Paper No. 14
Presentation Time: 11:30 AM


MANNING, Andrew H., U.S. Geological Survey, P.O. Box 25046, Mail Stop 973, Denver, CO 80225, TODD, Andrew S., Crustal Geophysics and Geochemistry Science Center, U.S. Geological Survey, P.O. Box 25046, Denver, CO 80225-0046, VERPLANCK, Philip L., U.S. Geological Survey, P.O. Box 25046, M.S. 973, Denver Federal Center, Denver, CO 80225-0046 and CAINE, Jonathan Saul, U.S. Geological Survey, Box 25046, DFC, MS 964, Denver, CO 80225-0046,

We compile a unique 30-year stream-water chemistry data set from a mineralized alpine watershed with naturally acidic, metal-rich water (acid-rock drainage) in the Colorado Front Range. Dissolved concentrations of Zn and other metals of ecological concern have increased by 100-400% (0.4-2.0 mg/L) since the early 1980’s during low-flow months, when metal concentrations are highest. SO4 and other major ions show similar increases. A lack of natural or anthropogenic land disturbances in the watershed during the study period suggests that climate change is the underlying cause. This trend is concerning given likely negative impacts on downstream ecosystems and water resources, and complications created for the establishment of attainable remediation objectives at nearby mine sites. We evaluate several possible climate-change-related causes of the increasing concentrations using three different approaches. First, local climate and streamflow indices are analyzed for trends during the sampling period. Second, groundwater hydrologic and geochemical data from a nearby USGS research watershed are examined for evidence of recent changes in subsurface sulfide weathering conditions. Third, schematic groundwater flow and reactive solute transport models are constructed using TOUGHREACT to test hypothetical effects of a changing climate on natural acid-rock drainage production. Preliminary results suggest that the most probable causes of the rising metal concentrations are melting permafrost (or reduction of seasonally frozen ground) and falling water tables resulting from decreased recharge.