Title:

Temporal Toxicity in Hydraulic Fracturing Wastewater from Black Shale Natural-Gas Wells Influenced by Production Times and Chemical Additives

The process of extracting hydrocarbon resources from low permeability formations such as black shales using horizontal drilling and hydraulic fracturing techniques is increasingly utilized around the globe. The hydraulic fracturing process generates large volumes of wastewater fluids known as flowback and produced waters (FPW). FPW contains a wide range of organic and inorganic constituents derived from both xenobiotic and geogenic origin, including known toxicants. Here, we assess the toxicity of 42 input media and produced fluid samples collected from four wells in the Utica Formation and Marcellus Shale using two distinct endpoint tests. Broad spectrum toxicity was assessed using a BioLuminescence Inhibition Assay employing the halotolerant bacterium Aliivibrio fischeri, while human cytotoxicity was evaluated using a N-acetylcysteine (NAC) thiol reactivity assay. The acute toxicity and thiol reactivity of early-stage flowback was higher than later produced fluids, with levels diminishing through time as the natural gas wells matured. These data show that both acute toxicity and cytotoxicity persists in produced waters up to nine months after hydraulic fracturing, with toxicity levels heavily influenced by specific well additives, as opposed to sample fraction or shale formation.

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