Document Type

Honors Thesis

Publication Date

Spring 5-8-2026

Abstract

This study investigates the hydrogeochemistry of the East Fork of the South Boulder River (EFSBR) in the northern Tobacco Root Mountains, Madison County, Montana. The upper 5 km of the main stem of the EFSBR is acidic (pH < 5.5), and dissolved concentrations of several trace metals including Al, Cd, Cu, and Zn exceed chronic aquatic life standards by 2-3 orders of magnitude. Unsurprisingly, the EFSBR appears to be devoid of trout. Since no mining has taken place at the source areas of the acidic headwater streams, this phenomenon is referred to as Natural Acid Rock Drainage (NARD). High mountains (up to 10,500’) bounding the EFSBR valley harbor a number of potential small rock glaciers and other periglacial features that appear to influence the hydrogeology and chemistry of the watershed. The steady flow of cold water in the hot summer months, during the same time period that neighboring drainages are drying up, is hypothesized to be sourced from melting ground ice high in the headwater catchment. Additionally, rock glaciers can act as efficient chemical reactors where they exist in conjunction with acid-generating mineralized bedrock, affecting the severity of acidic drainage and associated metal concentrations. Neutralization of the acidic water by alkaline tributaries results in the precipitation of aluminum hydroxysulfate (basaluminite), which covers the streambed with a vivid white mineral coating and can act as a natural sink through adsorption of other toxic metals to its surface. Pristine catchments exhibiting NARD can provide water quality data and serve as reference sites for restoration projects at sites experiencing Acid Mine Drainage where historical data does not exist. As climate change continues to alter the natural environment, especially in polar and high elevation regions, severity and instances of NARD are predicted to continue to increase on a global scale. A deeper understanding of the processes driving NARD from studies focused on areas such as the EFSBR will provide guidance for protecting vulnerable aquatic ecosystems and managing water resources in mountain communities into the future, as well as insights into necessary changes to established remediation methods in watersheds experiencing acid mine drainage.

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