A Review of Analytical Methods for Characterizing the Potential

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A Review of Analytical Methods for Characterizing the Potential Environmental Impacts of Unconventional Oil and Gas Development Ines C Santos, Zacariah Louis Hildenbrand, and Kevin A. Schug Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04750 • Publication Date (Web): 04 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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Analytical Chemistry

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A Review of Analytical Methods for Characterizing the Potential Environmental Impacts

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of Unconventional Oil and Gas Development

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Inês C. Santos1,2, Zacariah L. Hildenbrand2,3*, Kevin A. Schug1,2*

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AUTHOR INFORMATION:

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1. Department of Chemistry and Biochemistry, The University of Texas at Arlington, 700

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Planetarium Place, Arlington, TX 76019, USA 2. Affiliate of Collaborative Laboratories for Environmental Analysis and Remediation, The University of Texas at Arlington, Arlington, TX 76019, USA 3. Inform Environmental, LLC, 6060 N. Central Expressway, Suite 500, Dallas, TX 75206

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Correspondence should be addressed to: 700 Planetarium Pl.; Box 19065; Arlington, TX 76019-

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0065, USA. Tel.: +1 817 272 3541; Fax: +1 817 272 3808; e-mail address: [email protected]

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Correspondence should be addressed to: 6060 N. Central Expressway, Suite 500, Dallas, TX

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75206. [email protected]

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Keywords: Unconventional oil and gas extraction; analytical methodologies; environmental

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impacts; groundwater; produced water

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1. Introduction

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Unconventional oil and gas extraction (UOG) has expanded rapidly across the United

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States, as it has become an established technique for oil and gas extraction from low permeability

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shales. There are more than 900,000 active oil and gas wells in the United States, and more than

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130,000 have been drilled since 2010.1 The U.S. Energy Information Administration (EIA)

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estimates that in 2017, about 16.76 trillion cubic feet (Tcf) of dry natural gas was produced from

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shale resources in the United States, including the Bakken (North Dakota and Montana), Niobrara

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(Colorado), Marcellus and Utica (Pennsylvania, Ohio, and West Virginia), Haynesville (Louisiana

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and East Texas), Eagle Ford (South Texas), and Permian Basin (West Texas and Southeast New

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Mexico) shale plays, as shown in Figure 1.2

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Figure 1. Seven regions in the US that accounted for 92% of domestic oil and gas production from

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2011-2014. Source: U.S. Energy Information Administration (Oct 2008).3

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Hydraulic fracturing facilitates the extraction of oil and gas through the injection of

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aqueous solutions and suspensions at high pressure (480 – 850 bars); this process creates fractures

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in shale formations.4 The required fluids are comprised of 98-99.5% of water with the addition of

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sand and chemical additives.4,5 The selection of these chemical additives depends on the type of

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drilling mud used and on the geologic conditions. These chemicals include biocides to control

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microbial growth, friction reducers to reduce friction, gellants to increase viscosity, surfactants to

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reduce surface tension, oxygen scavengers to prevent corrosion, and proppants to keep the

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fractures open and to maintain a stable flow.6 Currently, Fracfocus.org7 contains the most

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comprehensive dataset on chemicals used in hydraulic fracturing. Some examples are given in

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Table 1. Nevertheless, to maintain intellectual propriety, oil and gas companies (and/or their

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suppliers) do not often provide complete information on the additives used, and this creates

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challenges in terms of determining the exposure safety, efficacy of treatment modalities, and/or

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the by-products present in waste water or contamination events.

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After hydraulic stimulation, waste fluid returns to the surface due to the high pressures in

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the well. This fluid, referred to as flowback water, is wastewater that contains high levels of

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dissolved solids, salts, and fracturing chemicals. When the flowback finishes, fluid that is within

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the oil or gas-producing formation can be recovered, which is called produced water. This

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produced water has high concentrations of salt and can contain harmful levels of metals and

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radioactivity.4,6 When they return to the surface, these waste streams must be either disposed of,

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treated, and/or reused.

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Table 1. Typical chemical additives used in hydraulic fracturing.7



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Additive type

Use

Main compound

Acid

Helps dissolve minerals and initiate cracks in the rock Prevent corrosion of the pipe

Hydrochloric acid

Percent by volume 0.07%

N,N-dimethyl formamide

0.05%

Decrease pumping friction

Polyacrylamide

0.05%

Clay stabilizer

Prevent clay from swelling

Potassium chloride

0.034%

Crosslinker

Borate salts

0.032%

Ethylene glycol

0.023%

Ammonium persulfate

0.02%

Ammonium bisulfite

0.004%

Biocide

Maintain fluid viscosity as temperature increases Prevent scale deposits in the pipe Promote breakdown of gelling agent Prevent precipitation of metal oxides Control bacterial growth

Glutaraldehyde

0.001%

Gellant

Improve proppant placement

Guar gum

0.5%

Corrosion inhibitor Friction reducer

Scale inhibitor Breaker Iron control

59 60 61

The environmental implications of UOG remains a complex and polarizing topic.8 Surface

