Measurements of organic compound emissions from a produced water disposal vault.

We measured organic compound emissions from a produced-water, evaporative disposal facility's oil-water separation vault in May 2022 and March-May 2023. Produced water is water pulled from the subsurface of a well along with the oil and natural gas; some produced water is disposed of by allowing it to evaporate from surface impoundments. The vault measured in this study separated residual oil from produced water before evaporative disposal. Because the vault's surface contained many potential small emission sources, we used a large plastic chamber to cover the entire vault and simultaneously capture all emissions. We also measured organic compounds in ambient air upwind and downwind of the vault and estimated emissions via a backward Lagrangian stochastic model (Windtrax). The total non-methane organic compound (TNMOC) emission rate from the vault ranged from 0.27 to 3.05 kg/h, averaging 1.99 kg/h in 2022 and 0.49 kg/h in 2023. The average TNMOC emission rate determined by the bLS method was 48% higher than the emission rate determined by the chamber method in 2023 (average of 0.73 kg/h). Still, the range of the chamber results fell within the range of TNMOC emissions from the model. Methanol emissions were much higher than the bLS method, averaging 85.3 g/hr, but were highly variable. We surmise that the water condensation on the chamber retained methanol and biased the results low. The extrapolated annual average emissions of methane, TNMOC, and methanol from the vault were 0.1, 15.5, and 1.4 U.S. tons/yr, respectively, within the range of emissions from uncontrolled oil storage tanks. The extrapolation considers bias in the chamber method and differences across the two years of measurements.Implications: The findings from our study indicate that emissions of non-methane organic compounds (TNMOC) from the oil-water separation vault at the produced-water evaporative disposal facility exhibit significant variability between years, with a notable decline in average emissions from 2022 to 2023. The higher emission rates recorded using the backward Lagrangian stochastic (bLS) model compared to the chamber method suggest that further investigation into measurement techniques is warranted to ensure accurate assessments of emissions. Additionally, the substantial variability in methanol emissions highlights the need for more controlled conditions during sampling to avoid potential biases. Overall, these results imply that while emissions from the vault are within the range of those from uncontrolled oil storage tanks, there is an ongoing necessity for improved monitoring and regulatory practices to mitigate environmental impacts associated with produced water disposal.