ISOTOPIC AND GEOCHEMICAL CHARACTERIZATION OF THE POWDER RIVER, WY AND MT: REFINING ASSIMILATIVE CAPACITY MODELS

The Powder River flows from headwaters west of Casper, Wyoming to its confluence with the Yellowstone River in Montana through an area of accelerating coal bed natural gas (CBNG) production. The ground water co-produced with the CBNG is typically discharged to the surface. The State of Montana has set numeric standards for electrical conductivity (EC) and sodium adsorption ratio (SAR) as measured at Moorhead MT USGS gauging station. Discharges of CBNG co-produced water in Wyoming are permitted so as not to exceed these limits using to an assimilative capacity model developed from water quality data collected at Moorhead. However, water quality may vary dramatically along the length of the river because of contributions from tributaries and on short timescales due to precipitation and snowmelt events. In order to better characterize Powder River water quality in space and time we have established 30 sampling sites of the main stem and several tributaries that are sampled at high and low flow, with a subset sampled monthly.

Powder River water is generally sodium-bicarbonate type, with calcium sulfate contributed by soluble salts flushed from the soil during precipitation events. EC is higher following precipitation events, and varies by more than a factor of 2 throughout the year. EC at the headwaters above CBNG development can exceed Montana standards. Although in compliance at Moorhead, SAR has exceeded the Montana standards both at the headwaters and near the confluence at low flow (September 2006). Water with higher SAR enters the main stem of the Powder River from tributaries including Beaver Creek (SAR = 21), although the impact on SAR of the main stem is modest.

Delta-18O and delta-D of Powder River water are isotopically light, indicating that the source of water may be dominated by precipitation from the Bighorn Mountains even in autumn. Many samples plot below the Global Meteoric Water Line, indicating that evaporation has affected the river water, particularly near Broadus, MT.

Results from this study will provide a foundation for assessing the effects of CBNG discharge on surface water chemistry and be useful in developing more detailed assimilative capacity models that protect the river and promote sustainable coalbed methane production.