http://onlinelibrary.wiley.com/doi/10.1111/j.1745-6584.2012.00933.x/abstract
Hydraulic fracturing of deep shale beds to develop natural gas has caused concern regarding the potential for various forms of water pollution. Two potential pathways—advective transport through bulk media and preferential flow through fractures—could allow the transport of contaminants from the fractured shale to aquifers. There is substantial geologic evidence that natural vertical flow drives contaminants, mostly brine, to near the surface from deep evaporite sources. Interpretative modeling shows that advective transport could require up to tens of thousands of years to move contaminants to the surface, but also that fracking the shale could reduce that transport time to tens or hundreds of years. Conductive faults or fracture zones, as found throughout the Marcellus shale region, could reduce the travel time further. Injection of up to 15,000,000 L of fluid into the shale generates high pressure at the well, which decreases with distance from the well and with time after injection as the fluid advects through the shale. The advection displaces native fluids, mostly brine, and fractures the bulk media widening existing fractures. Simulated pressure returns to pre-injection levels in about 300 d. The overall system requires from 3 to 6 years to reach a new equilibrium reflecting the significant changes caused by fracking the shale, which could allow advective transport to aquifers in less than 10 years. The rapid expansion of hydraulic fracturing requires that monitoring systems be employed to track the movement of contaminants and that gas wells have a reasonable offset from faults.
Directional drilling and hydraulic-fracturing technologies are dramatically increasing natural-gas extraction. In aquifers overlying the Marcellus and Utica shale formations of northeastern Pennsylvania and upstate New York, we document systematic evidence for methane contamination of drinking water associated with shalegas extraction. In active gas-extraction areas (one or more gas wells within 1 km), average and maximum methane concentrations in drinking-water wells increased with proximity to the nearest gas well and were 19.2 and 64 mg CH4 L-1 (n 1/4 26), a potential explosion hazard; in contrast, dissolved methane samples in neighboring nonextraction sites (no gas wells within 1 km) within similar geologic formations and hydrogeologic regimes averaged only 1.1 mg L-1 (P < 0.05; n 1/4 34). Average ?13 C-CH4 values of dissolved methane in shallow groundwater were significantly less negative for active than for nonactive sites (-37 AE 7? and -54 AE 11?, respectively; P < 0.0001). These ?13 C-CH4 data, coupled with the ratios of methane-to-higher-chain hydrocarbons, and ?2 H-CH4 values, are consistent with deeper thermogenic methane sources such as the Marcellus and Utica shales at the active sites and matched gas geochemistry from gas wells nearby. In contrast, lower-concentration samples from shallow groundwater at nonactive sites had isotopic signatures reflecting a more biogenic or mixed biogenic/ thermogenic methane source.
Federal environment officials investigating drinking water contamination near the ranching town of Pavillion, Wyo., have found that at least three water wells contain a chemical used in the natural gas drilling process of hydraulic fracturing. Scientists also found traces of other contaminants, including oil, gas or metals, in 11 of 39 wells tested there since March.
http://www.pnas.org/content/110/28/11250.abstract
We analyzed 141 drinking water wells across the Appalachian Plateaus physiographic province of northeastern Pennsylvania, examining natural gas concentrations and isotopic signatures with proximity to shale gas wells. Methane was detected in 82% of drinking water samples, with average concentrations six times higher for homes <1 km from natural gas wells (P = 0.0006). Ethane was 23 times higher in homes <1 km from gas wells (P = 0.0013); propane was detected in 10 water wells, all within approximately 1 km distance (P = 0.01). Of three factors previously proposed to influence gas concentrations in shallow groundwater (distances to gas wells, valley bottoms, and the Appalachian Structural Front, a proxy for tectonic deformation), distance to gas wells was highly significant for methane concentrations (P = 0.007; multiple regression), whereas distances to valley bottoms and the Appalachian Structural Front were not significant (P = 0.27 and P = 0.11, respectively). Distance to gas wells was also the most significant factor for Pearson and Spearman correlation analyses (P < 0.01). For ethane concentrations, distance to gas wells was the only statistically significant factor (P < 0.005). Isotopic signatures (δ13C-CH4, δ13C-C2H6, and δ2H-CH4), hydrocarbon ratios (methane to ethane and propane), and the ratio of the noble gas 4He to CH4 in groundwater were characteristic of a thermally postmature Marcellus-like source in some cases. Overall, our data suggest that some homeowners living <1 km from gas wells have drinking water contaminated with stray gases.
