A Duke University study of well water in northeastern Pennsylvania suggests that naturally occurring pathways could have allowed salts and gases from the Marcellus shale formation deep underground to migrate up into shallow drinking water aquifers.
The study found elevated levels of
salinity with similar geochemistry to deep Marcellus brine in drinking water
samples from three ground water aquifers, but no direct links between the
salinity and shale gas exploration in the region.
"This is a good news-bad news kind
of finding," says Avner Vengosh, professor of geochemistry and water
quality at Duke's Nicholas School of the Environment.
The good news, he notes, is that it's
unlikely that hydraulic fracturing for shale gas has caused the elevated
salinity. He says that the locations of the samples containing brine do not
correlate with the locations of shale-gas wells. The results from the new study
also are consistent with water-quality tests conducted in the aquifers in the
1980s before rapid shale-gas development began.
The bad news is that the geochemical
fingerprint of the salinity detected in well water from the Lock Haven,
Alluvium and Catskill aquifers suggests a network of natural pathways exists in
some locations, especially in valleys. These pathways allowed gases and
Marcellus brine to migrate up into shallow ground water aquifers from deeper
underground shale gas deposits.
"This could mean that some drinking
water supplies in northeastern Pennsylvania are at increased risk for
contamination, particularly from fugitive gases that leak from shale gas well
casings," Vengosh says.
Last May, the Duke team published the
first peer-reviewed paper that found elevated levels of methane contamination
in drinking water wells located within a kilometer of hydraulic fracturing, or
"fracking," shale gas wells, but found no evidence of contamination
from fracturing fluids or brines.
The new paper complements that study by
showing "there are likely pathways through which methane and brine could
flow," Vengosh says. The Duke team evaluated 426 samples from ground water
aquifers in six counties overlying the Marcellus shale formation in
The study appears in the online early
edition of theProceedings of the National Academy of Sciences.
It was funded by Duke's Nicholas School of the Environment.
"The small group of homes whose
water we sampled may be at higher risk of contamination, due to underlying
geology," says Nathaniel Warner, a PhD student at Duke who was lead author
on the study. "By identifying the geochemical fingerprint of Marcellus
brine, we can now more easily identify where these locations are and who these
homeowners might be."
Robert Jackson, Nicholas Professor of
Global Environmental Change and director of Duke's Center on Global Change,
co-authored the paper with Warner and Vengosh. He says, "These results
reinforce our earlier work showing no evidence of brine contamination from
shale gas exploration. They do, however, highlight locations and homeowners
more vulnerable to contamination, something we'll need to follow up."
The new findings also should help
address concerns about barium contamination in local drinking water, Warner says.
"Especially in valleys in the
region, elevated salinity is associated with barium contamination in the
water," Warner says. "Our study's findings suggest that homeowners
living in these areas are at higher risk of contamination from metals such as
barium and strontium."
The Marcellus shale formation is located
about a mile underground, and contains highly saline water that is naturally
enriched with salts, metals and radioactive elements.
Accelerated shale gas drilling and
hydrofracking in the northeastern Pennsylvania region in recent years has fueled
concerns about water contamination by methane, fracking fluids and wastewater
from the operations.
"As shale gas exploration is
becoming global – including in Poland, China, Australia and New Zealand – the take-home message of this study is that
pre-drilling water-quality monitoring is important for evaluating water-quality
baselines that can be used to detect future changes in water quality, and for
evaluating possible hydraulic short cuts and pathways between fluids and gases
in deep shale gas formations and shallow aquifers," says Vengosh.
"Such geochemical reconnaissance would provide a better risk assessment
for water contamination in newly developed shale gas exploration areas."