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.
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.