The Story of Groundwater in Olmsted County
A deep dive into the source of our drinking water
Groundwater in Olmsted County began as water from rain and snowmelt that soaks into the soil and moves into the rock layers below. The rock layers create formations that can hold water, known as aquifers or reservoirs. Olmsted County uses the groundwater stored in the aquifers for drinking water. Drinking water wells pull the water from underground to the surface.
Geology & Karst
Olmsted County has sedimentary bedrock – these are layers of limestone, dolostone, shale, and sandstone. Limestone and dolostone are carbonate rocks. Carbonate rocks are dissolvable by water. Our karst geology forms as water dissolves the rock and creates cracks and underground passageways. Surface water from rain, snowmelt, rivers, streams, and lakes can quickly make its way underground through the cracks and openings.
As you scroll through the story, click on the arrows to the right of an image to advance a slideshow. Click on an image or video to expand into a new window. If a video is embedded, select the triangle "play" button to begin viewing.
Watch this 10 minute video to see how groundwater moves through Southeast Minnesota (Produced by the Minnesota Department of Agriculture).
Explore a virtual tour map of “Karst in Minnesota” to learn more about the close connection between groundwater and surface water in Southeast MN. The Minnesota Pollution Control Agency produced the virtual tour map.
Have you noticed the different layers of rock as you drive around Olmsted County?
The illustration on the left, called a stratigraphic column, shows the rock layers, called bedrock unit formations. Each unit is made of marine sedimentary rock from the early Paleozoic age, 525 – 445 million years ago, when Minnesota was covered by a shallow ocean.
Imagine the rock formations under our feet stacked like the layers of a three tiered cake. The top layer (A) includes the uppermost bedrock unit formations of the Galena, followed by the middle formations (B) of the St. Peter and Prairie du Chien group, and finally the bottom tier of the cake (C); the Jordan, where we draw the majority of our drinking water.
Each of these formations are exposed at the surface somewhere in the County.
The lower aquifers of the St. Peter, Prairie du Chien, and Jordan, supply most of Olmsted County's drinking water. The map below illustrates the first encountered bedrock layer across the County. If you were to dig through the layer of glacial sediment, clay and sand under your feet, this is the first layer of bedrock you would encounter. Scroll in on the map to see what the first encountered bedrock layer is where you live.
Bedrock Geology
Drinking Water
Nearly all drinking water in Olmsted County is sourced from groundwater.
Decorah Edge
Our Natural Filter
Have you ever noticed water cascading from rock cuts along Highway 52?
If so, you've seen the workings of the Decorah Edge. Studies in the early 1990's found that there were few pollution problems in the lower aquifers overlain by the Decorah Shale. Later studies found that biological processes at the Decorah Edge filter pollutants from groundwater.
The Decorah Edge is called a confining unit because it hinders the downward movement of water into aquifers below it. The extent of the Decorah Edge in Olmsted County is shown in green on the map below.
Decorah Extent
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Groundwater Contamination
Water flowing across the landscape can make its way below ground through the cracks, crevices, and other karst features. Once underground, water, and any pollutants it may carry, can travel quickly through complex passageways.
The close connection between ground and surface water and the unpredictability of karst makes our groundwater very susceptible to pollution. This video from the MN Department of Agriculture illustrates how contaminants (like Nitrate) move in Southeastern Minnesota.
Olmsted County's groundwater is a high-quality resource. How we manage the land in sensitive areas has introduced contaminants into the groundwater system.
The upper carbonate aquifer (Galena) has not been used for new well construction since the county adopted its well ordinance in 1958 due to extensive pollution.
Naturally occurring and human-made pollutants are present in SE MN groundwater. Pollutants come from agricultural, industrial, and residential sources.
Can you identify potential sources of pollution in this illustration?
Nitrate-Nitrogen
Nitrate is a primary chemical component of fertilizer and manure that can contaminate drinking water supplies. While nitrate in drinking water above 10 ppm (the federal maximum contaminant level) can lead to a fatal condition called “blue baby syndrome” in infants, it can also be an indicator of other contaminants in the water.
The Minnesota Department of Health has established that a level of 3 ppm or above indicates that “human-made sources of nitrate have contaminated the water and the level could increase over time”.
A private well should be tested annually for nitrate.
Geologic conditions have a significant impact on nitrate transport in Olmsted County. In areas where the Decorah unit does not provide the extra level of protection, nitrate concentration is increasing in the deeper groundwater.
Of the private wells tested for nitrate at the Olmsted County Environmental Lab, sixty-five (65%) percent have a non-detectable level of nitrate. Fourteen percent (14%) have a detectable level of nitrate below 3 ppm.
Twenty percent (20%) of wells that have been tested are considered impacted by human contributed sources of nitrate (nitrogen fertilizer, septic systems, animal manure) with levels above 3 ppm, and four percent (4%) of wells exceed the health risk limit of 10 milligrams per liter or parts per million.
Nitrate - nitrogen can also be an indicator of other contaminants such as agricultural pesticides in the water.