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water,9–11 groundwater,12–20 soil,21–24 and air25–31 are potentially subject to contamination from

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surface spills, transport of fluids through micro-scale annular fissures in UOG gas wells, gas

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emissions, and from the physical mobilization of ions from scale/rust formations.9,20,32

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Understanding the causes of potential contamination is therefore important to develop responsible

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energy extraction and environmental stewardship practices. The development of analytical

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techniques to help provide this understanding is vital. This is a field that is still growing, as some

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factors still limit the research performed. Groundwater and produced water samples are often very

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difficult to obtain and funding for this type of research is currently inadequate. The majority of 4 ACS Paragon Plus Environment

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groundwater samples that have been analyzed as part of most published monitoring studies have

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been obtained mainly from residential wells, which requires the participation of landowners/ well

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owners. The variability in the willingness to participate, based on the different views and beliefs

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of those approached to obtain samples, is a confounding factor that may be perceived as

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experimental bias. Additionally, not all densely populated areas experiencing UOG have water

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wells, which hinders the ability to perform high-resolution mapping of those areas.

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The economics of this research also vary depending on who is performing it (academic,

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commercial, or governmental labs). The lack of funding in this field slows down the development

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of high-resolution techniques for the multiparametric determination of hydraulic fracturing

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compounds. Furthermore, the lack of communication by the industry as a whole (with some

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exceptions), also limits the development of environmental impact studies. A better line of

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communication between academic and private sector partners could provide insight on sampling

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times in order to align analyses with different aspects of the UOG operations process and what

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chemicals should be avoided and replaced. Overall, these are all difficulties that limit the research

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performed in this field and its advancement.

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The present article reviews the current state of the art in the analysis of potential hydraulic

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fracturing environmental impacts using modern analytical techniques. Initially, groundwater and

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produced water compositions are presented and compared as means to identify contamination

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events. Afterwards, the analytical methods developed so far to monitor both matrices are

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discussed, including sample preparation and analysis. Lastly, the current state of knowledge on

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the environmental impacts towards water, soil, and air contamination, as determined by various

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analytical studies, are reviewed.

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2. Groundwater and Produced Water Composition

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Groundwater is mainly rainwater that infiltrates through soil into flow systems in the

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underlying geologic materials. It contains a wide variety of dissolved inorganic constituents

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including major ions such as Na+, Mg2+, Ca2+, K+, Cl-, HCO3-, and SO42-, as a result of chemical

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interactions with geological materials, and to a lesser extent, contributions from the atmosphere.33

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According to the United States Environmental Protection Agency (US EPA),34 more than 13

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million households in the United States rely on private wells for drinking water. Because of this,

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fresh groundwater is one of the most valuable resources to be preserved and protected. Despite

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this need, from 2013 – 2016, the water use per UOG production well went up by approximately

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434% in the Permian Basin,35 with a median of 12 million gal/horizontal well stimulation.36

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Alternate water sources for fracturing, such as municipal wastewater, are thus being explored,

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alongside the potential for reutilization of produced water after treatment.37

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Produced water is much more complex compared to groundwater and is mainly composed

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of natural components of hydrocarbons that contain light distillates (naphtha, methane through

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pentane, and trace aromatics including benzene, toluene, xylenes, etc.), middle distillates

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(kerosene, gasoline components, and diesel components), and residues, including wax

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hydrocarbons with carbon chains from 18 to over 20. Salt content of produced water varies, from

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extremely freshwaters with less than 500 ppm total dissolved solids (TDS) to supersaturated brines

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(> 40,000 ppm TDS) from production wells that must be regularly treated with freshwater to

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remove halide scale deposits.38 Due to these high concentrations, dissolved salts are often the most

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difficult material to remove from oilfield-produced waters. Naturally occurring radioactive

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materials (NORM) are another common component of produced water.38,39 The different chemical

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compositions between groundwater and produced water can allow researchers to track 6 ACS Paragon Plus Environment

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contamination from UOG by identifying the release of unique chemicals used in these activities;

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however, it is necessary to differentiate between natural and anthropogenic origins. A comparison

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between the maximum contaminant limits for drinking water and desired water quality for water

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reuse in production well stimulation is also provided in Table 2.

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Table 2. Comparison of maximum contaminant limits (MCL) for drinking water and production

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well stimulation and chemical composition of produced water.38 Ion/Element

Drinking water MCL (ppm)6

pH Alkalinity Salinity Total dissolved solids (TDS) Total suspended solids (TSS) Total nitrogen Total organic carbon (TOC) Biochemical oxygen demand (BOD) Oil & grease Sodium Chloride Calcium Magnesium Potassium Sulfate Bromide Bromate Strontium Ammonium Bicarbonate

6.5-8.5

Production Well Stimulation MCL (ppm)7,8 6.0 – 8.0

500 500 44.3

250 75

30,000 – 50,000 2,000 2,000

250

500

0.01

300 7 ACS Paragon Plus Environment


Barium Iodide Iron Boron Carbonate Lithium E. coli

2

20

0.3

10 10

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0

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Contaminants are divided into those pertaining to chemical (inorganic, volatile organic,

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synthetic organic contaminants) and microbial constituents. All of these have an adverse health

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effect and so EPA set a maximum contaminant limit (MCL) for each contaminant in drinking water

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to protect drinking water sources. Clearly, produced water contains multiple contaminants at

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concentrations well above acceptable limits for drinking water and are therefore a risk to the

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surrounding environment. It is important to note that these standards only apply to groundwater

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used in public water systems but not to groundwater from private water wells, which increases the

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need for assessing potential contamination, in an effort to avoid adverse health effects.

