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Hartford, Connecticut Mobile Methane Leak Survey

Prepared July 18, 2016, revised 14 February, 2019

Tim Keyes PhD (1), Gale Ridge PhD (2), Martha Klein RN MPH (3), Nathan Phillips PhD(4), Bob Ackley (5), Yufeng Yang (6)

1 Evergreen Business Analytics, LLC

2 Steering Committee, 350 CT

3 Sierra Club Connecticut

4 Boston University, Department of Earth and Environment

5 Gas Safety, Inc.

6 Institut National des Sciences Appliquées (INSA), Lyon, France


A mobile methane leak survey was conducted on all public streets in Hartford, Connecticut (CT), from February 25 through March 31, 2016.  A total of 716 distinct methane leaks over 225 road miles in Hartford was detected, resulting in a leak frequency of 3.2 leaks per road mile.  This leak frequency compares to 4.3 leaks per mile previously found in Boston, Massachusetts (MA) (Phillips, et al. 2013).  The data collection method for the Hartford study was identical to the method used in Boston.  A preliminary estimate of methane leaks in Hartford is 0.86 metric tonnes leaked per day (or 313 metric tons per year; a metric tonne is 2,204.6 pounds, a U.S. ton is 2,000 pounds), equivalent to 42,840 cubic feet per day of natural gas.  This leakage rate represents an equivalent daily gas consumption of approximately 214 U.S. households. The data from this report, if it can be supplemented with an inventory map of leak-prone pipe location and information on pipeline operating pressures, will provide a spatial database that facilitates strategic repair and replacement of leak-prone pipe in Hartford.


Methane, the largest component of natural gas, is a powerful greenhouse gas (IPCC 2013), and when leaked from natural gas pipelines damages air quality, kills trees, and can become an explosion hazard such as seen in Massachusetts and New York (Korte, 2018; Santora, 2014).  Natural gas is 97 percent methane, the second most common greenhouse gas (GHG) in the atmosphere. Methane is an extremely potent GHG compared to carbon dioxide, and causes nearly 100 times the rate of global heating over the short term. The high leakage rate associated with methane extraction (“fracking”), and continued leaks throughout the system, including pipelines, compressor stations and metering stations, and intentional gas venting practices, contribute to the global warming potential of methane.

But of more immediate concern for most residents is the negative public health impacts that are caused by leaking methane.  Natural gas is fracked from the Marcellus Shale in Pennsylvania and Ohio, and includes other components in addition to methane.  Some of these are volatile organic compounds which lead to the formation of ground level ozone smog that promote asthma and exacerbates emphysema,  impairing lung function and other pre-existing respiratory problems: benzene, which is linked to cancer, respiratory illnesses, and birth defects; ethylbenzene, linked to neurological and blood disorders; and formaldehyde, which is linked to certain cancers and respiratory illnesses, and is leaked in substantial quantities from compressor stations.  See Raz et al (2014), Cohen (2015), Pun et al (2015), and Shi et al (2016). Natural gas from fracking represents a significant proportion of natural gas found the U.S. (U.S. Energy Information Administration, 2016).


The volume of natural gas leaked in cities and its climate and economic impacts can be substantial.  The recent study in Boston (McKain et al. 2015) estimated a gas loss rate from greater Boston at 2.7% of the total consumed natural gas, which resulted in an annual value loss of approximately $90 million, partially borne by ratepayers.  This represents approximately 10% of Massachusetts’ greenhouse gas emissions inventory. Estimates of tree damage from gas leaks in mid-sized municipalities in Massachusetts range from hundreds of thousands to millions of dollars. Urban natural gas pipelines are mostly made up of ‘distribution’ pipelines operating at low pressures (below 100 psi) compared to the high-pressure interstate transmission pipelines that deliver natural gas into high population areas and the “upstream” pipelines in production areas.  In older cities such as those in the eastern US, leak-prone cast iron, wrought iron, or bare steel gas pipelines can be up to a century old or more. These pipes often leak at decaying joint connections (typically spaced at 12-foot intervals), or due to corrosion or mechanical disturbance caused by freezing conditions. While an inventory of leak-prone pipes for Hartford was unavailable for this study, it is known that Connecticut has some of the highest total miles and percentage of leak-prone pipe in the U.S., (Pipeline Hazardous Materials and Safety Administration, 2015-17).  In general, academic research using precise measuring devices, such as the Picarro Cavity Ring-Down Spectrometer used in this study, documents more gas leaks than are reported by state regulatory agencies.

