Reconciling divergent estimates of oil and gas methane emissions.

Published estimates of methane emissions from atmospheric data (top-down approaches) exceed those from source-based inventories (bottom-up approaches), leading to conflicting claims about the climate implications of fuel switching from coal or petroleum to natural gas. Based on data from a coordinated campaign in the Barnett Shale oil and gas-producing region of Texas, we find that top-down and bottom-up estimates of both total and fossil methane emissions agree within statistical confidence intervals (relative differences are 10% for fossil methane and 0.1% for total methane). We reduced uncertainty in top-down estimates by using repeated mass balance measurements, as well as ethane as a fingerprint for source attribution. Similarly, our bottom-up estimate incorporates a more complete count of facilities than past inventories, which omitted a significant number of major sources, and more effectively accounts for the influence of large emission sources using a statistical estimator that integrates observations from multiple ground-based measurement datasets. Two percent of oil and gas facilities in the Barnett accounts for half of methane emissions at any given time, and high-emitting facilities appear to be spatiotemporally variable. Measured oil and gas methane emissions are 90% larger than estimates based on the US Environmental Protection Agency's Greenhouse Gas Inventory and correspond to 1.5% of natural gas production. This rate of methane loss increases the 20-y climate impacts of natural gas consumed in the region by roughly 50%.

Monitoring concentration and isotopic composition of methane in groundwater in the Utica Shale hydraulic fracturing region of Ohio.

Degradation of groundwater quality is a primary public concern in rural hydraulic fracturing areas. Previous studies have shown that natural gas methane (CH4) is present in groundwater near shale gas wells in the Marcellus Shale of Pennsylvania, but did not have pre-drilling baseline measurements. Here, we present the results of a free public water testing program in the Utica Shale of Ohio, where we measured CH4 concentration, CH4 stable isotopic composition, and pH and conductivity along temporal and spatial gradients of hydraulic fracturing activity. Dissolved CH4 ranged from 0.2 µg/L to 25 mg/L, and stable isotopic measurements indicated a predominantly biogenic carbonate reduction CH4 source. Radiocarbon dating of CH4 in combination with stable isotopic analysis of CH4 in three samples indicated that fossil C substrates are the source of CH4 in groundwater, with one 14C date indicative of modern biogenic carbonate reduction. We found no relationship between CH4 concentration or source in groundwater and proximity to active gas well sites. No significant changes in CH4 concentration, CH4 isotopic composition, pH, or conductivity in water wells were observed during the study period. These data indicate that high levels of biogenic CH4 can be present in groundwater wells independent of hydraulic fracturing activity and affirm the need for isotopic or other fingerprinting techniques for CH4 source identification. Continued monitoring of private drinking water wells is critical to ensure that groundwater quality is not altered as hydraulic fracturing activity continues in the region. Graphical abstract A shale gas well in rural Appalachian Ohio. Photo credit: Claire Botner.

Estimating Emissions of Toxic Hydrocarbons from Natural Gas Production Sites in the Barnett Shale Region of Northern Texas.

Oil and natural gas operations have continued to expand and move closer to densely populated areas, contributing to growing public concerns regarding exposure to hazardous air pollutants. During the Barnett Shale Coordinated Campaign in October, 2013, ground-based whole air samples collected downwind of oil and gas sites revealed enhancements in several potentially toxic volatile organic compounds (VOCs) when compared to background values. Molar emissions ratios relative to methane were determined for hexane, benzene, toluene, ethylbenzene, and xylene (BTEX compounds). Using methane leak rates measured from the Picarro mobile flux plane (MFP) system and a Barnett Shale regional methane emissions inventory, the rates of emission of these toxic gases were calculated. Benzene emissions ranged between 51 ± 4 and 60 ± 4 kg h-1. Hexane, the most abundantly emitted pollutant, ranged from 642 ± 45 to 1070 ± 340 kg h-1. While observed hydrocarbon enhancements fall below federal workplace standards, results may indicate a link between emissions from oil and natural gas operations and concerns about exposure to hazardous air pollutants. The larger public health risks associated with the production and distribution of natural gas are of particular importance and warrant further investigation, particularly as the use of natural gas increases in the United States and internationally.

Shallow Groundwater Conveyance of Geologically Derived Contaminants to Urban Creeks in Southern California.

