Using fixed-potential electrodes to quantify iron and manganese redox cycling in upland soils.

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 electric current. Electrode measurements were compared to soil porewater chemistry, pCO2, pO2, groundwater level, resistivity measurements, and precipitation. The fixed-potential electrodes recorded fluctuations over timescales from minutes to weeks. A consistently negative current was observed at 100 mV (interpreted as oxidation of Fe), while the 400-mV electrode fluctuated between negative and positive currents (Mn oxidation and reduction). When the water table rose above the electrodes, reduction was promoted, but above the water table, rainfall only stimulated oxidation. Precipitation frequency thus drove the multi-day reduction or oxidation events (return interval of 5-10 days). These measurements represent the first direct detections of frequency, period, and amplitude of oxidation and reduction events in unsaturated soils. Fixed-potential electrodes hold promise for accurately exploring the fast-changing biogeochemical impacts of metal redox cycling in soils and represent a significant advance for reactions that have been difficult to quantify.

Geochemical Evidence of Potential Groundwater Contamination with Human Health Risks Where Hydraulic Fracturing Overlaps with Extensive Legacy Hydrocarbon Extraction.

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 in SWPA may have lessened today's gas leakage, as no regional correlation was detected for [CH4] in SWPA. We identify hotspots where [Cl] and [CH4] increase by 3.6 and 3.0 mg/L, respectively, per UOG well drilled in SWPA. If the [Cl] correlations document contamination via brines leaked from wellbores, impoundments, or spills, we calculate that thallium concentrations could exceed EPA limits in the most densely developed hotspots, thus posing a potential human health risk.

Measurements of Atmospheric Methane Emissions from Stray Gas Migration: A Case Study from the Marcellus Shale.

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 eddy covariance measurements with the survey of the methane mole fraction to estimate total emissions over the study area (2,800 m2). Estimated at ∼6 kg CH4 day-1, emissions were spatially heterogeneous but showed no temporal trends over 6 months. The isotopic signature of the atmospheric CH4 source (δ13CH4) was equal to -29‰, consistent with methane of thermogenic origin and similar to the isotopic signature of the gas reported from the nearest shale gas well. While the magnitude of emissions from the potential leak is modest compared to large emitters identified among shale gas production sites, it is large compared to estimates of emissions from single abandoned wells. Since other areas of emissions have been identified close to this putatively leaking well, our estimate of emissions likely represents only a portion of total emissions from this event. More comprehensive quantification will require more extensive spatial and temporal sampling of the locations of gas migration to the surface as well as an investigation into the mechanisms of subsurface gas migration. This work highlights an example of atmospheric methane emissions from potential stray gas migration at a location far from a well pad, and further research should explore the frequency and mechanisms behind these types of events to inform careful and strategic natural gas development.

Microbial chemolithotrophic oxidation of pyrite in a subsurface shale weathering environment: Geologic considerations and potential mechanisms.

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 months at the inferred depth of in situ pyrite oxidation. The colonized materials were used as an inoculum for pyrite-oxidizing enrichment cultures. Microbial activity accelerated the release of sulfate across all conditions. 16S rRNA gene sequencing and metagenomic analysis revealed the dominance of a putative chemolithoautotrophic sulfur-oxidizing bacterium from the genus Thiobacillus in the enrichment cultures. Previously proposed models for aerobic microbial pyrite oxidation were assessed in terms of physical constraints, enrichment culture geochemistry, and metagenomic analysis. Although we conclude that subsurface pyrite oxidation at SSCHZO is largely abiotic, this work nonetheless yields new insight into the potential pathways by which aerobic microorganisms may accelerate pyrite oxidation at circumneutral pH. We propose a new "direct sulfur oxidation" pathway, whereby sulfhydryl-bearing outer membrane proteins mediate oxidation of pyrite surfaces through a persulfide intermediate, analogous to previously proposed mechanisms for direct microbial oxidation of elemental sulfur. The action of this and other direct microbial pyrite oxidation pathways have major implications for controls on pyrite weathering rates in circumneutral pH sedimentary environments where pore throat sizes permit widespread access of microorganisms to pyrite surfaces.

How Particle Size Influences Oxidation of Ancient Organic Matter during Weathering of Black Shale.

