BOARD00597

• Board Meeting Date: 3/29/1999
• Description: South Platte Conjunctive Use Project - Overview of Project Status
• Board Meetings - Doc Type: Memo

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Page 6 South Park Conjunctive Use Project

Elkhorn Thrust Fault Study Completed

This spring a hydro-geologic investigation of the Elkhorn
Thrust Fault was conducted as part of the ongoing engineering
studies to support the water rights applications for the South
Park Conjunctive Use Project. The abrupt rise of the granitic
uplands along the eastern edge of the South Park basin is a
clear delineation of the thrust fault front. After on site inspec-
tion, a suitable site-was selected on the southeastern comer of
the ranch near Indian Mountain subdivision, The area was
trenched with a backhoe to provide a more definitive descrip-
tion of the local geological contacts, Five monitoring wells
were drilled across the fault zone, See Figure 2, Two were
completed in the granitic rocks on the hanging wall (uphill)
side of the fault, A deep well was completed through the fault
zone to the South Park Formation side of the fault. Two wells
were completed in the South Park Formation materials on'the
footwall (downhill) side of the fault. Approximately 260' of
drill core of the fault zone was collected during drilling, From
that core, five
samples from
various depths
were selected,
, "Th.e five saDj'_,
pies were sent
to an independ-
ent geological
laboratory for
permeability
testing using a
triaxial per-
meameter, The
well penetrating the fault zone (PZ-4) and the monitoring well immediately to
the west, (PZ-5) were logged using standard geophysical equipment. Water
levels were measured in the wells after a period of stabilization,

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Elkhorn Thrust Fault Cross Sectiou

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(EAST)
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(HORl. & VERT.)

SP CUP
ElKHORN THRUST
fAULT INTERPRF:l1VE
CROSS SECTION

Figure 2.

The results indicated that the fault zone was approximately 300' feet thick at
the selected location, The samples subjected to the permeameter testing
showed permeabilities ranging from 3.lXIO-7 to 2.4XlO-9 cm/sec, [The sam-
ples tested would be classified as impermeable and suitable for use in dam con
struction or similar applications,] Water level measurements indicated that
water on the granitic side of the fault is being held there by the impermeable
fault zone, The results of these investigations support the values used in mod-
eling the fault zone in the Initial Surface and Ground Water Modeling Report
of the South Park Basin,

The results of the study also support the concept that the fault zone is a barrier
that protects wells in the Indian Mountain subdivision from the possible effect
from the proposed wells of the South Park Conjunctive Use Project,