Robert J. Poreda

Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction

Horizontal drilling and hydraulic fracturing are transforming energy production, but their potential environmental effects remain controversial. We analyzed 141 drinking water wells across the Appalachian Plateaus physiographic province of northeastern Pennsylvania, examining natural gas concentrations and isotopic signatures with proximity to shale gas wells. Methane was detected in 82% of drinking water samples, with average concentrations six times higher for homes <1 km from natural gas wells (P = 0.0006). Ethane was 23 times higher in homes <1 km from gas wells (P = 0.0013); propane was detected in 10 water wells, all within approximately 1 km distance (P = 0.01). Of three factors previously proposed to influence gas concentrations in shallow groundwater (distances to gas wells, valley bottoms, and the Appalachian Structural Front, a proxy for tectonic deformation), distance to gas wells was highly significant for methane concentrations (P = 0.007; multiple regression), whereas distances to valley bottoms and the Appalachian Structural Front were not significant (P = 0.27 and P = 0.11, respectively). Distance to gas wells was also the most significant factor for Pearson and Spearman correlation analyses (P < 0.01). For ethane concentrations, distance to gas wells was the only statistically significant factor (P < 0.005). Isotopic signatures (δ13C-CH4, δ13C-C2H6, and δ2H-CH4), hydrocarbon ratios (methane to ethane and propane), and the ratio of the noble gas 4He to CH4 in groundwater were characteristic of a thermally postmature Marcellus-like source in some cases. Overall, our data suggest that some homeowners living <1 km from gas wells have drinking water contaminated with stray gases.

Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

A carbon-rich black layer, dating to ≈12.9 ka, has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.

Early and Late Alkali Igneous Pulses and a High-3He Plume Origin for the Deccan Flood Basalts

Several alkalic igneous complexes of nephelinite-carbonatite affinities occur in extensional zones around a region of high heat flow and positive gravity anomaly within the continental flood basalt (CFB) province of Deccan, India. Biotites from two of the complexes yield 40Ar/39Ar dates of 68.53 � 0.16 and 68.57 � 0.08 million years. Biotite from a third complex, which intrudes the flood basalts, yields an 40Ar/39Ar date of 64.96 � 0.1 1 million years. The complexes thus represent early and late magmatism with respect to the main pulse of CFB volcanism 65 million years ago. Rocks from the older complexes show a 3He/4He ratio of 14.0 times the air ratio, an initial 87Sr/86Sr ratio of 0.70483, and other geochemical characteristics similar to ocean island basalts; the later alkalic pulse shows isotopic evidence of crustal contamination. The data document 3.5 million years of incubation of a primitive, high-3He mantle plume before the rapid eruption of the Deccan CFB.

Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett Shales

Significance Hydrocarbon production from unconventional sources is growing rapidly, accompanied by concerns about drinking-water contamination and other environmental risks. Using noble gas and hydrocarbon tracers, we distinguish natural sources of methane from anthropogenic contamination and evaluate the mechanisms that cause elevated hydrocarbon concentrations in drinking water near natural-gas wells. We document fugitive gases in eight clusters of domestic water wells overlying the Marcellus and Barnett Shales, including declining water quality through time over the Barnett. Gas geochemistry data implicate leaks through annulus cement (four cases), production casings (three cases), and underground well failure (one case) rather than gas migration induced by hydraulic fracturing deep underground. Determining the mechanisms of contamination will improve the safety and economics of shale-gas extraction. 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 < 0.01) and the proportions of atmospheric gases (air-saturated water; e.g., N2, 36Ar) were significantly lower (P < 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.

Source of radiogenic helium 4 in shallow aquifers: Implications for dating young groundwater

Radiogenic helium 4 (4Herad) has been used in numerous studies as a tracer of groundwater age in the range of 103–108 years. We have measured 4Herad along shallow groundwater flow paths at a variety of hydrogeologically distinct sites and postulate its use for dating groundwater as young as 101 years. Groundwater travel times and fluid velocities are particularly well documented at one site in northern Ontario because of detailed profiling of tritium, 3H/3He ratios, and chlorofluorocarbons (CFCs). Metamorphic rocks of the Canadian Shield (>1 Ga) that contain large quantities of 4He are the protolith of this unconsolidated aquifer and observed 4Herad values increase linearly with distance along a flow path and with increasing groundwater age. A solute transport model suggests that the aquifer solids are the source of 4Herad as vertical fluid velocities are too great to allow upward diffusion of 4Herad from the underlying shield rocks. The apparent rate of 4Herad release is 130 μcm3 m−3 yr−1 and is 300 times greater than can be supported by the in situ decay of U and Th series nuclides (i.e., the “steady state” approximation). Laboratory release experiments (conducted by sequentially heating the aquifer solids, measuring the amount of 4He released, and then extrapolating release rates to the in situ temperature) agree well with the field results and suggest that diffusion from aquifer solids is the source of 4Herad. The combined laboratory and field release data yield 4He diffusion coefficients that exhibit an Arrhenius temperature dependence that is similar to4Herad diffusion in quartz reported by other researchers. The 4Herad release rate at the Ontario site is extraordinarily similar to sites in Tennessee, Nebraska, and Germany in spite of major hydrogeologic differences. A model of 4He diffusion from spherical grains suggests that aquifer solids derived from old protoliths will release 4He at rates greater than supported by U/Th production for up to 50 million years in fine sands that have typical U/Th concentrations. Both observations and modeling suggest that 4He may be useful as a groundwater dating tool over a range of tens to hundreds of years. The latter is particularly important because no other groundwater dating techniques are accurate for waters ranging from 40 to about 500 years old.

