The work presented in this thesis builds upon previous research related to the biodegradation of organic constituents within hydraulic fracturing fluids (HFF). Hydraulic fracturing fluids, which contain approximately 1% chemical additives, are injected into deep shale plays to create cracks and improve the mobility and extraction of oil and natural gas. Although these fluids contain mostly organic compounds that are considered readily biodegradable as individuals, complex surfactant and glycol formulations are also used within these fluids to function as emulsifiers and wetting agents and may take longer to degrade in environmental systems. There are multiple routes where hydraulic fracturing fluids may be released to shallow subsurface and groundwater aquifers via spills or equipment failures and there is a large concern and uncertainty as to how the organic constituents within HFF will attenuate. If chemical additives within HFF, specifically surfactants, are accidently released to the environment and enter a groundwater aquifer that is used for drinking water, understanding the recalcitrance of these compounds is crucial in assessing the risk to public health. Here, analysis of the anaerobic biodegradation of glycol formulations, in addition to other nonionic surfactants, was conducted using batch soil and groundwater microcosms. Microcosms were used to test the degradation of five HFF related substrates: (1) a synthetic fracturing fluid (SFF) (2) a revert flow stimulation surfactant (3) a corrosion inhibitor (4) polypropylene glycol (PPG), and (5) propylene glycol. Changes in dissolved organic carbon (DOC), surfactant concentration (PPG, C8 and C10 ethoxylated alcohols, nonylphenol ethoxylates), metabolite concentration (propylene glycol, n-propanol, propionaldehyde, propionate, acetone, acetate) and other geochemical parameters (total and ferrous iron, sulfate, sulfide, and chloride) were measured over 50 days. Microbial community analysis and the abundance of key functional genes related to polypropylene glycol degradation were assessed using illumina sequencing.
About 90% of DOC in SFF and propylene glycol were degraded after a week lag period while less overall DOC removal (17% to 82%) occurred the PPG, Revert Flow, and corrosion inhibitor treatments by the end of the 50-day incubation. SFF had the highest maximum DOC degradation rate constant (0.160 day-1). Cell biomass increased 6-14 fold in biotic treatments. Within the SFF and Revert Flow treatments, C8 and C10 ethoxylated surfactants were below detection limits in solution phase by the end of the experiment. Sorption affinity of the ethoxylated alcohols varied; with a higher portion of C10 ethoxylates remaining in abiotic treatments (20-43%) than C8 ethoxylates (0-18%). Polypropylene glycol saw low biodegradation extents in the SFF, Revert Flow and PPG treatments (17%- 20%). Intermediate degradation product acetone increased in the SFF, corrosion inhibitor, and Revert Flow treatments as parent compounds were broken down, but did not accumulate in the propylene glycol treatment, suggesting that anaerobically, acetone is not the preferred degradation pathway for this monomer structure. Instead, propylene glycol was degraded to n-propanol, propionate, and acetate. All treatments showed high microbial diversity that changed little over the course of the experiment. Relative abundance of the diol dehydratase gene (large subunit) increased in the Revert Flow, corrosion inhibitor, SFF, and propylene glycol treatments, but not in the PPG or ambient groundwater treatments. At the end of the experiment, SFF and propylene glycol treatments contained large percentages of Firmicutes (37% and 15%) and Euryarchaeota (9.2% and 16%) phyla. Propylene glycol also contained a large portion of d-proteobacteria which contains several families capable of glycol degradation (Desulfobulbaceae and Syntrophaceae).
This research provides information on the biodegradation and sorption behavior of polypropylene glycols as individual constituents and as ingredients within hydraulic fracturing fluid and fluid additives. The data presented has implications for the attenuation potential of other surfactants in soil systems as well. In some cases, as much as 50% of total losses were attributed to abiotic changes (e.g. sorption) indicating that although these compounds may be removed from solution and not dispersed over great distances, they may not be completely mineralized. Furthermore, these compounds may be remobilized under certain conditions such as the interaction with other hydrophobic organics, which would allow for their transport through soils and surface waters.