Measuring concentrations of natural gas in three streams in Pennsylvania to estimate methane fluxes from the subsurface

Open Access
Author:
Grieve, Paul Luke
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 07, 2014
Committee Members:
  • Susan Louise Brantley, Thesis Advisor
Keywords:
  • Methane
  • Carbon isotopes
  • Stream monitoring
  • water chemistry
  • Groundwater
  • Northeastern Pennsylvania
  • Mass balance
Abstract:
The rapid pace of Marcellus Shale gas development in the northeastern United States has prompted expanded monitoring of water resources. A newly developed approach targets the monitoring of methane from gaining streams during base flow to estimate methane concentrations and fluxes from groundwater. We implemented this technique on three streams in northeastern Pennsylvania. Measured concentrations of dissolved methane in stream waters ranged between 1–28 µg/L, and in samples from piezometers inserted into stream beds reached as high as 4600 µg/L. δ13C CH4, δ13C C2H6, and δ2H CH4 were measured and used to characterize the source of methane as thermogenic or biogenic gas. The isotopic signatures of waters varied along stream reaches and varied seasonally. In contrast, the groundwater compositions inferred from piezometer waters were consistent. This consistency of the deeper waters supports the interpretation that piezometers sampled groundwater with thermogenic gas whose isotopic signature remained constant through the two seasons studied. A mass balance equation was used to estimate the influxes of methane from groundwater in gaining subreaches along three of the streams along with methane losses to the atmosphere. A numerical model was also used to simulate methane influx and degassing along three stream reaches. For streams in Susquehanna County, the fluxes to the atmosphere from an area equivalent to the stream footprint ranged from 1.9 to 11.6 mg m-2 d-1. In Sugar Run, methane was lost to the atmosphere at fluxes between 72 to 422 mg m-2 d-1, within the range of methane fluxes measured by aerial measurements in regions of intense drilling in Pennsylvania. In Susquehanna County, lineament analysis is consistent with the conclusion that methane may be migrating upward along faults or joints, but no such feature was identified near Sugar Run in Lycoming County. The gas isotopes are consistent with this in that the isotopic signatures collected in Susquehanna County are like those of gas deriving from the Catskill Formation. In Sugar Run, however, the gas data could be consistent with gases derived from the Marcellus Formation which lies at 4000 m depth. Given that no pre-development data are available, however, the measurements cannot be used to distinguish gas flows driven strictly by natural pressure gradients versus contributions related to shale gas wells. This work shows that the stream reach approach can provide watershed-scale estimates of groundwater composition and influx and, if completed before and after drilling, could provide a relatively cheap, aerially-extensive, and easily implemented technique to document contamination from shale gas development.