WATER-SUPPLY WELLS
4725.4050 APPLICABILITY.
Parts 4725.4050 to 4725.6050 are standards that apply to water-supply wells in
addition to the requirements in parts 4725.2010 to 4725.3875.
This portion of the rules applies only to water-supply wells. Construction of water-supply wells must
follow both the requirements in this section (Minnesota Rules, parts 4725.4050 through 4725.6050), and
the general well and boring rules (Minnesota Rules, parts 4725.2010 through 4725.3875). If a
requirement of the water supply rule, Minnesota Rules, parts 4725.4050 through 4725.6050 is
more restrictive than a requirement of the general rules regulating all wells and borings (Minnesota Rules,
parts 4725.2010 through 4725.3875), the more stringent standard must be followed.
Water-supply wells include all “wells” except monitoring and dewatering wells. Water-supply wells
include wells used for private and public drinking water, irrigation, heat pump water supply, industrial or
commercial use, air conditioning, and groundwater remediation.
STAT AUTH: MS s 103I.101; 103I.111; 103I.205; 103I.221; 103I.301; 103I.401;
103I.451; 103I.501; 103I.525; 103I.531; 103I.535; 103I.541; 103I.621; 144.05;
144.12; 144.383; 157.04; 157.08; 157.09; 157.13
HIST: 17 SR 2773; 33 SR 211
4725.4100 [Repealed, 17 SR 2773]
4725.4150 BENTONITE DRILLING FLUIDS.
Bentonite drilling fluids used to construct or repair a water-supply well must have
a measurable chlorine residual at all times during drilling or repair, except for a
remedial well where chlorine will interfere with water quality analysis or
remediation.
Bentonite drilling fluids must contain chlorine at all times during drilling. The rules do not require a
specific concentration, only that a measurable quantity exists. Chlorine concentrations can be easily
measured to 1 part per million (1 ppm) or less with chlorine test strips or with a chlorine test kit. Because
chlorine will be used up by organic and other material in the drilling fluid, chlorine will need to be added
to the drilling fluid as drilling progresses, possibly as often as each time a drill rod is added to the drill
string.
STAT AUTH: MS s 103I.101; 103I.111; 103I.205; 103I.221; 103I.301; 103I.401;
103I.451; 103I.501; 103I.525; 103I.531; 103I.535; 103I.541; 103I.621;
HIST: 33 SR 211
4725.4200 [Repealed, 17 SR 2773]
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4725.4250 LIMESTONE OR DOLOMITE WATER-SUPPLY WELLS.
Limestone or dolomite bedrock in Minnesota consists of deposits of magnesium or calcium carbonate
(lime) deposited in shallow seals from chemical precipitation or the remains of shells, and other sea life.
Over 400 million years, the formations fractured, eroded, and in some instances developed cavities and
caves. Limestone and dolomite can dissolve in weak acids, such as plant acids, creating interconnected
drainage systems, and in the extreme, sinkholes, caverns, and the cave systems of southeastern
Minnesota.
Water moves through sand or sandstone formations by slow movement through small pore spaces
between sand grains. Water moves through limestone or dolomite through fractures, solution cavities, and
in some cases, caves. A typical groundwater flow rate in sand is 100 to 200 feet in a year. Water can
move 100 to 200 feet per minute through heavily fractured limestone or dolomite. Water movement
through limestone and dolomite in southeastern Minnesota has been measured at over 3 miles in a day.
This very rapid water flow, without the natural filtration provided by flow through a fine-grained porous
media, makes wells completed in a limestone or dolomite highly susceptible to contamination. Since this
filtration which removes microorganisms and attenuates chemical contaminants is often minimal in the
limestone or dolomite, it must occur in the overlying materials before reaching the limestone or dolomite.
Since the limestone or dolomite is susceptible to contamination, the construction of nonpotable watersupply wells is also of concern, so that the construction does not spread contamination.
Subpart 1. Applicability. This part applies to water-supply wells, including
private drinking water supply, public drinking water supply, irrigation,
commercial, groundwater thermal exchange, and remedial wells completed in or
below limestone or dolomite. This part does not apply to borings, monitoring wells,
or dewatering wells.
Limestone or dolomite formations include the Cedar Valley through Galena groups, Platteville formation,
and Prairie du Chien group (Shakopee, New Richmond or Root Valley, and Oneota). The New Richmond
or Root Valley is a thin sandstone layer within the Prairie du Chien, which occurs in some locations in
southern Minnesota. It is considered part of the Prairie du Chien group and subject to the same restrictions
as the Prairie du Chien for water-supply well construction.
Subp. 2. Plastic casing. Plastic casing must not be used as an outside casing in a
water-supply well cased more than five feet into limestone or dolomite bedrock,
except that an inner plastic casing may be installed entirely inside an outer steel
casing in accordance with part 4725.2250, subpart 8.
