Results and lessons learned from a continuous injection tracer test in a small mountain stream receiving acid mine drainage

Metal-laden acid drainage from the reclaimed Landusky open-pit mine is thought to be the major source of environmental degradation to Swift Gulch, a small stream in the Little Rocky Mountains of north-central Montana. Ground water enters Swift Gulch through a series of Fe-stained springs located in a fault zone that extends to the southwest into the mine complex. The contamination threatens the nearby Fort Belknap Indian Reservation downstream. In this study, a detailed evaluation of the water chemistry of Swift Gulch during baseflow conditions was performed to assist in development of remedial plans. A continuous tracer injection test was performed in October 2007 using a combination of LiBr and Rhodamine WT dye. The tracer results were combined with field synoptic sampling of Swift Gulch to provide metal loading profiles for major and trace solutes, including Al, As, Fe(II), Fe(III), Mn, Ni, and Zn, along the entire 1,500 m length of the stream. Evidence from the tracer test indicates that nearly all of the metal contaminants in Swift Gulch enter the stream in a 500 m reach that is coincident with the fault zone. Iron enters the stream chiefly in its reduced Fe(II) state but is quickly oxidized to Fe(III) and then precipitated as ferric oxy-hydroxide. This reaction releases protons, and contributes to a drop in pH of Swift Gulch from values >6 to values <4 downstream. Several problems were encountered during the course of the tracer investigation. The rhodamine dye was unstable at the low pH of the stream and consequently was useless as a conservative tracer. Although Li⁺ was conservative, high background concentrations of this compound compromised its use as a tracer. Br^- proved to be the best tracer, and was used to quantify solute loads in the top 500 m of the drainage. However, even after 48 h of continuous injection, the concentrations of the Br^- tracer did not reach steady-state in the lower reach of the stream. These problems are good examples of the types of complications that arise during tracer tests of this type, and possible solutions are discussed.