There are three investor-owned natural gas utilities in Connecticut serving urban and suburban communities.  These are Connecticut Natural Gas (CNG;, Southern Connecticut Gas (SCG;, and Eversource/Yankee Gas Company (ES;  Gas service in Hartford is supplied by CNG, the smallest of the three main utilities in terms of miles of pipeline mains (with 296 miles of leak-prone cast iron and wrought iron mains in 2017 (14% of their total miles), per PHMSA data.  Hartford is the largest city within CNG service area. See Figure 1.

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Figure 1:  Mileage of cast iron pipe managed by Connecticut operators

There were 225.45 road miles in Hartford as of December 31, 2014 (Connecticut Dept of Transportation, 2014).  Strictly speaking, this study detected methane (CH4) leaks as a broader category than natural gas leaks.  Methane can originate from sources other than natural gas pipelines, including broken sewer mains, landfills, and wetlands.  Prior work in Boston showed that the vast majority of leaks detected from under streets and sidewalks bore a distinct chemical signature of natural gas methane.  Moreover, the spatial signature of wetland and landfill leaks is distinctly different from that of pipeline leaks. Pipe leaks are recognizable as abrupt and highly localized spikes in methane concentration, while wetland and landfill methane emissions are more diffuse gradual deviations from a baseline methane concentration. To perform a simple check of the source of leaks detected in this mobile survey, five leaks from the mobile survey were re-checked to verify that they were associated with natural gas pipelines.

Materials and Methods:

This survey was conducted during winter and early spring of 2016 (February 16 to March 31, 2016), a timing that suppressed alternative methane emission signals from possible wetland, landfill or other subsurface sources, because of cold temperatures.  Additionally, for comparison, data collected from the survey was placed precisely between the first and last dates of a PURA one-year reporting period (October 15, 2015 – October 15, 2016).

A mobile Picarro G2301 Cavity Ring-Down Spectrometer (Picarro, Inc., Santa Clara, CA; was used, installed in a van equipped with a geographic positioning system (GPS), and driven on 225.45 miles of public roads in Hartford, in both directions for all two-way streets.  A filtered inlet tube was placed outside the passenger side window at a height of 30 cm above pavement. The analyzer was periodically tested with 0 and 5 ppm CH4 test gas (Spec Air Specialty Gases, Auburn, ME;; reported precision ±10%) throughout the survey.

To distinguish discrete leaks from the spatially continuous raw methane concentration data, a modified Tau approach (Olewuezi et al., 2015) was used to perform outlier detection on the raw spatial methane concentration data.  This method is a statistical method for deciding whether to keep or discard suspected outliers in a population sample. A threshold methane level that meets the outlier category, indicating a leak, is calculated by the data set’s standard deviation and average.

To avoid double-counting leaks that were driven past multiple times, a procedure was used to eliminate multiple outliers within a spatial window of 30 meters radius from the highest peak methane concentration in the vicinity.  A spatial window was used from as small as 5 meters up to 30 meters. It was found that there was relative insensitivity of the total leak count in this range, while apparent leak count decreased substantially in window sizes above 30 meters.  Since vehicle lane widths are generally approximately 10 meters or less, the 30-meter window is large enough to prevent double-counting but small enough to avoid incorrectly combining separate actual leaks into one.

A quality control check was performed by auditing 5 randomly selected leaks from the driving survey and verifying and pinpointing subsurface methane concentrations using a ‘bang bar’ to penetrate the surface, in combination with a combustible gas indicator (Gas Sentry, Bascom-Turner, Inc. Norwood, MA;

Results and Discussion:

A total of 716 distinct methane leaks over 225.45 road miles in Hartford were detected, resulting in a leak frequency of 3.2 leaks per road mile.  This leak frequency compares to 4.3 leaks per mile previously discovered in Boston, MA (Phillips et al. 2013). Over a one-year period covering the same area (15 October, 2015 to 15 October, 2016), the Public Utilities Regulatory Authority (PURA) reported 138 leaks (data provided by PURA).   By identifying leaks known to PURA, this is approximately a 5-fold difference in leaks detected over a period of a few weeks, as compared to a year. Evidence suggests that CNG may be missing or not reporting a large percentage of its leaks.