In summer 2019, the MN Department of Agriculture (MDA) offered agricultural chemical testing for a total of 133 pesticide and pesticide break-down products (degradates). In total, 123 well owners took advantage of the follow-up monitoring that was offered.
Twenty-six (26) of 133 pesticides and pesticide break-down products were detected. No pesticide or pesticide degradate was detected at a level that exceeded a Health Risk Limit (HRL) or Risk Assessment Advice (RAA).
The cumulative health risk was provided to each well owner with the understanding that low levels of multiple pesticides in drinking water is not well understood at this time. Vulnerable populations such as infants, children, and pregnant/nursing women were cited to be at greatest risk.
The most commonly encountered pesticides or pesticide degradates are corn and soybean herbicides that are widely used in the County including:
- Metolachlor ESA (detected in 82% of wells tested), Didealkylatrazine (detected in 54% of wells tested), Alachlor ESA (detected in 47% of wells tested),
As the concentration of nitrate increases, the likelihood of detecting at least one pesticide also increases. For wells with nitrate concentrations above detection (0.40 mg/L nitrate), the detection frequency for detecting at least one pesticide was 91% ( MDA PWPS, 2019 ). Regardless of aquifer, well construction, or well use, the pesticide and nitrate relationship remains the same.
Other Contaminants
Chloride
Chloride is naturally present in SE MN groundwater due to the many minerals that exist in the state's bedrock aquifers. Natural weathering of rock can release chloride into groundwater. Natural background levels of chloride in southeastern Minnesota groundwater are very low and have been identified as less than 2 ppm (Tipping, 1994) and below detection (less than ~0.4 ppm) in Olmsted County's long-term groundwater monitoring network (Crawford, 2012).
In Olmsted County, chloride in groundwater originates from salt applied to roads in winter for deicing, brine applied for dust control in summer, water softening byproduct and infiltration from septic systems, and potassium chloride (potash) fertilizer. The federal secondary drinking water standard for chloride is 250 ppm due to aesthetic concerns in drinking water and its harmful affect on aquatic life.
Statewide, chloride concentrations are highest in shallow groundwater underlying commercial and industrial areas.
Chloride concentrations in groundwater within the Rochester central metropolitan area have increased since the 1980s, and higher average chloride concentrations are seen in areas where the Decorah confining layer does not provide the extra level of protection.
In Olmsted County, chloride has been used as a tracer for indicating contamination in private wells as it moves easily with infiltrating water.
Of the approximately 6,400 wells tested in Olmsted county since 1970, 33% have no detectable chloride (Crawford, 2015). Of those wells with enough data for trend analysis, 22% of them are trending upward.
Bacteria
Coliform bacteria may enter our groundwater system from nearby septic systems or feedlots. Karst features, such as the sinkhole shown here, can provide a direct connection for surface water to enter the groundwater. Bacteria can also be introduced when a well or plumbing distribution system is damaged or not properly maintained. Coliform bacteria are microbes that typically do not make a person ill; however, they are used as an indicator that other potentially harmful viruses, bacteria, and parasites may also be found in the water.
When coliform bacteria is present in the form of fecal coliform or E. coli, it can be harmful to human health.
In Olmsted County, 30% of wells sampled over the past five years had a positive coliform bacteria result, and ~2% of water samples tested had E. Coli bacteria which can cause serious gastrointestinal illness.
Private well owners should test their well for coliform bacteria every year based on the MN Department of Health's recommendations.
Manganese
Manganese occurs naturally in the bedrock and soil in SE MN. For infants, the health risk limit (HRL) is set at 100 micrograms per liter or part per billion (ug/L or ppb). Children and adults who drink water with high levels of manganese for extended time frames may have problems with memory, attention and motor skills. In Olmsted County, approximately seven percent of wells tested exceed the health risk limit for infants and an additional four percent of wells tested exceed the health risk limit for adults.
The MN Department of Health (MDH) recommends testing well water for manganese at least once to determine if treatment is needed and especially if an infant is in the household.
Based on a 2012 MN Department of Health (MDH) study, groundwater in the Paleozoic bedrock aquifers of SE MN tends to have low levels of manganese (below 100 ug/L), with higher concentrations located along the western boundary of the region.
While manganese data for Olmsted County is limited, this statewide map illustrates the probability of having manganese in groundwater at concentrations greater than 100 parts per billion (ppb) is very low.
Arsenic
Arsenic is a naturally occurring metal in our bedrock and soils of SE MN. Beginning in 2008, all newly constructed private wells in MN are required to test for arsenic. The federal drinking water limit for community water supply systems is 10 micrograms per liter of water (ug/L or ppb), however, there is no safe level of arsenic. Consuming water with arsenic levels lower than the standard over many years can still increase cancer risk.
Of wells tested in Olmsted County, 32% had a detectable level of arsenic, and 4.6% had concentrations > 2 micrograms per liter of water (ug/L or ppb).
It is recommended that your well is tested for arsenic at least once.
Click on the map to interact and view arsenic results in our area.
Arsenic in MN has a strong correlation with the Des Moines lobe till, a clay rich material which is generally found further west of Olmsted County.
Testing
Nitrogen Reduction Strategies
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Resources
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