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), we confirmed that the large particles of OM are preferentially depleted, leaving elongated pores of tens to hundreds of micrometers in length, while the smaller particulates of OM (ranging from ∼5 to ∼200 nm) are largely preserved during weathering. The retarded weathering of small OM particles is attributed to their close association with mineral surfaces in the shale matrix. The texture of OM in shale is underappreciated as an important factor that controls porosity generation and the weathering rate of OCpetro.

Detecting anomalous methane in groundwater within hydrocarbon production areas across the United States.

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 groundwater data from three other gas-producing states: New York (n=203), Texas (n=688), and Colorado (n=10,258). The applications show that 1.4%, 1.3%, 0%, and 0.9% of tested samples in these four states, respectively, have high [CH4] and are ≥50% likely to have been impacted by gas migrated from exploited reservoirs. If our approach is indeed successful in flagging anomalous CH4, we conclude that: i) the frequency of anomalous CH4 (# flagged water samples / total samples tested) in the Appalachian Basin is similar in areas where gas wells target unconventional as compared to conventional reservoirs, and ii) the frequency of anomalous CH4 in Pennsylvania is higher than in Texas + Colorado. We cannot, however, exclude the possibility that differences among regions might be affected by differences in data volumes. Machine learning models will become increasingly useful in informing decision-making for shale gas development.

Exploring the trend of stream sulfate concentrations as U.S. power plants shift from coal to shale gas.

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- concentrations could have decreased as much as 10.4%. Extrapolating the model to other states in the U.S., we found that as much as 46.1 thousand tons of SO2 emissions per state could have been avoided for a similar 30% coal-to-gas switch, with potential amelioration of water quality near power plants. The emission-control technology trend model provides a valuable tool for policy makers to assess the benefits of coal-to-gas shifts on water quality improvements as well as the effectiveness of emission control technologies.

Deep abiotic weathering of pyrite.

Pyrite is a ubiquitous iron sulfide mineral that is oxidized by trace oxygen. The mineral has been largely absent from global sediments since the rise in oxygen concentration in Earth's early atmosphere. We analyzed weathering in shale, the most common rock exposed at Earth's surface, with chemical and microscopic analysis. By looking across scales from 10-9 to 102 meters, we determined the factors that control pyrite oxidation. Under the atmosphere today, pyrite oxidation is rate-limited by diffusion of oxygen to the grain surface and regulated by large-scale erosion and clast-scale fracturing. We determined that neither iron- nor sulfur-oxidizing microorganisms control global pyrite weathering fluxes despite their ability to catalyze the reaction. This multiscale picture emphasizes that fracturing and erosion are as important as atmospheric oxygen in limiting pyrite reactivity over Earth's history.

Methane concentrations in streams reveal gas leak discharges in regions of oil, gas, and coal development.

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 more commonly observed type of groundwater seep known as abandoned mine drainage (AMD), we introduce the term, "gas leak discharge" (GLD) for these waters where they are not associated with coal mines. GLD and AMD, observed in all parts of the study area, are both CH4-rich. Surprisingly, the region of oldest and most productive oil/gas development did not show the highest median for stream [CH4]. Instead, the median was statistically highest where dense coal mining was accompanied by conventional and unconventional oil and gas development, emphasizing the importance of CH4 contamination from coal mines into streams.

Seismic refraction tracks porosity generation and possible CO2 production at depth under a headwater catchment.

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 table under valley and ridge. We attribute this gas largely to CO2 produced by 1) microbial respiration in soils as meteoric waters recharge into the subsurface and 2) the coupled carbonate dissolution and pyrite oxidation at depth in the rock. Bubbles may nucleate below the water table because waters depressurize as they flow from ridge to valley and because pores have dilated as the deep rock has been exhumed by erosion. Many of these observations are likely to also describe the weathering and flow path patterns in other headwater landscapes. Such combined geophysical and geochemical observations will help constrain models predicting flow, storage, and reaction of groundwater in bedrock systems.

Assessing Contamination of Stream Networks near Shale Gas Development Using a New Geospatial Tool.

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-analysis tool, GeoNet, that analyzes stream networks to detect statistically significant changes between background and potentially impacted sites. GeoNet was used on data in the Water Quality Portal for the Pennsylvania Marcellus region. With the most stringent statistical tests, GeoNet detected 0.2% to 2% of the known contamination incidents (Na ± Cl) in streams. With denser sensor networks, tools like GeoNet could allow real-time detection of polluting events.

