100
Part of the problem has to do with the complicated origins and migration of stray
gas in aquifers. As described in the previous chapter, biogenic methane can be generated by microbial processes in soils and shallow aquifers, and thermogenic methane comes from organic-bearing rocks like coal or shale exposed to high temperatures
during their burial history. Thus, stray gas can form within the aquifer from biological activity, enter groundwater from a compromised wellbore that extends kilometers deep into shale, or seep in from an organic-rich rock unit lying directly beneath
an aquifer. In many cases, gas from more than one origin will be present in an
aquifer, and the sources are sometimes indistinguishable.
A second part of the problem is related to how the data are collected for assessing
the presence of gas in groundwater. It is important to note that amount of methane
that dissolves in water, like carbon dioxide, is pressure-dependent. Just as soda
water will release bubbles when a bottle is uncapped, groundwater will release
methane when pressure is reduced. Depending on where in the water system a sample is collected: downhole, at the pump, or at the tap, the methane content can be
significantly different. A lack of uniformity on where and how samples have been
collected makes comparisons between different studies challenging, although several groups are working on standardizing stray gas sampling and analysis methods.
This risk and the concern of stray gas is the potential for methane to be released
in air. Methane dissolved in water is nontoxic and not hazardous. However, pumping a well reduces the pressure in an aquifer, which can result in methane gas
exsolving out of the groundwater and being released into the air. If it becomes
trapped in a confined space and reaches concentrations between the lower and upper
flammability limits of 5–15%, it may combust in a slow-motion explosion called a
deflagration. There have been more than a few instances where stray gas has accumulated in a basement or well vault and exploded, causing property destruction,
injuries, and even fatalities (Baldassare et al. 2014).
The dissolved methane content in groundwater from domestic water wells is
routinely measured by shale drillers as part of a pre-drilling baseline survey conducted on drinking water supplies within a radius of a kilometer or so from the shale
well site. This is done as a legal defense by the drilling industry to document existing contaminants in groundwater prior to drilling. If landowners seek compensation
for contamination of drinking water supply wells from “fracking,” the operators can
show that the contaminants often originated from other sources unrelated to the gas
well. Some states such as Pennsylvania have long required baseline groundwater
quality measurements as part of the drilling permit, but most operators now collect
it everywhere for liability protection.
There are a staggering number of existing or “legacy” contaminants in groundwater, including stray gas from natural sources, spilled fuel and other oily chemicals
like paint or motor oil, gasoline from leaking underground storage tanks, industrial
wastewater, pesticides and fertilizers applied to agricultural fields, chemical waste
buried in pits and trenches, and a host of others. The data collected by shale drillers on pre-existing chemicals in the groundwater are shared with the water well
owners so that everyone knows what is and is not present in their aquifers. Most
groundwater groups and county health agencies urge people to have their domestic
6 Fracking and Water
Part of the problem has to do with the complicated origins and migration of stray
gas in aquifers. As described in the previous chapter, biogenic methane can be generated by microbial processes in soils and shallow aquifers, and thermogenic methane comes from organic-bearing rocks like coal or shale exposed to high temperatures
during their burial history. Thus, stray gas can form within the aquifer from biological activity, enter groundwater from a compromised wellbore that extends kilometers deep into shale, or seep in from an organic-rich rock unit lying directly beneath
an aquifer. In many cases, gas from more than one origin will be present in an
aquifer, and the sources are sometimes indistinguishable.
A second part of the problem is related to how the data are collected for assessing
the presence of gas in groundwater. It is important to note that amount of methane
that dissolves in water, like carbon dioxide, is pressure-dependent. Just as soda
water will release bubbles when a bottle is uncapped, groundwater will release
methane when pressure is reduced. Depending on where in the water system a sample is collected: downhole, at the pump, or at the tap, the methane content can be
significantly different. A lack of uniformity on where and how samples have been
collected makes comparisons between different studies challenging, although several groups are working on standardizing stray gas sampling and analysis methods.
This risk and the concern of stray gas is the potential for methane to be released
in air. Methane dissolved in water is nontoxic and not hazardous. However, pumping a well reduces the pressure in an aquifer, which can result in methane gas
exsolving out of the groundwater and being released into the air. If it becomes
trapped in a confined space and reaches concentrations between the lower and upper
flammability limits of 5–15%, it may combust in a slow-motion explosion called a
deflagration. There have been more than a few instances where stray gas has accumulated in a basement or well vault and exploded, causing property destruction,
injuries, and even fatalities (Baldassare et al. 2014).
The dissolved methane content in groundwater from domestic water wells is
routinely measured by shale drillers as part of a pre-drilling baseline survey conducted on drinking water supplies within a radius of a kilometer or so from the shale
well site. This is done as a legal defense by the drilling industry to document existing contaminants in groundwater prior to drilling. If landowners seek compensation
for contamination of drinking water supply wells from “fracking,” the operators can
show that the contaminants often originated from other sources unrelated to the gas
well. Some states such as Pennsylvania have long required baseline groundwater
quality measurements as part of the drilling permit, but most operators now collect
it everywhere for liability protection.
There are a staggering number of existing or “legacy” contaminants in groundwater, including stray gas from natural sources, spilled fuel and other oily chemicals
like paint or motor oil, gasoline from leaking underground storage tanks, industrial
wastewater, pesticides and fertilizers applied to agricultural fields, chemical waste
buried in pits and trenches, and a host of others. The data collected by shale drillers on pre-existing chemicals in the groundwater are shared with the water well
owners so that everyone knows what is and is not present in their aquifers. Most
groundwater groups and county health agencies urge people to have their domestic
6 Fracking and Water