Noble Gases Identify the Mechanisms of Fugitive Gas Contamination in Drinking-Water Wells Overlying the Marcellus and Barnett Shales

Horizontal drilling and hydraulic fracturing have enhanced energy production but raised concerns about drinking-water contamination and other environmental impacts. Identifying the sources and mechanisms of contamination can help improve the environmental and economic sustainability of shale-gas extraction. We analyzed 113 and 20 samples from drinking-water wells overlying the Marcellus and Barnett Shales, respectively, examining hydrocarbon abundance and isotopic compositions (e.g., C2H6/CH4, δ13C-CH4) and providing, to our knowledge, the first comprehensive analyses of noble gases and their isotopes (e.g., 4He, 20Ne, 36Ar) in groundwater near shale-gas wells. We addressed two questions. (i) Are elevated levels of hydrocarbon gases in drinking-water aquifers near gas wells natural or anthropogenic? (ii) If fugitive gas contamination exists, what mechanisms cause it? Against a backdrop of naturally occurring salt- and gas-rich groundwater, we identified eight discrete clusters of fugitive gas contamination, seven in Pennsylvania and one in Texas that showed increased contamination through time. Where fugitive gas contamination occurred, the relative proportions of thermogenic hydrocarbon gas (e.g., CH4, 4He) were significantly higher (P \u3c 0.01) and the proportions of atmospheric gases (air-saturated water; e.g., N2, 36Ar) were significantly lower (P \u3c 0.01) relative to background groundwater. Noble gas isotope and hydrocarbon data link four contamination clusters to gas leakage from intermediate-depth strata through failures of annulus cement, three to target production gases that seem to implicate faulty production casings, and one to an underground gas well failure. Noble gas data appear to rule out gas contamination by upward migration from depth through overlying geological strata triggered by horizontal drilling or hydraulic fracturing

Upper- and mid-mantle interaction between the Samoan plume and the Tonga-Kermadec slabs

Mantle plumes are thought to play a key role in transferring heat from the core\u2013mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji\u2013Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (VSH4VSV) with thermo-mechanical calculations

Biogeochemical Stoichiometry of Antarctic Dry Valley Ecosystems

Among aquatic and terrestrial landscapes of the McMurdo Dry Valleys, Antarctica, ecosystem stoichiometry ranges from values near the Redfield ratios for C:N:P to nutrient concentrations in proportions far above or below ratios necessary to support balanced microbial growth. This polar desert provides an opportunity to evaluate stoichiometric approaches to understand nutrient cycling in an ecosystem where biological diversity and activity are low, and controls over the movement and mass balances of nutrients operate over 10–10⁶ years. The simple organisms (microbial and metazoan) comprising dry valley foodwebs adhere to strict biochemical requirements in the composition of their biomass, and when activated by availability of liquid water, they influence the chemical composition of their environment according to these ratios. Nitrogen and phosphorus varied significantly in terrestrial and aquatic ecosystems occurring on landscape surfaces across a wide range of exposure ages, indicating strong influences of landscape development and geochemistry on nutrient availability. Biota control the elemental ratio of stream waters, while geochemical stoichiometry (e.g., weathering, atmospheric deposition) evidently limits the distribution of soil invertebrates. We present a conceptual model describing transformations across dry valley landscapes facilitated by exchanges of liquid water and biotic processing of dissolved nutrients. We conclude that contemporary ecosystem stoichiometry of Antarctic Dry Valley soils, glaciers, streams, and lakes results from a combination of extant biological processes superimposed on a legacy of landscape processes and previous climates

Rare gases in Samoan xenoliths

The rare gas isotopic compositions of residual harzburgite xenoliths from Savai'i (SAV locality) and an unnamed seamount south of the Samoan chain (PPT locality) provide important constraints on the rare gas evolution of the mantle and atmosphere. Despite heterogeneous trace element compositions, the rare gas characteristics of the xenoliths from each of the two localities are strikingly similar. SAV and PPT xenoliths have ^3He/^4He ratios of 11.1 ± 0.5 R_A and 21.6 ± 1 R_A, respectively; this range is comparable to the ^3He/^4He ratios in Samoan lavas and clearly demonstrates that they have trapped gases from a relatively undegassed reservoir. The neon results are not consistent with mixing between MORB and a plume source with an atmospheric signature. Rather, the neon isotopes reflect either a variably degassed mantle (with a relative order of degassing of Loihi < PPT < Reunion < SAV < MORB), or mixing between the Loihi source and MORB. The data supports the conclusions of Honda et al. that the ^(20)Ne/^(22)Ne ratio in the mantle more closely resembles the solar ratio than the atmospheric one. ^(40)Ar/^(36)Ar ratios in the least contaminated samples range from 4,000 to 12,000 with the highest values in the 22 R_A PPT xenoliths. There is no evidence for atmospheric ^(40)Ar/^(36)Ar ratios in the mantle source of these samples, which indicates that the lower mantle may have ^(40)Ar/^(36)Ar ratios in excess of 5,000. Xenon isotopic anomalies in ^(129)Xe and ^(136)Xe are as high as 6%, or about half of the maximum MORB excess and are consistent with the less degassed nature of the Samoan mantle source. These results contradict previous suggestions that the high ^3He/^4He mantle has a near-atmospheric heavy rare gas isotopic composition

