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The sorption characteristics of inorganic and organic compounds found in hydraulic fracturing flowback and produced water: Implications for fate and transport
- Author / Creator
- Funk, Sean P
In recent years, due to increasing energy demands, hydraulic fracturing operations for recovering unconventional hydrocarbon resources has increased. Wastewater recovered is referred to as flowback and produced water (FPW), and is often saline, contains numerous organic and inorganic constituents, and may pose threats to groundwater resources. Hundreds of spills of FPW have been reported to the Alberta Energy Regulator each year. As such, the environmental risk that FPW may pose to shallow groundwater environments has emerged as a major concern in several jurisdictions. Recently, samples of FPW derived from hydraulic fracturing of the Duvernay Formation, near the Fox Creek, AB region, were characterized and found to contain a previously unidentified class of aryl phosphates (including diphenyl phosphate (DPP), triphenyl phosphate (TPP), among others). As an emerging contaminant in soil and groundwater systems, as well as being potentially harmful to aquatic ecosystems, it is important to determine the environmental fate of these aryl phosphates if spilled in near-surface environments. Batch sorption experiments and bioassay toxicological studies were conducted on DPP, with the aim of determining: 1) the sorption behavior of DPP onto various surficial sediments collected within the Fox Creek, AB region; and 2) the toxicity of DPP toward aquatic ecosystems. We report that the sorption of DPP onto both clay-rich soils and sandy sediment was low compared to that of other aryl phosphates, with an average log KOC value of 2.30 ± 0.42 (1σ). Therefore, the transport of DPP in groundwater would be rapid due to its low degree of sorption on surficial materials. We also determined the acute 96 h-LC50 of DPP on zebrafish embryos to be 50.0 ± 7.1 mg/L. From the results of our study, we infer that DPP may pose an environmental risk to aquatic ecosystems if released into the environment.
FPW is a highly complex fluid, containing numerous inorganic and organic constituents. Contaminants within this complex fluid may interact with each other, either synergistically or antagonistically, resulting in varied sorption behavior. Groundwater flow may also have an influence on how these contaminants sorb onto porous media. To quantify these impacts, batch and column experiments were conducted using FPW collected from hydraulic fracturing operations in the Duvernay Formation with soils collected from Fox Creek, AB. In our batch experiments, we found that for many of the dissolved inorganics (e.g. Sr, Cu, Ni), sorption was depressed relative to published literature values, likely a result of competition with other constituents within the fluid and the high ionic strength of the FPW. Differential sorption of polyethylene glycols (PEGs) was observed, with sorption increasing with increasing ethylene oxide numbers. Column experiments were also conducted to address how groundwater flow may influence sorption. Some dissolved inorganic constituents (e.g. Sr, Li, B) and PEGs were observed to exhibit depressed sorption compared to the batch experiments. We argue that sorption is non-instantaneous, characterized by two stages: a fast adsorption phase onto external sorption sites, followed by a slow absorption phase into internal sorption sites. Flow likely negated the latter process. However, the heavy metals (e.g. Cu) were observed to exhibit enhanced retention in the presence of flow. We argue that because the column experiments were conducted under anaerobic conditions, precipitation of heavy metal-bearing solids (possibly sulphides) may have acted to sequester these elements.
Modeling how contaminants are transported in the subsurface is a major problem that faces many hydrogeologists. To aid with this, we developed HYDROSCAPE, a MATLAB®-based software program that uses an analytical solution to the advection-dispersion equation to solve solute transport problems. The solution is heuristically modified in two important, novel ways, allowing the user to: 1) customize the source region; and 2) implement horizontal geological units within the domain (“simple geology”). Using HYDROSCAPE, three simple spill scenarios were simulated. The first two scenarios involved the introduction of FPW into an alluvial aquifer, the first with a source that is active for only 96 h, the second with a source that is continuous. The third scenario involved the downward percolation of FPW through an organic-rich, clay-rich soil horizon. Our results demonstrate the importance of considering source region history, the impact groundwater flow has on contaminant residence time, and how co-contaminant interactions affect the arrival order of contaminants in the subsurface.
- Graduation date
- Fall 2019
- Type of Item
- Doctor of Philosophy
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