Research links shale gas, legacy energy development to groundwater pollution
Fracking for natural gas in parts of Pennsylvania with a legacy of energy extraction may increase the risk of groundwater contamination, according to a team led by Penn State scientists.
The researchers found a possible link between elevated levels of chloride in groundwater and areas where horizontal drilling and fracking for shale gas overlapped with high-density older, conventional oil and gas wells and coal mining in southwestern Pennsylvania. The association was not observed in samples from northeastern Pennsylvania, which have also experienced the Marcellus Shale boom, but do not have the same long history of intensive energy extraction.
Elevated chloride levels in southwestern Pennsylvania were found in a few regional hot spots near high densities of unconventional wells, the scientists said. While these levels do not exceed safety standards, substances also present in shale gas waste fluids, such as thallium, if caused by leaks or spills from unconventional drilling, can exceed EPA limits in the most developed hot spots.
“Our results indicate a regional increase in groundwater chloride near unconventional wells in southwestern Pennsylvania, where the Marcellus Shale game overlaps with a long legacy of oil, gas and coal mining,” said Samuel Shaheen, a graduate student to Penn State, and lead author on the paper. “However, rather than each well producing more chloride, this regional impact is likely caused by hotspots where brine reaches groundwater through leaks or spills, or possibly, where subsurface features or well integrity allow leakage.”
The scientists said further research is needed to determine if there are potential impacts on drinking water. However, the study may lead to where that future work is taking place.
“One of the challenges in studying groundwater contamination from unconventional development is that we’ve drilled more than 10,000 of these wells in Pennsylvania,” Shaheen said. “The number of those likely to have contaminated someone’s drinking water source is quite small relative to the sheer number of wells drilled.”
The study, entitled “Geochemical Evidence of Groundwater Contamination and Potential Human Health Risks Where Hydraulic Fracturing Overlaps with Extensive Legacy Hydrocarbon Extraction,” appears in Environmental Science and Technology.
The team focused their research on southwestern Pennsylvania, an area that has seen extensive coal mining and drilling for oil and gas since the 1800s, and has more recently experienced the Marcellus Shale boom, including horizontal drilling and hydraulic fracturing to extract natural gas. .
Shale gas fracking uses large amounts of water, injected under high pressure, to open underground cracks and expose trapped gas. As a result, the wastewater generated by this process often contains large – and highly concentrated – amounts of deep brine salts, such as chloride, barium and strontium, as well as trace elements such as thallium and arsenic. Some of these substances pose potential risks to human health.
The team found elevations in chloride, barium and strontium concentrations localized to regional hotspots. These small but statistically significant increases were found closer to unconventional wells and in areas where more unconventional wells were located less than a mile from water samples.
“It’s not that every gas source is releasing the salts, it’s more that there are a handful of these problem areas where we’re seeing increasing concentrations nearby, and we’re detecting that across the region,” Shaheen said. “The number of unconventional wells that we think are causing problems is very low.”
Chloride can come from other sources, such as road salt, and the team is using methods to identify such effects. Some of the samples in the team’s dataset were collected at a time when regulations on drilling wastewater storage were less strict. Now wastewater must be stored in tanks instead of lined reservoirs.
In contrast to the increase in chloride, the team found lower frequencies of methane pollution in southwestern Pennsylvania compared to northeastern parts of the state. Methane can leak from wells that are not properly constructed or damaged. Methane itself is not toxic, but poses an explosion hazard in high concentrations in enclosed spaces and can lead to conditions that produce toxic elements such as arsenic in water.
“We as geochemists paid a lot of attention to the methane problem because of the potential for explosions or fires, but here we saw potentially bigger problems on the salt side of things,” Shaheen said.
The lower frequency of methane pollution initially surprised Shaheen and the rest of the team because of the significant methane emissions from coal and the high incidence of coal mining in the region. But because of the region’s long history of conventional energy extraction, methane may have previously been extracted through conventional drilling or leaked into the atmosphere through coal mines or oil and gas infrastructure, the scientists said.
“While we thought it would cause more problems, this overlap with conventional oil and gas drilling and coal mining could have actually released some of the methane that normally causes these problems and reduced the frequency of methane migration during unconventional drilling,” Shaheen said. .
The team used a dataset that consisted of 6,991 groundwater samples, with 4,325 collected from household wells and 2,666 from wells. They also used advanced data mining techniques, such as machine learning, to find contamination that is usually uncommon in large data sets.
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Samuel W. Shaheen et al, Geochemical evidence of potential groundwater contamination with human health risks where hydraulic fracturing overlaps with extensive legacy hydrocarbon extraction, Environmental Science and Technology (2022). DOI: 10.1021/acs.est.2c00001
Quote: Research links shale gas, legacy energy development to groundwater pollution (2022, September 21) retrieved September 21, 2022 from https://phys.org/news/2022-09-links-shale-gas-legacy-energy.html
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