Mantle neon and atmospheric contamination

The apparent distinction between atmospheric and mantle ^(20)Ne/^(22)Ne ratios may provide a technique to quantify air contamination in mantle-derived materials. In the absence of mantle nuclear reactions, which produce either ^(20)Ne or ^(22)Ne in substantial quantities, it is likely that the entire mantle is characterized by a single, uniform ^(20)Ne/^(22)Ne ratio; a value of around 12.5 is suggested by analyses of MORBs, OIBs, diamonds and xenoliths. If this premise is correct, then any measured ^(20)Ne/^(22)Ne ratios in mantle samples that are lower than this must result from addition of an air component, with ^(20)Ne/^(22)Ne= 9.8. This is most likely a syn- or post-eruptive contaminant. The degree of air contamination inferred from ^(20)Ne/^(22)Ne ratios is generally small for diamonds, but is increasingly significant for MORBs and OIBs; many OIB's may carry > 90% air neon. We calculated “air-neon corrected” ^(21)Ne/^(22)Ne and ^(40)Ar/^(36)Ar ratios for the highly degassed MORB mantle and for the less degassed (high ^3He/^4He) “plume” reservoir. The inferred MORB composition is indistinguishable from measurements of some gas-rich glasses. The calculated plume composition is similar to the least air-like measurements from ocean islands, but is less air-like than has been proposed previously. This plume composition is not consistent with a completely undegassed reservoir. From these corrected mantle compositions, we calculated the relative time-integrated rare gas abundances in the mantle, using a simple evolutionary model, which simultaneously considers the isotopic compositions of He, Ne, Ar and Xe. The model shows that both MORB and plume reservoirs have evolved with nearly solar elemental abundances. This provides strong support to suggestions based on Ne isotopes that the Earth accreted with gases nearly solar in composition. Importantly, the inferred mantle Ne/Ar ratios are much higher than atmospheric, which is consistent with simultaneous fractionation of both the atmospheric neon isotope ratio and the Ne/Ar ratio by massive hydrodynamic escape. Mixing between MORB and plume reservoirs (with our calculated elemental and isotopic abundances), plus varying amounts of added air, can account for the rare gases in nearly all mantle-derived rocks

Noble gases in deformed xenoliths from an ocean island: characterization of a metasomatic fluid

New noble gas measurements have been made on Samoan ultramafic xenoliths in order to characterize the composition and nature of entrapment of a postulated mantle metasomatic agent. The new measurements were performed on gases extracted from severely tectonized harzburgites and dunites by both bulk crushing and laser microprobe. The tectonized specimens have the highest noble gas concentrations yet reported from mantle materials and attest to deformation in a highly gas-charged environment. The noble gas isotopic systematics are similar to those observed in undeformed specimens from the same locality, and are consistent with mixing between a mantle component (e.g. ^3He/^4He = 12 R_A, ^(40)Ar/^(36) Ar > 10,000) and an atmospheric contaminant. Within the xenoliths, the mantle component is spatially associated with features previously attributed to metasomatism (e.g. HAURI et al., 1993). Although this metasomatic component has many characteristics suggesting derivation from material returned to the mantle by subduction, its relatively high ^3He/^4He ratio is enigmatic. Whatever its source, this fluid appears to have existed within the Samoan mantle over fairy large temporal and spatial scales, and plays an important role in the geochemistry of Samoan basalts. Just as with the mantle component, the deformed xenoliths are also enriched in the atmospheric contaminant. This enrichment suggests pervasive penetration of air into the ubiquitous micro fractures and decrepitated fluid inclusions of the deformed specimens. In addition to source and contamination effects, the noble gases within these xenoliths record variable degrees of elemental fractionation. While the gas-rich (deformed) xenoliths have ^4He-^(21)Ne*-^(40)Ar* systematics close to long-term closed-system behavior, the comparatively gas-poor samples have lost up to 90% of their helium without concomitant loss of neon and argon. This likely represents diffusive loss of helium after fluid inclusion entrapment