GC ensures integrity of drinking water in fracking locations: multiple gas chromatography methods are being deployed to monitor a suite of fracking-related contaminants that may be present in drinking water.
This article focuses on natural gas, which is the cleanest-burning of the naturally occurring hydrocarbons, with much lower emissions of undesirable pollutants and greenhouse gases than conventional fossil fuels such as coal and crude oil.
Horizontal drilling and hydraulic fracturing, also known as fracking, is used to extract natural gas and oil from shale rock layers deep below the earth's surface. This is done by first drilling vertically down approximately a mile and a half below the surface and then turning the drill horizontally and drilling another several thousand feet through the shale deposit. The well is encased with cement to ensure groundwater protection, and then a fracking fluid containing water, sand, surfactants and various additives is pumped down the well at very high pressure, which fractures the shale rock to release the natural gas. This horizontal drilling process makes it possible to extract natural gas in large quantities from shale rock, which was previously unrecoverable with conventional vertical drilling methods.
However, this technology presents a number of environmental challenges. The wells are drilled vertically through aquifers a few hundred feet under the surface, on their way to the deep shale rock deposits a further seven thousand feet below. Herein lies the challenge: in the process of drilling the wells and preparing them for production, there is the potential for contamination of the drinking water aquifers with methane, propane and ethane, together with the fracking fluid itself. It is also probable that methane already exists at a low concentration in the aquifer from diffusion of these gases from the naturally occurring decay of biological materials in the ground. As a result, to ensure an uncontaminated supply of drinking water in these areas, it is important not only to confirm the absence of contaminants in the aquifer before and during the drilling process, but also after the well goes into production.
Natural gas reservoirs
There were approximately 500,000 natural gas wells drilled in the U.S in 2014, which produced almost 26 trillion cubic feet of gas. Currently, this exploration for natural gas is being carried out in 33 states of which Texas, Pennsylvania, West Virginia, Oklahoma and Colorado represent the leading five with almost 300,000 wells drilled between them. These states are taking advantage of huge reservoirs of gas found in the Barnett, Haynesville-Bossier and Woodford shale gas formations in the south of the country, the Niobrara field in the Rock Mountains region and the Marcellus shale, found in and around the east coast region.
Clearly, this increased revenue from natural gas is beneficial for the economic development of states with these large shale gas formations. However, with the continued debate over the environmental impact and safety of hydraulic fracturing, this increased revenue presents many challenges in balancing growth with public health concerns of residents. Fortunately, many state and federal regulatory agencies have put into place a number of methods to determine and measure the potential impact on residents who live near fracking operations.
Safe drinking water
In the goal of ensuring safe drinking water, two methods are currently being deployed to monitor a suite of fracking-related volatile organic compounds and hydrocarbon gases using gas chromatography. One of these methods is used to measure toxic organic compounds (VOCs), which can be associated with the fracking fluid in water, using capillary column gas chromatography (GC) fitted with a purge and trap sample introduction system coupled with mass spectroscopy (MS) detection according to EPA Method 524.2. Using this approach, the sample is purged of its volatile components, which are then trapped on a sorbent material, heated and back-flushed into a GC column, where they are separated and eluted into a mass spectrometer for detection.
The second method involves the determination of the volatile hydrocarbon gases in drinking water. This solution uses headspace coupled with gas chromatography, and was adapted from EPA RSK175, the analysis of dissolved gases in drinking water. Headspace is a term used to describe the gas space above the sample in a headspace vial, where the volatile components diffuse into the gas phase, forming the headspace gas. The hydrocarbon gases, including methane, are efficiently partitioned into the headspace gas volume above the liquid samples and separated, as demonstrated by the chromatogram of a 100 [micro]L independent (second source) quality control standard of seven hydrocarbons in drinking water shown in Figure 1. Let's take a more detailed look at the GC methodology (using a PerkinElmer TurboMatrix Headspace Clarus 580 Gas Chromatograph with flame ionization detection) developed for this analysis.
A five-point calibration curve was created to establish method linearity and reporting limits. Five headspace vials were prepared with 10 mL of deionized (DI) water then capped using PTFE silicone septa. A 20, 50, 200, 1,000 and 1,500 [micro]L volume of the hydrocarbon standard (Air Liquide Specialty Gases) diluted in nitrogen was inserted manually through theseptum into the water of each of the five vials attaining concentration values. A blank is also analyzed, and the responses, if any, are inserted into the calibration curve as a sixth point.
To demonstrate linearity, the correlation coefficients for the seven hydrocarbons are shown in Table 1. Because the concentration risk of the gases is at high concentration, the objective for this analysis was to achieve a reporting limit of 0.13 ppm for methane, which has been easily accomplished. However, it should be emphasized that this method is capable of achieving a reporting limit of 0.8 ppb if required.
Recoveries of an independent quality control standard (Natural Gas Standard No. 3, Restek Corp.) were in the range of 95 to 99 percent for six of the hydrocarbons. The concentration values for the 100 [micro]L spike of this quality control are shown in Table 2. Ethylene is not contained in this second source standard, therefore, a separate standard was prepared from the Air Liquide standard to confirm the concentration of ethylene.
The high visibility of fracking in the media has created an unprecedented demand for the testing of water samples for contaminants associated with hydraulic fracturing. As a result, many state and county public health labs offer a testing program based on the methodology described in this study. In fact, a recent study carried out by a large county public health laboratory, showed that of the more than 200 private wells sampled, there was no sign of either contamination from the fracking fluid or hydrocarbons from the natural gas itself, which may give some residents peace of mind that their drinking water supplies are safe for consumption.
by Lee Marotta, Sr. Field Application Scientist, PerkinElmer Instruments, Shelton, Conn., and Robert Thomas, Scientific Solutions, Gaithersburg, Md.
Table 1: Correlation coefficients (r2) for the seven gases rounded to five decimal places Analyte Correlation coefficient r2 Methane 1.00000 Ethylene 0.99996 Ethane 0.99998 Propane 0.99976 N-Butane 0.99997 Isopentane 0.99996 N-Pentane 1.00000 Table 2: Concentration values of an independent quality control standard. * Note: concentration for ethylene was obtained from the Air Liquide standard Amount spiked 100 [micro]L in 10 mL Methan 0.6577 Ethylene * 0.1173 Ethane 0.1245 Propane 0.1826 N-Butane 0.2383 Isopentane 0.2988 N-Pentane 0.2988
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|Author:||Marotta, Lee; Thomas, Robert|
|Date:||Feb 1, 2015|
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