In California alone, there are currently over 200 instances on the EPA's list of impaired water bodies with unknown sources of excessive salinity or trace contaminants. This investigation focuses on Orange County, CA, a region that has undergone extensive hydrological modification, relies heavily on imported water for municipal supply, and has come under regulatory scrutiny for elevated TDS, sulfate, Cd, Ni, and Se. A survey of shallow groundwater weeps and springs, discharging directly to urban creeks, reveals high concentrations of TDS, sulfate, Cd, Ni, Zn, Cu, and Se that are often far in excess of water quality standards. Isotopic (δ(34)S and δ(18)O) and geochemical evidence indicate that the source of sulfate and TDS is weathering of sulfide minerals in the Capistrano Formation marine mudstone and dissolution of secondary minerals formed during past periods of sulfide oxidation, rather than anthropogenic inputs. The relative availability of carbonate minerals along the flow path appears to control pH, which then influences trace metal mobility to surface waters. Stable isotopes of H2O indicate that despite widespread use of imported water, meteoric recharge dominates shallow groundwater inputs with municipal sources contributing only 13-29% of discharge. These findings highlight the importance of understanding the hydrogeological setting to properly apportion contaminant sources and conveyances.

Integrating Source Apportionment Tracers into a Bottom-up Inventory of Methane Emissions in the Barnett Shale Hydraulic Fracturing Region.

A growing dependence on natural gas for energy may exacerbate emissions of the greenhouse gas methane (CH4). Identifying fingerprints of these emissions is critical to our understanding of potential impacts. Here, we compare stable isotopic and alkane ratio tracers of natural gas, agricultural, and urban CH4 sources in the Barnett Shale hydraulic fracturing region near Fort Worth, Texas. Thermogenic and biogenic sources were compositionally distinct, and emissions from oil wells were enriched in alkanes and isotopically depleted relative to natural gas wells. Emissions from natural gas production varied in δ(13)C and alkane ratio composition, with δD-CH4 representing the most consistent tracer of natural gas sources. We integrated our data into a bottom-up inventory of CH4 for the region, resulting in an inventory of ethane (C2H6) sources for comparison to top-down estimates of CH4 and C2H6 emissions. Methane emissions in the Barnett are a complex mixture of urban, agricultural, and fossil fuel sources, which makes source apportionment challenging. For example, spatial heterogeneity in gas composition and high C2H6/CH4 ratios in emissions from conventional oil production add uncertainty to top-down models of source apportionment. Future top-down studies may benefit from the addition of δD-CH4 to distinguish thermogenic and biogenic sources.

Direct measurements show decreasing methane emissions from natural gas local distribution systems in the United States.

Fugitive losses from natural gas distribution systems are a significant source of anthropogenic methane. Here, we report on a national sampling program to measure methane emissions from 13 urban distribution systems across the U.S. Emission factors were derived from direct measurements at 230 underground pipeline leaks and 229 metering and regulating facilities using stratified random sampling. When these new emission factors are combined with estimates for customer meters, maintenance, and upsets, and current pipeline miles and numbers of facilities, the total estimate is 393 Gg/yr with a 95% upper confidence limit of 854 Gg/yr (0.10% to 0.22% of the methane delivered nationwide). This fraction includes emissions from city gates to the customer meter, but does not include other urban sources or those downstream of customer meters. The upper confidence limit accounts for the skewed distribution of measurements, where a few large emitters accounted for most of the emissions. This emission estimate is 36% to 70% less than the 2011 EPA inventory, (based largely on 1990s emission data), and reflects significant upgrades at metering and regulating stations, improvements in leak detection and maintenance activities, as well as potential effects from differences in methodologies between the two studies.

Sensor transition failure in the high flow sampler: Implications for methane emission inventories of natural gas infrastructure.

UNLABELLED: Quantification of leaks from natural gas (NG) infrastructure is a key step in reducing emissions of the greenhouse gas methane (CH4), particularly as NG becomes a larger component of domestic energy supply. The U.S. Environmental Protection Agency (EPA) requires measurement and reporting of emissions of CH4 from NG transmission, storage, and processing facilities, and the high-flow sampler (or high-volume sampler) is one of the tools approved for this by the EPA. The Bacharach Hi-Flow Sampler (BHFS) is the only commercially available high-flow instrument, and it is also used throughout the NG supply chain for directed inspection and maintenance, emission factor development, and greenhouse gas reduction programs. Here we document failure of the BHFS to transition from a catalytic oxidation sensor used to measure low NG (~5% or less) concentrations to a thermal conductivity sensor for higher concentrations (from ~5% to 100%), resulting in underestimation of NG emission rates. Our analysis includes both our own field testing and analysis of data from two other studies (Modrak et al., 2012; City of Fort Worth, 2011). Although this failure is not completely understood, and although we do not know if all BHFS models are similarly affected, sensor transition failure has been observed under one or more of these conditions: (1) Calibration is more than ~2 weeks old; (2) firmware is out of date; or (3) the composition of the NG source is less than ~91% CH4. The extent to which this issue has affected recent emission studies is uncertain, but the analysis presented here suggests that the problem could be widespread. Furthermore, it is critical that this problem be resolved before the onset of regulations on CH4 emissions from the oil and gas industry, as the BHFS is a popular instrument for these measurements. IMPLICATIONS: An instrument commonly used to measure leaks in natural gas infrastructure has a critical sensor transition failure issue that results in underestimation of leaks, with implications for greenhouse gas emissions estimates as well as safety.

High methane emissions from a midlatitude reservoir draining an agricultural watershed.

Reservoirs are a globally significant source of methane (CH4), although most measurements have been made in tropical and boreal systems draining undeveloped watersheds. To assess the magnitude of CH4 emissions from reservoirs in midlatitude agricultural regions, we measured CH4 and carbon dioxide (CO2) emission rates from William H. Harsha Lake (Ohio, U.S.A.), an agricultural impacted reservoir, over a 13 month period. The reservoir was a strong source of CH4 throughout the year, emitting on average 176 ± 36 mg C m(-2) d(-1), the highest reservoir CH4 emissions profile documented in the United States to date. Contrary to our initial hypothesis, the largest CH4 emissions were during summer stratified conditions, not during fall turnover. The river-reservoir transition zone emitted CH4 at rates an order of magnitude higher than the rest of the reservoir, and total carbon emissions (i.e., CH4 + CO2) were also greater at the transition zone, indicating that the river delta supported greater carbon mineralization rates than elsewhere. Midlatitude agricultural impacted reservoirs may be a larger source of CH4 to the atmosphere than currently recognized, particularly if river deltas are consistent CH4 hot spots. We estimate that CH4 emissions from agricultural reservoirs could be a significant component of anthropogenic CH4 emissions in the U.S.A.

Nitrous oxide emissions from wastewater treatment and water reclamation plants in southern California.

Nitrous oxide (N2O) is a long-lived and potent greenhouse gas produced during microbial nitrification and denitrification. In developed countries, centralized water reclamation plants often use these processes for N removal before effluent is used for irrigation or discharged to surface water, thus making this treatment a potentially large source of N2O in urban areas. In the arid but densely populated southwestern United States, water reclamation for irrigation is an important alternative to long-distance water importation. We measured N2O concentrations and fluxes from several wastewater treatment processes in urban southern California. We found that N removal during water reclamation may lead to in situ N2O emission rates that are three or more times greater than traditional treatment processes (C oxidation only). In the water reclamation plants tested, N2O production was a greater percentage of total N removed (1.2%) than traditional treatment processes (C oxidation only) (0.4%). We also measured stable isotope ratios (δN and δO) of emitted N2O and found distinct δN signatures of N2O from denitrification (0.0 ± 4.0 ‰) and nitrification reactors (-24.5 ± 2.2 ‰), respectively. These isotope data confirm that both nitrification and denitrification contribute to N2O emissions within the same treatment plant. Our estimates indicate that N2O emissions from biological N removal for water reclamation may be several orders of magnitude greater than N2O emissions from agricultural activities in highly urbanized southern California. Our results suggest that wastewater treatment that includes biological nitrogen removal can significantly increase urban N2O emissions.

Measurements show that marginal wells are a disproportionate source of methane relative to production.

Oil and natural gas wells are a prominent source of the greenhouse gas methane (CH4), but most measurements are from newer, high producing wells. There are nearly 700,000 marginal "stripper" wells in the US, which produce less than 15 barrels of oil equivalent (BOE) d-1. We made direct measurements of CH4 and volatile organic carbon (VOC) emissions from marginal oil and gas wells in the Appalachian Basin of southeastern Ohio, all producing < 1 BOE d-1. Methane and VOC emissions followed a skewed distribution, with many wells having zero or low emissions and a few wells responsible for the majority of emissions. The average CH4 emission rate from marginal wells was 128 g h-1 (median: 18 g h-1; range: 0- 907 g h-1). Follow-up measurements at five wells indicated high emissions were not episodic. Some wells were emitting all or more of the reported gas produced at each well, or venting gas from wells with no reported gas production. Measurements were made from wellheads only, not tanks, so our estimates may be conservative. Stochastic processes such as maintenance may be the main driver of emissions. Marginal wells are a disproportionate source of CH4 and VOCs relative to oil and gas production. We estimate that oil and gas wells in this lowest production category emit approximately 11% of total annual CH4 from oil and gas production in the EPA greenhouse gas inventory, although they produce about 0.2% of oil and 0.4% of gas in the US per year. Implications: Low producing marginal wells are the most abundant type of oil and gas well in the United States, and a surprising number of them are venting all or more of their reported produced gas to the atmosphere. This makes marginal wells a disproportionate greenhouse gas emissions source compared to their energy return, and a good target for environmental mitigation.

