Impacts of Hydrologic Change on Geochemistry in the Upper Snake River, a High Mountain Acid Rock Drainage Stream

Crouch, Caitlin Marie

Graduate Thesis Or Dissertation

Impacts of Hydrologic Change on Geochemistry in the Upper Snake River, a High Mountain Acid Rock Drainage Stream Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/gh93gz82j

Abstract

The Upper Snake River watershed in Colorado is in an alpine catchment impacted by acid rock drainage originating from the natural weathering of pyrite. A compilation of historic low flow stream chemistry data from this site indicates that zinc concentrations have increased six-fold over the past 30 years, with a more rapid increase in the past decade. Over this time period, pH has also decreased substantially (from approximately 4.0 to 2.6). Observed increases in metals concentrations are correlated with earlier snowmelt and peak streamflow. I hypothesize that an increase in zinc concentrations is due to changes in groundwater caused by climate change and associated earlier peak snowmelt (by 2-3 weeks), resulting in a lower groundwater table and new subsurface material being exposed to weathering conditions for the first time. Observed increases in background metals concentrations may pose a growing danger to aquatic ecosystems and as such have implications for mitigation of former mining sites. The current research further investigates the established source of metal-rich inflows from the northeastern slope of the catchment using a tracer injection study with synoptic sampling. Results of the tracer study indicate two primary zones of trace metals loading. The first is on the steep, rocky, alpine slopes where stream discharge and trace metal loads increase rapidly. This is likely the source of increased weathering and decreasing pH. Discharge and metal loads increase dramatically again through a gently-sloped meadow and through bog iron ore in the bottom of the river valley, which is in the last 60-m reach of the tributary before it flows into the Upper Snake River. The tracer study indicated a significant increase in hyporheic exchange along this final reach. I hypothesize that decreasing pH over the past three decades reached a threshold, mobilizing zinc sorbed to iron hydroxides in the hyporheic zone, meadow, and bog iron ore. This mobilization of metals from the meadow is likely to have begun over the past several years, during which time the metals concentrations have increased exponentially.

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