1. Introduction
Public concern about the potential impacts of hydraulic fracturing (HF) is often focused more on the fluids injected into the ground than the fate of the large volume of resulting wastewater, even though the latter may be more important in preventing environmental contamination [
1]. Wastewater from oil and gas production consists of formation water as well as fluids injected into the well for various purposes. When HF is used, particularly large volumes of wastewater are generated due to the injection of up to more than 10 million liters of HF fluid into the well for stimulation [
2]. After HF is completed, the wastewater generated initially reflects the chemical characteristics of HF fluid, having relatively high total organic carbon (TOC) content without a concomitantly high conductivity. Later, as the well begins to produce hydrocarbons, the wastewater bares a closer resemblance to the formation water, having relatively low TOC content and high conductivity [
3], though it may still have a moderate TOC content when it is in contact with oil.
Clark and Veil [
4] published one of the most comprehensive studies estimating the volume of wastewater generated annually in the US using data from 2007. They estimated that about 3.3 trillion liters of wastewater from both conventional and unconventional drilling operations are generated annually in the US; the present value is likely higher. The potential environmental impact of such a large volume of waste, if not properly handled, cannot be overstated, though this is not a new problem in many areas across the country. Options for handling wastewater include disposal and treatment for reuse or other industrial applications. Disposal of wastewater in disposal wells (DWs) can have high associated transportation costs, may induce seismic activity [
5], can contaminate nearby aquifers [
6], and is not always compatible with the regional geology. In the US, wastewater management practices vary regionally depending on local resources. For example, in Pennsylvania, a state where only a few DWs can provide satisfactory containment, most of the wastewater generated from HF is reused as HF fluid [
7]. In contrast, in the Barnett Shale of North Texas, the wastewater is mostly disposed of in DWs [
8]. In general, the use of DWs is often the least expensive wastewater management option. The cost of treatment as well as the optimum management strategy is governed of course by the wastewater composition [
9].
High total dissolved solids, radioactivity, and the proprietary nature of fracturing fluids present challenges in the treatment and reuse of wastewater. The chemical content of the waste is highly variable and depends both on geological factors and the composition of the HF fluid used, which varies regionally and across operators. Analytical methods to assess the composition/quality of wastewater can allow the best management practices and the selection/development of best treatment methods.
Characterization of wastewater from HF operations has been reported in the literature in various degrees of detail [
3,
10,
11,
12,
13]. Orem
et al. [
3] characterized produced water and formation water samples from various shale and coalbed methane plays using TOC analysis, high-performance liquid chromatography (HPLC) for volatile fatty acids, and gas chromatography-mass spectrometry (GC-MS) for extractable hydrocarbons. Maguire-Boyle and Barron [
10] analyzed produced water from the Marcellus (PA), Eagle Ford (TX), and Barnett (NM) shale plays using TOC analysis, GC-MS for organic acids and volatile organic compounds, inductively coupled plasma-optical emission spectroscopy (ICP-OES) for metals, and conductivity and pH analysis. Thurman
et al. [
11] analyzed ethoxylated surfactants in flowback water and produced water using liquid chromatography-quadrupole-time-of-flight-mass spectrometry (LC-QTOF-MS). Lester
et al., characterized a single composite HF flowback water sample from the Denver-Julesburg basin of Colorado using a suite of analytical techniques including TOC analysis, ion chromatography, inductively coupled plasma-mass spectrometry (ICP-MS), GC-MS, LC-QTOF-MS and spectrofluorometry [
13].
We aimed in this pilot study to develop and apply a broad suite of modern analytical techniques to characterize selected wastewater samples from HF operations in West Texas, a region in which there exists little to no literature of detailed chemical analyses of wastewater from UDO, as thoroughly as possible. Three wastewater samples, obtained from various sources, were analyzed for volatile and semi-volatile compounds with GC-MS, for metals with ICP-OES, for non-volatile organic compounds with high performance liquid chromatography–high resolution mass spectrometry (HPLC-HRMS), and high performance ion chromatography (IC). Nonspecific measurements included TOC/total nitrogen (TN) content, conductivity, and pH. Several compounds identified among the wastewater samples are known common components of HF fluid, including 2-butoxyethanol, cocamide diethanolamines, and
o-xylene [
14].
