STRONTIUM ISOTOPES AS INDICATORS OF REGIONAL VARIATION IN GROUNDWATER SYSTEMS IN AN AREA OF COALBED METHANE PRODUCTION, POWDER RIVER BASIN, WYOMING

Analysis of groundwater samples collected from producing coalbed methane wells and adjacent sand horizons in the Powder River Basin reveal significant regional variation in strontium ratio and geochemistry. In the Decker, Montana area in the northwestern Powder River Basin, coal waters have low ⁸⁷Sr/⁸⁶Sr ratios (0.7079-0.7112). Waters are sodium-bicarbonate type and have relatively high TDS (503-3408 mg/L).

In the Gillette/Wright area of the basin, Wyodak-Anderson coal waters have markedly different strontium characteristics. Coal waters have ⁸⁷Sr/⁸⁶Sr ratios from 0.7114 near the recharge zone to 0.71510 deeper in the basin. TDS of these sodium-bicarbonate type waters (370-1940 mg/L) increase from south to north and east to west. The trends suggest an increase in ⁸⁷Sr/⁸⁶Sr ratio and TDS with increased water residence time.

Sandstone and shale aquifer waters in the Gillette/Wright area have strontium isotope ratios that are distinct from the adjacent coal aquifer waters. Waters from deeper sandstones are sodium-bicarbonate type with ⁸⁷Sr/⁸⁶Sr ratios=0.71258-0.71271; those from shallow sands and shales are calcium-sulfate type with lower ⁸⁷Sr/⁸⁶Sr ratios (0.7108 to 0.7126).

This variation suggests that there is a less radiogenic source of strontium in waters from coal in the north compared to the south, and different strontium sources in coal and sand. The main host of strontium in sandstones is likely carbonate cement while the source in coal appears to be primarily derived from atmospheric dust trapped by vegetation. The proportion of this aerosol source entering groundwater may be a function of residence time. Coal horizons in the south are relatively undisturbed, permitting long flow times along relatively uniform paths. The northern area is marked by northeast trending faults that could serve to streamline water flow through coal horizons, thus limiting time of interaction with strontium-bearing material in coal and resulting in lower ⁸⁷Sr/⁸⁶Sr values.