Street-level emissions of methane and nitrous oxide from the wastewater collection system in Cincinnati, Ohio.

Recent studies have indicated that urban streets can be hotspots for emissions of methane (CH4) from leaky natural gas lines, particularly in cities with older natural gas distribution systems. The objective of the current study was to determine whether leaking sewer pipes could also be a source of street-level CH4 as well as nitrous oxide (N2O) in Cincinnati, Ohio, a city with a relatively new gas pipeline network. To do this, we measured the carbon (δ13C) and hydrogen (δ2H) stable isotopic composition of CH4 to distinguish between biogenic CH4 from sewer gas and thermogenic CH4 from leaking natural gas pipelines and measured CH4 and N2O flux rates and concentrations at sites from a previous study of street-level CH4 enhancements (77 out of 104 sites) as well as additional sites found through surveying sewer grates and utility manholes (27 out of 104 sites). The average isotopic signatures for δ13C-CH4 and δ2H-CH4 were -48.5‰ ± 6.0‰ and -302‰ ± 142‰. The measured flux rates ranged from 0.0 to 282.5 mg CH4 day-1 and 0.0-14.1 mg N2O day-1 (n = 43). The average CH4 and N2O concentrations measured in our study were 4.0 ±â€¯7.6 ppm and 392 ±â€¯158 ppb, respectively (n = 104). 72% of sites where fluxes were measured were a source of biogenic CH4. Overall, 47% of the sampled sites had biogenic CH4, while only 13% of our sites had solely thermogenic CH4. The other sites were either a source of both biogenic and thermogenic CH4 (13%), and a relatively large portion of sites had an unresolved source (29%). Overall, this survey of emissions across a large urban area indicates that production and emission of biogenic CH4 and N2O is considerable, although CH4 fluxes are lower than those reported for cities with leaky natural gas distribution systems.

Assessment of methane emissions from the U.S. oil and gas supply chain.

Methane emissions from the U.S. oil and natural gas supply chain were estimated by using ground-based, facility-scale measurements and validated with aircraft observations in areas accounting for ~30% of U.S. gas production. When scaled up nationally, our facility-based estimate of 2015 supply chain emissions is 13 ± 2 teragrams per year, equivalent to 2.3% of gross U.S. gas production. This value is ~60% higher than the U.S. Environmental Protection Agency inventory estimate, likely because existing inventory methods miss emissions released during abnormal operating conditions. Methane emissions of this magnitude, per unit of natural gas consumed, produce radiative forcing over a 20-year time horizon comparable to the CO2 from natural gas combustion. Substantial emission reductions are feasible through rapid detection of the root causes of high emissions and deployment of less failure-prone systems.

Impact of direct greenhouse gas emissions on the carbon footprint of water reclamation processes employing nitrification-denitrification.

Water reclamation has the potential to reduce water supply demands from aquifers and more energy-intensive water production methods (e.g., seawater desalination). However, water reclamation via biological nitrification-denitrification is also associated with the direct emission of the greenhouse gases (GHGs) CO2, N2O, and CH4. We quantified these direct emissions from the nitrification-denitrification reactors of a water reclamation plant in Southern California, and measured the (14)C content of the CO2 to distinguish between short- and long-lived carbon. The total emissions were 1.5 (±0.2) g-fossil CO2 m(-3) of wastewater treated, 0.5 (±0.1) g-CO2-eq of CH4 m(-3), and 1.8 (±0.5) g-CO2-eq of N2O m(-3), for a total of 3.9 (±0.5) g-CO2-eqm(-3). This demonstrated that water reclamation can be a source of GHGs from long lived carbon, and thus a candidate for GHG reduction credit. From the (14)C measurements, we found that between 11.4% and 15.1% of the CO2 directly emitted was derived from fossil sources, which challenges past assumptions that the direct CO2 emissions from water reclamation contain only modern carbon. A comparison of our direct emission measurements with estimates of indirect emissions from several water production methods, however, showed that the direct emissions from water reclamation constitute only a small fraction of the plant's total GHG footprint.