2. Materials and Methods
2.1. Sampling
Samples were collected in amber high-density polyethylene bottles with limited headspace, stored and shipped to the University of Texas at Arlington on ice, and refrigerated in the laboratory at 4 °C prior to analyses. Sample 1 was taken directly from effluent of an HF well in Odessa, Texas. Sample 2 was obtained from a DW near Reagan County, Texas, taken about 3 months after disposal began in the well. Sample 3 was taken from a waste pit in Nolan County, Texas, within weeks of the pit being filled. To the best of our knowledge, Samples 2 and 3 only contain wastewater from wells that had been stimulated by HF.
2.2. Reagents and Standards
Ammonium acetate (99.999%) (SKU No. 372331) and HPLC grade ethyl acetate (SKU No. 34858) were purchased from Sigma-Aldrich Corp. (St. Louis, MO, USA) LC-MS grade water (cat. No. LC-365), isopropanol (cat. No. 9827), and acetonitrile (cat. No. zh30000LCMS) were purchased from Honeywell International, Inc. (Morristown, NJ, USA), Avantor Performance Materials, and PHARMCO-AAPER, respectively. Deionized (DI) water was supplied by a model 1102D High-Purity deionized water system using a continuously recirculating loop (Aries Filterworks, West Berlin, NJ, USA). Nitric acid (OmniTrace®) (cat. No. NX0407-2) was purchased from EMD Millipore (Darmstadt, Germany). The metals standard solution containing all metals with the exceptions of Sr and Zr for ICP-OES (cat. No. QCS-26) was purchased from High-Purity Standards, Inc. (Charleston, SC, USA). The Sr and Zr standard solution used for ICP-OES consisted of strontium chloride hexahydrate (cat. No. AC31508-1000) and zirconium (IV) oxychloride octahydrate (cat. No. AC20837), purchased from Thermo Fisher Scientific, Inc. (Waltham, MA, USA) in 2% nitric acid in DI water. The following chemicals used to prepare the standard solution for IC: potassium nitrate (cat. No. A14527) from Alfa Aesar; potassium perchlorate (cat. No. 7053) and sodium chloride (cat. No. 7581-06) from Mallinckrodt Pharmaceuticals; potassium sulfate (cat. No. P-305) and sodium acetate (cat. No. S-210) from Fisher Scientific Co. (Hampton, NH, USA); and potassium formate (cat. No. S-5044) from Merck and Co., Inc. (Kenilworth, NJ, USA).
2.3. Gas Chromatography-Mass Spectrometry
The GC-MS analysis protocol was adapted from Fontenot
et al. [
15]. A GCMS-TQ8030 gas chromatograph-mass spectrometer equipped with an SHRXI-5MS (30 m × 0.25 mm × 0.25 µm) column and an AOC-20i/s autoinjector/autosampler (all from Shimadzu Scientific Instruments, Inc., Columbia MD, USA; hereinafter SSI) was used to identify volatile and semi-volatile organic compounds. Liquid-liquid extraction was used to separate organics from the samples. Five milliliters of sample (or DI water blank) were vortexed for 30 s with 2 mL ethyl acetate. Sample 1 was available in limited volume and was diluted 10x before extraction. Operational details: Injection volume, 2 µL; injection port temperature, 300 °C; split ratio, 20:1; carrier gas, helium; linear velocity, 35 cm/s; ionization mode, electron ionization; transfer line temperature, 260 °C; ion source, 260 °C. The oven temperature ramp program was 0 →3→4.5→11→13 min; 40→40→70→330→330 °C.
