Produced water from coal-bed natural gas (CBNG) production may contain sodium bicarbonate (NaHCO₃) at concentrations that can harm aquatic life, according to a new study by the US Geological Survey; Montana Fish, Wildlife and Parks; the Bureau of Land Management and the US Environmental Protection Agency.

The study area included the Tongue and Powder Rivers in Montana and Wyoming, where several types of experiments and assessments were used for 13 aquatic species.

Salts such as sodium bicarbonate are found naturally in the water along the coal-bed natural gas seams. When excessive amounts of this water are discharged into freshwater streams and rivers, however, the results can adversely affect the ability of fish and other aquatic organisms to survive.

The water extracted along with coal-bed natural gas is called produced water. Produced water is a by-product of the coal-bed natural gas extraction. Companies may dispose of produced water in several ways: discharging it directly into watersheds; treating and then discharging it; injecting it into deep wells; discharging it to drip irrigation systems; or capturing it in evaporation ponds. Produced water is not the same as water injected during hydraulic fracturing.

The aquatic species tested had difficulty surviving in waters in which sodium bicarbonate was found at levels from about 1,120 to greater than 8,000 milligrams (mg) of sodium bicarbonate per liter. Results varied across species and depended upon the age of the organism. Chronic toxicity was observed at concentrations that ranged from 450 to 800 mg of sodium bicarbonate per liter. The specific concentration depended on the sensitivity of the four species of invertebrates and fish exposed. The Tongue River has a natural baseline of approximately 280 mg of sodium bicarbonate per liter.

Deionization treatment practices employed in the Tongue and Powder River watersheds appear to reduce the concentrations of sodium bicarbonate and reduce the toxicity of untreated effluent water. Areas with concentrations likely to cause significant mortality in the Tongue and Powder River Basins appear to be limited to tributaries and parts of mixing zones with considerable additions of untreated discharge.

Results from this study may have applications beyond coal-bed natural gas production, according to USGS. Other energy production practices also provide a potential pathway for salts to enter surface water via produced water and sediment transport. Conventional and unconventional (including hydraulic fracturing) oil and gas extraction practices often result in large volumes of produced water that contain elevated salts and dissolved solids from naturally occurring sources. The addition of sulfates and bicarbonates to surface waters may also result from the disturbances associated with mountain top removal mining. The current data can also be used to separate effects of saline discharges from those potentially posed by other constituents.

Additional consultation for this report was provided by the State of Montana, State of Wyoming, Montana State University, University of Wyoming, and others as part of a Powder River Aquatic Task Group.

Resources

• Farag, A.M., and Harper, D.D., eds, 2012, The potential effects of sodium bicarbonate, a major constituent from coalbed natural gas production, on aquatic life: U.S. Geological Survey, Scientific Investigations Report 2012–5008, 101 p.