Decorah Edge: A critical water supply component

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Decorah Edge A critical water supply component
Terry Lee Environmental Services Coordinator
Olmsted County, MN 2116 Campus Drive
SE Rochester, MN 55904 [phone] Email
lee.terry_at_co.olmsted.mn.us
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Decorah Edge

• Its the source of about half of Olmsted Countys
  drinking water.
• It functions as a natural water filter that may
  be worth as much as 5 million per year.
• Much of it will be impacted by development in the
  next 25 years.
• Development impacts may be significant and
  irreversible.

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Groundwater Recharge in Olmsted County
Precipitation (30 in/yr.)
Evapo-transpiration (20 in/yr.)
Runoff (4 in/yr.)
Groundwater Recharge (6 in/yr.)
About 1/5 of the annual rainfall infiltrates
through the soils and reaches the underlying
aquifers (aquifers are rock layers that store and
transmit water)
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Step 1. Rainfall infiltrates through soils and
into the cracks and crevices of the underlying
bedrock. This water recharges the Upper Carbonate
Aquifer.
Decorah Edge Groundwater Recharge Area
Step 2. Water in the aquifer is perched on the
clay-rich impermeable shale. Water flows from
the Upper Carbonate Aquifer downhill or towards
an area of lower pressure.
Impermeable Decorah Shale
Step 3. When the flowing water in the Upper
Carbonate Aquifer reaches a breach in the shale,
it again continues its downward migration and
recharges the underlying Lower Carbonate Aquifer.
This breach typically occurs along hillsides at
the edge of the shale.
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Cracks and crevices in the Upper Carbonate
Aquifer store water and allow it to flow freely
The impermeable Decorah shale prevents the
downward flow of groundwater
Water drains from the Upper Carbonate Aquifer
through the soils overlying the Decorah shale
Decorah Edge Exposure on Hwy 52 South
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Improved Geologic Mapping Was Needed to Assess
the Decorah Edge Groundwater Recharge Processes
2001 The 1996 124,000 scale map fitted to the
1200 scale digital elevation model
1988 1100,000 scale map produced as part of the
County Geologic Atlas
1996 124,000 scale map produced at the request
of Rochester Public Utilities
60 acre site north of Century High School
Source Minnesota Geological Survey, 1988 1996
Maps
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Researchers used the improved geology maps to
develop a 3-D illustration of the Decorah Edge
bedrock layers at a 200 acre site north of
Century High School
Cummingsville Limestone (65)
Decorah Shale (40)
Platteville Limestone (25)
St. Peter Sandstone (100)
Glenwood Shale (6)
40 is approximately the height of a four story
building
() maximum thickness of bedrock formations
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Researchers found that Decorah Edge areas
contained some of the most diverse wetlands in
the state
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Development on the Decorah Edge in the City of
Rochester
Even as late as 1977 very little development had
occurred on the hillslopes underlain by the
Decorah shale. This was likely due to their being
steep and wet.
Source Rochester-Olmsted County Planning
Dept.
Most development prior to 1977 had been on the
sands and gravels or the underlying St. Peter
sandstone and Shakopee limestone
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Decorah Shale
Groundwater Flow Direction
Homes and streets in NE Rochester built before
Decorah Edge groundwater recharge processes were
understood
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Water Being Pumped To The Storm Sewer From The
Basement Of A Home Located On The Decorah Edge
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Decorah Shale
Heavy equipment stuck in a residential lot on
Rocky Creek Drive (2005)
New subdivision in NE Rochester being constructed
on the Decorah Edge
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Typical Costs of Basement Repairs
Baseboard Drainage 2,000 Subfloor Drainage
4,000 Sump Pump Installation 600 Excavation
Tiling - Damp-proofing 6,000 -
Water-proofing 9,000 Wall Anchoring 3,000
Source 1999 survey of Rochester waterproofing
contractors. Costs do not include basement
cleanup, repairs, or rug and furniture
replacement.
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Unrecovered City Costs/Household/Year if Basement
Drainage is Directed to the Rochester Sanitary
Sewer

• Sump Discharges . 750
• Sub-Floor Drains . 225
• Gravity Basement Drainage 22
• Rain Gutters . 120

