PERMFILE46021

• Permit No: M2000158
• IBM Index Class Name: Permit File
• Doc Date: 3/23/2001
• Doc Name: POTENTIAL GROUND WATER RESOURCE IMPACTS ASSOCIATED WITH STARTER PIT DEWATERING
• From: DENNIS MCGRANE
• To: GREG ROUSE
• Media Type: D
• Archive: No

Memorandum to Greg Roush• •
March 23, 2001
Page 6
recharge amount is more than triple the maximum theoretical dewatering rate (2500 gpm, Table
3) and would significantly limit aquifer dewatering.
Canal RechazQe
Leakance from the Westem Mutual Ditch and Farmers Independent Ditch will also maintain
water levels. The Westem Mutual Ditch flows along the eastern border within 600 feet of the
proposed pit. Using similar aquifer properties described above, a canal length of around 2.5
miles, an average canal width of 5 feet and a 6-month wetting period, we can expect an average
of around 700 gpm of leakage to the aquifer. This recharge amount is approximately 28% of the
maximum theoretical dewatering rate (2500 gpm, Table 3) and would reduce aquifer dewatering.
Aquifer Subflow
Aquifer Subflow is water flowing through the aquifer in response to the natural aquifer gradient.
Hurr's water table map shows Subflow moving from south to north at an average gradient of
around 0.003. Using Dazcy's Law (Q = KIA) where: Q =Aquifer Subflow (gpm); I =Average
ground water gradient (.003); and A =aquifer cross-sectional azea (1.2 million squaze feet), we
calculate approximately 5,300 gpm moving through the aquifer from up gradient. This is more
than double the maximum theoretical dewatering rate (2500 gpm, Table 3) and would
significantly limit aquifer dewatering.
We conclude that the maximum dewatering rate (2,500 gpm, Table 3) is only 17 percent of the
amount of potential recharge to the area. Therefore the drawdown cone will likely be
significantly contained and minimal impacts to other users will likely occur.
IMPACT
The Colorado Division of Water Resources requires that all new wells be located 600 feet from
existing wells. One purpose of the 600-foot rule is to prevent the drawdown caused by one well
to adversely affect the performance of the other. Tables 2 and3 show that pit dewatering could
cause water levels to decline up to 8 feet at a distance of around 600 feet from the starter pit.
Based on mitigating factors discussed above, we believe the actual drawdown would likely be
less than half that amount (4 feet).
We realize that a decline in the aquifer saturated thickness could cause a proportional drop in the
maximum well yield for an existing pump system well. Assuming an average saturated
thickness of 50 feet in areas where agricultural wells are present, and an expected water level
decline of 4 feet (Table 3), we can expect a drop in the maximum well yield of less than 10
for wells located within 600 feet of the proposed starter pit. In our opinion, this reduction in the
maximum well yield would not be noticeable for most well users and' could easily be mitigated
by extending the pumping time.
LEONARD RICE CONSULTING WATER ENGINEERS, INC.
2000 CLAY STREET. SuIrE 300, DENVER, COLORADO 5021 ~-5~ ~ B • PRONE (SOS) 455'8588 • FAX [phone]