A preliminary estimate of the leakage rate from the leaks found during the survey was made using the leak size distribution data from the Boston study (Hendrick et al. 2016).  Assuming leaked natural gas volume from the Hartford pipes had the same distribution found in the Boston study, with a log-normal average of 1.2 kg CH4 per day, a preliminary estimate would find 0.86 metric tonnes/42,840 cubic feet of gas was lost to the atmosphere each day, resulting in 313 metric tonnes per year.  Although peak methane concentrations observed from the mobile survey offered a rough indication of leak size, it is not a reliable indicator of this, because shifting wind speed and direction influences leaked gas concentrations from moment to moment.

The 0.86 metric tonnes of methane loss per day in Hartford compares to the estimate of 4.0 metric tonnes of methane loss per day in in Boston.  Note Boston has 3.5 times more public road miles than Hartford with 790 miles (Phillips, et al. 2013). The Hartford leak rates represent an equivalent daily gas consumption of approximately 214 U.S. households.  This is a small fraction of the total households in Hartford, which is approximately 45,800 (U.S. Census Bureau, 2018). The benefits of repairing gas leaks for air quality, tree health, and public and property safety add impetus for the City of Hartford to address the problem.

From data provided to the DOT’s Pipeline Hazardous Materials Safety Administration (PHMSA) (2015-17), in 2015 Connecticut ranked 6th among U.S. states in terms of the total number of miles of leak-prone, cast iron and wrought iron gas distribution pipeline, and first in miles of cast iron and wrought iron pipe in percentage of total miles (16.9%) of distribution pipeline.  By comparison, Massachusetts ranked 3rd and 4th in these categories, respectively. From 2005-15, according to the U.S. Energy Information Administration (EIA, 2016), CNG reduced its inventory of leak-prone, cast iron and wrought iron pipe by 24%, about the same as the reduction attained by Yankee Gas Services Co, and at twice the rate over the same period by the largest gas utility in the state, Southern Connecticut Gas Co (12% reduction from 2005-15).

The data from the two sources indicate that Hartford is substantially less prone to gas leaks than Boston, and may be in better shape than cities served by Southern Connecticut Gas Co.  This may be attributed to better efforts by CNG to repair and replace leak-prone pipe in Hartford compared to National Grid, the gas utility serving most of Boston, or Southern Connecticut Gas, which has reduced its leak-prone pipe miles by half the rate of CNG during the last decade (PHMSA, 2015-17).  The worst leaks in Hartford were noted along the length of Main Street. Figures 2 and 3 show the frequency of leaks found by PURA and those found in this study, by street name.

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Figure 2:  Frequency distribution by street name from leak reports recorded by PURA between October 15, 2015 and October 15, 2016, a one year, four season period.  A total of 138 gas leaks reported in the city of Hartford.

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Figure 3:  Frequency distribution by street name of Leaks from this study between February 25 through March 31, 2016.  A total of 716 gas leaks were recorded in the city of Hartford.

In 2014 “An Act Concerning Lost and Unaccounted for Gas” (PA 14-152) was passed by the Connecticut State legislature. This two-paragraph act permits Connecticut natural gas distribution companies to charge Connecticut ratepayers hidden fees not clearly identified in their bills for lost, unused gas.  These companies are permitted to estimate the volume of “lost and unaccounted for” natural gas escaping from their equipment, and charge customers to recoup the revenue. This provides a disincentive for repair and/or replacement of faulty pipes and joints in a timely manner, because it lacks incentive to do so.  As written earlier, it is estimated the State of Massachusetts currently loses approximately $90 million/year in lost value of leaked gas to the atmosphere. Though lacking reliable data, it can be inferred that the Connecticut natural gas pipeline system is also losing millions of dollars per year of gas revenue to the atmosphere.


