are not well understood. We investigated this question by examining the chemical weathering and porosity profiles from catchments developed on marine shale located in Pennsylvania, USA (Susquehanna Shale Hills Critical Zone Observatory, SSHCZO); California, USA (Eel River Critical Zone Observatory, ERCZO); and Taiwan (Fushan Experimental Forest). The protolith compositions, protolith porosities, and the depths of regolith at these sites are roughly similar while the catchments are characterized by large differences in erosion rate (1–3 mm yr−1 in Fushan ≫ 0.2–0.4 mm yr−1 in ERCZO ≫ 0.01–0.025 mm yr−1 in SSHCZO). The natural experiment did not totally isolate erosion as a variable: mean annual …

As natural gas has grown in importance as a global energy source, leakage of methane (CH4) from wells has sometimes been noted. Leakage of this greenhouse gas is important because it affects groundwater quality and, when emitted to the atmosphere, climate. We hypothesized that streams might be most contaminated by CH4 in the northern Appalachian Basin in regions with the longest history of hydrocarbon extraction activities. To test this, we searched for CH4-contaminated streams in the basin. Methane concentrations ([CH4]) for 529 stream sites are reported in New York, West Virginia and (mostly) Pennsylvania. Despite targeting contaminated areas, the median [CH4], 1.1 μg/L, was lower than a recently identified threshold indicating potential contamination, 4.0 μg/L. [CH4] values were higher in a few streams because they receive high-[CH4] groundwaters, often from upwelling seeps. By analogy to the …

Geophysical tools have proven to be useful to extend our observations of the Critical Zone from point measurements (ie soil pits, boreholes) to larger areas efficiently. However, most of the geophysical surveys are along 2D transects, which might not be representative for the entire watershed. Here, we present our progress exploring the subsurface structure at Shale Hills watershed using a three-dimensional seismic refraction survey consisting of 4200 geophones with 2 m spacing over a 180 x 160 m grid. In general, we observe that where the underlying bedrock is mostly shale, the regolith thickness is roughly constant for transects parallel to the channel. But a previous study based on multiple 2D seismic transects showed an asymmetric pattern of regolith depth with relatively thicker regolith profile on the north-facing slopes. This asymmetric pattern of regolith thickness was attributed to frost-cracking driven by …

All communities must maintain the quality of their water resources at the same time that they use land for a range of other anthropogenic activities. If such activities pollute drinking water or harm ecosystems, human health can be deleteriously impacted over long periods of time. However, even though many water quality data are measured worldwide, it is often difficult to find sufficient data to assess problems at the spatial scale needed. This is especially true when land use includes impact-causing activities such as mining, energy exploitation, road use, agriculture, and urbanization dispersed across a large region. We have been collating groundwater and surface water quality data for hydrocarbon basins. We started with the basin that was first exploited for coal, oil, and gas in the United States (ie, the Appalachian Basin). These data are being published in the Shale Network database (https://doi. org/10.4211/his …

Silicate weathering regulates climate on geological time scales as a net, climate-sensitive sink of atmospheric CO2. On the other hand, sulfuric acid produced through sulfide dissolution affects silicate weathering, diminishing its effectiveness as a climate regulator based on evidence from river chemistry. This study takes a theoretical approach to quantitatively examine the effect of sulfide dissolution, coupling a one-dimensional model of pyrite weathering to that of albite transformation to kaolinite. The coupled model reveals that when the reaction front of sulfide is deeper than that of silicates reacting with CO2, the silicate-weathering feedback on climate is not directly affected by sulfide dissolution, but only indirectly through oceanic processes such as reverse weathering and carbonate deposition. In turn, when sulfide dissolution occurs within zones of CO2-silicate reactions close to the surface, the feedback …

In weathered bedrock aquifers, groundwater is stored in pores and fractures that open as rocks are exhumed and minerals interact with meteoric fluids. Little is known about this storage because geochemical and geophysical observations are limited to pits, boreholes, or outcrops or to inferences based on indirect measurements between these sites. We trained a rock physics model to borehole observations in a well-constrained ridge and valley landscape and then interpreted spatial variations in seismic refraction velocities. We discovered that P-wave velocities track where a porosity-generating reaction initiates in shale in three boreholes across the landscape. Specifically, velocities of 2.7 ± 0.2 km/s correspond with growth of porosity from dissolution of chlorite, the most reactive of the abundant minerals in the shale. In addition, sonic velocities are consistent with the presence of gas bubbles beneath the water …