Table 1 displays the MS program used for the GC-MS analysis. Based on initial studies of ingredients used in hydraulic fracturing fluid and previous experience, several compounds were thought as likely to be present
a priori, and the selected ion monitoring mode was used for their detection mode (
Table 2). The scan mode was used for the detection of unexpected compounds and the impartation of structural information.
Table 1.
Mass spectrometer (MS) program for gas chromatography-mass spectrometry (GC-MS).
Table 1.
Mass spectrometer (MS) program for gas chromatography-mass spectrometry (GC-MS).
Start Time (min) | End Time (min) | Acqu. Mode | Event Time (s) | m/z |
---|
0.60 | 2.25 | Scan | 0.20 | 40.00–100.00 |
0.60 | 2.25 | SIM | 0.10 | 31.10, 55.10, 29.10 |
2.88 | 5.78 | Scan | 0.20 | 40.00–200.00 |
2.88 | 5.78 | SIM | 0.10 | 78.10, 56.10, 31.10, 45.10, 91.10, 44.10 |
5.78 | 6.35 | Scan | 0.20 | 40.00–200.00 |
5.78 | 6.35 | SIM | 0.10 | 91.10, 57.10, 29.10, 105.10, 44.00 |
6.35 | 7.30 | Scan | 0.20 | 40.00–250.00 |
6.35 | 7.30 | SIM | 0.10 | 45.10, 57.10, 59.10, 68.10, 73.00, 91.10, 105.15, 103.00 |
7.30 | 8.50 | Scan | 0.20 | 40.00–300.00 |
7.30 | 8.50 | SIM | 0.10 | 128.10, 142.15 |
8.50 | 11.00 | Scan | 0.20 | 40.00–400.00 |
8.50 | 11.00 | SIM | 0.10 | 45.10, 144.15, 213.10 |
11.00 | 13.00 | Scan | 0.20 | 40.00–400.00 |
Table 2.
Gas chromatography-mass spectrometry (GC-MS) targeted compounds with associated CAS number and selected-ion monitoring (SIM) ion.
Table 2.
Gas chromatography-mass spectrometry (GC-MS) targeted compounds with associated CAS number and selected-ion monitoring (SIM) ion.
Compound | CAS Number | SIM Ion | Compound | CAS Number | SIM Ion |
---|
Acetaldehyde | 75-07-0 | 29.10 | Mesitylene | 108-67-8 | 105.15 |
Acetophenone | 98-86-2 | 105.10 | Methanol | 67-56-1 | 31.10, 29.10 |
Benzene | 71-43-2 | 78.10 | 1-Methylnaphthalene | 90-12-0 | 142.15 |
Benzyl Chloride | 100-44-7 | 91.10 | 2-Methylnaphthalene | 91-57-6 | 142.15 |
Bisphenol A | 80-05-7 | 213.10 | Naphthalene | 91-20-3 | 128.10 |
2-Butoxyethanol | 111-76-2 | 57.10 | 1-Naphthol | 90-15-3 | 144.15 |
n-Butanol | 71-63-3 | 56.10 | 2-Naphthol | 135-19-3 | 144.15 |
Cumene | 98-82-8 | 105.10 | 1,2-Propanediol | 57-55-6 | 45.10 |
Dimethylformamide | 68-12-2 | 44.10 | n-Propanol | 71-23-8 | 31.10 |
d-Limonene | 5989-27-5 | 68.10 | 2-Propyn-1-ol | 107-19-7 | 55.10 |
Ethanol | 64-17-5 | 31.10, 29.10 | Toluene | 108-88-3 | 91.10 |
Ethylbenzene | 100-41-4 | 91.10 | 1,2,4-Trimethyl Benzene | 95-63-6 | 105.10 |
Ethylene Glycol | 107-21-1 | 31.10 | o-Xylene | 95-47-6 | 91.10 |
2-Ethylhexanol | 104-76-7 | 57.10 | m-Xylene | 108-38-3 | 91.10 |
Glutaraldehyde | 111-30-8 | 44.10 | p-Xylene | 106-42-3 | 91.10 |
Isopropanol | 67-63-0 | 45.10, 29.10 | | | |
The data were analyzed with GCMSsolutions (ver. 4.20, SSI). Mass spectra were generated by averaging the chromatographic peaks from the inflection points and subtracting an average mass spectrum outside of the peak. Responses at any other m/z value where the maximum intensity did not temporally coincide with the chromatographic peak under investigation were removed. A mass spectral similarity index value (0–100) was software generated based on comparison with a library of mass spectra (National Institute of Standards and Technology, ver. 2011, Washington, DC, USA). Except for two bisphenol F isomers (similarity index 85), all other reported compounds had a similarity index of ≥90. In addition, the retention times of toluene and o-xylene were known. Compounds for which we could not decipher the chemical formulas or functional groups are not reported.
