Metal Transport in Mineralized Mountain Watersheds

Science Center Objects

The central objective of this project is to develop a greater understanding of deep bedrock groundwater circulation and its contribution to surface water metal loads in mineralized mountain blocks composed of sedimentary rocks. This work is being performed in cooperation with Lawrence Berkeley National Laboratory as part of a broader research program aimed at understanding processes controlling the export of water, metals, carbon, and nitrogen from Rocky Mountain headwater catchments under changing climate and land-use conditions.

scientist drilling bedrock piezometer

Drilling bedrock piezometer RP9 to a depth of 6 ft in Redwell Basin, Gunnison County, Colorado, using handheld rock coring drill. (Credit: Richard Wanty, U.S. Geological Survey. Public domain.)

Science Issue and Relevance

Understanding the manner in which hydrothermally altered rocks containing abundant sulfide minerals ('mineralized rocks') naturally release metal-rich water to the environment in mountain watersheds is essential for managing surface water resources with locally mineralized mountain headwaters, such as the Colorado River.  Understanding how these sulfide weathering processes are influenced by changing climate and land-use conditions is particularly important for managing to protect future water quality and ecosystem health, and for establishing reasonable clean-up standards at active and abandoned mine sites in light of potentially changing background metal levels. Most studies of the environmental effects of sulfide weathering in mineralized watersheds are limited to sampling surface waters and springs because groundwater wells are not available. For this reason, our understanding of the deeper (>10s of meters) hydrogeology and geochemistry is severely limited. This study and its combined use of geophysical methods, geologic characterization, water chemistry data, and bedrock wells of varying depth (up to 250 feet) will address multiple fundamental, yet poorly studied, aspects of deep bedrock groundwater flow and metals transport in mineralized mountain blocks. Even though bedrock groundwater discharge may be a relatively small component of the annual surface water budget, its contribution to stream metal loads may be disproportionately large due to higher concentrations in this groundwater. This phenomenon has been understudied, and recent work suggests that it may be widely under-appreciated.

Monitoring well

Completion of monitoring well MW2.1. Valves were installed to hold back artesian flow under a pressure of about 15 psi. (Credit: Andrew Manning, U.S. Geological Survey. Public domain.)

Methodology to Address Issue

This project combines geophysical, hydrologic, and geochemical methods with geological and borehole data to accomplish the research objectives. Our interdisciplinary approach is necessitated by the inherent complexity of hydrogeochemical systems in mineralized mountain blocks. Work will be focused in Redwell Basin, a road-accessed (allowing drilling) alpine watershed near Crested Butte, Colorado containing extensive mineralized bedrock. Redwell basin overlies the unmined but well-delineated Mt. Emmons porphyry Mo deposit (one of the largest in the USA), produces both natural and mining-related acid-rock drainage, and is within the upper East River watershed where most research activities funded under  Lawrence Berkeley National Laboratory Watershed Function Science Focus Area are occurring. Two deep boreholes will be drilled in the upper part of Redwell Basin and completed with multi-level monitoring wells to depths up to 250 feet. Multiple shallow piezometers will also be drilled and installed in groundwater discharge zones. These wells and piezometers will provide a window to the deeper groundwater system, and enable the collection of geologic, geophysical, hydraulic, and geochemical data that will be used in combination with numerical modeling to determine fluxes of groundwater and dissolved constituents through the bedrock. Geophysical surveys will be used to generalize spatially discrete borehole and outcrop observations over broader areas. This integrated research approach should help fill significant knowledge gaps regarding the hydrogeochemical significance of bedrock flow systems for shallow ecosystems in the upper East River area, and the effects of mineralization on the hydraulic properties of the rocks.

drilling and core samples

Drilling MW1 in upper Redwell Basin, Gunnison County, CO. Example sections of MW1 drill core are also shown, from both the shallow oxic zone where sulfide are oxidized (depth <50 ft) and the deeper anoxic zone where sulfides remain unoxidized. (Credit: Andrew Manning and Jeff Mauk, U.S. Geological Survey. Public domain.)