Journal of Environmental Engineering

Hydraulic fracturing is a method of oil and gas extraction from shale in which substantial volumes of water return to the surface containing chemicals and microorganisms. This paper begins to address the microbial composition and aqueous chemistry and the potential for intrinsic and enhanced bioremediation of these waters. The waters from a gas and oil shale in the Marcellus and Bakken regions, respectively, were analyzed for inorganic elements, organic chemicals, microbial taxonomic composition, and biodegradative capabilities. The waters were highly saline, reaching NaCl concentrations up to 3.5 N, but no significant levels of radioactive elements were detected. More than 1,000 organic compounds were separated and identified by comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry. The major classes of organic compounds, in order of decreasing abundance, were aliphatics, cycloaliphatics, single-ring aromatics, and polycyclic aromatic compounds. The bacterial genera found natively in the waters were identified by sequencing the 16S rRNA genes within the extracted DNA. The major genera identified included strains known to thrive under saline conditions (Halanaerobium, Marinobacter, Oceanimonas, Streptohalobacillus) and degrade petroleum hydrocarbons (Thauera, Pseudomonas, Marinobacterium, Williamsia, Colwellia). Microbial populations were extracted from the Bakken shale waters, encapsulated within silica gels, and then reintroduced into their waters of origin. Both intrinsic biodegradation from the free native microorganisms and enhanced biodegradation with the addition of encapsulated bacteria were observed. In total, this paper begins to better define the properties of waters derived from hydraulic fracturing and suggests a potential for the application of bioremediation to remove organic contaminants.

The development of the Marcellus Shale gas play in Pennsylvania and the northeastern United States has resulted in significant amounts of water and wastes transported by truck over roadways. This study used geographic information systems (GIS) to quantify truck travel distances via both the preferred routes (minimum distance while also favoring higher-order roads) as well as, where available, the likely actual distances for freshwater and waste transport between pertinent locations (e.g., gas wells, treatment facilities, freshwater sources). Results show that truck travel distances in the Susquehanna River Basin are greater than those used in prior life-cycle assessments of tight shale gas. When compared to likely actual transport distances, if policies were instituted to constrain truck travel to the closest destination and higher-order roads, transport mileage reductions of 40–80% could be realized. Using reasonable assumptions of current practices, greenhouse gas (GHG) emissions associated with water and waste hauling were calculated to be 70–157 MT ${\mathrm{CO}}_2\mathrm{eq}$ per gas well. Furthermore, empty so-called backhaul trips, such as to freshwater withdrawal sites or returning from deep well injection sites, were found to increase emissions by an additional 30%, underscoring the importance of including return trips in the analysis. The results should inform future life-cycle assessments of tight shale gases in managed watersheds and help local and regional governments plan for impacts of transportation on local infrastructure.

Flowback water from natural gas extraction in Marcellus Shale contains very high concentrations of inorganic salts and organic chemicals. Potential reuse of this water in subsequent hydraulic-fracturing operations may be limited by high concentrations of divalent cations (e.g., Ba, Sr, and Ca). Kinetics of barite and celestite precipitation in flowback waters from different well sites was evaluated in this study. Ba reacted rapidly with sulfate and reached equilibrium within 30 min, whereas Sr reacted slowly and took days to reach equilibrium. Equilibrium concentrations of Ba and Sr predicted by thermodynamics models were compared with experimental results. Activity corrections based on the Pitzer equation provided the best agreement with experimental data for both Ba and Sr. Comparison of barite and celestite precipitation kinetics in actual and synthetic flowback water revealed that there was no observable impact of organics and other minor components in actual flowback water on barite precipitation rate. This was primarily due to the fact that barite precipitation occurred relatively quickly at the high saturation levels utilized in this study. By contrast, lattice poisoning and complexation with organic matter had a profound impact on the comparatively slower celestite precipitation. The presence of organic matter in actual flowback water increased Ba and Sr concentrations in solution, and contributed to the discrepancy between measured and predicted equilibrium concentrations.

We present the details of a numerical model simulating the migration of pressurized air used for pneumatic drilling of a well in an aquifer. We used an incident that occurred in West Virginia during June 2012 as a basis for making the simulations realistic. We developed a 3D conceptual model using the multipurpose model TOUGH2 to simulate the events during this incident. Input parameters for the model were obtained from field measurements, and a number of reasonable assumptions were made for other parameters. Our results showed that compressed air from a drilling well is capable of creating a high pressure gradient in groundwater at hundreds of meters from the drill hole, even if the air leakage from the drilling well occurs in a confined aquifer, and even if the leakage duration is only 2 h. Therefore, one way to prevent the pressure buildup in the surrounding aquifers is through emplacement of observation wells before drilling, which would alert the drillers to any unusual pressure buildup inside the confined aquifer. However, air leakage in unconfined aquifers seems to have a much smaller spatial extent (less than tens of meters). Sensitivity analysis revealed that air pressure, fracture permeability, and injection time are critical parameters for the propagation of air.

Fossil fuel extraction activities generate wastewaters that are often high in total dissolved solids (TDS) and specific constituents that can affect drinking water, if these wastewaters enter surface waters. Control of TDS in source waters is difficult without identification of the potential sources of high TDS wastewater associated with fossil fuel activities. Characteristics of natural waters, oil and gas-produced waters, and coal-related wastewaters were analyzed to extract information about constituent concentrations and anion ratios. Statistical analysis of the anion ratios indicates that the ${\mathrm{SO}}_4/\mathrm{Cl}$ ratio is higher in coal-related wastewaters than in oil and gas-produced waters, suggesting that wastewaters can be distinguished based on this ratio. An approach that compared the ${\mathrm{SO}}_4/\mathrm{Cl}$ ratio with bromide concentration for the wastewaters can serve to separate oil and gas-produced waters from brine treatment plant discharges, and from the various coal-related wastewaters. This method was applied to surface water quality data collected from two tributaries in Southwestern Pennsylvania from September 2009 to September 2012. Results show that this constituent and ratio method, combined with mixing curve calculations, can be used to identify water quality changes in these two tributaries. Similar mixing models, when applied to regionally relevant high TDS wastewater data, may be used in other areas experiencing water quality changes resulting from fossil fuel extraction activities.

Prerequisite to detailed risk analyses of flowback and produced water management from unconventional resource extraction is the thorough characterization of wastewater management strategies, treatment technologies, prices, and future developments. This expert elicitation compares professional responses on current practices and future trends in wastewater management from Pennsylvania’s Marcellus formation across the oil and gas sector, the wastewater treatment sector, and the regulatory sector. Although expert judgments were highly influenced by the respondent’s role in unconventional resource development, the results of this expert elicitation suggest that water reuse is not inhibited by high concentrations of total dissolved solids, that waste transport accounts for the majority of the cost associated with off-site wastewater treatment and disposal, and that prices for commercial wastewater treatment are likely to drop over the next five years. Taken together, these results indicate that long-term water reuse is a viable strategy for oil and gas wastewater management among companies with continuous or near continuous drilling operations and that future risk analyses of oil and gas wastewater management should concentrate on reuse activity. The results also suggest that expanding economical water reuse practices to companies that drill fewer sequential or spatially clustered wells may require regulatory or policy intervention.