Abstract

The production water from coalbed natural gas (CBNG) extraction contains many constituents. The U.S. Environmental Protection Agency has established aquatic life criteria for some of these constituents, and it is therefore possible to evaluate their risk to aquatic life. However, of the major ions associated with produced waters, chloride is the only one with an established aquatic life criterion (U.S. Environmental Protection Agency, 1988).

The focus of this research was NaHCO₃, a compound that is a major constituent of coalbed natural gas produced waters in the Tongue and Powder River Basins. This project included laboratory experiments, field in situ experiments, a field mixing zone study, and a fishery presence/absence assessment.

Though this investigation focuses on the Tongue and Powder River Basins, the information is applicable to other watersheds where sodium bicarbonate is a principle component of product water either from CBNG or from traditional or unconventional oil and gas development. These data can also be used to separate effects of saline discharges from those potentially posed by other constituents. Finally, this research effort and the additional collaboration with USGS Water Resources and Mapping, Bureau of Land Management, US Environmental Protection Agency, State of Montana, State of Wyoming, Montana State University, University of Wyoming, and others as part of a Powder River Aquatic Task Group, can be used as a model for successful approaches to studying landscapes with energy development.

The laboratory acute toxicity experiments were completed with a suite of organisms, including 7 species of fish, 5 species of invertebrates, and 1 amphibian species. Experiments performed on these multiple species resulted in LC50s that ranged from 1,120 to greater than (>) 8,000 milligrams sodium bicarbonate per liter (mg NaHCO₃/L) (also defined as 769 to >8,000 milligrams bicarbonate per liter (mg HCO₃-/L) or total alkalinity expressed as 608 to >4,181 milligrams calcium carbonate per liter (mg CaCO₃/L)) that varied across species and lifestage within a species. The age at which fish were exposed to NaHCO₃ significantly affected the severity of toxic responses for some organisms. The chronic toxicity of NaHCO₃ was defined in experiments that lasted from 7–60 days post-hatch. For these experiments, sublethal effects such as growth and reproduction, in addition to significant reductions in survival were included in the final determination of effects. Chronic toxicity was observed at concentrations that ranged from 450 to 800mg NaHCO₃/L (also defined as 430 to 657 mg HCO₃-/L or total alkalinity expressed as 354 to 539 mg CaCO₃/L) and the specific concentration depended on the sensitivity of the four species of invertebrates and fish exposed. Sublethal investigations during chronic studies revealed percent decrease in the activity of sodium-potassium adenosine triphosphatase (Na/K ATPase, an enzyme involved in ionoregulation) and the age of the fish at the onset of the decrease may affect the ability of fathead minnow to survive exposures to NaHCO₃. A database of toxicity evaluations of NaHCO₃ on aquatic life has been constructed. Using these data, sample acute and chronic criteria of 459 and 381 mg NaHCO₃/L, respectively, can be calculated for the protection of aquatic life. The final derivation and implementation of such criteria is, of course, left to the discretion of the concerned management agencies.

A combination of in situ experiments, static-renewal experiments performed simultaneously with in situ experiments, and static renewal experiments performed with site water in the laboratory, demonstrated that untreated coalbed natural gas (CBNG) product water from the Tongue and Powder River Basins reduces survival of fathead minnow and pallid sturgeon. More precisely, the survival of early-lifestage fathead minnow, especially those less than 6-days post hatch (dph), likely is reduced significantly in the field when concentrations of NaHCO₃ rise above 1,500 mg/L. However, age was not a factor for pallid sturgeon and they were sensitive to product water regardless of age.

Treatment with the Higgins Loop™ technology and dilution of untreated water increased survival in the laboratory. Both of these situations reduced ammonia in addition to the concentrations of NaHCO₃. These experiments addressed the acute toxicity of effluent waters being added to the main stem rivers, but did not address issues related to the volumes of water that may be added to the watershed. Mixing zones of the three outfalls studied ranged from approximately 800–1,200 m below the confluence and the areas within these mixing zones with acutely lethal concentrations of NaHCO₃ (as defined by the presence of concentrated dye) are limited. The areas with concentrations of NaHCO₃ more than the concentrations likely to cause significant mortality, and more than the calculated sample water-quality criteria in the Tongue and Powder River Basins appear to be limited to tributaries and parts of mixing zones with considerable additions of untreated effluent.

First posted June 14, 2012