The
potential contamination of shallow groundwater with inorganic
constituents is a major environmental concern associated with shale
gas extraction through hydraulic fracturing. However, the impact of
shale gas development on groundwater quality is a highly controversial
issue. The only way to reliably assess whether groundwater quality
has been impacted by shale gas development is to collect pre-development
baseline data against which subsequent changes in groundwater quality
can be compared. The objective of this paper is to provide a conceptual
and methodological framework for establishing a baseline of inorganic
groundwater quality in shale gas areas, which is becoming standard
practice as a prerequisite for evaluating shale gas development impacts
on shallow aquifers. For this purpose, this paper first reviews the
potential sources of inorganic contaminants in shallow groundwater
from shale gas areas. Then, it reviews the previous baseline studies
of groundwater geochemistry in shale gas areas, showing that a comprehensive
baseline assessment includes documenting the natural sources of salinity,
potential geogenic contamination, and potential anthropogenic influences
from legacy contamination and surface land use activities that are
not related to shale gas development. Based on this knowledge, best
practices are identified in terms of baseline sampling, selection
of inorganic baseline parameters, and definition of threshold levels.
Significance
Natural gas is a key fossil fuel as the world transitions away from coal toward less polluting energy sources in an attempt to minimize the impact of global climate change. Historically, the origin of natural gas produced from conventional reservoirs has been determined based on gas compositional data and stable isotope fingerprints of methane, ethane, and higher
n
-alkanes, revealing three dominant sources of natural gas: microbial, thermogenic, and abiotic. In our detailed synthesis of published natural gas data from a variety of unconventional hydrocarbon reservoirs worldwide, we demonstrate that there is a previously overlooked source of natural gas that is generated by radiolysis of organic matter in shales.
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