Water in a Dry Land: A Problematic Gift

by Larry C. Munn

By now I realize that virtually every generalization I've heard about water produced from coalbed methane development (CBM) was wrong, at least in part.

Perhaps the most important of these generalizations is that CBM development will provide a plenitude of high-quality water for agriculture in the Powder River Basin. At first glance, it would seem that additional water will be a blessing for agriculture at the dry end of the Great Plains. And for some it may prove to be an asset. But there are significant problems lurking for both agriculturists and the environment as CBM development rapidly expands in northeastern Wyoming.

Salty Soils, Salty Streams

In a nutshell, product water coming from the coal seams of the Powder River Basin's Wasatch and Fort Union formations is to a greater or lesser extent laden with salt. Scientists use electrical conductivity (EC) as a convenient, accurate measure of salinity in water. Salinity in CBM product water produces conductivity readings from 750 to above 4500*. A reading of 750 is already the upper limit recommended for human consumption, and a reading of 1200 creates serious irrigation problems for clayey soils. While livestock and wildlife can tolerate higher salt levels in drinking water after they've acclimatized to it, using CBM product water with salt levels much above the human drinking-water threshold for irrigation will inevitably cause salinization of the soils.

Compounding this hazard is the fact that most of the salt in CBM product water is sodium bicarbonate, which slows the infiltration of water into the soil. The measure traditionally used to evaluate a sodicity hazard is the Sodium Adsorption Ratio (SAR), and SARs above 8 in irrigation water can be problematic. Observed SARs of CBM product water in the Powder River Basin range from less than 1 to greater than 40. High salinity will overcome the reduction in infiltration caused by high SARs, but frankly, you do not want salty water entering the soil anyway.

Compounding Existing Problems

Salty CBM product water used for irrigation may also dissolve and mobilize salts already naturally occurring in the Powder River Basin's soils. This combination of new salt brought to the surface and naturally occurring soil salts will inevitably generate either a marked salinization of soils or salt loading of the streams flowing out of the development area.

Because non-salt adapted (hallophytic) plants must exclude salt at the root surface to prevent direct salt damage when taking up water from the soil, using vegetation to soak up added CBM product water from soils only ensures continued salt build-up over time. Eventually, desirable native plants will die and be replaced by more salt-tolerant plants (like greasewood), weeds or bare ground. Continued watering with salty water requires that the salts be leached below the root zone at least once annually, but this will simply move the salt from the soils to the streams.

An Overwhelming Deluge

Some CBM product water is of good quality and will be welcomed by ranchers in the region. However, much of this water will create a wastewater disposal problem, and should be viewed-and regulated-accordingly. CBM wells will be producing gas and water 24 hours a day, all year long. Ranchers and farmers may be able to use this water for seasonal irrigation, but how will they dispose of it during the rest of the year?

With an estimated 70,000 CBM wells planned for the Powder River Basin, the likely impact of salt loading will be severe. While controlled additions of good-quality water early in the growing season may enhance native plant growth, even irrigation with high-quality snowmelt water will mobilize naturally occurring salts. Once a landscape in a dry climate is salinized or made sodic, it may remain so for hundreds of years or longer.

The Powder River Basin is a part of Wyoming's heritage and has for a long time provided wonderful habitat for grazing animals, both domestic and wild, as well as for humans. I would like to think it will remain so after the activity and money associated with CBM development are gone. But I am worried that the pace and scale of CBM development may simply overwhelm efforts to safeguard the future quality of life in the Powder River Basin.

* The standard conductivity units used to report electrical conductivity are milli-mhos per centimeter or the Systems International equivalent, micro-Siemens per centimeter.

Larry C. Munn is Professor of Soil Science in the Department of Renewable Resources at the University of Wyoming.