Hartford, Connecticut Mobile Methane Leak Survey

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.

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Materials and Methods:

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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).

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A mobile Picarro G2301 Cavity Ring-Down Spectrometer (Picarro, Inc., Santa Clara, CA; www.picarro.com) 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; www.mainespecialtygases.com; reported precision ±10%) throughout the survey.

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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.

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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.

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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; https://www.bascomturner.com).

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Results and Discussion:

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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.

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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.

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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.

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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).

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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.