Natural gas production in the United States has increased rapidly over the past decade, along with concerns about methane (CH4) fugitive emissions and its climate impacts. Quantification of CH4emissions from oil and natural gas (O&NG) operations is important for establishing scientifically sound policies for mitigating greenhouse gases. We use the aircraft mass balance approach for three flight experiments in August and September 2015 to estimate CH4emissions from O&NG operations over the southwestern Marcellus Shale. We estimate a mean CH4emission rate as 21.2 kg/s with 28% coming from O&NG operations. The mean CH4emission rate from O&NG operations was estimated to be 1.1% of total NG production. The individual best‐estimate emission rates from the three flight experiments ranged from 0.78 to 1.5%, with overall limits of 0% and 3.5%. These emission rates are at the low end of other top‐down studies, but consistent with the few observational studies in the Marcellus Shale region as well as the U.S. Environmental Protection Agency CH4inventory. A substantial source of CH4(~70% of observed CH4emissions) was found to contain little ethane, possibly due to coalbed CH4emitted either directly from coal mines or from wells drilled through coalbed layers in O&NG operations. Recent regulations requiring capture of gas from the completion‐venting step of hydraulic fracturing appear to have reduced the atmospheric release of CH4. Our study suggests that for a 20‐year time scale, energy derived from the combustion of natural gas extracted from this region likely exerts a net climate benefit compared to coal.
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.
more » « less- Award ID(s):
- 1639150
- NSF-PAR ID:
- 10079874
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 115
- Issue:
- 49
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. 12349-12358
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
The amount of radon in natural gas varies with its source. Little has been published about the radon from shale gas to date, making estimates of its impact on radon‐induced lung cancer speculative. We measured radon in natural gas pipelines carrying gas from the Marcellus Shale in Pennsylvania and West Virginia. Radon concentrations ranged from 1,520 to 2,750 Bq/m3(41–74 pCi/L), and the throughput‐weighted average was 1,983 Bq/m3(54 pCi/L). Potential radon exposure due to the use of Marcellus Shale gas for cooking and space heating using vent‐free heaters or gas ranges in northeastern U.S. homes and apartments was assessed. Though the measured radon concentrations are higher than what has been previously reported, it is unlikely that exposure from natural gas cooking would exceed 1.2 Bq/m3(<1% of the U.S. Environmental Protection Agency's action level). Using worst‐case assumptions, we estimate the excess lifetime (70 years) lung cancer risk associated with cooking to be 1.8×10−4(interval spanning 95% of simulation results: 8.5×10−5, 3.4×10−4). The risk profile for supplemental heating with unvented gas appliances is similar. Individuals using unvented gas appliances to provide primary heating may face lifetime risks as high as 3.9×10−3. Under current housing stock and gas consumption assumptions, expected levels of residential radon exposure due to unvented combustion of Marcellus Shale natural gas in the Northeast United States do not result in a detectable change in the lung cancer death rates.
-
more » « less
Abstract Growing evidence suggests microbial respiration of dissolved organic carbon (DOC) may be a principal driver of subsurface dissolution and cave formation in eogenetic carbonate rock. Analyses of samples of vadose zone gasses, and geochemical and hydrological data collected from shallow, uncased wells on San Salvador Island, Bahamas, suggest tidally varying water tables may help fuel microbial respiration and dissolution through oxygenation. Respiration of soil organic carbon transported to water tables generates dysaerobic to anaerobic groundwater, limiting aerobic microbial processes. Positive correlations of carbon dioxide (CO2), radon‐222 (222Rn) and water table elevation indicate, however, that tidal pumping of water tables pulls atmospheric air that is rich in oxygen, and low in CO2and222Rn, into contact with the tidal capillary fringe during falling tides. Ratios of CO2and O2in vadose gas relative to the atmosphere indicate this atmospheric oxygen fuels respiration within newly‐exposed, wetted bedrock. Deficits of expected CO2relative to O2concentrations indicate some respired CO2is likely removed by carbonate mineral dissolution. Tidal pumping also appears capable of transferring oxygen to the freshwater lens, where it could also contribute to respiration and dissolution; dissolved oxygen concentrations at the water table are at least 5% saturated and decline to anaerobic conditions 1–2 m below. Our results demonstrate how tidal pumping of air to vadose zones can drive mineral dissolution reactions that are focused near water tables and may contribute to the formation of laterally continuous vuggy horizons and potentially caves. © 2020 John Wiley & Sons, Ltd.
