An investigation of acidic head-water streams in the Judith Mountains, Montana, USA

• Three small streams transition from strongly acidic to neutral pH.
• Acid drainage is caused by natural weathering of pyrite-rich igneous intrusions.
• Concentrations of Al, Cd, Cu, Pb, Tl, Zn exceed water quality standards.
• Ferricrete deposits have similar metal chemistry to modern in-stream sediment.
• The study area is a useful reference site for mining-impacted acidic streams.

Acid rock drainage exists in three headwater streams (Armells Creek, Collar Gulch, Chicago Gulch) in the Judith Mountains, central Montana, USA. The streams drain opposing sides of a central, pyrite rich, hydrothermally altered, granite-porphyry intrusion. Although significant production of precious metals has occurred elsewhere in the Judith Mountains, no present day mining is taking place and the drainages of interest have not been heavily impacted by historical mining activities. All three streams are acidic (pH < 4) in their headwaters and become pH-neutral with distance downstream due to the influx of alkaline groundwater and tributary flows. Concentrations of dissolved aluminum, cadmium, copper, lead, thallium, zinc and fluoride ion are locally well above regulatory standards. Detailed synoptic sampling using the continuous tracer injection method shows that metal loading is diffuse and is associated with weathering of the pyrite-rich bedrock, with negligible contributions of acid or metals from legacy mine waste. Each stream is precipitating hydrous ferric and aluminum oxides in discrete zones due to a combination of oxidation and acid-neutralization processes. An abundance of terraced ferricrete deposits shows that acid rock drainage existed prior to human disturbance. Furthermore, a comparison of the trace-metal content (Cu/Fe ratio) of modern, in-stream precipitates vs. ancient (undated) ferricrete deposits suggests that the chemistry of the streams has not changed significantly due to anthropogenic activity. The geochemistry of headwater streams in the Judith Mountains may provide a useful baseline comparison to nearby regions in central Montana with similar geology that have been heavily disturbed by historic and modern mining.