Search for: All records

Produced water (PW) is the largest waste stream associated with oil and gas (O&G) operations and contains petroleum hydrocarbons, heavy metals, salts, naturally occurring radioactive materials and any remaining chemical additives. In some areas in Wyoming, constructed wetlands (CWs) are used to polish PW downstream of National Pollutant Discharge Elimination System (NPDES) PW release points. In recent years, there has been increased interest in finding lower cost options, such as CWs, for PW treatment. The goal of this study was to understand the efficacy of removal and environmental fate of O&G organic chemical additives in CW systems used to treat PW released for agricultural beneficial reuse. To achieve this goal, we analyzed water and sediment samples for organic O&G chemical additives and conducted 16S rRNA gene sequencing for microbial community characterization on three such systems in Wyoming, USA. Three surfactants (polyethylene glycols, polypropylene glycols, and nonylphenol ethoxylates) and one biocide (alkyldimethylammonium chloride) were detected in all three PW discharges and >94% removal of all species from PW was achieved after treatment in two CWs in series. These O&G extraction additives were detected in all sediment samples collected downstream of PW discharges. Chemical and microbial analyses indicated that sorption and biodegradation were the main attenuation mechanisms for these species. Additionally, all three discharges showed a trend of increasingly diverse, but similar, microbial communities with greater distance from NPDES PW discharge points. Results of this study can be used to inform design and management of constructed wetlands for produced water treatment. 

Oil and gas (O&G) extraction generates large volumes of produced water (PW) in regions that are often water-stressed. In Wyoming, generators are permitted under the National Pollutant Discharge Elimination System (NPDES) program to discharge O&G PW for beneficial use. In one Wyoming study region, downstream of the NPDES facilities exist naturally occurring wetlands referred to herein as produced water retention ponds (PWRPs). Previously, it was found that dissolved radium (Ra) and organic contaminants are removed within 30 km of the discharges and higher-resolution sampling was required to understand contaminant attenuation mechanisms. In this study, we sampled three NPDES discharge facilities, five PWRPs, and a reference background wetland not impacted by O&G PW disposal. Water samples, grab sediments, sediment cores and vegetation were collected. No inorganic PW constituents were abated through the PWRP series but Ra was shown to accumulate within PWRP grab sediments, upwards of 2721 Bq kg −1 , compared to downstream sites. Ra mineral association with depth in the sediment profile is likely controlled by the S cycle under varying microbial communities and redox conditions. Under anoxic conditions, common in wetlands, Ra was available as an exchangeable ion, similar to Ca, Ba and Sr, and S was mostly water-soluble. 226 Ra concentration ratios in vegetation samples, normalizing vegetation Ra to sediment Ra, indicated that ratios were highest in sediments containing less exchangeable 226 Ra. Sequential leaching data paired with redox potentials suggest that oxic conditions are necessary to contain Ra in recalcitrant sediment minerals and prevent mobility and bioavailability. 

Unconventional oil and gas residual solid wastes are generally disposed in municipal waste landfills (RCRA Subtitle D), but they contain valuable raw materials such as proppant sands. A novel process for recovering raw materials from hydraulic fracturing residual waste is presented. Specifically, a novel hydroacoustic cavitation system, combined with physical separation devices, can create a distinct stream of highly concentrated sand, and another distinct stream of clay from the residual solid waste by the dispersive energy of cavitation conjoined with ultrasonics, ozone and hydrogen peroxide. This combination cleaned the sand grains, by removing previously aggregated clays and residues from the sand surfaces. When these unit operations were followed by a hydrocyclone and spiral, the solids could be separated by particle size, yielding primarily cleaned sand in one flow stream; clays and fine particles in another; and silts in yet a third stream. Consequently, the separation of particle sizes also affected radium distribution – the sand grains had low radium activities, as lows as 0.207 Bq g −1 (5.6 pCi g −1 ). In contrast, the clays had elevated radium activities, as high as 1.85–3.7 Bq g −1 (50–100 pCi g −1 ) – and much of this radium was affiliated with organics and salts that could be separated from the clays. We propose that the reclaimed sand could be reused as hydraulic fracturing proppant. The separation of sand from silt and clay could reduce the volume and radium masses of wastes that are disposed in landfills. This could represent a significant savings to facilities handling oil and gas waste, as much as $100 000–300 000 per year. Disposing the radium-enriched salts and organics downhole will mitigate radium release to the surface. Additionally, the reclaimed sand could have market value, and this could represent as much as a third of the cost savings. Tests that employed the toxicity characteristic leaching protocol (TCLP) on these separated solids streams determined that this novel treatment diminished the risk of radium mobility for the reclaimed sand, clays or disposed material, rendering them better suited for landfilling. 

High concentrations of barium (Ba), strontium (Sr) and radium (Ra) are present in both the liquid and suspended solid portions of wastewater produced from hydraulic fracturing. These high concentrations often require special treatment in which the solid and liquid portions are separated and then independently treated prior to disposal. The solids are typically disposed in landfills while the liquids are further treated, recycled for future hydraulic fracturing, or disposed via injection wells. Finding optimal treatment methods of both the solid and the liquid fractions requires a thorough understanding of potential Ra mobility from both the raw suspended solids and mineral precipitates formed during treatment. Using a sequential extraction procedure, we found that, without treatment, more than 50% of Ra-226 in the suspended solids was associated with soluble salts and readily exchangeable fractions. When the liquid portion of the wastewater was treated by mixing with acid mine drainage (AMD), which contained high sulfate concentrations, approximately 80–97% of the total Ra-226 in the mixture solution is found in the insoluble sulfate fraction of the precipitate. The activity of Ra-226 sequestered in the precipitated solid sulfate fractions is positively correlated with the Sr/Ba ratio of the wastewater-AMD solution. We discuss implications of these findings for effective long-term management of elevated radium in both solid and liquid wastes. 

In the western U.S., produced water from oil and gas wells discharged to surface water augments downstream supplies used for irrigation and livestock watering. Here we investigate six permitted discharges on three neighboring tributary systems in Wyoming. During 2013–16, we evaluated radium activities of the permitted discharges and the potential for radium accumulation in associated stream sediments. Radium activities of the sediments at the points of discharge ranged from approximately 200–3600 Bq kg −1 with elevated activities above the background of 74 Bq kg −1 over 30 km downstream of one permitted discharge. Sediment as deep as 30 cm near the point of discharge had radium activities elevated above background. X-ray diffraction and targeted sequential extraction of radium in sediments indicate that radium is likely coprecipitated with carbonate and, to a lesser extent, sulfate minerals. PHREEQC modeling predicts radium coprecipitation with aragonite and barite, but over-estimates the latter compared to observations of downstream sediment, where carbonate predominates. Mass-balance calculations indicate over 3 billion Bq of radium activity ( 226 Ra + 228 Ra) is discharged each year from five of the discharges, combined, with only 5 percent of the annual load retained in stream sediments within 100 m of the effluent discharges; the remaining 95 percent of the radium is transported farther downstream as sediment-associated and aqueous species.