Water quality assessment downstream of oil and gas produced water discharges intended for beneficial reuse in arid regions

Produced water (PW) is the largest waste stream associated with oil and gas extraction and contains organics, salts, metals and radioactive materials. In the United States, west of the 98th meridian, the National Pollutant Discharge Elimination System exemption allows for release of PW to surface waters for agricultural beneficial reuse if it is “of good enough quality”. Due to the complex and variable composition of PW, the downstream impacts of these releases are not fully understood. In this study, a detailed chemical analysis was conducted on a stream composed of PW 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. Inorganics detected at the discharge were within regulatory effluent limits. While some inorganic species (i.e., strontium, barium and radium) decreased in concentration downstream due to co-precipitation, concentrations of many inorganic species including sodium, sulfate and boron increased due to water evaporation. Consequently, downstream water quality changes need to be considered to adequately evaluate the potential impact of discharged PW. Regulatory health thresholds for humans, livestock, and aquatic species for most chemical species present at the discharge are still lacking. As a result, toxicity tests are necessary to determine the potential health impacts to downstream users.