The State of Connecticut possesses an energy policy which is committed to expanding public and corporate use of natural gas.  Additionally, it supports the expansion of large capacity high pressure Interstate transmission pipelines through Connecticut from frack gas providers to consumers, international exporters, and business entities in other regions of the U.S. and Canada.  Pressure is maintained using compressor stations spaced at intervals along the transmission pipelines. During the summer of 2015, PURA held a series of hearings addressing expansion of the Algonquin Gas Transmission pipeline through Oxford Connecticut, by the Atlantic Bridge Project.  During the PURA Oxford hearing a citizen testified they had purchased a house located in a valley below a gas compressor station in Oxford. On several occasions during calm cold weather in winter the citizen had experienced a strong odor of gas around the home which had sickened the family. This is the result of temperature inversion which acted like a lid, trapping escaped gas at ground level causing the gas to pool in the valley.  The testimony supports gas loss in other sections of the gas supply system not yet surveyed (personal communication).


PURA is responsible for intrastate gas pipeline safety oversight, and is authorized by the federal Department of Transportation/Pipeline Hazardous Materials and Safety Administration (PHMSA)/Office of Pipeline Safety to monitor interstate pipeline safety. PURA claims it records and monitors all leaks that are reported to it. A concern is that Eversource, CNG, and SCG have been reported to check their pipes remotely and electronically which may result in underestimation of gas leak events.


Given the results from the study, the rate of methane leaks appears to be much higher than PURA records show, and the gas expansion plan should be re-evaluated in light of this.  The solution to the problem is legislative, and there should be a zero-leak tolerance policy maintained for natural gas pipelines, as there is for petroleum pipelines (Clean Water Act 33 U.S.C. 1251 et seq.).


PURA categorizes gas leaks into three classes or grades.  Grade 1 (existing hazardous leak), Grade 2 (potential future hazardous leak), and Grade 3 (non-hazardous at the time of detection, and expected to remain non-hazardous).  Grades 1 and 2 are reported to PURA, but not 3. According to PURA, Grade 3 leaks are categorized as too small to report (personal communication from Karl Baker, Public Utilities Supervisor of Technical Analysis, PURA).  Additionally, PURA does not require the volume of natural gas loss in Grades 1 and 2 to be reported. Grade 3 leaks are often numerous and may be seen as problematic, because a) they may progress into a higher Grade (i.e., are mis-classified), b) the accumulated number of small leaks become equal in volume loss as fewer higher volume leaks and c), the often more widely distributed Grade 3 leaks, may cause more human and environmental harm over a wider geographic area.  Figures 4 and 5 plot the location of leaks found in this study, and leaks reported to PURA.

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Figure 4:  Locations of methane leaks round in Hartford; data plotted in blue (716 leaks) gathered in this study February 25 through March 31, 2016, and in red by PURA (138 leaks of Grade 1 and 2) October 15, 2015 – October 15, 2016.

From evidence provided in this study, it may be suggested that a proactive alternative to the PURA model of chiefly reactive reporting may be both economically and environmentally compatible.  This study identified natural gas leaks using vehicle mounted sensing equipment, while the PURA model appears to largely be dependent on voluntary human reporting. Considering the relative ease that testing equipment can be mobilized, a proactive approach may be superior in terms of public health, the environment, and long-term cost-benefits to both the State of Connecticut and gas companies. The study illustrates a means to improve gas industry system-wide performance as well as enhance public understanding of the efficacy of mobile gas leak street-level monitoring.

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Figure 5:  Locations of PURA Methane Leaks Found in Hartford, 2011-2016 (1,069 leaks, 321 of Grade 1 and 748 of Grade 2; Grade 1 leaks are red filled circles, Grade 2 are not filled).

Future improvements on this study would include obtaining the complete pipeline inventory and map, and a map of the operating pressures of the pipes in Hartford from CNG.  These data may help explain why certain roadways in Hartford possess a higher spatial density of leaks than others, and would allow for an estimate of the likely rankings of leak rates from particular lengths of pipeline.  Among the low-pressure distribution pipelines, operating pressures can vary substantially, from 0.5 psi to 60 psi or more. A pipe will leak at a rate that is proportional to the pipeline operating pressure, so leaks found in zones of higher operating pressure will be expected to leak higher volumes of natural gas.  Proactive measures and management of all gas pipeline system defects, from small to large, with transparency taking into account both the frequency and severity of leaks, using established risk management procedures is recommended.


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