Pyrite, a ubiquitous iron sulfide mineral, oxidizes to produce sulfuric acid (H 2 SO 4). Stream chemistry is often used to investigate pyrite oxidation because rivers integrate the byproducts of weathering. When studying larger river systems, distinguishing pyrite-derived sulfate is difficult because there are many sources of sulfate in streams (eg, atmosphere, evaporites, agriculture). Here, we utilize stream chemistry and machine learning to distinguish pyrite-derived sulfate in Shavers Creek, the main stream in the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Central Pennsylvania. We also utilize public datasets from the Water Quality Portal (WQP) to upscale our study to the Susquehanna River Basin in order to evaluate pyrite oxidation on a broader spatial scale. Our machine learning model deciphered that in Shavers Creek and the Susquehanna River there are three main sources of riverine …

Geophysical surveys and observations of corestones and outcrops reveal that the fractures in rock at meters to tens of meters under the Río Icacos watershed in the Luquillo Critical Zone Observatory, Puerto Rico are complex and laterally discontinuous. Weathering weakens the minerals along fracture surfaces until particles detach and erode. Such subsurface physical erosion is known as seepage erosion. However, neither the geomorphological nor geochemical significance of seepage erosion has not been well documented. To elucidate seepage erosion at Río Icacos, we sampled sediments from a seep (seep sediments) emitting between corestones at~ 10 m depth at Río Icacos during Aug-Nov, 2016.

More than 1 million wells may have been completed using hydraulic fracturing techniques in the USA alone; however, there have been few case studies exploring the impacts on water resources due to methane migration. This study evaluated the results of three investigations initiated by the US Environmental Protection Agency, that were subsequently described in published studies at Dimock in Pennsylvania, Parker-Hood County in Texas, and Pavillion in Wyoming, as well as another study completed at Sugar Run in northeast Pennsylvania. In addition, earlier investigations at Shaws Corner in Pennsylvania, Jackson County in West Virginia, Garfield County in Colorado, and Bainbridge in Ohio are summarized. The most common cause of incidents was the presence of uncemented sections of production casings in wells that allowed gas migration from intermediate depths to shallow freshwater aquifers. In three cases, an inadequate depth of the primary top of cement (TOC) also contributed to impacts. Sources of methane were best identified through analyses of isotopes on samples from production casings, annular spaces, and water wells. In Dimock, some isotope signatures changed with time, after the completion of remedial actions. In Parker-Hood County, where impacts were not related to gas well activity, noble gas analyses were also needed to determine the source of gas. At Pavillion, where maximum methane concentrations in water wells were< 1 mg/L, no significant impacts were documented. For all the sites, most or all of the fugitive gas incidents may have been prevented by fully cementing production casings to the land surface.

Weathering of bedrock creates and occludes permeability, affecting subsurface water flow. Often, weathering intensifies above the water table. On the contrary, weathering can also commence below the water table. To explore relationships between weathering and the water table, a simplified weathering model for an eroding hillslope was formulated that takes into account both saturated and unsaturated subsurface water flow (but does not fully account for changes in dissolved gas chemistry). The phreatic line was calculated using solutions to mathematical treatments for both zones. In the model, the infiltration rate at the hill surface sets both the original and the eventual steady‐state position of the water table with respect to the weathering reaction front. Depending on parameters, the weathering front can locate either above or below the water table at steady state. Erosion also affects the water table position by …

Using large groundwater data sets and data mining techniques, the potential water quality impacts of shale gas extraction can be assessed on a regional scale. However, such methods have not previously been implemented in shale plays where shale gas extraction overlaps with extensive urbanization, legacy oil and gas drilling, and/or coal mining. Such overlap may complicate the attribution of contaminant sources or interact with shale gas drilling to exacerbate groundwater contamination.

Metals in soils can be used as electron acceptors for anoxic respiration and as electron donors for lithotrophic microbes. These reversible redox reactions influence carbon mineralization and fixation, and they are regulated by variations in soil moisture and resultant oxygen dynamics. Recent work from the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Central Pennsylvania confirms the importance of water content to soil iron redox, indicating widespread iron redox cycling in response to precipitation. In this study, we tested the ability of electrochemical monitoring to serve as a real-time nondestructive measurement of dynamic metal redox reactions in the valley floor of a shale watershed at the SSHCZO. Graphite electrodes were installed at 50 cm and 70 cm and poised at 100 mV and 400 mV (vs. SHE) to mimic iron oxides and manganese oxides, respectively. Current through the electrode …

Weathering partially controls concentrations of important gases in the atmosphere including CO 2 and O 2. For example, pyrite, the most common iron sulfide in Earth's crust, oxidizes rapidly today at Earth's surface, drawing oxygen out of the atmosphere. Currently, we have little ability to model weathering accurately because our approach to understanding such mineral-water-gas reactions is to study separate parts of different systems at different spatial scales from that of electron microscopy to seismic mapping to integration of riverine chemistry across watersheds. Once measured, we have no way to put these observations into one model that crosses scales. We have used a critical zone observatory to create such a model that crosses scales from nanometers to tens of kilometers in documenting how pyrite oxidizes. Our study site is a catchment developed on weathering grey shale that contains trace pyrite that …