If plastic casing is used to complete a well in limestone or dolomite bedrock the open hole must be drilled
prior to casing installation. As required by Minnesota Rules, part 4725.2650, subpart 9a, the plastic casing
must be extended 5 feet into the limestone or dolomite bedrock, and the annular space surrounding the
casing in bedrock must be filled with neat-cement or cement sand grout. There is no bore hole size
requirement if the limestone or dolomite formation is the uppermost bedrock. The rule is designed to
assure a solid seal into the bedrock, but not allow plastic casing to extend so far that the casing may be
deformed by excessive heat generated from cement grout in a large cavity.
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Subp. 3. Bore hole size. A casing that extends more than ten feet into limestone
or dolomite bedrock must be installed in a bore hole a minimum of 3.0 inches
larger, or 3.5 inches larger for casings deeper than 100 feet and larger than 12
inches inside diameter, than the outer diameter of the casing or couplings,
whichever is larger.
Subp. 4. Use of limestone or dolomite for potable supply.
A. A water-supply well used to provide potable water must not be completed in
limestone or dolomite bedrock unless the limestone or dolomite bedrock is overlain
by at least 50 feet of unconsolidated material, sandstone, or shale that extends in
all directions around the well for a minimum one mile radius. Limestone or
dolomite bedrock includes the Cedar Valley through Galena groups, Platteville
formation, and the Prairie du Chien group.
Subpart 4, A, concerns potable water-supply wells only.
In order to finish a potable water well in a limestone or dolomite formation, the limestone or dolomite
formation must have at least 50 feet of unconsolidated materials (glacial drift, alluvium) or noncarbonate
rock (sandstone or shale) over the limestone or dolomite formation at the drill site and everywhere
within a 1-mile radius of the well site.
Many of the MGS’s county atlases contain maps that illustrate well construction in limestone areas. In
addition, the MDH Well Management Section has produced maps available to contractors, which
illustrate well construction in some limestone areas not contained in the county atlas series. Geologic
atlases and MDH limestone well construction maps will be available for additional counties in the future.
Any well connected to plumbing fixtures (toilets, sinks, hose bibs, etc.) in a dwelling, or connected to
plumbing fixtures where water is available for human consumption or human contact is considered a
potable supply.
B. The commissioner may establish limestone and dolomite bedrock well
construction maps identifying areas of known or suspected contamination, areas
with unique hydrologic or geologic conditions, or areas where protective
conditions exist, including low permeability overlying materials, favorable
groundwater gradients, or reduced contaminant loading in recharge areas. The
conditions in item A do not apply in areas designated as approved for drilling on
the limestone and dolomite bedrock well construction maps published by the
commissioner.
Subp. 5. Water-supply well completed in limestone or dolomite. Where a potable
water-supply well meeting the conditions of subpart 4 or a non-potable watersupply well, is completed in limestone or dolomite bedrock, the following apply.
A. If the static water level in the well is more than ten feet above the limestone
or dolomite:
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(1) steel casing must be installed into the limestone or dolomite by driving the
casing with a drive shoe through unconsolidated materials or sandstone into, but
not more than ten feet into, the limestone or dolomite; or
(2) a bore hole must be drilled into the limestone or dolomite and steel casing
installed to the bottom of the bore hole. If the bore hole extends more than ten feet
into the limestone or dolomite, the bore hole must be a minimum of 3.0 inches
larger, or 3.5 inches larger for casings deeper than 100 feet and larger than 12
inches inside diameter, than the outside diameter of the casing or couplings,
whichever is larger. The annular space surrounding the casing must be grouted
from the bottom of the casing to the top of the bedrock with neat-cement grout or
cement-sand grout. The unconsolidated materials portion of the annular space
must be grouted according to part 4725.3050, subparts 1 to 3.
B. If the static water level in the well is less than ten feet above the limestone or
dolomite, a bore hole must be drilled a minimum of 3.0 inches larger, or 3.5 inches
larger for casings deeper than 100 feet and larger than 12 inches inside diameter,
than the outside diameter of the casing or couplings, whichever is larger. The bore
hole must extend a minimum of 20 feet below the static water level. Steel casing
must be installed to the bottom of the bore hole. The annular space from the
bottom of the casing to the top of bedrock must be filled with neat-cement grout or
cement-sand grout. The unconsolidated materials portion of the annular space
must be grouted according to part 4725.3050, subparts 1 to 3.
A nonpotable water-supply well, such as an irrigation well, completed in limestone or dolomite does not
require 50 feet of unconsolidated material or firm insoluble rock that extends around the well or boring
for a 1-mile radius.
