B. Water Supply Update, PW024
CITY OF
CHANHASSEN
7700 Market Boulevard
PO Box 147
Chanhassen, MN 55317
Administration
Phone: 952.227.1100
Fax: 952,227.1110
Building Inspections
Phone: 952,227.1180
Fax: 952.227.1190
Engineering
Phone: 952.227.1160
Fax: 952.227.1170
Finance
Phone: 952.227.1140
Fax: 952.227.1110
Park & Recreation
Phone: 952.227.1120
Fax: 952,227.1110
Recreation Center
2310 Coulter Boulevard
Phone: 952.227.1400
Fax: 952.227.1404
Planning &
Natural Resources
Phone: 952.227.1130
Fax: 952,227.1110
Public Works
1591 Park Road
Phone: 952.227.1300
Fax: 952.227.1310
Senior Center
Phone: 952.227.1125
Fax: 952.227.1110
Web Site
www.ci.chanhassen.mn.us
8
-
MEMORANDUM
TO:
Todd Gerhardt, City Manager
Paul Oehme, P.E., Dir. of Public Works/City Engineer V O.
FROM:
DATE:
dXv#
Water Supply Update - PW024
September 10, 2007
SUBJ:
BACKGROUND
Water Supply Update
Barr Engineering Company was retained by the City to look into the possible cause of
low yields in City Well Nos. 2, 5, and 6 that began in mid-summer 2007 and continue
to persist. WellNos. 5 and 6 pump from a sand-gravel deposit that sits on top of
gravelly clay. Well No.2 pumps from the underlying Prairie du Chien-Jordan
bedrock aquifer. Barr evaluated the geology, pumping and groundwater flow
conditions in the eastern part of Chanhassen anc:t. modified an existing computer
model of groundwater flow of the area.
Barr has preliminarily concluded that there is likely a connection1:>etween the Prairie
du Chien-Jordan aquifer and the sand-gravel deposit in the vicinity of Well Nos. 2, 5,
and 6 where the gravelly clay layer that separates the sand-gravel deposits from the
bedrock is thin or absent. This sand-gravel unit covers about 6-12 square miles but in
most areas, it is only a few feet thick (it is thickest in the vicinity of Well Nos. 5 and
6). Above normal groundwater pumping from the Prairie du Chien-Jordan aquifer in
southwestern Hennepin County and southeastern Carver County as caused a mod~st
(and probably temporary) drop in pressures inthe Prairie du Chien-Jordan aquifer:
However, lower water levels in the Prairie du Chien-Jordan aquifer have likely
resulted in a slow drop in water levels within the sand-gravel deposit in the form of
leakage where the gravelly clay separating layer is thin or absent. The sand-gravel
unit does not receive much recharge from above because it is overlain by thick clay
deposits - most of its recharge has likely come as upward flow from the Prairie du
Chien-Jordan aquifer during times when water levels in the Prairiedu Chien-Jordan
aquifer were higher. This is probably not a condition that would have revealed itself
by testing.
Well No. 11 continues to supply water from the sand-gravel unit but this well is
screened deeper and can tap into water in the bottom of the sand-gravel aquifer. Well
Nos. 5 and 6 have very little available draw down and will not yield much water
unless regional groundwater levels rebound. Some rebound may occur this winter but
it is unlikely that storage within the sand and gravel aquifer will be replenished before
next summer's high demand period. Because of the close proximity of Well Nos. 2,
5, and 6 future pumping of Well No.2 will likely cause significant well interference
effects that will render Well Nos. 5 and 6 unusable in the future.
The City of Chanhassen · A growing community with clean lakes, quality schools, a charming downtown, thriving businesses, winding trails, and beautiful parks. A great place to live, work, and play,
T odd Gerhardt
Water Supply Update
September 10, 2007
It is preliminarily recommended that the pump in Well No.2 be lowered to near the
bottom of the well (it is currently set with the pump intake at the top of the Prairie du
Chien). This should restore the yield of this well for future use. However, future
pumping of Well No.2 will likely render Well Nos. 5 and 6 unusable. An additional
Prairie du Chien-Jordan aquifer well should be drilled to make up for some of the loss
from Well Nos. 5 and 6. This new well should be preferably at least 1,000 feet from
another high-capacity well. The Prairie du Chien-Jordan aquifer is generally a much
more reliable source of water than the glacially deposited sand-gravel aquifers in the
area. It is also recommended that the so-called "Junior High School Well" (a City
standby well in the Prairie du Chien-Jordan aquifer) be put into service with the pump
installed as deep as practical. The City will need to work with the Department of
Natural Resources and Department of Health staff in permitting these wells.
Sprinkling Ordinance Recommended Changes
Staff would like to have the City Council consider changes to the City Code regarding
Lawn Sprinkling. Based on the finding from this summer's water supply problem, staff
would like to request two changes to the code.
1. Prohibit lawn sprinkling from lO a.m. to 5 p.m. daily. The normal daily high
water demand periods are from 3:30 a.m. to 9 a.m. and then again from 6:00 p.m.
to 11 p.m. During these peak demands, water is drawn out of the water towers.
The time between 9 a.in. and 6 p.m. is critical for the system to catch up before
the next peak demand occurs. By restricting sprinkling during the daytime hours,
the system would have time to catch up and hopefully stop pumping for a time.
This should allow the aquifers some time to recharge and not be constantly
pumped. This change is required because, currently, demand during the day does
not allow the water towers to fill up and the system to rest before the next peak
period. Also, during the summer days, it is not recommended to water during the
day since a substantial amount of the water is lost to evaporation.
As a reference, other communities that do not allow watering during the day are:
Bumsville, Eden Prairie, Edina, Hopkins, Lakeville, Minnetonka, Plymouth, St.
Louis Park and Savage.
2. Staff is also recommending changing the odd/even sprinkling restriction to year-
round. Currently, the odd/even restriction is from May 1 to September 30. This
change would make it easier for property owners to follow the ordinance and not
potentially have to change settings on sprinkler systems seasonally. Based on
research, established lawns do not need to be watered on a daily basis. Watering
every other day is sufficient to keep a healthy lawn. New sod or seed would be
exempt from this restriction by a no-fee permit.
This change would also make our ordinance more consistent with other
surrounding communities. Other communities that have a year-round odd/even
lawn sprinkling ordinance include: Eden Prairie, Edina, Minnetonka, Savage,
Eagan and Bumsville.
G:\ENGIPUBLlCIPW024- 2007 water suppy issueslbkgd 091007 update wrksession .doc
The Hydrologic Cycle...
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Feet above mean sea level
MINNEAPOLIS
750
600
450
300
150
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Miles 0
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