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Regarding microbial contaminants, Cryptosporidium, Giardia lamblia, Legionella, Total

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Coliforms, and enteric viruses are the standards set by the EPA to guarantee the quality and safety

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of drinking water. Nevertheless, there are other potential pathogenic microorganisms, which are

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not regulated and can be isolated from potable water.40 As an example, Salmonella and Shigella

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are known to cause outbreaks in the U.S. and should be included in the standards for better water

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quality. In fact, the majority of waterborne disease outbreaks have been previously linked to

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groundwater (76%) provided by community systems (36%), noncommunity systems (39%), and

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individual wells (25%). These outbreaks were mainly caused by untreated groundwater, treatment

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failures or deficiencies, and problems in the distribution system.40


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In order to assess the potential environmental impact of PW and evaluate handling and

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disposal options, we need to fully understand the chemistry and microbiology of these fluids. In

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the review by Liden et al.,37 produced water reuse was thoroughly discussed including the concerns

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regarding reuse applications and what technologies could be used to remove these contaminants.

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Additionally, monitoring surface and groundwater quality is of utmost importance to control

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environmental impacts and decrease health risks.

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3. Risk Assessment

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Environment analysis follows the common steps of any analytical process: (1) Sampling

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and sample preparation; (2) measurements; and (3) data processing. Current EPA methods are

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specific for a certain analyte (targeted analysis) and therefore, it is necessary to use multiple

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methods to get a complete chemical coverage. Multiple methods decrease the throughput of the

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analysis, which is critical for quality measurements and the ability to carry out large-scale studies.

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Additionally, these targeted methods do not account for the presence of unknown compounds that

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are therefore overlooked. In this sense, the development of targeted and untargeted methods, with

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high throughput and high sensitivity tailored to UOG activities, are necessary. Sample extraction

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and analysis of groundwater and produced water are discussed in the next sections.

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Before talking about sample preparation and analysis, the importance of blanks throughout

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the analytical process needs to be mentioned. Sample blanks increase confidence in the analysis

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performed by making sure no contaminations occurred during sample collection, handling, or

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preparation. A variety of blanks provide valuable insight to any environmental investigation. These

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include analytical blanks, field blanks, travel blanks, and storage blanks. The analytical blank

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mimics the sample matrix but without the analyte. This will reveal contamination from preparation, 9 ACS Paragon Plus Environment


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consumables, and solvent impurities. The field blank and travel blank are blank samples prepared

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with matrix in the lab prior to sample collection. The field blank is opened during sample collection

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to represent any contamination from the ambient air. The travel blank is not opened in the field

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but it travels with the sample and accounts for any contamination from sample containers and

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traveling conditions. The storage blank is prepared with matrix in the lab after sample collection

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and stored in close proximity with the sample to account for contamination during short-term

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storage.41

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3.1. Sample Extraction

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Sample pretreatment is one of the critical steps in the analysis of environmental samples

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and identification of contaminants. Environmental samples are complex and analytes may be

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present at trace levels. Even more challenging is the analysis of produced water, due to its complex

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matrix, especially the high salt content, and due to the fact that its composition can change

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depending on the shale geology, location, depth, and lifetime of the well from which it is obtained.

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Therefore, sample cleanup is extremely important to isolate and/or preconcentrate the analyte and

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to remove possible matrix interferences.42

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Liquid-liquid extraction (LLE) is a straightforward and widely used approach for isolating

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desired analytes based on their affinity (and that of potential interferences) for immiscible liquid

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phases brought into contact. LLE is becoming less popular with the advent of much more

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technologically-advanced techniques, but it is still important for the extraction of organic

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pollutants or metals before, for example, gas chromatographic analysis. For the primarily aqueous-

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based samples encountered in the field of research considered here, a small volume of organic

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solvent is added to the sample to facilitate extraction of desired components into the organic 10 ACS Paragon Plus Environment

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fraction. The analyte can be further concentrated, if necessary, by evaporation.42 LLE is laborious,

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time consuming, and uses large quantities of organic solvents. Nevertheless, this technique

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provides good throughput, is cheap, and improves sensitivity to allow measurements of analyte

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abundances below the EPA MCL values in groundwater samples. In the work by Thacker et al.,43

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LLE was used to separate organics from UOG wastewater samples before gas chromatography-

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mass spectrometry (GC-MS) analysis.

