No abstract

The water dimer has been studied by accurate ab initio calculations. The main purpose of the calculations was to investigate the magnitude of, and how to eliminate the basis set superposition errors at different levels of theory. At the Hartree–Fock level the superposition errors are insignificant with the largest basis sets, and the counterpoise method works well with all the basis sets used in this study. At the correlated level superposition errors are still significant even for very large basis sets, and the standard counterpoise technique leads to overcorrection. The most important result of the present study is that the local correlation methods gives essentially the correct result for the correlation contribution to the association energy even with modest basis sets. The association energy at the MP4(SDQ) level is predicted to be 4.8 kcal/mol. The correlation contribution to the association energy is 1.2 kcal/mol which can be decomposed into an attractive intermolecular contribution of 1.8 kcal/mol and a repulsive intramolecular contribution of 0.6 kcal/mol. Ionic terms contribute about 30% to the dispersion force at the equilibrium distance. If the effect of triple substitutions is taken into account the association energy is estimated to be around 5.1 kcal/mol.

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 km 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.

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.

This study places constraints on the source and transport mechanisms of methane found in groundwater within the Barnett Shale footprint in Texas using dissolved noble gases, with particular emphasis on Kr andXe. Dissolved methane concentrations are positively correlated with crustal He,Ne, and Ar and suggest that noble gases and methane originate from common sedimentary strata, likely the Strawn Group. In contrast to most samples, four water wells with the highest dissolved methane concentrations unequivocally show strong depletion of all atmospheric noble gases (Ne, Ar,Kr, Xe) with respect to air-saturated water (ASW). This is consistent with predicted noble gas concentrations in a water phase in contact with a gas phase with initial ASW composition at 18 °C-25 °C and it suggests an in situ, highly localized gas source. All of these four water wells tap into the Strawn Group and it is likely that small gas accumulations known to be present in the shallow subsurface were reached. Additionally, lack of correlation ofKr/Ar and Xe/Ar fractionation levels along with He/Ne with distance to the nearest gas production wells does not support the notion that methane present in these groundwaters migrated from nearby production wells either conventional or using hydraulic fracturing techniques.