Surface spills are believed to be the most likely transport pathway for oil- and gas-related organic compounds to contaminate groundwater resources. In the Denver-Julesburg (DJ) Basin of Colorado, ~90 spills are reported annually that contaminate groundwater with either benzene, toluene, ethylbenzene or xylenes (BTEX). Of these spills, produced water was released in &gt;60%. In the DJ Basin, flowback and produced waters from unconventional wells typically have high BTEX concentrations (&gt;10 ppm) and ppm-range concentrations of the surfactants polyethylene glycols (PEGs), polypropylene glycols (PPGs) and linear alkyl ethoxylates (LAEs).<br/><br/>The objective of this study was to measure the biodegradation kinetics of the BTEX compounds and three classes of surfactants (PEGs, PPGs, and LAEs) under conditions simulating a surface spill of flowback water. Groundwater microcosms were constructed using a 1:5 ratio of alluvial aquifer sediment to a groundwater solution. A 14-day flowback water collected directly from the separator tank of a DJ Basin production well was mixed at a 1:10 ratio with a synthetic groundwater. The synthetic groundwater was consistent with the local alluvial aquifer and had high concentrations of nitrate (17 ppm) and sulfate (450 ppm). Microorganisms present in the system were either native to the sediment or present in the flowback sample, which was not filtered or sterilized. A control sterilized with sodium azide was used to distinguish biodegradation. All microcosm solutions were initially oxic, capped under normal atmospheric conditions (10% v/v headspace), and allowed to proceed to more reducing conditions. The BTEX compounds were entirely degraded within 4 d under mixed oxic and nitrate-reducing conditions. The PEGs and LAEs were 80 – 90% degraded in 2 d under oxic conditions and persisted at low levels (10-15% remaining) under anoxic conditions (4 – 47 d). The PPGs were found to be more persistent: 30 – 40% transformed to carboxylated degradates under oxic conditions, and no additional removal was observed under anoxic conditions. These results indicate that the BTEX compounds may be rapidly biodegraded under shallow oxic groundwater conditions in the event of a spill of flowback water. Additionally, this study improves our knowledge of the environmental behavior of PPG surfactants, which may have applications as indicator compounds of oil- and gas-related wastewaters accidentally released to groundwater, particularly under anoxic conditions.

Biodegradation of organic additives and hydrocarbons from flowback water of the Denver-Julesburg Basin: A laboratory study of shallow groundwater conditions

253rd National Meeting (2017)

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