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Solid phase extraction (SPE) remains the most popular means of extraction and

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concentration. SPE is based on passing the sample across a packed bed of sorbent, and it is useful

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for analyte extraction, concentration, and sample clean up. It is frequently used to extract

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semivolatile and nonvolatile analytes from a complex matrix. This technique, compared to LLE,

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needs less solvent and has proved to be effective in the isolation and preconcentration of a wide

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variety of contaminants in the environment, but is more expensive. Cluff et al,44 extracted

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ethoxylated surfactants using C18 SPE before analysis using liquid chromatography (LC)-MS.

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Solventless extractions such as SPME and purge-and-trap techniques, usually coupled with

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GC, are a “greener” alternative for the extraction of volatile and semi-volatile analytes. With these

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methods, analytes are trapped on a selective sorbent and then desorbed to the analytical column.

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Headspace analysis (HS) is another solventless extraction technique that can be used for the

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determination of volatile analytes. The sample is heated and agitated to liberate the gas to an open

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headspace in the vial, which is then sampled. HS sampling greatly reduces contamination of the

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GC instrument injection port and column by nonvolatile components, relative to direct injection

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approaches.

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Dorman et al.45 have used a combination of approaches for the preparation of wastewater

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and extraction of organic compounds. The authors extracted and concentrated wastewater to a final



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volume of 2 mL from a starting volume of 500 mL, giving them a preconcentration factor of 250.

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A modified USEPA 3510c extraction technique was used. Each sample was serially extracted,

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three times, under acidic conditions (pH = 2) and three times under basic conditions (pH = 11)

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using dichloromethane as the extraction solvent. If an emulsion occurred, the sample was

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centrifuged for 3 min at 3000 rpm in order to achieve necessary separation of phases. Kuderna–

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Danish evaporation was then used to concentrate samples to a final volume of 2 mL. All sample

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extracts were analyzed using comprehensive multidimensional gas chromatography (GCxGC)-

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MS.

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One new trend is the use of ionic liquids (ILs) to aid extraction processes. Ionic liquids are

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organic salts with a low melting point that are a good alternative to traditional solvents used in

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industry, and they are being utilized in both extraction, such as for hollow fiber – liquid phase

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microextraction, and chromatography applications.46 Recently, ILs were used in headspace

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analysis, as a co-solvent, for the extraction of benzene, toluene, ethyl benzene, and xylenes

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(BTEX) from soil samples.21 The use of hydrophilic room temperature ILs, mixed with the soil in

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the HS vial prior to heating, was found to help homogenize the matrix and facilitate liberation of

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BTEX with minimal matrix effects. Relative to the established EPA method 5021A, where water

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is used as a cosolvent, this new HS-GC – mass spectrometry (GC-MS) was demonstrated to

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alleviate the need for soil matrix matching in calibration procedures.23

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Samples such as produced water are extremely complex and, as a result, it is difficult to

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perform target analysis without interferences. Additionally, multiple compounds are of interest

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that are present at variable concentrations making this analysis even more challenging. Therefore,

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researchers usually resort to the use of standard methods to be able to handle the complexity or

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complications that arise from these new types of samples with various targets. Nevertheless, as the


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field becomes more mature, researchers are developing new and specific sample preparation

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strategies to overcome some of the challenges posed by this type of sample. A good example of

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innovation in the field of sample preparation is the use of ionic liquids to suppress matrix

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interferences.

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3.2. Sample Analysis

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Water quality is one major concern and monitoring parameters, such as pH, total dissolved

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solids, VOCs, anions, cations, hydrocarbons, and bacteria is required. Many of these parameters

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have specified maximum contaminant level (MCL) limits, as delineated previously in Table 2.

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These measurements are particularly challenging due to the variety of analytes at variable

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concentrations and also due to the presence of unknowns (both unexpected and without standards).

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Mass spectrometry (MS), however, is a valuable tool in this field for the targeted and non-targeted

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analysis as it allows the identification of knowns and unknows, respectively, as discussed below.

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Several analytical methodologies have been developed and approved by the U.S. EPA to

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analyze drinking water and wastewater (Figure 2), to ensure compliance with regulations. The

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problem is that many, if not all, of the currently approved methods have not been validated in

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matrices with dissolved salt content as high as 350,000 mg/L total dissolved solids (TDS). For this

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reason, there are still no standardized methods to analyze produced water. Nevertheless, in the past

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decade, additional analytical methods have been developed to study the impacts of UOG activities

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on groundwater, surface water, soil, and air quality and to characterize these same environments.

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It is important to mention that, though they are not EPA approved methods, they do follow the best

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practices from those methods. For example, how certain types of water and wastewater samples

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should be preserved (acid for metals, chloroform for anions, no headspace) and for how long they 13 ACS Paragon Plus Environment


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should be stored (maximum 2 weeks in fridge for organics, 24 hours for microbes, etc.) are

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important parameters to maintain when new methods are developed.

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Chromatography is a separation technique used to isolate the analyte from other

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components present in the sample. GC is mainly used for the separation of volatile and semi-

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volatile compounds while LC is used for semi-volatile and non-volatile compounds. GC is the

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technique of choice for environmental analysis; however, LC has grown in popularity, particularly

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in the analysis of produced water, as it increases the number of compounds that can be analyzed,

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compared to GC.

