MBMG - Ground Water Investigations Program

Coal Aquifer Contribution to Powder River Basin Streams

Background

Montana coalbed-methane production withdrew 207 million barrels (26,700 acre-feet) of groundwater from coal aquifers from 2007 through 2015. Groundwater withdrawal during coalbed-methane production, upgradient from where the produced coalbed subcrops to streams, has the potential to reduce baseflow to those streams. The ability to quantify the impact to surface water from upgradient energy development will aid in the acquisition of the necessary permits, allowing energy producers to use the coproduced water for beneficial purposes, rather than treat it as a waste product.

Purpose

The purpose of this project was to compare isotope-geochemistry tracing techniques to traditional in-stream flow measurements to identify and quantify the contribution of coal aquifer baseflow to Powder River Basin streams. Isotopes of carbon and strontium were chosen for analysis because they have been shown in the literature to effectively fingerprint the groundwater contribution to surface water in the Powder River Basin.

Project Scope

In the winter of 2012 and 2013, water samples were collected from, and flow rates measured on, Rosebud Creek, Hanging Woman Creek, Otter Creek, the Tongue River, and the Powder River from sites above and below coal outcrops. Groundwater samples were collected from coal aquifers close to where they outcropped. Isotope mixing calculations solved for the percent contribution from coal aquifers to the streams. These values were compared to measured streamflows.

Results

Isotopic tracing of baseflow from coal aquifers was successfully applied to two watersheds in the Powder River Basin: Otter Creek and the Powder River. The Otter Creek site was successful because of the creek’s small size and because it crosses a large coal outcrop. The Knobloch coal aquifer supplies approximately 0.5 cfs to Otter Creek. The Powder River study site was successful because the river crosses numerous small coals over the sampled reach. These numerous small coals supply approximately 6 cfs to the Powder River between the state line and Broadus.

Using isotopes as a coal aquifer baseflow indicator was not successful on Rosebud Creek, the Tongue River, or Hanging Woman Creek. Access to Rosebud Creek was difficult and the wide spacing in the sample collection points missed significant surface tributaries. The high flow rate of the Tongue River prevented the isotopic signature of the baseflow from exceeding the measurement threshold. The natural high salinity of Hanging Woman Creek masked the contribution and the large alluvial valley resulted in dispersing and diluting the signature of the baseflow.