PROJ02106

• Contract/PO #: FS0063FX
• Contractor Name: Groundwater Appropriators of the South Platte
• Contract Type: Miscellaneous
• Water District: 0
• Loan Projects - Doc Type: Feasibility Study

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F. Geotechnical
A series of geotechnical subsurface explorations were completed by Earth Engineering
Consultants. The initial field exploration consisted of shallow borings extending to a maximum
depth of 6 feet. A total of 13 shallow borings were completed within and adjacent to the
proposed site. The main purpose of this initial exploration was to determine the location and
extent of clay overburden on the property and if the clays were suitable materials for
construction of a dam.

The report describes the underlying material as consisting of 3-6 inches of topsoil and vegetation
underlain by a dark gray brown lean clay ranging in depth from approximately 3-6 feet. The
lean clays were underlain by granular fme to medium sands with varying amounts of silts and
clays. The depth and consistency of the underlying granular material was not evaluated in the
initial evaluation.

After further review of the initial evaluation, a second series of borings that extended down into
the bedrock formation was obtained to evaluate the underlying alluvial material. It was also
considered important to confirm that the underlying material was of the same composition
described by the USGS. A series of six borings were completed to a depth sufficient to
encounter the underlying bedrock, which generally is found at a depth of 50 feet in this area.
Two of the six borings completed to the bedrock were used to perform field slug tests for
comparison with published USGS geologic information. The slug tests indicated permeability's
ranging from lxl0-s -lxl0-3 cmls. The slug tests were performed in the lean fine clays and the
underlying fine granular material. The site is underlain by the Brule formation ranging from a
depth of 42-50 feet. The field data from the borings indicated bedrock was encountered at
approximately 3,520' msl, which corresponds to USGS published data for this area.

Groundwater levels at the time of testing varied from a depth of 2 feet on the north side of the
site, to approximately 10 feet in the southern most portions. Fluctuations in groundwater levels
can occur depending on hydrological and seasonal conditions. The location of groundwater will
be significant in construction ofthe reservoir. A high groundwater table will require the use of
dewatering trenches to allow for the effective keying ofthe dam core. Additional information
will need to be developed during the preliminary design phase to more accurately locate the
normal groundwater table. This information is also important for the 404 permit phase ofthe
project. It is recommended that the current routing of irrigation tailwater upgradient of the
property be evaluated to see if any reductions in flows entering the property are possible.

G. Compact Call Issues
Other potential opportunities beyond the water well augmentation aspects of the proposed
reservoir is to re-regulate the flows in the South Platte River to increase the certainty of
compliance with Nebraska's compact entitlement. The South Platte River Compact between the
States of Colorado and Nebraska was adopted in 1921 to remove all causes of present and future
controversy between the two states. The compact language states that "between the first day of
April and fifteenth day of October of each year, Colorado shall not permit diversions from the
Lower Section of the river, to supply Colorado appropriations having adjudicated dates of
priority subsequent to the fourteenth day of June, 1897, to an extent that will diminish the flow
of the river at the Interstate Station (Julesburg Gage), on any day, below a mean flow of 120
cubic feet of water per second oftime".

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