The relationship between the water table and the surface flow of a losing stream, lower Medano Creek, Great Sand Dunes National Monument, Colorado |
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| Authors: | G L Hadlock T E Lachmar J P McCalpin |
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| Institution: | (1) Radian Corporation, 4021 South 700 East, Suite 600, Salt Lake City, UT 84107, USA, US;(2) Department of Geology, Utah State University, Logan, UT 84322-4505, USA, US;(3) GEO-HAZ Consulting, Inc., P.O. Box 1377, Estes Park, CO 80517, USA, US |
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| Abstract: | Proposed groundwater withdrawals in the San Luis Valley of Colorado may lower the water table in Great Sand Dunes National
Monument. In response, the National Park Service initiated a study that has produced a generalized conceptual model of the
hydrologic system in order to assess whether a lowering of the water table might decrease the surface flow of lower Medano
Creek. Based upon information obtained during the drilling of several boreholes, there appear to be five important hydrostratigraphic
units underlying lower Medano Creek within the upper 30 m of the ground surface: 1. a perched aquifer overlying an aquitard
located between about 5 and 6 m below the ground surface; 2. the aquitard itself; 3. an unconfined aquifer located between
the upper and lower aquitards; 4. an aquitard located between about 27 and 29 m below the ground surface; and 5. a confined
underlying the lower aquitard. Because the areal extent of the aquitards cannot be determined from the borehole data, a detailed
conceptual model of the hydrogeologic system underlying lower Medano Creek cannot be developed. However, a generalized conceptual
model can be envisioned that consists of a complex system of interlayered aquifers and leaky aquitards, with each aquifer
having a unique hydraulic head. Water levels in the perched aquifer rise rapidly to their annual maximum levels in response
to the arrival of the flow terminus of Medano Creek during the spring runoff event, and the location of the flow terminus
is directly dependent upon the discharge of the creek. Water levels in the deeper, non-perched aquifers do not appear to fluctuate
significantly in response to the arrival of the flow terminus, demonstrating that it is unlikely that the proposed groundwater
withdrawals will decrease the surface flow of lower Medano Creek.
Received: 27 December 1995 · Accepted: 20 February 1996 |
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| Keywords: | Hydrogeology Surface water Groundwater Streamflow |
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