Hydraulic fracturing is a well stimulation technique used to improve the flow of oil and gas from petroleum-bearing rock formations. A fluid mixture containing water, proppant, and chemical additives is injected into a well at high pressure to fracture the rock. Hydraulic pressure is then removed, allowing oil, gas, and formation water to flow into the well through the fractures held open by the proppant. The portion of the injected fluid that returns to the surface once pressure is released is called flowback water. The native formation water that also surfaces during the extraction is generally referred to as produced water (Thurman, Ferrer, Blotevogel, & Borch, 2014). Average injection water volumes range from
The potential for large quantities of FPW production combined with the widespread spatial overlap between domestic extraction sites and agricultural land (Figure 1) have sparked interest in FPW reuse for agricultural irrigation, especially in drought-stricken areas. Small operations reusing treated and diluted enhanced oil recovery (EOR)-produced water for irrigation are already underway in California (Brost, 2002), and researchers have studied the reuse of coalbed methane FPW for irrigation in Wyoming (Engle, 2011). Additionally, natural gas production in the U.S. is estimated to grow by 65% in the next 30 years, accounting for 39% of the domestic energy production by 2050 (U.S. Energy Information Administration, 2018). The overlap of land uses and predictions of continued energy growth make the evaluation of FPW reuse safety an increasingly important research topic for sustainable water management.
Hydraulic fracturing FPW can contain both additive chemicals and naturally occurring constituents (California Council on Science and Technology [CCST], 2015; Thurman et al., 2014). Recent analytical laboratory studies have shown that of the median 14 additive chemicals used in each hydraulic fracture (U.S. Environmental Protection Agency [U.S. EPA], 2015), biocides and surfactants are identifiable in some FPW samples (Ferrer & Thurman, 2015; Thurman et al., 2014). In addition to chemical additives, naturally occurring contaminants can also be present in FPW (Abualfaraj, Gurian, & Olson; 2014, CCST, 2015; Lester et al., 2015). These naturally occurring constituents include metals, radionuclides, and polycyclic aromatic hydrocarbons. Several constituents brought up from the subsurface in the hydraulic fracturing process, such as the metals evaluated in this research, can potentially pose a health risk when water is not tested or treated appropriately for its end use (Vengosh, Jackson, Warner, Darrah, & Kondash, 2014).
Currently, there is limited published literature addressing the health implications associated with FPW reuse for agricultural irrigation. This investigation is designed to quantify the plant uptake and health impacts associated with concentrations of the naturally occurring toxic metals arsenic and cadmium, which are found in hydraulic fracturing flowback water. Results from this study can be combined with future research evaluating additive chemical uptake in plants to obtain a more thorough understanding of hydraulic fracturing FPW agricultural reuse safety.
Constituent and Crop Selection
A literature review of regional hydraulic fracturing FPW samples formed the basis for constituent and concentration selection. At the time of this experiment, the most extensive compilation of flowback water observations (219 flowback water samples) was documented by four agencies in Pennsylvania, West Virginia, and New York from March 2008-December 2010 (Abualfaraj et al., 2014). Researchers in California also published analytical results from 48 FPW samples (CCST, 2015) and researchers in Colorado published analytical results from one flowback water sample (Lester et al., 2015). Constituents were assessed based on the availability of health data, severity of toxicity, and whether their concentrations were near or exceeded either the national drinking water standards or agricultural water quality thresholds in California (Table 1). Based on the evaluated parameters, arsenic and cadmium were selected for the experiment.
The selected concentrations of 77 [micro]g/L for arsenic and 12 [micro]g/L for cadmium represent the median of 219 arsenic samples and 218 cadmium samples reported in the Northeastern regional study (Abualfaraj et al., 2014). The selected concentrations fall within the concentration range of the 48 FPW samples reported in California (CCST, 2015) and within 15% of the arsenic concentration reported in Colorado (Lester et al., 2015). The widely used corrosion inhibitor tetrasodium ethylenediaminetetraacetic acid (EDTA) was also selected as a constituent of concern based on its ability to act as a chelating agent potentially able to increase the uptake of cationic metals into plants (Chen, Li, & Shen, 2004). EDTA was added to irrigation water at a concentration of 37 mg/L based on the median use concentration reported in the U.S. EPA chemical disclosure registry FracFocus (U.S. EPA, 2015).
The selected chemicals were applied through irrigation water to the staple crop wheat. The incorporation of a staple crop was an integral experimental parameter because it is not easily removed from the human diet. Additionally, due to the high consumption rate of a staple crop, small amounts of chemical contamination can pose a risk to human health. Organic hard red spring wheat (Triticum aestivum) from the Sustainable Grains Company was chosen as the target crop primarily because wheat was reported as the single most consumed grain crop by humans and livestock worldwide (Pimentel & Pimentel, 2008). Additionally, previous research shows that wheat can accumulate selected metals including arsenic (Bhattacharya, Samal, Majumdar, & Santra, 2010) and cadmium (Mortvedt, Mays, & Osborn, 1981). Wheat is also a representative member of the Poales plant order, having the ability to move organic contaminants such as EDTA from their roots to their shoots through acropetal translocation (Collins & Willey, 2009). Hard red spring wheat was specifically selected because it is the dominant type of spring wheat in the U.S., representing 12.6 of the 13.5 million acres of spring wheat planted in 2015 (U.S. Department of Agriculture [USDA], 2015).
Experimental Design and Protocol
The greenhouse experiment consisted of a completely randomized design (CRD) with three treatments and eight replications per treatment. A random number generator was used to develop the CRD grid. The experimental treatments consisted of a control, treatment 1, and treatment 2. The control plants were irrigated with reverse osmosis fertilized water with no hydraulic fracturing flowback water constituents. Treatment 1 plants were irrigated with reverse osmosis fertilized water collected at the greenhouse and amended with arsenic, cadmium, and EDTA in the laboratory. Treatment 2 plants were irrigated with reverse osmosis fertilized water collected at the greenhouse and amended with only arsenic and cadmium in the laboratory.
Ron's Mix soil, available through the University of California, Davis (UC Davis) Orchard Park greenhouse facility, was used throughout the experiment. Sample analysis of the base soil indicated that the total arsenic and cadmium concentrations in the soil were 3.84 and 0.253 mg/kg, respectively. Grains were planted 4 cm apart using templates at a depth of 5 cm. Individual experimental units were planted with 59 seeds and grain was aggregated from the mature wheat plants in each experimental unit after 76 days of growth before laboratory analyses.
Water sample analysis of base water indicated total arsenic and cadmium concentrations were lower than the method detection limits of 0.010 and 0.0020 mg/L, respectively. Fertilizers Growmore 4-18-38...