DEVELOPMENT AND APPLICATION OF A NEAR REAL-TIME IN-SITU ANALYZER TO DETERMINE NA, MG, AND CA IN STREAMS AFFECTED BY COAL BED METHANE DISCHARGE WATERS

Coal bed methane (CBM) development is proceeding rapidly in the Tongue River basin in SE Montana and NE Wyoming, USA. CBM extraction generates large volumes of water with high Na levels (~100-500 mg/L) and these waters are routinely spread over the ground surface, stored in impoundments or discharged into surface drainages. Sodium Adsorption Ratio (SAR=(Na/((Ca+Mg)/2)^1/2)) is a measure of the suitability of irrigation water for sustained soil and crop health and a SAR limit of 3 has been established for the Tongue River during the summer irrigation period. Tongue River CBM discharge water has a SAR value of 30-40 and there is concern that large volumes of CBM discharge water could enter the Tongue River and degrade irrigation water quality. The US Geological Survey (USGS) has initiated a comprehensive monitoring program to examine any changes or trends in surface water quality in the Tongue River and provide real time information to concerned stakeholders and resource managers.

Traditional water quality studies involve field collection followed by laboratory analysis, typically resulting in ~10-20 sample analyses/year. This low frequency sampling cannot provide real-time information and often misses important episodic events such as rainstorms and dam releases that can dramatically affect SAR values and irrigation water quality. To address this problem in the Tongue River watershed, we are developing a relatively low cost (<$3,000) in-situ automated SAR field analyzer for hourly analysis of Na, Mg, and Ca. The SAR field analyzer measures Na by ion selective electrode and determines Mg and Ca by spectrophotometry. The SAR field analyzer is self-calibrating and an onboard computer performs instrument control, data storage, and transfers data to an existing USGS gage station telemetry system. In contrast to traditional sampling, the SAR field analyzer reduces sampling costs, dramatically increases the number of samples analyzed, and provides near real time data. A description of the instrumentation and preliminary in-situ field results will be presented.

The SAR field analyzer will contribute to the USGS assessment of the impact of CBM development in the Tongue River basin and provide near real time water quality information to concerned stakeholders and resource managers.