Bowdoin National Wildlife Refuge (NWR) was established in 1936 to provide habitat for breeding and migrating waterfowl. The refuge is located about 11 km to the east of Malta, in the Milk River Valley of Phillips County, Mont. (fig. 1). Characteristic of refuges located in the arid North- Central United States, Bowdoin NWR is at risk to increased salinization and accumulation of trace elements that can threaten wetland biota (DuBois and others, 1992; Nimick, 1997; Laubhan and others, 2006). Bowdoin NWR has been the subject of various investigations and model simulations (Lambing and others, 1988; Hamilton and others, 1989; Kendy, 1999; Bauder and others, 2007) that suggest that salt concentrations in lakes and marshes are increasing and that the long-term accumulation of salts could pose a significant threat to flora and fauna.