A validation of the 3H/3He method for determining groundwater recharge

Tritium and He isotopes have been measured at a site where groundwater flow is nearly vertical for a travel time of 100 years and where recharge rates are spatially variable. Because the mid-1960s 3H peak (arising from aboveground testing of thermonuclear devices) is well-defined, the vertical groundwater velocity is known with unusual accuracy at this site. Utilizing 3H and its stable daughter 3He to determine groundwater ages, we compute a recharge rate of 0.16 m/yr, which agrees to within about 5% of the value based on the depth of the 3H peak (measured both in 1986 and 1991) and two-dimensional modeling in an area of high recharge. Zero 3H/3He age occurs at a depth that is approximately equal to the average depth of the annual low water table, even though the capillary fringe extends to land surface during most of the year at the study site. In an area of low recharge (0.05 m/yr) where the 3H peak (and hence the vertical velocity) is also well-defined, the 3H/3He results could not be used to compute recharge because samples were not collected sufficiently far above the 3H peak; however, modeling indicates that the 3H/3He age gradient near the water table is an accurate measure of vertical velocities in the low-recharge area. Because 3H and 3He have different diffusion coefficients, and because the amount of mechanical mixing is different in the area of high recharge than in the low-recharge area, we have separated the dispersive effects of mechanical mixing from molecular diffusion. We estimate a longitudinal dispersivity of 0.07 m and effective diffusion coefficients for 3H (3HHO) and 3He of 2.4×10−5 and 1.3×10−4 m2/day, respectively. Although the 3H/3He age gradient is an excellent indicator of vertical groundwater velocities above the mid-1960s 3H peak, dispersive mixing and diffusive loss of 3He perturb the age gradient near and below the 3H peak.

High-3He plume origin and temporal-spatial evolution of the Siberian flood basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a 40Ar/39Ar plateau age of 253.3 � 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed 3He/4He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-3He SFBP plume with a suboceanic-type upper mantle beneath Siberia.

Tritium and helium: 3 as groundwater age tracers in the Borden Aquifer

Vertical profiles of 3H and He isotope ratios have been measured in groundwater from the well-characterized Borden aquifer, Ontario. The sum of 3H and tritiogenic 3He (3He*) is used as an equivalent nondecaying tracer, while the ratio of 3He* to 3H is used to compute groundwater ages. The mid-1960s 3H peak is not always apparent using the 3H data because both dispersion and radioactive decay have significantly reduced the magnitude of the 3H bomb peak. However, the sum of 3H and 3He* clearly defines the bomb peak at several locations. The accuracy of the 3H/3He dating method depends on the ability of the saturated zone to retain 3He* against diffusive loss at the water table and on the amount of dispersive mixing that occurs within the saturated zone of shallow unconfined aquifers. Helium 3 confinement is strong while dispersive mixing is weak in the Borden aquifer, resulting in an excellent delineation of groundwater travel times. Computed 3H/3He age profiles are compared with travel times predicted using a previously calibrated flow model. Although the 3H/3He age profiles are vertically offset from the modeled travel times, the travel time and 3H/3He age gradients compare exceptionally well. Recharge rates have been computed using the 3H/3He age gradients and vary from 62 cm yr−1 beneath the Borden landfill to 14 cm yr−1 north of the landfill. The 3H/3He-computed recharge agrees well with the recharge function used in previous flow modeling.

Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: A He‐Pb‐Nd‐Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges

He-Pb-Nd-Sr isotope systematics in basalts from the Mid-Atlantic Ridge (MAR) from 65°N to 78°N are reported for mapping the zone of influence of the Jan Mayen and Iceland mantle plumes in the Arctic. The geographical variation and the two distinct trends observed in the He-Pb-Nd-Sr isotope space clearly indicate that the boundary between the zone of influence of the “low 3He/4He” Jan Mayen plume and the “high 3He/4He” Iceland mantle plume is in the vicinity of the Spar FZ. Modeling indicates that the dispersion of the Jan Mayen plume is not purely radial, but extends preferentially northward, probably because of decreasing spreading rate and the cascading of the buoyant mantle plume across the Jan Mayen fracture zone (FZ) due to the large denivellation (change of level) of the base of the lithosphere caused by the large age offset (∼20 Myr). The incompatible parent and daughter (PD) element concentrations and their ratios for the basalt population from the Jan Mayen plume are highly coherent with the Pb-Nd-Sr isotope ratios and show essentially the same geographical pattern in spite of large variations in the mean degree of fusion and extent of fractional crystallization. In contrast, over the southern Kolbeinsey Ridge, unusual decouplings are observed between He-Pb-Nd-Sr isotopic systematics and incompatible element variations, as well as inferred melting conditions. These decouplings are best explained by a modified version of the dispersion model of the Iceland plume by Mertz et al. [1991] which was based on more limited isotopic data, and the fluid dynamic models of Ito et al. [1999]. In addition to binary mixing of the Iceland plume with the depleted asthenosphere a combination of (1) defluidization of the Iceland mantle plume occurring deeper than the dominant zone of dry decompression melting for enhancing He and heat transport along the southern Kolbeinsey Ridge and (2) fractional melting accompanying the northward dispersion and decompression of the Iceland mantle plume is required in order to explain the difference in wavelength between the He gradient and the Pb-Nd-Sr isotope gradients observed in mapping the dispersion of the Iceland plume along the MAR. Note that the Tjornes TZ does not act as a dam against the northward dispersion of the Iceland plume, contrary to what the rare earth element variation, by itself, previously suggested. Finally, over the Knipovich Ridge the large scatter and lack of any systematics between the Pb-Nd-Sr isotope and related parent/daughter ratios along this immature, discontinuous, and shear-dominated ridge, running parallel and close to the Svalbard continental break (74°–79°N), suggest the involvement in the melting of randomly distributed continental mantle lithosphere schlierens present in the depleted upper mantle source of the Knipovich Ridge basalts.

The groundwater geochemistry of the Bengal Basin: Weathering, chemsorption, and trace metal flux to the oceans

Sixty-eight groundwater samples from the Ganges-Brahmaputra floodplain in the Bengal Basin were analyzed to assess the groundwater geochemistry, the subsurface hydrology, the buffering effects of sediments on trace metal concentrations and their isotopic compositions, and the magnitude of the subsurface trace element flux to the Bay of Bengal and to the global ocean. Samples obtained from depths of 10 to 350 m were measured for major and trace elements, dissolved gas, and tritium. On the basis of the 3He/3H ages, the groundwater at depth (30–150 m) appears to be continually replenished, indicating that this recharge of groundwater to depth must ultimately be balanced by a significant quantity of submarine discharge into the Bay of Bengal. Using the 3He/3H groundwater age–depth relationship to calculate a recharge rate of 60 ± 20 cm/yr, we estimate a subsurface discharge into the Bay of Bengal of 1.5 ± 0.5 × 1011 m3/yr, or 15% of the surface Ganges-Brahmaputra river (GBR) flux. Several trace elements, especially Sr and Ba, display elevated concentrations averaging 7 to 9 times the surface GBR water values. The submarine groundwater fluxes of Sr and Ba to the oceans are 8.2 ± 2 × 108 and 1.5 ± 0.3 × 108 mol/yr, or 3.3 and 1.2%, respectively, of the world total, or equal to the surface GBR Sr and Ba estimated fluxes. Our groundwater flux for Ba agrees with the estimate of Moore (1997) (3 × 108–3 × 109 mol/yr), on the basis of measured Ba and Ra excesses in the Bay of Bengal. Other trace metals, such as U and Mo, are at low but measurable levels and are not major contributors to the global flux in this river system. A comparison of the Sr and Ba concentrations, plus 87Sr/86Sr ratios in groundwater to the oxalate extractable fractions of a coastal sediment core, suggests that weathering of carbonates and minor silicates, coupled with cation exchange plus adsorption and desorption reactions, controls the trace element concentrations and 87Sr/86Sr isotopic compositions in both the groundwater and river water. Our data also imply that other coastal floodplains (e.g., the Mekong and the Irrawaddy rivers) that have high precipitation rates and rapid accumulation of immature sediments are likely to make significant contributions to the global oceanic trace metal budgets and have an impact on the Sr isotopic evolution in seawater.