Assessment of water quality, toxicity and treatment strategies downstream of NPDES oil and gas produced water discharges intended for beneficial reuse
Date
2019
Authors
McLaughlin, Molly Cook, author
Borch, Thomas, advisor
Blotevogel, Jens, advisor
Argueso, Juan Lucas, committee member
Mouser, Paula, committee member
Sale, Tom, committee member
Journal Title
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Abstract
Produced water is the largest waste stream associated with oil and gas operations. This complex fluid contains petroleum hydrocarbons, heavy metals, salts, naturally occurring radioactive materials (NORMs) and any remaining chemical additives. In the United States, west of the 98th meridian, the federal National Pollutant Discharge Elimination System (NPDES) exemption allows release of produced water for agricultural beneficial reuse if it is of "good enough quality." Due to the complex and variable composition of produced water as well as the variations in permit effluent limits and treatment approaches, the downstream impacts of NPDES produced water releases are not fully understood. The goal of this dissertation was to determine if the current NPDES produced water permit effluent limits are adequate and if not, to identify additional steps that can be taken to improve water quality. As a first step towards this goal, a detailed chemical and toxicological analysis was conducted on a stream composed of produced water released for agricultural beneficial reuse. Over 50 geogenic and anthropogenic organic chemicals not specified in the effluent limits were detected at the discharge including hydrocarbons, halogenated compounds, and surfactants. Most were removed within 15 km of the discharge due to volatilization, biodegradation, and sorption to sediment. Additionally, the attenuation rate increased substantially in a wetland downstream of the discharge point. Tens of inorganic species were also detected in the watershed, including many sourced from produced water. In contrast to organic chemicals, the concentrations of most inorganic species increased downstream due to water evaporation. This included contaminants of concern such as boron, selenium and total dissolved solids (TDS). An assessment of regulatory health thresholds revealed that eight of the organic species detected at the discharge were listed by the U.S. Environmental Protection Agency (EPA) and the International Agency for Research on Cancer (IARC) to be known, probable or possible carcinogens. Mutagenicity of this water was assessed using a yeast mutation assay that analyzed copy number variation (CNV) duplications, CNV deletions, forward point mutations and reversion point mutations. These mutations are established as having a role in human disease, including cancer. Higher rates of mutation were observed at the discharge point and decreased with distance downstream. This correlated with the concentrations of known carcinogens detected in the stream including benzene and radium. Mutation rate increases were most prominent for CNV duplications and were higher than mutation rates observed in mixtures of known composition containing all detected organic carcinogens in the discharge. In addition, samples were evaluated for acute toxicity in Daphnia magna and developmental toxicity in zebrafish (Danio rerio). Acute toxicity was minimal, and no developmental toxicity was observed. Finally, in response to the observation that attenuation of organic chemicals increased in wetlands, constructed wetlands downstream of three different NPDES produced water discharges, including the discharge of focus in the chemical and toxicological analysis, were evaluated for their viability to polish produced water. The results showed that wetlands are effective at attenuating commonly used non-ionic surfactants, as well as a commonly used biocide. Attenuation was not only due to degradation, but also accumulation in sediments. Sediment accumulation has the potential to limit the lifetime of the wetlands or increase the frequency with which sediment must be excavated. The results of this dissertation identified multiple improvements that can be made to NPDES produced water regulations. Current regulations apply to the discharge site only. This dissertation shows that downstream changes in water quality must be considered to adequately evaluate potential impacts of produced water discharges, as exemplified by the increasing concentrations of inorganic species downstream. Secondly, toxicological results showed that chemical analysis alone is insufficient to assess impacts of these releases and that a thorough assessment of chronic toxicity is necessary to fully assess produced water for beneficial reuse. Current regulations require acute toxicity testing, but no assessment of chronic toxicity. Finally, prior to widespread implementation of constructed wetlands for produced water treatment, additional research is needed to assess the impact of oil and gas chemical additives on the maintenance schedules of these systems, as well as the long-term impact to soil health. If these waters can be reused safely and economically, many stakeholders stand to benefit. If this practice is expanded prematurely, the quality and health of water, soil, crops and downstream users could be negatively impacted. The research contained in this dissertation is one step in a life-cycle analysis of the costs, impacts and benefits associated with oil and gas extraction.
Description
Rights Access
Subject
chemical fate and transport
produced water
beneficial reuse
toxicity
oil and gas