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LC work has been performed by Ferrer et al.47 for the analysis of hydraulic fracturing

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additives including gels, biocides, and surfactants. Optimization of mobile and stationary phases

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are constantly being made to improve separation of complex mixtures. Furthermore, in cases where

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high resolution is needed, GCxGC or comprehensive multidimensional liquid chromatography

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(LCxLC) can be used. For this purpose, two analytical columns are coupled for an orthogonal

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separation which provides increased peak capacity and resolution. Nevertheless, this technique is

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expensive and requires significant additional analytical expertise to operate, relative to traditional

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one-dimensional chromatographic separations.48 Prebihalo et al.45 used GCxGC to achieve the

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level of sensitivity and selectivity necessary for detecting compounds in complex samples, such as

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wastewater.

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Figure 2. Analytical and EPA approved methods for the analysis of wastewater (adapted from

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Liden, T.; Santos, I.C.; Hildenbrand, Z.L.; Schug, K.A. Analytical Methods for the Comprehensive

278

Characterization of Produced Water, in Ahuja, A. (Ed.), Evaluating Water Quality to Prevent Future

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Disasters. Elsevier. (In Press; publication expected May 2019)).49

280 281

When characterizing water and wastewater, there are four primary categories to evaluate:

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Basic water quality metrics and bulk measurements; organic compounds; biological constituents;

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and inorganic species.

284 285

3.2.1. Bulk measurements 15 ACS Paragon Plus Environment


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Bulk measurements, such as those for determining total dissolved solids (TDS) and total

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organic carbon (TOC), are an important aspect of any water analysis as they allow for a quick

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assessment of overall water quality in a cost-effective way. Basic groundwater measurements

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include pH, temperature, total suspended solids, salinity, alkalinity, and others. Measurements

290

such as pH are important for both environmental impact analysis as well as optimizing fracturing

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fluids. Hardness and alkalinity provide general information on the water chemistry, such as the

292

potential for unwanted scale buildup. These measurements can be made on site with a probe, or in

293

the lab by titrations, or with analyzers such as the total organic carbon analyzer (TOC).38 Methods

294

used to monitor alkalinity levels include titrations and colorimetric testing, such as in

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Environmental Protection Agency (EPA) methods 310.1 and 310.2, respectively, as well as in EPA

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method 310.1 for the quantification of alkalinity in produced water. Total organic carbon is usually

297

measured using a bench-top total organic carbon analyzer.

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These methods are simple to perform but careful must be given to the presence of

299

interferences, particularly in produced water due to the high levels of salt contained therein. Since

300

these bulk parameters do not usually represent constituents at trace levels, dilutions are performed

301

before analysis to avoid interferences. Bulk measurements do not provide information on specific

302

analytes, but they do give general information on water status. For increased resolution, the

303

application of more specific analytical measurements to target specific constituent classes can be

304

pursued.

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3.2.2. Ion Composition

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Produced water and groundwater contain ions that are mobilized from the subsurface

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environment. High concentrations of chloride and bromide can be found in wastewaters and these 16 ACS Paragon Plus Environment

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309

are a concern to drinking water utilities because the former can interfere with treatment, while the

310

latter can form toxic disinfection byproducts (DBPs) during water treatment. Sulfate is also of

311

concern as it can be reduced to hydrogen sulfide and corrode infrastructure. Additionally, the levels

312

of calcium, barium, and strontium are especially important to measure as they contribute to scaling,

313

problems in water pumps, pipes, and other pieces of equipment resulting in the poor performance

314

of water recycled for future fracturing events.39 For the analysis of these pertinent anions, ion

315

chromatography is the preferred analytical technique.50 Several standard methods for the analysis

316

of anions have been established for domestic and industrial waste, including SM 4110, EPA 300.0

317

and 300.1. However, for produced water, due to the high concentration of anions, particularly

318

chloride, sample dilution is necessary to avoid inaccurate results. In the work by Coday et al.,51

319

PW samples from the Niobrara shale formation were diluted 30 times to bring the chloride

320

concentration below 500 mg/L in order to accurately determine the anion concentrations.

321

Besides anions, metal concentrations in flowback and produced waters are of concern due

322

to the potential implications for drinking water supplies if the waste fluids contact surface water

323

and groundwater sources. Certain metals are toxic to the environment, and they are also known to

324

cause scaling during the well stimulation process. Inductively coupled plasma - optical emission

325

spectroscopy (ICP-OES) and inductively coupled plasma - mass spectrometry (ICP-MS) are

326

crucial techniques for the monitoring of metal contaminants. Both allow a simultaneous

327

determination of a wide assortment of metals from a single sample. These are prescribed for use

328

in standard methods SM 3120B, EPA 200.7, ASTM D1976, EPA 200.8, SM 3125, ASTM 5673,

329

and USGS I-4020-05. Flame atomic absorption (FAA) or graphite furnace atomic absorption

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(GFAA) can also be used using the EPA method 265.2 or 267.2; however, they are less sensitive

331

and only allow for the detection of one metal at a time.52 Minimal sample preparation is required