-
Streams and rivers are significant sources of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) globally, and watershed management can alter greenhouse gas (GHG) emissions from streams. We hypothesized that urban infrastructure significantly alters downstream water quality and contributes to variability in GHG saturation and emissions. We measured gas saturation and estimated emission rates in headwaters of two urban stream networks (Red Run and Dead Run) of the Baltimore Ecosystem Study Long-Term Ecological Research project. We identified four combinations of stormwater and sanitary infrastructure present in these watersheds, including: (1) stream burial, (2) inline stormwater wetlands, (3) riparian/floodplain preservation, and (4) septic systems. We selected two first-order catchments in each of these categories and measured GHG concentrations, emissions, and dissolved inorganic and organic carbon (DIC and DOC) and nutrient concentrations biweekly for 1 year. From a water quality perspective, the DOC : NO3− ratio of streamwater was significantly different across infrastructure categories. Multiple linear regressions including DOC : NO3− and other variables (dissolved oxygen, DO; total dissolved nitrogen, TDN; and temperature) explained much of the statistical variation in nitrous oxide (N2O, r2 = 0.78), carbon dioxide (CO2, r2 = 0.78), and methane (CH4, r2 = 0.50) saturation in stream water. We measured N2O saturation ratios, which were among the highest reported in the literature for streams, ranging from 1.1 to 47 across all sites and dates. N2O saturation ratios were highest in streams draining watersheds with septic systems and strongly correlated with TDN. The CO2 saturation ratio was highly correlated with the N2O saturation ratio across all sites and dates, and the CO2 saturation ratio ranged from 1.1 to 73. CH4 was always supersaturated, with saturation ratios ranging from 3.0 to 2157. Longitudinal surveys extending form headwaters to third-order outlets of Red Run and Dead Run took place in spring and fall. Linear regressions of these data yielded significant negative relationships between each gas with increasing watershed size as well as consistent relationships between solutes (TDN or DOC, and DOC : TDN ratio) and gas saturation. Despite a decline in gas saturation between the headwaters and stream outlet, streams remained saturated with GHGs throughout the drainage network, suggesting that urban streams are continuous sources of CO2, CH4, and N2O. Our results suggest that infrastructure decisions can have significant effects on downstream water quality and greenhouse gases, and watershed management strategies may need to consider coupled impacts on urban water and air quality.more » « less
-
more » « less
Abstract Recent advances in shale gas development have largely outpaced efforts to manage associated waste streams that pose significant environmental risks. Wastewater management presents significant challenges in the Marcellus shale, where increasing fluid volumes concomitant with expanding development will threaten to overwhelm existing infrastructure over the next decade. In this work, we forecast growth in drilling, flowback, and produced fluid volumes through 2025 based on historic data and consider conventional and alternative disposal options to meet future demands. The results indicate that nearly 12 million m3(74 MMbbl) of wastewater will be generated annually by 2025. Even assuming wastewater recycling rates in the region rebound, meeting increased demands for wastewater that cannot be reused due to poor quality or logistics would require significant capital investment to expand existing disposal pathways, namely treatment and discharge at centralized facilities or dedicated brine injection in Ohio. Here, we demonstrate the logistical and environmental advantages of an alternative strategy: repurposing depleted oil and gas wells for dedicated injection of wastewater that cannot otherwise be reused or recycled. Hubs of depleted wells could accommodate projected increases in wastewater volumes more efficiently than existing disposal options, primarily because the proximity of depleted wells to active production sites would substantially reduce wastewater transport distances and associated costs. This study highlights the need to reevaluate regional-scale shale wastewater management practices in the context of evolving wastewater qualities and quantities, as strategic planning will result in more socially and economically favorable options while avoiding adverse environmental impacts that have overshadowed the environmental benefits of natural gas expansion in the energy sector.
