A CLASS STUDY OF HYDROGEN AND OXYGEN ISOTOPIC COMPOSITION OF NATURAL WATERS IN BEAVERHEAD COUNTY, MONTANA

Students in three Environmental Geochemistry classes at University of Montana Western (2007, 2009, 2011) studied the stable-isotope composition of hydrogen and oxygen in precipitation, surface water, and groundwater in Beaverhead County, Montana. Twelve snow samples and two rain samples were collected to construct a local meteoric water line (LMWL). Linear regression analysis derived an equation for the winter LMWL of ∂D = 7.39∂¹⁸O - 6.62, r² = 0.98, and the addition of two summer rain samples modified the equation considerably, to ∂D = 7.79∂¹⁸O +2.52, r² = 0.99. This work contributes new information and begins to establish a baseline from which to monitor future change, especially as it may relate to climate change. We compared the LMWL from this study with other local and regional analyses, e.g., Gammons et al. (2006) LMWL for Butte, Montana (∂D = 7.31∂¹⁸O – 7.5, r² = 0.987) and Benjamin et. al. (2004), for a broader area of southeastern Idaho, western Wyoming, and southcentral Montana (∂D = 7.95∂¹⁸O +8.09, r² = 0.98).

An additional 29 samples of surface-waters and groundwaters were also analyzed for stable hydrogen and oxygen isotopes. The isotope data, combined with reconnaissance-level analyses of major dissolved ions, contribute to an understanding of surface and groundwater interactions. For example, two local hot springs appear to be sourced by colder/higher-altitude precipitation than other springs and wells. Some wells and warm or hot springs may record interaction of groundwater with ¹⁸O in carbonate rocks and organic H₂S.

This project provided an experiential focus for the class, as students collected samples, did lab chemistry, analyzed their data, and constructed PowerPoint presentations of the results. Because most of the class data is from winter sampling, one of the 2011 students is scheduled to expand the work with a Senior Thesis, conducted in the spring and summer of 2012, in part to determine if there is a seasonal difference in the LMWL.