http://pubs.acs.org/doi/abs/10.1021/es4011724
Natural gas has become a leading source of alternative energy with the advent of techniques to economically extract gas reserves from deep shale formations. Here, we present an assessment of private well water quality in aquifers overlying the Barnett Shale formation of North Texas. We evaluated samples from 100 private drinking water wells using analytical chemistry techniques. Analyses revealed that arsenic, selenium, strontium and total dissolved solids (TDS) exceeded the Environmental Protection Agency’s Drinking Water Maximum Contaminant Limit (MCL) in some samples from private water wells located within 3 km of active natural gas wells. Lower levels of arsenic, selenium, strontium, and barium were detected at reference sites outside the Barnett Shale region as well as sites within the Barnett Shale region located more than 3 km from active natural gas wells. Methanol and ethanol were also detected in 29% of samples. Samples exceeding MCL levels were randomly distributed within areas of active natural gas extraction, and the spatial patterns in our data suggest that elevated constituent levels could be due to a variety of factors including mobilization of natural constituents, hydrogeochemical changes from lowering of the water table, or industrial accidents such as faulty gas well casings.
https://s3.amazonaws.com/propublica/assets/natural_gas/ohio_methane_report_080901.pdf
The DMRM determined that accumulation and confinement of deep, high-pressure gas in the surface-production casing annulus of the English #1 well, between November 13 and December 15, 2007 resulted in over-pressurization of the annulus. This over-pressurized condition resulted in the invasion, or migration, of natural gas from the annulus of the well into natural fractures in the bedrock below the base of the cemented surface casing. This gas migrated vertically through fractures into the overlying aquifers and discharged, or exited, the aquifers through local water wells.
https://s3.amazonaws.com/propublica/assets/methane/thyne_review.pdf
The currently available water quality data is sufficient to establish the range of natural background water chemistry and delineate the impact of petroleum activities. Impacts from petroleum activity are not usually present at levels that exceed regulatory limits. The sub-regulatory impacts most clearly delineated are elevated methane and chloride in groundwater wells. There is a temporal trend of increasing methane in groundwater samples over the last seven years coincident with the increased number of gas wells installed in the study area. Pre-drilling values for methane in groundwater establish natural background was less than 1ppm, except in cases of biogenic methane that are confined to pond and stream bottoms. The cases of biogenic methane can be readily identified by stale isotopic values of the methane. The isotopic data for the methane samples show the most of the samples with elevated methane are thermogenic in origin.
More conclusive identification of the origin of methane can be made by determination of the inorganic carbon isotopic value. Concurrent with the increasing methane concentration there has been an increase in groundwater wells with elevated chloride that can be correlated to the number of gas wells. Chloride is derived from produced water*. The increasing methane and chloride will not trigger regulatory action since there is no regulated limit on methane and the majority of chloride values are below regulatory limits, however, as more gas wells are drilled the chloride value may reach the regulatory limit.
* Note: "Produced water" is industry jargon for the mixture of fracturing fluids that is released just after a well is inundated with chemical slurry, and is known to contain dozens of physiologically and environmentally hazardous compounds. This study did not seek out levels of these potential contaminants, and focused only on methane and chlorine, but the presence of chlorine from produced water is a strong indicator that other compounds in the fracturing fluids are also present.
Some further reading: