2020-04-30_HYDROLOGY - M2008017

• Permit No: M2008017
• IBM Index Class Name: Hydrology
• Doc Date: 4/30/2020
• Doc Name Note: GW Modeling Rpt
• Doc Name: Hydrology Report
• From: Operator
• To: DRMS
• Permit Index Doc Type: Hydrology Report
• Email Name: ECS
• Email Name: MAC
• Media Type: D
• Archive: No

Derr Pit - Groundwater Modeling Report
January 31, 2020 Page 6 of 24
McGrane Water Engineering, LLC
1669 Apple Valley Rd. Lyons, CO 80540 Phone: [phone]
E-Mail: [e-mail] Web: ttp://www.mcgranewaterengineering.com
due to measurement error and annual and seasonal differences between measurements, so we do
not believe the effort would result in more accurate model forecasts.
Modeling Approach
We conducted three “steady state” runs using the model; the first (DerrSS3) establishes the
“predevelopment” water table (Figure 2) and aquifer thickness (Figure 5). We then use
predevelopment run heads (ie. water table elevations) as input into the “impact” runs
(DerrSS3_wPit and DerrSS3_wNDerrPit) with the pit cells off, to simulate post-slurry wall
conditions. We calculated the change in water levels caused by slurry walls by subtracting the
post slurry wall water table elevations from the predevelopment water table elevations. Through
this process, we were able to create contour lines showing regional water table changes and
tabulate “impacts” at individual wells.
Water Level Changes Caused by Slurry Walls
After both the Loloff and Derr pit slurry walls are installed, the model predicts that water levels
will rise approximately 2.5 feet on the upgradient sides of the pits (Figure 9) and drop less than
one (-1) foot on the downgradient (south) side of the pits.
With the addition of the Derr Amendment slurry wall, the model predicts that water levels will
rise approximately 3 feet on the upgradient sides of the pits (Figure 10) and drop up to -2.5 on
the downgradient (south and east) sides near existing wells. Water levels between the Derr and
the amendment slurry walls are expected to decline between -4 to -5 feet, but there are no private
wells in that area.
Predicted Changes at Private Wells
Table 2 shows that if the Loloff and Derr slurry walls are installed, then ten wells could expect
water levels to increase or decrease at least +/- 0.5 feet compared to predevelopment conditions.
The maximum upgradient increase is approximately 1.6 ft at the Parker domestic well. The
maximum downgradient decrease is -0.9 ft at the Taylor and Baab domestic wells. Table 2 also
compares the modeled drawdown from predevelopment conditions to actual based on January,
2020 data (discussed below).
Table 2 –Change in Water Levels Caused by Loloff and Derr Slurry Walls
Well Location Well Owner (type)Well
Permit
Depth
(ft)
Modeled
Predevelop-
ment Depth to
Water (ft)
Modeled
Change in
Water Levels
(ft)
Modeled
Predicted
Depth* (ft)
Depth to
Water (ft)
1/9/2020
Current
Drawdown* from
Predevelopment
Conditions (ft)
Koehler (Domestic)314643 52 40.5 0.8 39.7 49.2 -8.7
Koehler (Stock)314644 52 39.2 0.7 38.5 41.8 -2.6
Davis (Irrigation)14960-R 101 38.8 1.0 37.8 47.2 -8.4
Parker (Domestic)44673 72 35.9 1.6 34.3 42.5 -6.6
Harrell (Domestic)226878-A 50 10.1 0.7 9.4 10.9 -0.8
Harrell (Irrigation)287278 30 11.3 0.9 10.4 10.2 1.1
Ruland, Bud 421-WCB 87 23.5*1.4 22.1*ND ND
Taylor (Domestic)223885-A 44 9.9 -0.9 10.8 19.3 -9.4
Baab, A C (domestic)620-WCB 45 9.3*-0.9 10.2*ND ND
Orona (Domestic)28174 ND 11.3*-0.7 12.0*ND ND
Notes:ND = No Data
*Calculated based on DEM data ground elevations
Upgradient
Wells
Downgradient
Wells