Source 1999 estimate based on typical flow
volumes. Only gravity drainage systems like the
Beaver System are allowed to drain to sanitary
sewer under the current Rochester Ordinance.
Rain gutters are shown for flow comparison
purposes.
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Aquifers in Olmsted County
The Decorah shale separates the Upper Carbonate
Aquifer from the Lower Carbonate Aquifer
Lower Carbonate Aquifer is the source of water
for all wells drilled since the late-1950s
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Geologic cross-section shown in next 3 slides
Township Boundaries
Upper Carbonate Aquifer hasnt been used for new
wells since the County adopted a Well Code in
1960
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Aquifers and Groundwater Flows in the
Rochester Area
The Upper Carbonate Aquifer Recharges the Lower
Carbonate Aquifer at the Decorah Edge
Upper Carbonate Aquifer
Lower Aquifer
The Lower Carbonate Aquifer drains into the
Zumbro River, or is withdrawn by water supply
wells and discharged into the Zumbro at the Water
Reclamation Plant
Aquifers and recharge areas are as much a part of
the Rochester water supply as are wells, pumps,
valves, pipes, and towers.
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Percent of the City of Rochester Aquifer
Recharge by Source (1987-88)
Zumbro Valley Outside Rochester - 24
Decorah Edge - 49
Buried Valley - 2
Sewered Area - 9
Leakage Through Decorah Shale - 7
Stream Leakage - 9
A US Geological Study completed in 2000
attributed a slightly higher percent of overall
recharge to the Decorah Edge (62)
Source US Geological Survey, 1990
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Groundwater Travel Times in the Rochester Area
Maximum residence time in Upper Carbonate Aquifer
1 to 25 yrs (most is 10 to 15 yrs)
Maximum time for water entering the Lower
Carbonate Aquifer to reach a Rochester Water
Supply Well 50 to 200 yrs
Source US Geological Survey, 2000
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Most of Olmsted County is characterized as
a Karst terrain
Karst terrains are very susceptible to
groundwater pollution -- contaminants can enter
the ground and move rapidly through cracks and
crevices.
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Nitrate is the Most Common Chemical Pollutant
Found in Drinking Water in Olmsted County

• High nitrate concentrations are toxic for infants
  in their first three months of life, consequently
  the EPA has established a 10 ppm nitrate limit
  for public drinking water supplies.
• The Upper Carbonate Aquifer can no longer be used
  in the County as a source of drinking water due
  to nitrate pollution.
• There are now areas of the County where the Lower
  Carbonate Aquifer has nitrate pollution levels
  that limit its use for drinking water supplies.

NO3-N
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Nitrate Pollution Probability
The probability of nitrate pollution increases
from west to east
Low
Medium
High
Winona
Olmsted
Dodge
Since nitrate sources are abundant throughout SE
MN, the probability of nitrate pollution is
largely determined by how long it takes nitrate
to leach down into underlying aquifers
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,,, Nitrate has already reached underlying
aquifers in three communities east of Olmsted
County
Lewiston
St. Charles
Utica
In Lewiston, high nitrate water is blended with
high radium water from a deeper well -- the
blended water has 6.4 ppm nitrate and 4.3 pCi/L
radium. Utica may soon be forced into a similar
compromise.
Drinking Water Standard
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Nitrate Pollution Probability in Olmsted County
Low
Medium
High
Winona
Olmsted
Dodge
Decorah is Absent
The highest probability of nitrate pollution
occurs in areas where the Decorah shale is absent
and water recharges directly into the Lower
Carbonate Aquifer
Decorah is Present
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Nitrogen Fertilizer Sales Trends in Minnesota
1960-2003
Much of the groundwater being pumped by Rochester
Public Utilities recharged prior to the use of
commercial fertilizer and before there was
significant development on the Decorah Edge
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Examples of U.S. chemical consumption (tons/yr)
Chemical use has increased significantly since
World War II. The full impact on the City water
supply of this increase will not likely be known
for several more decades.
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This aquifer was abandoned in the 1950s due to
extensive pollution
Upper Aquifer
Studies in the late-1980s found that half of
Rochesters municipal water comes from Upper
Aquifer discharges at the Decorah Edge
Decorah Shale
Lower Aquifer
Water quality studies in the early-1990s found
few pollution problems in the underlying aquifer.
Later studies found that biological processes
at the Decorah Edge filter pollutants from
groundwater.
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Decorah Edge Water Monitoring Study
Decorah Shale
General Direction of Groundwater Flow
Glenwood Shale
Wetland
Groundwater Monitoring Points
The U.S. Geological Survey has been monitoring
changes in water quality at a Decorah Edge study
site in NE Rochester since 2003.
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Nitrate Monitoring Results for Transect 4 at
Decorah Edge Study Site
Nitrate decreased from 9.8 ppm to 0.7 ppm as
recharge water flowed over the Decorah shale (May
2004)
17.2
9.8
6.6
Cummingsville
0.7
Decorah
Platteville
2.6
Glenwood
1.9
St. Peter
The Lower Carbonate Aquifer (St. Peter-Prairie du
Chien-Jordan) provides nearly all of the drinking
water in Olmsted County
The drinking water standard for nitrate is 10 ppm
higher levels are toxic for infants
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Rochester Water Supply Area
Decorah Edge
Area of the Lower Carbonate Aquifer that supplies
the City of Rochesters municipal wells
Fillmore County adopted a Decorah Edge Zoning
Amendment in 2003
Karst area of the Upper Carbonate Aquifer that
drains to the Decorah Edge
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By 2045 most of the Rochester Decorah Edge
recharge area will be fully developed
Ancient river valley filled with clay-rich
glacial till recharge rates are very low in
this setting
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Rochester Population Growth and Water Demand
(1983-2003)
Overall water demand has increased 50 since 1983
and future population growth will put even
greater demands on local aquifers
1983
2003
1993
Source Rochester Public Utilities
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Decorah Edge Summary

• The Decorah Edge is a critical recharge area for
  drinking water supplies in Olmsted County and
  current development practices have the potential
  to reduce both the quantity and quality of the
  recharge.
• The proposed ordinance amendments would guide
  development in the Decorah Edge area so that
  recharge processes will be preserved.