2.4. Inductively Coupled Plasma-Optical Emission Spectroscopy
An ICPE-9000 (SSI) equipped with a CETAC ASX-520 autosampler (Teledyne Technologies, Inc., Omaha, NE, USA), a mini-torch nebulizer, and argon plasma was used to measure concentrations of various metals. Samples were diluted 10× (Sample 2) or 20× (Samples 1 and 3) with 2% by volume ICP-grade aqueous nitric acid solution. Standard addition was performed for quantitation with 0, 100, and 200 ppb spiked concentrations. Blanks consisted of 2% nitric acid in DI water treated in the same manner. The determined blank concentrations, which were all below 10% of the sample concentrations, were subtracted from that of the sample concentrations.
2.7. Total Organic Carbon/Total Nitrogen Analysis
A TOC-L total organic carbon analyzer outfitted with the TNM-L total nitrogen module and an ASI-L autosampler (all from SSI) were used for Total Carbon (TC), Inorganic Carbon (IC), and TN measurements. Samples (100x dilution for Sample 1) were filtered with 0.45 µm PTFE syringe filters as needed and filled a 40 mL volatile organic analysis autosampler vial. TOC was calculated as the difference between TC and IC.
2.8. Conductivity and pH Analysis
An HI2020 Edge multi-parameter meter equipped with an HI11310 pH electrode and an HI763100 conductivity probe (all from HANNA Instruments, Woonsocket, RI, USA) was used to measure the pH and conductivity of filtered and undiluted samples in our laboratory at 20 °C.
4. Conclusions
Information gained from detailed chemical analyses of wastewater can be used to manage it more appropriately, develop targeted treatment methods, source it, and assess the relative health risk associated with exposure to it. The methods described in this paper may also be of value for the assessment of remediation strategies, potentially contaminated groundwater, and other industrial wastewaters.
This report is not an exhaustive characterization of wastewater. Organic speciation through GC-MS and HPLC-HRMS data from methods intended for surveying generally unknown samples has also been presented here. In all cases, it is important to incorporate best practices from standard methods available for various targeted analyses; it is also, however, important to maintain the ability to discover unexpected analytes that were not targeted. More information of wastewater constituents can be gained through additional advanced analytical chemistry, primarily for specific classes or functional groups of organic compounds, trace metal detection, and complementary separation and detection techniques.
For future studies, we have a number of recommendations based on our experience. Though not always easily obtained, it would be most desirable to only analyze samples whose history is known in sufficient detail to permit a deeper discussion of the context of measured parameters and a more direct comparison to wastewater from other studies. We also recommend taking into consideration the volumes of sample required for method development and sample preparation when sampling to ensure that all desired chemical analyses can be performed. The volumes necessary for method development and analysis are related to the concentration of the analyte targets, the sensitivity of the analytical methods, and the selected sample preparation techniques. Many EPA methods for the analysis of volatile and semi-volatile organic compounds in water-based samples require up to 1 liter of sample to concentrate analytes prior to GC-MS analysis. Metal and ion analysis methods typically do not require preconcentration steps, therefore less initial volume required. Storage space for these samples is also a practical consideration in the volume collected.