Is Food Irrigated with Oilfield-Produced Water in the California Central Valley Safe to Eat? A Probabilistic Human Health Risk Assessment Evaluating Trace Metals Exposure

Pathways for Potential Exposure to Onshore Oil and Gas Wastewater: What We Need to Know to Protect Human Health

Produced water is a chemically complex waste stream generated during oil and gas development. Roughly four trillion liters were generated onshore in the United States in 2021 (ALL Consulting, 2022, https://www.gwpc.org/wp-content/uploads/2021/09/2021_Produced_Water_Volumes.pdf). Efforts are underway to expand historic uses of produced water to offset freshwater needs in water-stressed regions, avoid induced seismic activity associated with its disposal, and extract commodities. Understanding the potential exposures from current and proposed produced water uses and management practices can help to inform health-protective practices. This review summarizes what is known about potential human exposure to produced water from onshore oil and gas development in the United States. We synthesize 236 publications to create a conceptual model of potential human exposure that illustrates the current state of scientific inquiry and knowledge. Exposure to produced water can occur following its release to the environment through spills or leaks during its handling and management. Exposure can also arise from authorized releases, including permitted discharges to surface water, crop irrigation, and road treatment. Knowledge gaps include understanding the variable composition and toxicity of produced water released to the environment, the performance of treatment methods, migration pathways through the environment that can result in human exposure, and the significance of the exposures for human and ecosystem health. Reducing these uncertainties may help in realizing the benefits of produced water use while simultaneously protecting human health.

Treated water from oil and gas extraction as an unconventional water resource for agriculture in the Anadarko Basin

The energy industry generates large volumes of produced water (PW) as a byproduct of oil and gas extraction. In the central United States, PW disposal occurs through deep well injection, which can increase seismic activity. The treatment of PW for use in agriculture is an alternative to current disposal practices that can also provide supplemental water in regions where limited freshwater sources can affect agricultural production. This paper assesses the potential for developing PW as a water source for agriculture in the Anadarko basin, a major oil and gas field spanning parts of Kansas, Oklahoma, Colorado, and Texas. From 2011 to 2019, assessment of state oil and gas databases indicated that PW generation in the Anadarko Basin averaged 428 million m3/yr. A techno-economic analysis of PW treatment was combined with geographical information on PW availability and composition to assess the costs and energy requirements to recover this PW as a non-conventional water resource for agriculture. The volume of freshwater economically extractable from PW was estimated to be between 58 million m3 per year using reverse osmosis (RO) treatment only and 82 million m3 per year using a combination of RO and mechanical vapor compression to treat higher salinity waters. These volumes could meet 1-2 % and 49-70 % of the irrigation and livestock water demands in the basin, respectively. PW recovery could also modestly contribute to mitigating the decline of the Ogallala aquifer by ~2 %. RO treatment costs and energy requirements, 0.3-1.5 $/m3 and 1.01-2.65 kWh/m3, respectively, are similar to those for deep well injection. Treatment of higher salinity waters increases costs and energy requirements substantially and is likely not economically feasible in most cases. The approach presented here provides a valuable framework for assessing PW as a supplemental water source in regions facing similar challenges.

Historic and Contemporary Surface Disposal of Produced Water Likely Inputs Arsenic and Selenium to Surficial Aquifers

Oil and gas development generates large amounts of wastewater (i.e., produced water), which in California has been partially disposed of in unlined percolation/evaporation ponds since the mid-20th century. Although produced water is known to contain multiple environmental contaminants (e.g., radium and trace metals), prior to 2015, detailed chemical characterizations of pondwaters were the exception rather than the norm. Using a state-run database, we synthesized samples (n = 1688) collected from produced water ponds within the southern San Joaquin Valley of California, one of the most productive agricultural regions in the world, to examine regional trends in pondwater arsenic and selenium concentrations. We filled crucial knowledge gaps resulting from historical pondwater monitoring by constructing random forest regression models using commonly measured analytes (boron, chloride, and total dissolved solids) and geospatial data (e.g., soil physiochemical data) to predict arsenic and selenium concentrations in historical samples. Our analysis suggests that both arsenic and selenium levels are elevated in pondwaters and thus this disposal practice may have contributed substantial amounts of arsenic and selenium to aquifers having beneficial uses. We further use our models to identify areas where additional monitoring infrastructure would better constrain the extent of legacy contamination and potential threats to groundwater quality.

Potential for Biomass Production and Remediation by Cultivation of the Marine Model Diatom Phaeodactylum tricornutum in Oil Field Produced Wastewater Media

While oilfield produced water (PW) is one of the largest, unclaimed wastewater streams of the oil industry, it could potentially be used as a cultivation medium for microalgae. Microalgae could help with the remediation of this water while also delivering biomass that can be transformed into valuable byproducts such as biofuels. The coupling of these two purposes is expected to cut production costs of biofuels while aiding environmental protection. In this study, we compared the cultivation capacity of the marine model diatom Phaeodactylum tricornutum in media at varying salinities and in media composed of PW from two oilfields in the Central Valley of California that differed drastically in the concentration of inorganic and organic constituents. Specifically, we measured the carrying capacity of these media, the maximum growth rates of P. tricornutum, its cellular lipid accumulation capacity, and its capacity to remediate the most polluted PW source. Our study shows that P. tricornutum can successfully adjust to the tested cultivation media through processes of short-term acclimation and long-term adaptation. Furthermore, the cultivation of P. tricornutum in the most heavily polluted PW source led to significant increases in cell yield and improved photosynthetic capacity during the stationary phase, which could be attributed chiefly to the higher levels of nitrate present in this PW source. Chemical water analyses also demonstrated the capability of P. tricornutum to remediate major nutrient content and potentially harmful elements like fluorine and copper. Because P. tricornutum is amenable to advanced genetic engineering, which could be taken advantage of to improve its cultivation resilience and productivity in an economic setting, we propose this study as a step towards essential follow-up studies that will identify the genetic regulation behind its growth in oilfield PW media and its remediation of the PW constituents.