Jonathan Saul Caine, Task Leader
Expansion of the Denver metropolitan area into the foothills of the Rocky Mountain Front Range has resulted in a growing reliance on groundwater from individual domestic wells. The Turkey Creek Watershed, exemplifying this trend, is underlain by complexly deformed and fractured crystalline bedrock where groundwater resources are poorly understood and concerns regarding groundwater mining and degradation have arisen. As part of a pilot project to establish quantitative bounds on the groundwater resource, an outcrop-based geological characterization and numerical modeling study of the brittle structures and their controls on the flow system suggests that groundwater storage, flow, and contaminant transport is primarily controlled by a heterogeneous array of fracture networks. Inspection of well permit data and field observations leads to a conceptual model where three dominant lithologic groups underlying sparse surface deposits form the aquifer system-metamorphic rocks, a complex array of granitic intrusive rocks, and major brittle fault zones. Pervasive but variable jointing of each lithologic group forms the 'background' permeability structure and is an important component of the bulk storage capacity. This 'background' is cut by brittle fault zones of varying architectural styles and by pegmatite dikes, both with much higher fracture intensities relative to 'background' that likely make them spatially complex conduits. Probabilistic, discrete fracture network and finite element modeling was used to estimate porosity and permeability at the outcrop scale. Results from this study reveal a complex aquifer system where the upper limits on estimated hydraulic properties suggest limited storage capacity and permeability as compared with many sedimentary rock and surficial deposit aquifers.
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Outcrop Scale Features: 
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Regional to Map Scale Features 
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Handcart Gulch in the Colorado Rocky Mountain Front Range hosts a naturally acidic (pH 3-4), metal-rich stream due to the presence of a small, unmined hydrothermal metal deposit. Fundamental questions regarding the role of ground water in transporting metals and acid in the vicinity of such sulfide deposits remain unanswered. Handcart Gulch provides a unique opportunity to address these questions because the watershed contains four deep boreholes (1200 to 3500 feet deep), along with nine new shallower boreholes drilled near the trunk stream. Field activities performed in 2003 include the collection of basic geologic data, a stream tracer-dilution study, geophysical well-logging, and pumping tests. Ground and surface water samples were collected and analyzed for dissolved noble gases, 3H, CFCs, major ions, and multiple isotopes. Rock samples were also collected to characterize their mineralogy and chemistry. Preliminary results include: (1) High ambient ground-water flow rates (from down-hole flow meter data), high fracture intensities, and considerable hydraulic conductivities were observed in the bedrock near the stream; (2) Artesian conditions exist near the stream due to a ferricrete deposit that behaves as a confining unit; (3) Springs near the stream generally discharge shallow-circulating ground water with 3H/3He ages of <2 years, whereas bedrock well waters have ages of 10 to 25 years; (4) Significant (100-foot range) seasonal water table fluctuations occur in the upper part of the watershed; (5) Age data and temperature profiles hold considerable promise for constraining ground-water and solute fluxes through the application of a coupled heat and fluid flow model.
A. Location and Groundwater Flow Model Domain
B. Geology, Brittle Structures, Hydrothermal Alteration and Mineralization
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