Amphibians Exposed to Oil and Gas Co-Produced Wastewaters: Differentiating the Actual and the Perceived Inorganic Contaminant Hazards — Prairie Pothole Region Completed

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Energy production is a national priority that has been addressed in part through rapid increases in domestic oil and gas production. During resource extraction of oil and gas, brines is rich in chloride and other major ions, metals, and other chemicals, are co-produced. The Williston Basin, one of North America's largest oil production areas, overlaps the Prairie Pothole Region, an area of small, shallow, isolated wetlands that offers important habitat for aquatic species and migratory birds. Co-produced wastewaters, resulting from spills or historical oil and gas extraction disposal practices, persist in some of the wetlands, resulting in altered water and sediment chemistry. Because the brines from the Williston Basin generated during oil and gas production can contain high levels of chloride, it is often the primary element of concern for biological effects.

Although the effects of brines in general, and chloride specifically, on amphibians have been documented in wetlands affected by runoff of road salts in urban areas, there is little information about effects on resident wildlife including amphibians in wetlands near oil and gas production. Amphibians were chosen as the focus of the study because their populations are declining worldwide, and they are sensitive to changes in the water and sediment chemistry owing to their permeable skin. Understanding the effects of multiple stressors on amphibian survival is important to determine if mitigation or restoration is needed, and if it is needed, to prioritize strategies to protect wildlife health to continue sustainable energy development.

Therefore, U.S. Geological Survey (USGS) Energy Science Team and Amphibian Research and Monitoring Initiative scientists completed field studies to determine if the larval amphibian abundance of boreal chorus frogs (Pseudacris maculata), barred tiger salamanders (Ambystoma mavortium), and northern leopard frogs (Lithobates [Rana] pipiens) were related to two potential stressors: chloride concentrations in wetlands affected by historic discharges of oil and gas co-produced wastewaters and metals concentrations in amphibian tissues and wetland sediments.

Field studies were completed during 2015 to 2017 at 33 freshwater wetlands primarily affected by historic disposal of oil and gas wastewaters in the Prairie Pothole Region. Few (less than 5 percent) of the surface waters in the region are naturally dominated by chloride, so scientists were able to identify wetlands affected by historic oil and gas wastewaters by measuring chloride concentrations and specific conductance, and by reviewing historic and current well locations.

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Chloride concentrations in the wetlands sampled ranged from 2.6 to 17,150 milligrams per liter, and 20 of the 33 wetlands had mean concentrations that exceeded 230 milligrams per liter, which is the U.S. Environmental Protection Agency’s Aquatic Life Criteria for chronic chloride toxicity. Amphibian abundance was lower in wetlands with greater chloride concentrations for all three species studied. The abundance of boreal chorus frog larvae declined most rapidly in response to increased chloride concentrations, followed by the northern leopard frog and barred tiger salamander, highlighting differences in species sensitivity.

Metals have also been identified as co-contaminants in wastewaters and are perceived hazards for biota, but in this study most of the metals detected in sediments and amphibian tissues were not directly linked with the higher chloride concentrations, indicating that they originated from other sources. Exceptions include sodium and strontium in wetland sediments, and selenium and vanadium in amphibian tissues. Metal tissue concentrations were higher in tadpoles that graze compared to predatory salamanders, indicating frequent contact with the sediments could lead to greater ingestion of metal-laden materials. Metals in the sediments and tissues represent an additional potential stressor, other than elevated chloride concentrations from oil and gas brines that may have influenced amphibian abundance and survival.

The results of this combination of studies provide information to understand some of the many possible stressors, including chloride and metals, that may affect resident amphibians in the environment. Questions remain about the combined effects of pathogens, other contaminants, and physical stressors on amphibians and other aquatic organisms. The Energy Science Team of the USGS continues to address these and other questions related to toxicant exposure.

The USGS Toxic Substances Hydrology and Contaminant Biology combined programs, as well as the Amphibian Research and Monitoring Initiative, supported these integrated science studies.