Coal aquifer contribution to streams in the Powder River Basin, Montana

- Coalbed methane and coalmine production can reduce groundwater baseflow to streams.
- Baseflow from coal aquifers is quantified using geochemical and isotope signatures.
- Quantification was successful in two southeastern Montana streams.

SummaryGroundwater contributions to streams can be reduced by groundwater withdrawal associated with coalbed methane and coal mine production. Quantifying the groundwater contribution to streams aids the assessment of potential impacts to in-stream flow and provides information necessary for energy producers to use coproduced water for beneficial purposes, rather than treating it as a waste product. Stream flow, field parameters, common ions, and isotopes of carbon and strontium were measured on Otter Creek and the Powder River in southeastern Montana. Direct streamflow measurements were ineffective because of the magnitude and nature of coalbed contribution. The coal groundwater contribution did not exceed the geochemical detection threshold on two nearby streams. Geochemical models based on isotopic data proved to be the most effective analytical method, resulting in baseflow measurements from coal aquifers of 28-275 l s−1.