Microbial chemolithotrophy mediates oxidative weathering of granitic bedrock.

The flux of solutes from the chemical weathering of the continental crust supplies a steady supply of essential nutrients necessary for the maintenance of Earth's biosphere. Promotion of weathering by microorganisms is a well-documented phenomenon and is most often attributed to heterotrophic microbial metabolism for the purposes of nutrient acquisition. Here, we demonstrate the role of chemolithotrophic ferrous iron [Fe(II)]-oxidizing bacteria in biogeochemical weathering of subsurface Fe(II)-silicate minerals at the Luquillo Critical Zone Observatory in Puerto Rico. Under chemolithotrophic growth conditions, mineral-derived Fe(II) in the Rio Blanco Quartz Diorite served as the primary energy source for microbial growth. An enrichment in homologs to gene clusters involved in extracellular electron transfer was associated with dramatically accelerated rates of mineral oxidation and adenosine triphosphate generation relative to sterile diorite suspensions. Transmission electron microscopy and energy-dispersive spectroscopy revealed the accumulation of nanoparticulate Fe-oxyhydroxides on mineral surfaces only under biotic conditions. Microbially oxidized quartz diorite showed greater susceptibility to proton-promoted dissolution, which has important implications for weathering reactions in situ. Collectively, our results suggest that chemolithotrophic Fe(II)-oxidizing bacteria are likely contributors in the transformation of rock to regolith.

Exploring How to Use Groundwater Chemistry to Identify Migration of Methane near Shale Gas Wells in the Appalachian Basin.

Methane (CH4) enters waters in hydrocarbon-rich basins because of natural processes and problems related to oil and gas wells. As a redox-active greenhouse gas, CH4 degrades water or emits to the atmosphere and contributes to climate change. To detect if methane migrated from hydrocarbon wells (i.e., anomalous methane), we examined 20 751 methane-containing groundwaters from the Upper Appalachian Basin (AB). We looked for concentrations (mg/L) that indicated AB brine salts (chloride concentrations ([Cl]) > 30; [Ca]/[Na] < 0.52) to detect natural methane, and we looked for concentrations of redox-active species ([SO4] ≥ 6; [Fe] ≥ 0.3) to detect anomalous methane. These indicators highlight natural contamination by methane-containing brines or recent onset of microbial oxidation of methane coupled to iron- or sulfate-reduction. We hypothesized that only waters recently contaminated by methane still exhibit high iron and sulfate concentrations. Of the AB samples, 17 (0.08%) from 12 sites indicated potential contamination. All were located in areas with high densities of shale-gas or conventional oil/gas wells. In contrast, in southwestern Pennsylvania where brines are shallow and coal, oil, and gas all have been extracted extensively, no sites of recent methane migration were detectable. Such indicators may help screen for contamination in some areas even without predrill measurements.

Links between physical and chemical weathering inferred from a 65-m-deep borehole through Earth's critical zone.

As bedrock weathers to regolith - defined here as weathered rock, saprolite, and soil - porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Co-located geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from saprolite to weathered bedrock is more gradational, over a depth range of 11-18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth's surface.

Assessing changes in groundwater chemistry in landscapes with more than 100 years of oil and gas development.

With recent improvements in high-volume hydraulic fracturing (HVHF, known to the public as fracking), vast new reservoirs of natural gas and oil are now being tapped. As HVHF has expanded into the populous northeastern USA, some residents have become concerned about impacts on water quality. Scientists have addressed this concern by investigating individual case studies or by statistically assessing the rate of problems. In general, however, lack of access to new or historical water quality data hinders the latter assessments. We introduce a new statistical approach to assess water quality datasets - especially sets that differ in data volume and variance - and apply the technique to one region of intense shale gas development in northeastern Pennsylvania (PA) and one with fewer shale gas wells in northwestern PA. The new analysis for the intensely developed region corroborates an earlier analysis based on a different statistical test: in that area, changes in groundwater chemistry show no degradation despite that area's dense development of shale gas. In contrast, in the region with fewer shale gas wells, we observe slight but statistically significant increases in concentrations in some solutes in groundwaters. One potential explanation for the slight changes in groundwater chemistry in that area (northwestern PA) is that it is the regional focus of the earliest commercial development of conventional oil and gas (O&G) in the USA. Alternate explanations include the use of brines from conventional O&G wells as well as other salt mixtures on roads in that area for dust abatement or de-icing, respectively.

