Crustal Geophysics and Geochemistry Science Center

Handcart Gulch — Integrated Headwaters Research on Hydrogeologic and Geochemical Processes and Monitoring of Environmental Change

Watershed Site Details

Site Hydrogeological Characterization

Handcart Gulch is an alpine watershed along the Continental Divide. It contains an unmined mineral occurrence composed primarily of pyrite with trace metals including copper and molybdenum. Although the geology of Handcart Gulch is typical of many hydrothermal mineral deposits in the intermountain West, the area is unique because of a set of deep and shallow wells used to conduct an integrated study of the processes involved with the liberation, transport, and fate of naturally occurring acid-rock drainage in a mountain watershed. Commonly, mountainous watersheds are underlain by complexly deformed crystalline rocks where the occurrence, storage, and flow of ground water are poorly understood. These environments also are characterized by high to extreme hydraulic gradients and heterogeneous networks of fractures and faults that control the ground-water flow system and contaminant transport. Integrating geological, hydrological, geochemical, and geophysical data in a series of numerical flow models, we find that ground-water flow at the watershed scale can be conceptualized using a relatively simple equivalent porous media approach. We also find that ground water accounts for a significant component of discharge to the trunk stream, about 37 percent of the total annual streamflow. The work to date (2008) suggests that, in spite of low bulk permeability and various geologic complexities, hydrologic contributions from fractured crystalline bedrock rock should be carefully considered when evaluating alpine ground-water flow systems.

Figure 3.  Location map of Handcart Gulch study site - click on image for large version.

Figure 3: Location of Handcart Gulch with major physiographic province boundaries and the location of the study area within the Colorado Mineral Belt and Montezuma Mining District, which is characteristic of many such districts in the Mineral Belt. Also shown is a visible satellite image draped on a tilted digital elevation model. The image shows the extreme topographic relief (over 800 meters with a gradient of about 0.1) and the red-orange weathering of the unmined mineral occurrence. The study area (red) and numerical ground-water flow model domains (red and green) are also shown. Webster Pass and Red Cone Mountain are shown for reference. [Click on image for large version.]

Figure 4. Photo of Red Cone Peak - click on image for larger version.

Figure 4: Red Cone mountain looking east from Webster Pass at the Continental Divide 3,688 meters (12,100 feet), the location of 335 meter deep monitoring well WP1, and upper reaches of Handcart Gulch, Colorado. This is the headwater drainage of the South Platte river and is representative of many Rocky Mountain alpine watersheds. Note the red staining from the oxidation of hydrothermal pyrite at concentrations of nearly 8 percent of the rock mass. This is the primary and natural source of low-pH water in the watershed. The locations of research monitoring wells holes WP3, WP4, and HC1 are shown as ovals. Alpine tundra, rock glacier lichen, fir and spruce ecosystems are also noted. [Click on image for large version.]

Figure 5. Hydrothermal alteration and geologic structure map - click on image for large version.

Figure 5: Hydrothermal alteration and geologic structure map draped on a digital elevation model as shown in Figure 3. Note: Webster Pass is at the headwaters of Handcart Gulch and the northwest edge of the study area for reference. [Click on image for large version.]

Testing Conceptual Hypotheses

Understanding how ground water transports metals and acid to mountain streams is critical in order to effectively manage mountain watersheds affected by acid-rock and acid-mine drainage. The transport of dissolved constituents in ground water is best represented by a coupled numerical ground-water flow and solute-transport model. Building such a numerical model requires first developing a conceptual model of the ground-water flow system that includes its most fundamental characteristics. This chapter describes the development of a conceptual model of ground-water flow in Handcart Gulch, an alpine watershed in the Front Range of Colorado affected by acid-rock drain-age. New applications of ground-water age and temperature data that utilize the site's unique monitoring-well network were essential in this process and resulted in a conceptual model considerably more accurate and defensible than what could have been developed using standard hydrologic and chemical data alone. See Manning and Caine, 2008; and Link to Verplanck et al., 2008, Chapter J in USGS Circular 1328 for more details.

Figure 6. - wo conceptual models of groundwater flow in an alpine watershed - click on image for large version.

Figure 6: Schematic diagrams illustrating two different conceptual models of ground-water flow in an alpine watershed underlain by fractured crystalline rock. (A) Uniform flow system in which aquifer thickness, z, recharge rate, R, and porosity, n, are relatively uniform throughout the watershed. Mean residence times, τ, of samples from wells along the creek that integrate all flow paths to the creek (flow-weighted samples) are uniform and equal to zn/R (Haitjema,1995). (B) Complex flow system in which z, R, and n vary throughout the watershed, resulting in variations in τ along the creek. K, hydraulic conductivity. [Click on image for large version.]

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