Page 18 of 63

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for ICP-based elemental determination techniques, apart from filtration and/or dilution of the

333

sample, and the addition of acids, prior to analysis.50 The high temperature of an ICP torch

334

alleviates many interferences, which can hamper other techniques like GFAA and FAA. This is

335

particularly important in the analysis of metals in PW as interferences are a major issue. There are

336

a variety of different interferences possible in PW as this is such a complex sample. These include

337

spectral interferences (signal overlap from different metals) and chemical interferences (other

338

components and their effect on effective atomization). Nevertheless, the high temperature of the

339

torch results in more effective excitation of atoms preventing the formation of non-emitting and

340

refractory oxides. The choice of ICP-OES versus ICP-MS is generally dictated by the metals

341

desired to be measured. Most metals are abundant enough to be measured by ICP-OES; however,

342

to reach EPA MCL for arsenic in drinking water, an ICP-MS is generally required to achieve the

343

necessary sensitivity.

344 345

3.2.3. Volatile and Semivolatile Organic Compounds

346

Volatile and semivolatile organic compounds, such as natural gas constituents (methane,

347

ethane, propane, etc.), BTEX (benzene, toluene, ethyl benzene, and xylenes), diesel range organics

348

(DRO), or organic solvents are of particular interest, as these can be related with groundwater

349

contamination. GC-MS is the technique of choice for analysis of these compounds, due to their

350

volatile and semi-volatile nature. These compounds are traditional petrochemical compounds of

351

interest and therefore, there are either EPA or ASTM methods with which many researchers are

352

familiar. VOCs such as methanol, ethanol, and propargyl alcohol can be analyzed using HS-GC,

353

which reduces the required sample preparation and minimizes matrix interferences. This is

354

particularly important in the analysis of produced water. 18 ACS Paragon Plus Environment

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Several detectors can be coupled with GC, including MS, electron capture detector (ECD),

356

flame ionization detector (FID), thermal conductivity detector (TCD), and others. MS is the most

357

sensitive detector and provides qualitive information. EI is the typical ionization technique used.

358

EI generates a collection of diagnostic fragment ions, which are useful for qualitative analysis in

359

conjunction with the availability of mass spectral libraries. FID does not provide qualitative

360

information, but it is able to provide quantitative information based on an analyte’s retention time.

361

Recently, a new GC detector, based on vacuum ultraviolet spectroscopic absorption (VUV)

362

was developed.53,54 VUV measures gas phase absorption in the VUV and ultraviolet wavelength

363

regions (120–240 nm) and has been previously used to monitor dissolved gases in water. This

364

universal detector allowed the identification and quantification of C1-C5 hydrocarbons, along with

365

N2, O2, and CO2.55 Due to the unique VUV spectrum, deconvolution of co-eluting analytes is

366

possible.

367 368

3.2.4. Trace and Ultra-trace Analysis

369

Liquid chromatography – mass spectrometry (LC-MS) is an ideal tool to analyze the

370

nonvolatile species involved in UOG activities. These species include surfactants, fatty amines, or

371

high molecular weight ionic polyacrylamide friction reducers. Surfactants are used as a wetting

372

agent that, when used along with other chemicals, can significantly increase the productivity of a

373

well. To date, few studies have been performed that use LC-MS to characterize produced water or

374

to monitor groundwater located near UOG activities. This could be due to the lack of libraries for

375

MS data when using electrospray ionization (ESI)-MS, cost of instrumentation, or the matrix

376

complexity of produced water. Additionally, as previously mentioned, it is very difficult to obtain

377

produced water samples and adequate funding to perform such studies. Nevertheless, polyethylene 19 ACS Paragon Plus Environment


Page 20 of 63

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glycols (PEG) and polypropylene glycols (PPG) have been identified in produced water using LC-

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MS by some researchers.56,57 The nonionic composition of the aforementioned surfactants function

380

as weatherizers, emulsifiers, corrosion inhibitors, and wetting agents. Benzothiazole and other

381

heterocyclic compounds have also been identified in wastewater from gas production using LC-

382

MS.58

383

Tandem mass spectrometry (MS/MS) can provide further confidence in identification or

384

provide structural information by fragmenting compounds in the mass analyzer. Fragmentation

385

can be generated by ion-trap, triple-quadrupole, or quadrupole-time-of-flight MS detectors;

386

however low-resolution mass spectrometers do not acquire nominal masses and so their

387

application in non-targeted analysis is limited. One of the hottest trends continues to be the use of

388

high-resolution mass spectrometry (HRMS) with liquid chromatography (LC) for non-targeted

389

analysis, more specifically, to identify unknown contaminants. Ultra-HRMS such as Fourier

390

transform - ion cyclotron resonance (FT-ICR)-MS has high enough resolution and mass accuracy

391

to resolve nominally isobaric compounds and therefore provide an identification based on accurate

392

mass. Molecular investigations with these high-end mass spectrometers have predominantly been

393

applied to produced water characterization.41,47,57,59,60 Ethoxylated compounds used in hydraulic

394

fracturing have been analyzed using HRMS.60 Additionally, Luek et al.59 identified halogenated

395

organic compounds in hydraulic fracturing wastewaters using ultra-HRMS. Figure 3 shows how

396

complex the mass spectra of produced water is; however, using a high-resolution mass

397

spectrometer, each peak observed is actually resolved and, using MS/MS data, identification is

398

possible. In the future, HRMS can have a significant impact in the identification of unknown

399

transformation products and metabolites, which are important to monitor.