We present a model of chemical reaction within hills to explore how evolving porosity (and by inference, permeability) affects flow pathways and weathering. The model consists of hydrologic and reactive‐transport equations that describe alteration of ferrous minerals and feldspar. These reactions were chosen because previous work emphasized that oxygen‐ and acid‐driven weathering affects porosity differently in felsic and mafic rocks. A parameter controlling the order of the fronts is presented. In the absence of erosion, the two reaction fronts move at different velocities and the relative depths depend on geochemical conditions and starting composition. In turn, the fronts and associated changes in porosity drastically affect both the vertical and lateral velocities of water flow. For these cases, estimates of weathering advance rates based on simple models that posit unidirectional constant‐velocity advection do not …

Chemical spills in streams can impact ecosystem or human health. Typically, the public learns of spills from reports from industry, media, or government rather than monitoring data. For example, ∼1300 spills (76 ≥ 400 gallons or ∼1500 L) were reported from 2007 to 2014 by the regulator for natural gas wellpads in the Marcellus shale region of Pennsylvania (U.S.), a region of extensive drilling and hydraulic fracturing. Only one such incident of stream contamination in Pennsylvania has been documented with water quality data in peer-reviewed literature. This could indicate that spills (1) were small or contained on wellpads, (2) were diluted, biodegraded, or obscured by other contaminants, (3) were not detected because of sparse monitoring, or (4) were not detected because of the difficulties of inspecting data for complex stream networks. As a first step in addressing the last problem, we developed a geospatial …

Data sharing benefits the researcher, the scientific community, and most importantly, the public by enabling more impactful analysis of data and greater transparency in scientific research. However, like many other scientists, the low-temperature geochemistry (LTG) community has generally not developed protocols and standards for publishing, citing, and versioning datasets. This paper is the product of a group of LTG and data scientists convened to strategize about the future management of LTG data. The group observed that the current landscape of sites for LTG – the data-scape -- is a “street bazaar” of data repositories. This was deemed appropriate because LTG scientists target many different scientific questions and produce data with different structures and volumes described by copious and complex metadata. Nonetheless, the group agreed that publication of LTG science must be accompanied by sharing of data in publicly accessible repositories. To enable this for sample-based data, samples should be registered with globally unique persistent identifiers. LTG scientists should be able to use both highly structured databases designed for specialized types of data or generalized, unstructured, and non-targeted data repositories. The group strategized that the overall data informatics paradigm should shift from “build data repository, data will come” to “publish data online, cybertools will find.” In other words, the most important need within the growing and complex data-scape is for increasingly powerful tools for searching and cross-referencing data across the proliferating data repositories. This strategy requires increasing emphasis on …

In agricultural areas, phosphate fertilizer acts as a major source of non-point source pollution. These fertilizers may contaminate surface and groundwaters by accumulating heavy metals such as uranium and strontium. Because fertilizers are not the only source of these heavy metals in watersheds, chemical data alone are not able to discriminate between different sources of pollution, and subsequently struggle to indicate corrective action. Coupled isotope ratios 234 U/238 U and 87 Sr/86 Sr can help provide a useful and novel approach to identifying and quantifying the origins of heavy metal contamination.

The spatial distribution of weathered rock across landscapes strongly influences how water and solutes are routed throughout the landscape, but we know little about this distribution. To understand the controls on the evolution of weathering under hilly and mountainous regions, we investigated the spatial distribution of the oxidation front for pyrite at the Shale Hills catchment in central Pennsylvania using geochemical measurements on materials recovered from boreholes across the catchment as well as geophysical measurements. As a subcatchment of the Susquehanna Shale Hills Critical Zone Observatory, intensive field monitoring and measurements of surface water and groundwater hydrology and aqueous chemistry in the past decade allow us to test how the reaction front recorded in the illite/chlorite/quartz-dominated shale is related to flow paths and solute fluxes.

One goal in critical zone (CZ) science is to project the response of Earth's near‐surface fluxes of water, sediments, and nutrients to perturbations in climate and human actions, an approach that is increasingly known as earthcasting. However, earthcasting requires knowledge of the present and a deep understanding of the past and, more importantly, a validation through collection of data and simulations of past processes. This so‐called hindcasting pairs past climate and present‐day critical zone structure to understand how the function of Earth's living skin has evolved with time. The combined approach of hindcasting and earthcasting illuminates strategies for managing the critical zone and generates new hypotheses to be tested in the field and laboratory. Here we: (a) present a road map to earthcasting and hindcasting, (b) review several examples of these projections, and (c) explore a recent earthcast and …