by Shannon Julius
Drilling into shale is a difficult task, as gases are under high pressure and can easily damage the well’s integrity if drilling is done incorrectly. Such damage allows natural gases, particularly methane, to “migrate” through cement seals and into groundwater, which happens with approximately 1–3% of wells in Pennsylvania. The high toxicity of fracturing fluid raises the concern of fluid migration accompanying methane migration, and research has yet to determine the extent to which fracturing fluid can affect groundwater. However, it is highly likely that most of the unrecovered fracturing fluid is absorbed by the shale formation. The remaining fracturing fluid is recovered as wastewater, which must be disposed of properly in order to maintain surface water quality. These aspects of hydraulic fracturing create environmental and social concerns with respect to regional water quality. Vidic et al. (2013) reviewed current research related to chemical aspects of water quality in regions of shale gas well drilling, focusing on the Marcellus shale region. The three major areas of concern were methane migration into groundwater, fracturing fluid contamination of groundwater, and wastewater management.
Vidic et al. found reports that indicate that 1–2% of shale gas wells have a faulty cement seal around well casings that are meant to keep methane out of aquifers. This percentage could be slightly higher in actuality, as the Pennsylvania Department of Environmental Protection (PADEP) issued notices of violation to 3.4% of Pennsylvania wells for well construction problems from 2008 to 2013. Damage to well casings can occur when drilling into areas of existing high pressure gases. If drilling pressure is too high, drilling can fracture the formation in undesired places. If pressure is too low, gases can infiltrate the cement seal before it hardens. Either of these possibilities will cause gas to “migrate,” to move through the area that has been drilled and potentially end up in nearby groundwater.
Methane alone is not toxic to ingest, but certain aquatic bacteria will remove oxygen from the water if methane is present. Low oxygen conditions can increase the solubility of elements such as arsenic or iron as well as support the life of anaerobic bacteria which create sulfide, an inorganic molecule that can contribute to air and water quality issues. When methane turns returns to a gaseous state after being dissolved, it can cause water turbidity or, in high concentrations, explode.
Dissolved methane levels have been shown to be higher in drinking water wells less than 1 kilometer from active Marcellus gas wells than from wells farther away. However, methane can be formed naturally by subsurface organic matter at high temperatures (“thermogenic” source) or by bacterial processes at shallow depths (“biogenic” source). Shale gas wells extract thermogenic natural gas, so determining whether gas is thermogenic or biogenic is the first step towards figuring out if stray gases may have originated from a shale gas well. With the widespread lack of pre-drilling data, researchers must look closely at groundwater chemistry in order to determine if the source of dissolved gases is thermogenic or biogenic. Methods include measuring the concentrations of other natural gases and looking at the isotopic signatures of hydrogen, oxygen, and carbon. In Pennsylvania, natural methane levels happen to be relatively high, so there is controversy over whether Marcellus wells are to blame for any methane present in groundwater. Compounding the problem is the history of conventional gas and oil drilling; these wells could have been responsible for methane migration in the region before public concern over hydraulic fracturing led to groundwater research.
Fracturing fluid is a potentially hazardous water contaminant. On average, 90% of the 2–7 million gallons of fracturing fluid used for each well does not resurface after the injection process, and it is largely unknown where this fluid ends up. Studies of the Marcellus shale formation show that the shale is very dry, and it is likely that most fluid absorbs into the shale. There is the possibility that water that is not absorbed could move along abandoned oil and gas well shafts that are in the same region. However, study of 233 drinking wells in the shale gas region of Pennsylvania found no fluid contamination of groundwater, and even sites with dissolved methane present had no evidence of fracturing fluid. No studies included in this review had reliable evidence that contamination from fracturing fluid has happened in the past or will happen in the future.
The water that is recovered from hydraulic fracturing is a combination of injected fracturing fluid and water from underground formations. The average volume of wastewater from Marcellus wells in Pennsylvania was 26 million barrels per year between 2008 and 2011, and wastewater from Marcellus shale gas wells made up 79% of all oil and gas wastewater requiring management in Pennsylvania in 2011. Marcellus shale wastewater is high in total dissolved solids (TDS) and radiogenic compounds. Currently almost all flowback water (recovered fracturing fluid) is captured at the surface and used in another fracturing operation after dilution or treatment. This practice of reuse greatly reduces the volume of wastewater that must be treated and disposed of, but will do so only while new wells are made at high rates. After some time, there will be more wastewater from operating wells than new well drilling will require. Reuse concentrates certain contaminants, including radioactive radium, which makes it increasingly difficult to properly dispose of reused wastewater.
Most final disposal happens in injection disposal wells, but this may not be an appropriate solution in Pennsylvania due to low injection well capacity. Currently there are only five operating injection disposal wells in Pennsylvania that are licenced to handle the level of contamination present in shale gas wastewater, and construction of new disposal wells would require regulator permission and expensive construction. Additionally, injection wells are known to induce seismic events. Until recently, municipal waste treatment facilities (also known as publicly operated treatment works or POTWs) have been used to treat wastewater, but these did not have the capacity to treat water high in TDS. Thus, use of POTWs led to an increase of TDS in waterways, which evoked significant public resistance to POTW use. Eventually PADEP created discharge limits in order to discourage the use of POTWs.
Wastewater can contaminate surface water if it is released into the environment without adequate treatment from waste treatment facilities or when wastewater is spilled from storage facilities or trucks. There are a few ways to test for surface water contamination by wastewater. Researchers look for high concentrations of sodium, calcium, and chlorine, which are present in wastewater but can originate from many sources. High concentrations of strontium, bromine, barium, and a high ratio of certain strontium isotopes are stronger evidence of wastewater contamination. Despite these indicators, research that attempts to identify the source of surface water contamination finds difficulty connecting results to Marcellus wells because there is little information available regarding pre-shale gas era water quality. To increase availability of data, PADEP started a surface water monitoring program in 2011, but currently research is underfunded and more research into proper disposal methods is necessary.
Vidic, R., Brantley, S., Vandenbossche, J., Yoxtheimer, D., Abad, J., 2013. Impact of shale gas development on regional water quality. Science 340, 777–892. Full paper at: http://bit.ly/1nuRjqH