New Research on Fracking and Contamination of Drinking Water

Scientists have found and investigated methane in drinking water wells near fracked gas wells in the Marcellus Shale. Fracking or hydraulic fracturing as it is more properly known involves the pressurized injection of fluids commonly made up of mostly water and chemical additives into a geologic formation. The pressure used exceeds the rock strength and the fluid opens or enlarges fractures in the rock. As the formation is fractured, a “propping agent,” such as sand or ceramic beads, is pumped into the fractures to keep them from closing as the pumping pressure is released. The fracturing fluids (water and chemical additives) are partially recovered and returned to the surface or deep well injected. Natural gas will flow from pores and fractures in the rock into the wells allowing for enhanced access to the methane reserve.

Over the past few years, the use of hydraulic fracturing for gas extraction has increased and has expanded over a wide diversity of geographic regions and geologic formations beyond its original use in old oil and gas fields to revitalize them. By January of 2013, the daily production of methane (CH4) in the United States had increased 30% from January 2005 to about 70 billion cubic feet of gas each day. As fracking has expanded at what seems a breakneck speed in some regions, so has a public and regulatory concern about the possible environmental consequences of fracking and horizontal drilling. These concerns include air pollution from the operation of heavy equipment, human health effects for workers and people living near well pads from chemical exposure, noise and dust, induced seismicity from the disposal of fracking fluids, and increased greenhouse gas emissions from poor well head control and continued use of hydrocarbons.

However, the biggest health concern remains the potential for drinking water contamination from fracturing fluids, natural formation waters, and stray gases. While geologists and engineers believed that in hydraulic fracturing the intervening layers of rock prevent a fissure from extending into the water table, this had not been studied and there were reported instances of contamination of drinking water wells in areas that had been fracked. Only in the past three years has the potential to contaminate drinking water wells been studied. In a small group of studies (listed below) that were primarily in the Marcellus region of Pennsylvania, peer-reviewed studies found no evidence of salts, metals, or radioactivity beyond naturally occurring concentrations in drinking water wells near shale gas wells. However, in the latest studies they did find increased levels of methane in groundwater wells.

Methane gas occurs naturally in groundwater aquifers in most geological sedimentary basins. Methane gas exists in a dissolved state in the groundwater underground and will “bubble out” when pumped to the surface. For those on private water well supplies, spurting taps is a typical indication of this phenomenon. Methane present in groundwater can be a result of biogenic activity or can be from coal gas beds or from deeper shale gas. Biogenic methane is produced by subsurface bacteria and commonly occurs naturally in groundwater aquifers used for water well supplies. Thermogenic methane gas is produced at greater depths through high pressure and temperature processes and is characteristic of deep oil and gas reservoirs that conventional and shale gas wells tap into. Methane gas typically contains trace amounts of ethane. The proportion of methane to ethane in a gas can help determine its origin. Biogenic gas typically contains above 1,000 times more methane than ethane, but thermogenic gas has higher levels of ethane. In addition, isotope data can also be used to help determine whether a gas is biogenic or thermogenic. In the most recent research paper from the scientists at Duke University, University of Rochester and California State Polytechnic University (1) used these ratios to examine the occurrence and source of methane in drinking water wells in northeastern Pennsylvania.

A total of 81 samples from drinking water wells were collected in six counties in Pennsylvania (Bradford, Lackawanna, Sullivan, Susquehanna, Wayne, and Wyoming), and results were combined with 60 previous samples from a 2011 study by Stephen G. Osborn et al. (2). Dissolved methane was detected in the drinking water of 82% of the houses sampled (115 of 141 samples). Methane concentrations in drinking water wells of the homes closest to the gas wells were six times higher on average than concentrations for homes farther away. All of the 12 houses where CH4 concentrations were greater than 28 mg/L (the threshold for immediate remediation set by the US Department of the Interior) were well within a mile of an active shale gas well. Concentrations of ethane (C2H6) and propane (C3H8) were also higher in drinking water of homes near the shale gas wells.

The scientists concluded that the combined results suggest that natural gas, derived at least in part from thermogenic sources (the shale gas) was present in some of the shallow water wells less than a mile away from natural gas wells. The scientist pointed out that the two simplest explanations for the higher dissolved gas concentrations measured in the drinking water are faulty or inadequate steel casings and/or imperfections in the cement sealing (also known as the grouting) between casings and rock that keep fluids from moving up the outside of the well. In 2010, the Pennsylvania Department of Environmental Protection (DEP) issued 90 violations for faulty casing and cementing on 64 Marcellus shale gas wells; 119 violations were issued in 2011.

The scientist believed based on their isotopic analysis and previous studies that the cause of the elevated levels of methane (CH4) in the groundwater was due to imperfections in the cement grouting on the wells. Faulty cement grouting can allow methane and other gases from intermediate layers to flow into, up, and out of the void between the steel casing and the grouting into shallow drinking water layers. The geochemical and isotopic compositions of stray gas contamination in this scenario would not fully match the target shale gas, and no fracturing chemicals or deep formation waters would be expected, because a direct connection to the deepest layers does not exist; and this is consistent with their findings. Faulty grouting is believed to be the most likely cause of the scientists’ findings. Legacy or abandoned oil and gas wells (and even abandoned water wells) though a potential source of contamination, were unlikely to be the cause in this instance. Historical drilling activity was negligible within the study area making this mechanism unlikely there. Though, in 2000, the Pennsylvania DEP estimated that it had records for 141,000 of the 325,000 oil and gas wells that had historically been drilled in the state.

In another study by Duke University and the US Geological Survey no evidence of drinking water contamination from methane from shale gas was found in a part of the Fayetteville Shale in Arkansas (7). That shale has a less fractured geology than the Marcellus and good confining layers above and below the drinking water aquifers. Ultimately, we need to understand why, in some cases, shale gas extraction contaminates groundwater and how to ensure that contamination does not happen with a high level of certainty in susceptible geology. Well construction and maintenance needs to be studied, optimized and carefully regulated before further expansion of shale gas development.

1. Jackson, RB, Vengosh, A, Darrah, TH, Warner,  NR, Down, A, Poreda, RJ, Osborn, SG, Zhao, K, Karr,JD (2013) Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction PNAS 2013 ; published ahead of print June24, 2013, doi:10.1073/pnas.1221635110
2. Osborn SG, Vengosh A, Warner NR, Jackson RB (2011) Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing. Proc Natl Acad Sci USA 108(20):8172–8176.
3. DiGiulio DC, Wilkin RT, Miller C, Oberley G (2011) Investigation of Ground Water Contamination Near Pavillion, Wyoming (US Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, Ada, OK), p 74820.
4. Warner NR, et al. (2012) Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania. Proc Natl Acad Sci USA 109(30):11961–11966.
5. Chapman EC, et al. (2012) Geochemical and strontium isotope characterization of produced waters from Marcellus Shale natural gas extraction. Environ Sci Technol 46(6):3545–3553.
6. Boyer EW, et al. (2012) The Impact of Marcellus Gas Drilling on Rural Drinking Water Supplies (The Center for Rural Pennsylvania, Harrisburg, PA)
7. Kresse TM, et al. (2012) Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas, 2011 (USGS), US Geological Survey Scientific Report 2012–5273 (Lafayette Publishing Service Center, Lafayette, LA).