Scott A. Hynek

Stream Measurements Locate Thermogenic Methane Fluxes in Groundwater Discharge in an Area of Shale-Gas Development

The environmental impacts of shale-gas development on water resources, including methane migration to shallow groundwater, have been difficult to assess. Monitoring around gas wells is generally limited to domestic water-supply wells, which often are not situated along predominant groundwater flow paths. A new concept is tested here: combining stream hydrocarbon and noble-gas measurements with reach mass-balance modeling to estimate thermogenic methane concentrations and fluxes in groundwater discharging to streams and to constrain methane sources. In the Marcellus Formation shale-gas play of northern Pennsylvania (U.S.A.), we sampled methane in 15 streams as a reconnaissance tool to locate methane-laden groundwater discharge: concentrations up to 69 μg L(-1) were observed, with four streams ≥ 5 μg L(-1). Geochemical analyses of water from one stream with high methane (Sugar Run, Lycoming County) were consistent with Middle Devonian gases. After sampling was completed, we learned of a state regulator investigation of stray-gas migration from a nearby Marcellus Formation gas well. Modeling indicates a groundwater thermogenic methane flux of about 0.5 kg d(-1) discharging into Sugar Run, possibly from this fugitive gas source. Since flow paths often coalesce into gaining streams, stream methane monitoring provides the first watershed-scale method to assess groundwater contamination from shale-gas development.

Strontium isotopes in tap water from the coterminous USA

Strontium isotope analysis has proven useful in geo-location investigations of organic and inorganic materials and may complement the region-of-origin information provided by hydrogen and oxygen stable isotope analysis. In this study, we analyzed 99 drinking (tap) water samples collected from 95 municipal water systems across the USA to investigate the potential that 87 Sr/ 86 Sr can be used to provenance samples from managed hydrological systems. Results from a leaching and exchange experiment demonstrated that non-ideal storage conditions did not prohibit Sr isotope analysis of previously archived water samples stored in glass. Tap water samples were analyzed via multi-collector inductively coupled plasma mass spectrometry, which was preceded by a novel, automated, in-line Sr purification method. Measured tap water 87 Sr/ 86 Sr was compared to expected 87 Sr/ 86 Sr for collection location, which was predicted using four published isotope landscape (isoscape) models: age of bedrock (bedrock model), age plus major and minor lithology of bedrock (major bedrock model), weathering of Sr from rock (local water model), and surface fluxes within watershed (catchment model). The geologic history of the geographic regions represented by collected tap waters was diverse and we therefore expected significant covariation in measured and modeled 87 Sr/ 86 Sr values. Tap water exhibited large ranges in both Sr concentration (0-1.9 mg/L) and 87 Sr/ 86 Sr (0.7037-0.7320). Measured tap water 87 Sr/ 86 Sr ratios were significantly and positively correlated with predictions based on bedrock and catchment models. However, these bedrock and catchment models explained relatively little of the tap water Sr isotopic variation (;10% and 17%, respectively), suggesting that the factors affecting drinking water 87 Sr/ 86 Sr are complex and more numerous than the variables included in current water models. This could be due to the reliance of some municipal water systems on groundwater, rather than surficial water sources; the natural movement of water across distinct geologic gradients; and/or the managed transport of water from source to point-of-use. Although published isoscape models for predicting Sr isotopic variation within the continental USA are reasonable approaches for estimating surface water 87 Sr/ 86 Sr, additional efforts are needed to develop a prediction model specifically for tap water 87 Sr/ 86 Sr.

Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment

The hydrologic connectivity between hillslopes and streams impacts the geomorphological evolution of catchments. Here, we propose a conceptual model for hydrogeomorphological evolution of the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), a first-order catchment developed on shale in central Pennsylvania, U.S.A. At SSHCZO, the majority of available water (the difference between incoming meteoric water and outgoing evapotranspiration) flows laterally to the catchment outlet as interflow, while the rest is transported by regional groundwater flow. Interflow, shallow hillslope flow, is limited to the upper 5 to 8 m of highly fractured bedrock, thought to have formed during periglacial conditions in the late Pleistocene. In contrast, groundwater flowpaths are influenced by the primary permeability of the varying stratigraphic units. Both flowpaths respond to weathering-related secondary permeability. O2-rich interflow mixes with deep O2-poor groundwater under the catchment outlet at depths of 5–8 m. Penetration of this oxygenated interflow under the valley results in pyrite oxidation, release of sulfuric acid, dissolution of minerals, and weakening of bedrock. This is hypothesized to enhance channel incision and, in turn, to promote drainage of deep groundwater from the ridges. Drainage subsequently lowers the catchment water table, advancing the cascade of reactions that produce regolith. Weathering in the catchment is characterized by both sharp and diffuse reaction fronts. Relatively sharp fronts (pyrite, carbonate) mark where vertical, unsaturated flow changes to horizontal, saturated flow, while diffuse fronts (illite, chlorite, feldspar) mark where flow is largely vertical and unsaturated. According to this model, catchment morphology reflects subsurface pyrite reactions due to mixing of interflow and groundwater flow under the valley floor that ultimately results in clay weathering and regolith production nearer the land surface.

Regional groundwater flow and accumulation of a massive evaporite deposit at the margin of the Chilean Altiplano

Focused groundwater discharge in closed basins provides opportunities to investigate mechanisms for closing hydrologic and solute budgets in arid regions. The Salar de Atacama (SdA), adjacent to the Central Andean Plateau in the hyperarid Atacama Desert, provides an extreme example of halite (>1800 km3) and lithium brine (~5,000 ppm) accumulation spanning late Miocene to present. Minimum long-term water discharge needed to sustain halite accumulation over this timescale at SdA is 9–20 times greater than modern recharge (and double wet-climate paleo recharge) within the topographic watershed. Closing this imbalance requires sourcing water from recharge on the orogenic plateau in an area over 4 times larger than the topographic watershed. Prolonged water discharge at SdA requires long residence times, deep water tables in recharge zones coupled with persistent near surface water tables in discharge areas, and large contributing areas characterized by strong gradients in landscape and climate resulting from plateau uplift.

Gravel-capped benches above northern tributaries of the Escalante River, south-central Utah

Andesitic boulder deposits mantle straths cut in sedimentary bedrock high above the northern tributaries of the Escalante River in south-central Utah. The andesitic gravel deposits are derived from the southern escarpments of Boulder Mountain and Aquarius Plateau. The sedimentology and geomorphic expression of these deposits suggest they are from slurry-flow mass movements that have been reworked by fluvial processes. The andesitic boulders are significantly tougher than the local sedimentary bedrock and cause boulder armoring and topographic inversion. The andesitic boulders are also effective tools for fluvial incision when transported across weaker bedrock. Cosmogenic 3 He exposure-age dating of some of the largest boulders exposed on the treads of four different deposits range from 303 ± 48 to 1395 ± 241 ka. The tallest boulders exposed on the deposit surfaces tend to yield the oldest exposure ages, suggesting that boulder erosion and deposit erosion are controlling the exposure-age populations and indicating that even the oldest exposure ages from a given deposit are likely minimum age estimates. Using the oldest exposure age from each surface, we estimate maximum Escalante River northern tributary incision rates of 151–323 m Ma −1 for the period since 0.6–1.4 Ma.