400


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401

402 403

Figure 3. Ultrahigh resolution mass spectra of FB1 (top) and FB2 (bottom) flowback water

404

samples showing exact masses and molecular formula assignments between m/z 266.75-267.05.

405

Reproduced from Luek, J. L.; Schmitt-Kopplin, P.; Mouser, P. J.; Petty, W. T.; Richardson, S. D.;

406

Gonsior, M. Environ. Sci. Technol. 2017, 51 (10), 5377–5385 (ref 58). Copyright 2017 American

407

Chemical Society.59

408 409

3.2.5. Bacteria

410

The identification of microorganisms is also important in the assessment of water quality.

411

These can cause disease or negatively impact UOG activities, by way of propagating corrosion or

412

commodity souring. Conventional techniques, such as plating and genotypic methods, are

413

preferred for the identification of microorganisms; however, with the advent of soft ionization

414

techniques such as matrix assisted laser/desorption ionization (MALDI), intact bacterial colonies

415

can be analyzed and identified based on their protein profile.61–63 Intact bacteria does not literally

416

mean intact but that colonies have not been treated for the removal of any cellular components.

417

This technique, which is usually coupled with a time-of-flight (TOF) analyzer, not only provides



Page 22 of 63

418

identification but it also delivers insight into the metabolic states of the detected cells. Beyond the

419

initial capital cost, which is not trivial, subsequent analyses are cheaper and faster than nucleic

420

acid techniques. Due to its detection limit and to ensure purity, the culturing of bacteria before

421

analysis is required, and therefore, the appropriate culture media needs to be chosen. Preferably,

422

broad-spectrum media should be used for the isolation of major bacterial groups. Of course, only

423

culturable microorganisms can be analyzed by this technique, and this is certainly a drawback.

424

However, the need for culturing is a common step for other conventional identification methods,

425

as described before, and this step also ensures that only viable (i.e., live) organisms in the sample

426

are identified. Our group successfully used MALDI-TOF MS to identify the microbial community

427

of groundwater and produced water.39,64,65 Figure 4 shows the workflow for the identification of

428

microorganisms using MALDI.

429

430 431

Figure 4. Workflow for bacterial identification using MALDI-TOF MS. 22 ACS Paragon Plus Environment

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432 433

The identification of environmental microorganisms using MALDI-TOF MS still poses

434

some challenges. This technique requires the cultivation of bacteria prior analysis to obtain the

435

number of cells needed and to obtain pure colonies. This limits the analysis of uncultivable bacteria

436

and of complex bacterial mixtures. However, in order to analyze complex mixtures, developments

437

in this field have been made as shown by Yang et al.66 In their work, the authors proposed a

438

framework to characterize polybacterial samples based on their MALDI-TOF spectra, using a

439

reference database containing 1081 mass spectra of pure cultures of 1081 strains/480 species/64

440

genera. Recently, MS imaging (MSI) was used for the untargeted analysis and chemical imaging

441

of complex microbial communities, opening a whole new avenue to better understand

442

microorganism and their interactions.67 Additionally, the identification of environmental

443

microorganisms is limited by the available spectral libraries. As it is known, these libraries were

444

designed for a clinical setting and are thus clinically-biased. Therefore, some of the environmental

445

microorganisms isolated are unidentifiable with these libraries. Due to this reason, researchers are

446

creating their own libraries to facilitate environmental identifications. .68

447

Nucleic acid-based techniques such as DNA/RNA sequencing also play an important role

448

in the identification of bacteria has these can give an insight of unknown microorganisms as well

449

as unculturable microorganisms.69 The most recent technology, next-generation sequencing allows

450

several sequencing reactions to be run in parallel, and this provides for faster and cheaper analysis.

451

While viable and useful, these methods do not provide any information on the metabolic state of

452

the microorganism. Mass spectrometry can thus be a good complementary tool to genomic

453

techniques to study the proteome, metabolome, and lipidomics of microorganisms. An example of

454

complementary work was shown by Santos et al.70 where MALDI-TOF MS and GC-VUV were



Page 24 of 63

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used to study changes in the protein and fatty acid profiles of microorganisms when exposed to

456

common hydraulic fracturing components. This method does is outside of the box of the traditional

457

standard methods where different parameters are measured using different analytical tools. These

458

works are paving the way towards the development of more nontraditional methods for a

459

comprehensive analysis of UOG activities and their environmental impacts.

460 461

4. Hydraulic Fracturing Environmental Impact

462

The main concern that stems from hydraulic fracturing is the potential environmental

463

impact. Are these activities risk-free? Researchers already know these activities are not risk free

464

and again, stressing out the need for high-resolution analytical methods that allow a comprehensive

465

characterization of this industrialized process and its environmental impacts is always important.