Detecting and explaining why aquifers occasionally become degraded near hydraulically fractured shale gas wells.

Extensive development of shale gas has generated some concerns about environmental impacts such as the migration of natural gas into water resources. We studied high gas concentrations in waters at a site near Marcellus Shale gas wells to determine the geological explanations and geochemical implications. The local geology may explain why methane has discharged for 7 years into groundwater, a stream, and the atmosphere. Gas may migrate easily near the gas wells in this location where the Marcellus Shale dips significantly, is shallow (∼1 km), and is more fractured. Methane and ethane concentrations in local water wells increased after gas development compared with predrilling concentrations reported in the region. Noble gas and isotopic evidence are consistent with the upward migration of gas from the Marcellus Formation in a free-gas phase. This upflow results in microbially mediated oxidation near the surface. Iron concentrations also increased following the increase of natural gas concentrations in domestic water wells. After several months, both iron and SO42- concentrations dropped. These observations are attributed to iron and SO42- reduction associated with newly elevated concentrations of methane. These temporal trends, as well as data from other areas with reported leaks, document a way to distinguish newly migrated methane from preexisting sources of gas. This study thus documents both geologically risky areas and geochemical signatures of iron and SO42- that could distinguish newly leaked methane from older methane sources in aquifers.

Big Groundwater Data Sets Reveal Possible Rare Contamination Amid Otherwise Improved Water Quality for Some Analytes in a Region of Marcellus Shale Development.

Eleven thousand groundwater samples collected in the 2010s in an area of Marcellus shale-gas development are analyzed to assess spatial and temporal patterns of water quality. Using a new data mining technique, we confirm previous observations that methane concentrations in groundwater tend to be naturally elevated in valleys and near faults, but we also show that methane is also more concentrated near an anticline. Data mining also highlights waters with elevated methane that are not otherwise explained by geologic features. These slightly elevated concentrations occur near 7 out of the 1,385 shale-gas wells and near some conventional gas wells in the study area. For ten analytes for which uncensored data are abundant in this 3,000 km2 rural region, concentrations are unchanged or improved as compared to samples analyzed prior to 1990. Specifically, TDS, Fe, Mn, sulfate, and pH show small but statistically significant improvement, and As, Pb, Ba, Cl, and Na show no change. Evidence from this rural area could document improved groundwater quality caused by decreased acid rain (pH, sulfate) since the imposition of the Clean Air Act or decreased steel production (Fe, Mn). Such improvements have not been reported in groundwater in more developed areas of the U.S.

Detecting the effects of coal mining, acid rain, and natural gas extraction in Appalachian basin streams in Pennsylvania (USA) through analysis of barium and sulfate concentrations.

To understand how extraction of different energy sources impacts water resources requires assessment of how water chemistry has changed in comparison with the background values of pristine streams. With such understanding, we can develop better water quality standards and ecological interpretations. However, determination of pristine background chemistry is difficult in areas with heavy human impact. To learn to do this, we compiled a master dataset of sulfate and barium concentrations ([SO4], [Ba]) in Pennsylvania (PA, USA) streams from publically available sources. These elements were chosen because they can represent contamination related to oil/gas and coal, respectively. We applied changepoint analysis (i.e., likelihood ratio test) to identify pristine streams, which we defined as streams with a low variability in concentrations as measured over years. From these pristine streams, we estimated the baseline concentrations for major bedrock types in PA. Overall, we found that 48,471 data values are available for [SO4] from 1904 to 2014 and 3243 data for [Ba] from 1963 to 2014. Statewide [SO4] baseline was estimated to be 15.8 ± 9.6 mg/L, but values range from 12.4 to 26.7 mg/L for different bedrock types. The statewide [Ba] baseline is 27.7 ± 10.6 µg/L and values range from 25.8 to 38.7 µg/L. Results show that most increases in [SO4] from the baseline occurred in areas with intensive coal mining activities, confirming previous studies. Sulfate inputs from acid rain were also documented. Slight increases in [Ba] since 2007 and higher [Ba] in areas with higher densities of gas wells when compared to other areas could document impacts from shale gas development, the prevalence of basin brines, or decreases in acid rain and its coupled effects on [Ba] related to barite solubility. The largest impacts on PA stream [Ba] and [SO4] are related to releases from coal mining or burning rather than oil and gas development.