Abstract

Strontium isotopes are effective in identifying and quantifying environmental changes due to the discharge of coal bed natural gas (CBNG) produced water. The process of extracting natural gas from coal beds includes the depressurization of the coal through the removal of the coal aquifer water. In the Powder River Basin of northeast Wyoming and southwest Montana, this water is generally discharged to the surface including on- and off-channel impoundments and is used as an irrigation source. Through natural processes including salt dissolution, sulfur reduction and methanogenesis, coal aquifer water attains a sodium-bicarbonate composition. The interaction of a sodium dominated water with the local environment and ecology has the potential to affect the quality of the groundwater and soil physical, chemical, and biological properties.

Strontium isotopes can uniquely fingerprint CBNG produced water as compared to the local surface and groundwater. The ratio of Sr-87 to Sr-86 has been used to identify the presence of CBNG produced water in near surface aquifers, approximate its contribution to the aquifer and estimate the rate of infiltration in an on-channel CBNG discharge pond. Additionally, changes to the ⁸⁷Sr/⁸⁶Sr ratio indicated the dissolution of local salts from the soil of the stream bank. The chemical similarity of strontium to calcium allows the strontium isotope ratio to be used in calcium cycle studies as well. To prevent the sodification of fields irrigated with CBNG produced water, gypsum may be applied to the soil, which is intended to provide additional calcium that would prevent the incorporation of sodium to the soil cation exchange complex. The gypsum strontium isotope ratio is measurably different from the strontium isotope ratio of the local soil. This allows the incorporation of the gypsum source of strontium (and therefore calcium) onto the soil cation exchange complex to be identified and its contribution quantified. Approximately 50% of the strontium on the cation exchange complex at the surface (0-5 cm) of irrigated soils is from the gypsum amendment. Additionally, because the strontium isotope ratio is not fractionated in biological uptake, the incorporation of strontium from gypsum has been identified in vegetation from irrigated fields.