BOARD00597

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

Page 2 South Park Conjunctive Use Project

wells for project purposes would average 6,200 acre-feet per year,

Throughout the process of developing these reports, it has become increasingly clear that Aurora's commitment to de-
velop the project based on sustainable water yields significantly influences the operation of the project. Although up to 47
wells were identified in the application, the modeled operational scenario uses only 12 wells with an average annual
pumping rate of about 6,200 acre-feet.

Possible effects were predicted by comparing natural, existing conditions that would exist without SPCUP to effects that
would result from the operation of the SPCUp,

Possible Project Effects

Effects ofthe SPCUP operations are potentially felt in two areas: surface water
and ground water, Potential effects upon surface water systems are:

I, Accretions and or depletions to streamflow due to pumping and recharge
operations;
2, Accretions to streamflows attributed to leakage lI$ diverted water is con-
veyed to the recharge areas in the collection system;
3, Reductions in streamflow due to direct diversions into the surface water
collection system, These diversions are made in priority, and, therefore
do not impair senior water users,

Sustainability is
achieved by balancing
the lllllOunt of water
witllllrml'n fi"()11I the aq-
u~fer with the amount
of water recharged into
the aquifer,

,1:h,e effects of recharge andpumlJing openitions and diteh losses are the items
discussed in detail, because they have the greatest potential to affect existing water rights holders inhey are not properly
augmented, There is little measurable effect on Jefferson'and Michigan Creeks, Significant depletions occur on Tarryall
Creek and Park Gulch, There are no significant effects on local water users, Downstream water users can be augmented
(protected) by releases from the project. However, the effects on the Colorado Water Conservation Water Board's Mini-
mum Instream Flow Rights are noticeable on Tarryall Creek and will need to be augmented at the point of depletion with
releases from the project.

The SPCUP operations could potentially affect ground water users depending on the location of the wells and the aquifer
that is affected, The four main aquifers that could be potentially affected based on location are: '

. Precambrian Aquifer--Granitic Uplands (Indian Mountain)
. Alluvial Aquifer (along stream reaches)
. Upper South Park Aquifer
. Lower South Park Aquifer,

Precambrian Aquifer--Granitic Uplands
The modeling has shown that the maximum drawdown in the granitic aquifer (wells in Indian Mountain subdivision)
would be about two feet. The area of drawdown is confined to a narrow band, between 1000 and 2500 feet wide, east of
the Elkhom Thrust Fault, which separates the Precambrian aquifer from the South Park Aquifers, This small amount of
drawdown would be difficult to detect amid seasonal variation and well-to-well interference that is common in granitic
aquifers, The impacts to any wells in the area may be considered minor as long as the drawdown is less than a few feet.

Alluvial Aquifer
In the area of Jefferson and Michigan Hill, the modeling suggests that drawdowns would be less than a foot due to
SPCUP operations, In some areas north and west of Michigan Hill the well water level actually rises about a foot due to
ditch losses from the surface water collection system, In the South Park Aquifers south of the Project area, drawdowns of
approximately 2 to 3 feet are possible south of the Elkhorn Ranch, This drawdown decreases to zero south of Seven-Mile