Groundwater Monitoring for Shale Gas: Adaptation of Concepts from DNAPL Site Investigations

Shale gas development has been a substantial and growing activity for more than a decade but only limited groundwater monitoring has been done to date, nearly all of which has been sampling of domestic wells. This has caused confusion and debate about the results. There is recognition of need to move beyond this type of sampling to science- based monitoring serving multiple objectives and relevant questions. The challenge for the hydrogeological community is to figure out how such monitoring should best be done. This presentation examines some concepts based primarily on experience drawn from investigations of DNAPL sites on fractured sedimentary rock.

The main challenges for shale gas development are fractured sedimentary rock monitoring spanning a much larger depth range than that of conventional practice. The concerns about shale gas pertain to varied contaminant types from both shallow and deep sources. Shallow sources including fracking chemicals, fuels and flowback fluids and contaminants such as natural gas and salinity originating at depth below the fresh groundwater zone, most likely from the intermediate zone and less so from the deep zone where the fracking occurs. But the further challenge draws from multiple objectives for assessing performance, sentry and receptor monitoring, each with their own design and baseline data needs. There is much experience from contaminated site investigations to guide the shallow monitoring but not the deeper monitoring into the Intermediate zone. Rather than conventional monitoring wells where one well is installed in each drill hole, shale gas monitoring can derive effectiveness from use of depth- discrete, multilevel systems (MLS's) in single holes for which there are many versions to select from depending on the depth and nature of the hydrogeology  (e.g FLUTe, Westbay-Schlumberger and Solinst- Waterloo and CMT systems). Experience at DNAPL sites shows that effective monitoring first requires subsurface characterization to guide vertical placement of ports and seals and aid interpretation. Experience at DNAPL sites in sedimentary rock covers the depth range down to 400 m and is further supported with experience from deep sedimentary rock investigations for radioactive waste repositories and CO2 injection sites.