466

The health concerns associated with UOG are usually related with the direct or indirect exposure

467

of workers and residents living near these activities to the pollutants, chemicals, and nonchemical

468

stressors that may be present in the air or water (Figure 5).71 Some of the chemicals used in

469

hydraulic fracturing are known to be hazardous to human health and chronic exposure effects for

470

some of them include cancer, immune system compromises, changes in body weight, changes in

471

blood chemistry, cardiotoxicity, neurotoxicity, liver and kidney toxicity, and reproductive and

472

developmental toxicity. However, to understand whether these chemicals can affect human health,

473

chemical concentrations are necessary and still, more evidence is necessary to relate UOG

474

activities with health risks.

475


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476 477

Figure 5. Direct and indirect health effects from UOG (adapted from Stigler-Granados, Advances

478

in Chemical Pollution, Environmental Management and Protection, 2017).72

479 480

4.1. Groundwater and Surface Water Contamination

481

Proximity to hydraulically fractured production wells may increase the risk of

482

contamination since well casing failure, out-of-zone fracking, and the formation of micro annuli

483

can result in the migration of gas, produced water, and chemical additives. In 2016, the EPA

484

examined the proximity of hydraulically fractured wells to public water supplies; however, citing

485

a lack of aggregated groundwater well construction data, the EPA did not assess the proximity of

486

hydraulically fractured wells to private, self-supply groundwater wells. In a study by Jasechko and

487

Perrone,73 the authors determined that approximately half of all hydraulically fractured wells

488

stimulated in 2014 exist within 2-3 km of one or more domestic groundwater wells. As previously 25 ACS Paragon Plus Environment


Page 26 of 63

489

mentioned, more than 13 million households in the United States rely on private wells for drinking

490

water. However, EPA does not regulate private wells nor provides standards for these thus making

491

it difficult to control contamination events.

492

Water contamination can occur through different mechanisms, such as wastewater spills,

493

leaks from faulty casing and cementing, gas migration, and waste water disposal.6,74 Wastewater

494

spills, although infrequent, can occur and have been reported across the United States. Median

495

spill volumes range from 340 gallons (1,300 liters) to 1,000 gallons (3,800 liters) per spill.

496

Documented cases have shown that spills of produced water have reached surface water (e.g.

497

creeks, ponds, or wetlands) and groundwater sources. Groundwater quality near UOG activities

498

has been fairly extensively studied.9,13–20,75,76 Contamination of groundwater with methane,

499

inorganic metals and salts has been identified and attributed to spills and the transport of fluids

500

through microscale fissures in UOG gas wells.

501

The presence of dissolved gases, such as methane, in groundwater poses a potential

502

flammability or explosion hazard to homes with private domestic wells. Two sources of methane

503

can manifest its presence in groundwater. One is biogenic methane, a by-product of bacterial

504

metabolism, and the other is thermogenic methane, which is the primary target of UOG recovery.

505

This methane gas is formed by the presence of decomposing organic matter under high

506

temperatures and pressures over a long period of time (that is, from deep geological formations).

507

Because of the different implications for each type of natural gas, methane measured in shallow

508

groundwater must include further investigations to distinguish between biogenic or thermogenic

509

origins.

510

The origin of the measured methane can be determined either through isotopic abundances

511

of carbon-13 (13C), deuterium (2H), or the ratio of methane to higher-chain hydrocarbons (ethane,


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512

propane, and butane).17 Osborn et al.17 used both methods to study methane contamination of

513

groundwater. High concentration of methane coupled with the isotopic pattern were consistent

514

with deeper thermogenic methane sources such as the Marcellus and Utica shales at the active sites

515

and matched gas geochemistry from gas wells nearby. In contrast, lower concentration of methane

516

and the isotopic signatures in samples from shallow groundwater at nonactive sites showed a more

517

biogenic or mixed biogenic/ thermogenic methane source. The authors were not able to find

518

evidence for contamination of drinking-water samples with deep saline brines or fracturing fluids.

519

Darrah et al.77 used noble gas elemental and isotopic tracers to help differentiate between natural

520

geological migration of hydrocarbon gases and anthropogenic contamination as shown in Figure

521

6. Noble gases analysis is ideal to track gas migration since these are not subject to microbial

522

degradation, oxidation, and weathering. Using this technique, the authors demonstrated that eight

523

discrete clusters of groundwater wells exhibited evidence for fugitive gas contamination. This

524

contamination was consistent with well integrity problems such as poor cementation and poorly

525

constructed wells (e.g., improper, faulty, or failing production casings).

526



Page 28 of 63

527 528

Figure 6. 4He/CH4 vs.

20Ne/36Ar

(Upper Left) and 4He/CH4 vs. δ13C-CH4 (Lower Left) and

529

C2H6+/CH4 vs. δ13C-CH4 (Upper Right) and 4He/40Ar* vs. 4He/20Ne (Lower Right) of produced

530

gases and groundwater in the MSA (Left)and BSA (Right) at distances >1 km (triangles) and

A Review of Analytical Methods for Characterizing the Potential

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