THE RELATIONSHIP BETWEEN THE WATER-TABLE AND THE SURFACE FLOW OF A LOSING STREAM, LOWER MEDANO CREEK, GREAT-SAND-DUNES-NATIONAL-MONUMENT, COLORADO

Proposed groundwater withdrawals in the San Luis Valley of Colorado ma y lower the water table in Great Sand Dunes National Monument. In resp onse, the National Park Service initiated a study that has produced a generalized conceptual model of the hydrologic system in order to asse ss whether a lowering of the water table might decrease the surface fl ow of lower Medano Creek. Based upon information obtained during the d rilling of several boreholes, there appear to be five important hydros tratigraphic units underlying lower Medano Creek within the upper 30 m of the ground surface: 1. a perched aquifer overlying an aquitard loc ated 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. Becau se the areal extent of the aquitards cannot be determined from the bor ehole data, a detailed conceptual model of the hydrogeologic system un derlying lower Medano Creek cannot be developed. However, a generalize d conceptual model can be envisioned that consists of a complex system of interlayered aquifers and leaky aquitards, with each aquifer havin g a unique hydraulic head. Water levels in the perched aquifer rise ra pidly to their annual maximum levels in response to the arrival of the now 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 f low terminus, demonstrating that it is unlikely that the proposed grou ndwater withdrawals will decrease the surface flow of lower Medano Cre ek.