2024-04-08_PERMIT FILE - M2024015

• Permit No: M2024015
• IBM Index Class Name: Permit File
• Doc Date: 4/8/2024
• Doc Name: Application
• From: J & T Consulting, Inc
• To: DRMS
• Email Name: BFB
• Email Name: EL1
• Media Type: D
• Archive: No

SVRC Groundwater Evaluation
January 2024 Page 1 of 58
BACKGROUND AND PURPOSE
The proposed Sweet Valley Reservoir Complex (SVRC) is located approximately 2 miles west of
Gilcrest, Colorado in Section 30, Township 4 North, Range 66 West(6th PM). The site is located
approximately one mile east of the South Platte River (SPR) which flows northeast along this
stretch. The SVRC consists of two proposed lined pits, each approximately 40 acres. The mine
owner/applicant is the Central Colorado Water Conservancy District(CCWCD).
As part of the mine permit application process, the mine consultant, J&T Consulting, Inc. (J&T)
requested that McGrane Water Engineers, LLC. (MWE) determine the potential impacts of
installing pit liners at the SVRC. Impacts typically include a rise in the water table("mounding")
on the up-gradient side of the slurry wall and a decline in the water table ("shadowing") on the
down gradient side. Water level increases to within 10 feet of the surface on the up-gradient side
of the pit could also cause flooding of low-lying structures such as home basements. A decline in
water levels on the down gradient side could reduce the aquifer saturated thickness and well yields
if the decline is more than approximately 10 percent of the saturated aquifer thickness.
Water Level History
The Colorado Geological Survey (CGS) has documented a rise in alluvial aquifer groundwater
levels in the vicinity of Gilcrest, Colorado over the past several decades (Topper and Others,
2014). The higher than normal water table is a concern among residence and regulators due to
potential flooding of basements and water logging damage to agriculture. We created a `'recent
Pre-pit" water table based on CGS contours that represent 2012 conditions and modified them to
consider more recent 2021 and 2023 water levels from SVRC and other pit monitoring wells. We
conclude that although water levels in the pit vicinity have likely increased approximately 10 feet
over past decades, they are still 10-15 feet below ground level at and upgradient from the pits.
Other Planned Pits
Other gravel mining operations ("Monarch"Pit and"Section 20" Pit)are planned and applications
have submitted to DRMS. Applicants for both pits have proposed installing infiltration drains to
mitigate mounding on the upgradient sides, and recharge ponds, trenches or subsurface
"exfiltration" drains (to eliminate evaporation)downgradient to mitigate shadowing. Upgradient
drains are also planned for the SVRC as discussed below.
The study area surrounding the pits covers approximately 24 square miles and includes
approximately 404 permitted and constructed wells (excluding abandoned wells and monitoring
wells) per the State's well database (CDWR, 2023). Figure 1 shows the permitted wells (sorted
by use),proposed pit boundaries(white),study area and natural features provided by Google Earth.
Due to the increased density of wells closer to Gilcrest, we did not consider wells shown in
yellow in our hydrology data set, but we did consider them in our water level impact (aka
"change") analysis.
Report Structure
This report includes an:
1. Executive Summary of the study, modeling results and recommendations;
2. Appendix A that provides the hydrogeologic evaluation and description of the model
conceptualization, construction and the SVRC impact and mitigation analyses;
McGrane Water Engineering,LLC
1669 Apple Valley Rd. •Lyons,CO 80540 • Phone:[phone];email:[e-mail]