WSP12268

• File Number: 8509
• Description: San Luis Valley
• State: CO
• Basin: Rio Grande
• Water Division: 3
• Date: 6/1/1987
• Title: Interim Task 5 Report Deep Well Testing & field Investigation - San Luis Valley Confined Aquifer Study - Phase 1
• Water Supply Pro - Doc Type: Report/Study

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~has been derived from analysis of geophysical logs and water samples from the
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(~Hooper Pool Well at Hooper, and the Carroll Well and A1amosa Geothermal Well
~ at Alamosa (Figure 2.1). At these locations, which are thought to be at or
near rift-related fault zones, this conclusion is fairly well documented,
Away from these areas, water quality in the deep confined aquifer can be
inferred only from analysis of sparsely-located geophysical logs, with no
supporting water-quality data. Therefore, extension of this conclusion to
other areas of the Valley is not possible without corroborating data in non-
rift-related areas.

In the San Juan foothills of the Del Norte area, hydraulic conductivity
of approximately the upper 1700 feet of the Conejos Formation is estimated to
be on the order of 10-3 em/sec. At that hydraulic conductivity, the
transmissivity of the interval 180-1710 feet in Well No. 1-19 SFC, about five
miles south of Del Norte, is expected to be approximately 30,000 gpd/ft. This
has been estimated by comparison with hydraulic conductivity analyzed in a
Conejos Formation well drilled through very similar lithologies (Huntley,
1976). Accuracy of this hydraulic conductivity estimate is expected to be
good for similar Conejos lithologies. Where the Conejos is composed
predominantly of much finer-grained materials, as appears to be the case
beneath the San Luis Valley, 10-3 cm/sec may be considered to be an upper
limit of hydraulic conductivity.

Transmissivity of the Conejos Formation is expected to decrease
considerably to the east, from the San Juan foothills into the San Luis
Valley. The effect of this would be to force ground water moving down-
gradient (into the Valley) into pathways less resistant to ground water flow.
The most likely pathway appears upward, into HSU-2. This inference is based
on analysis of seismic-reflection sections and geophysical logs, Since no
direct evidence to support or deny this hypothesis is presently available, it
should not be considered as a definite conclusion.

There may be a component of ground water movement from sub-basins of the
Rio Grande (e.g., San Francisco Creek and Pinos Creek) southward into the
drainage basins of the Alamosa River, La Jara Creek, and the upper reaches of

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