Groundwater flow in an intermountain basin: Hydrological, geophysical, and geological exploration of South Park, Colorado

Ball, Lyndsay Brooke

Graduate Thesis Or Dissertation

Groundwater flow in an intermountain basin: Hydrological, geophysical, and geological exploration of South Park, Colorado Public Deposited

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Citations

Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/sq87bt72z

Abstract

Groundwater in the intermountain basins of the American West is increasingly of interest with respect to water supply, ecosystem integrity, and contaminant and heat transport processes. These basins are defined by their heterogeneity through large topographic relief, substantial climatic variability, and permeability distributions made complex through variations in lithology and deformation over the orogenic history of these regions, leading to folded and faulted aquifers. This dissertation focuses on the influence of these heterogeneities on the groundwater flow system of the South Park basin in central Colorado, USA. The influence of faults on shallow groundwater flow was examined at two locations along the mapped trace of the Elkhorn fault, a Laramide reverse fault that juxtaposes crystalline and sedimentary rocks in eastern South Park. At the first location, electromagnetic, resistivity, self-potential, and hydraulic data were collected at an existing well field straddling the fault trace. Integrated analysis suggested the fault behaves as combined conduit barrier to groundwater in flow the upper 60 m. A second location along the mapped trace was selected through additional geophysical exploration. New boreholes were drilled to make direct geologic, hydrologic, and geophysical observations of the fault zone. However, these boreholes did not intersect the Elkhorn fault despite passing through rocks with similar electrical resistivity signatures to the first study location. Analyses of drill core and geophysical data indicate that the mineralogical composition of the crystalline rocks strongly influences their resistivity values, and the resistivity contrasts associated with the rock juxtaposition created by the Elkhorn fault is not unique. A steady-state, three-dimensional groundwater flow model of the South Park basin was developed to explore the influence of complex topography, recharge, and permeability structure on regional groundwater flow. Geologic complexity and the associated permeability structure are controlling factors in recharge and discharge patterns. Most groundwater circulates in the upper 1 km in local and intermediate flow systems, although mountain block recharge does make notable contributions to the main valley. Regional scale flow processes were found to be relatively insensitive to heterogeneity in recharge or fault zone permeability, although both can have localized impacts to groundwater flow.

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