Subp. 6. Water-supply well completed below limestone or dolomite. A watersupply well completed below limestone or dolomite where the conditions of subpart
4 apply must be constructed by drilling a bore hole a minimum of 3.0 inches
larger, or 3.5 inches larger for casings deeper than 100 feet and larger than 12
inches inside diameter, than the outside diameter of the casing or couplings a
minimum of ten feet below the limestone or dolomite, and a minimum of ten feet
below the static water level. Steel casing must be installed to the bottom of the bore
hole, and the annular space from the bottom of the casing to the top of bedrock
must be filled with neat-cement grout or cement-sand grout. The unconsolidated
materials portion of the annular space must be grouted according to part
4725.3050, subparts 1 to 3.
Subpart 6 concerns water-supply wells completed below a limestone or dolomite formation where the
limestone or dolomite cannot be used for a potable water supply. This subpart applies to wells completed
in the St. Peter sandstone underlying the Platteville limestone, or wells completed in the Jordan sandstone
underlying the Prairie du Chien limestone. The casing must extend at least 10 feet into the St. Peter
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sandstone or Jordan sandstone and 10 feet below the static water level. Since these wells are completed in
a sandstone formation, only 10 feet of sandstone or equivalent is necessary to remove microbiological
contaminants that may be in the limestone or dolomite.
Subparts 5 and 6 of this part allow the annular space in unconsolidated materials to be sealed with
cuttings taken from the bore hole except for the top 50 feet which must be sealed by pumping neat
cement, sand-cement, or bentonite grout into the annular space.
Subp. 7. Remedial well in limestone or dolomite. A remedial well is exempt from
the requirement in subpart 5 to extend the casing 20 feet below the static water
level if the well screen or open hole intersects the water table, the casing
terminates no more than ten feet above the static water level, and all casing
installed in limestone or dolomite is grouted with neat-cement grout or cementsand grout.
STAT AUTH: MS s 103I.101; 103I.111; 103I.205; 103I.221; 103I.301; 103I.401;
103I.451; 103I.501; 103I.525; 103I.531; 103I.535; 103I.541; 103I.621;
HIST: 33 SR 211
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295
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4725.4300 [Repealed, 17 SR 2773]
4725.4350 WATER-SUPPLY WELL DISTANCE FROM WATER BODIES;
PROTECTIONS IN FLOOD AREAS.
Subpart 1. Distance from water bodies. The minimum horizontal distance
between a water-supply well and the ordinary high water level of a stream, river,
pond, storm water retention pond, or lake is 35 feet. The isolation distance does
not apply to:
A. an area protected by a flood control structure accepted by the United States
Federal Emergency Management Agency (FEMA), as designated on a FEMA flood
map;
B. a wetland, grassed waterway, depression, culvert, or ditch holding water less
than six months of the year; or
C. an artificial pond holding less than 5,000 gallons of water.
The ordinary high-water level is often the top of a stream or lake bank, denoted by trees such as oaks or
maples which will not survive flooding. The existing water level or shoreline is often not the ordinary
high-water level. The ordinary high-water level of many lakes is available from the DNR or local
governments.
It is important to measure the distance horizontally, not along the slope.
Artificial ornamental ponds holding less than 5,000 gallons are not included in the 35-foot distance
requirement. Ditches, swales, grassed waterways, or culverts are not required to be 35 feet from a well
unless the depression contains water more than six months of the year.
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Subp. 2. Flood protection. A water-supply well must be constructed to prevent
the entry of flood water into the well by:
A. extending the casing at least five feet above the regional flood level;
B. installing a watertight seal and extending the casing ten feet above the
established ground surface, if the regional flood level is more than five feet above
the established ground surface;
C. installing an outer, neat-cement grouted protective casing in accordance
with part 4725.6755 subpart 2, item B, extending the protective casing and well
casing a minimum of two feet above the established ground surface, and installing
a waterproof threaded cap or a waterproof compression seal with drawbolts and a
one-piece top plate on both casings; or
D. extending the casing a minimum of two feet above the established ground
surface, installing a sealed spool, or flowing well pitless unit, and installing a
waterproof, nonvented compression seal.
The requirements in this subpart do not apply to a water-supply well located in
an area protected by a flood control structure accepted by the United States
Federal Emergency Management Agency (FEMA), as designated on a FEMA flood
map
“Regional flood level” is defined in Minnesota Statutes, section 103F.111, subdivision 10, to mean a
flood that is representative of large floods known to have occurred generally in the state and reasonably
characteristic of what can be expected to occur on an average frequency in the magnitude of a 100-yearrecurrence interval. The regional flood is often referred to as the “100-year flood.” This is the flood used
for most building codes and flood insurance determinations.
“Established ground surface” is defined in Minnesota Rules, part 4725.0100, subpart 28, to mean the
intended or actual finished grade (elevation) of the surface of the ground at the site of a well or boring.
The requirement to extend the casing 5 feet above the regional flood level applies only to water-supply
wells located within the designated 100-year flood plain. This is the area marked on FEMA maps.
Maps designating the areas of regional flood plains and flood level information may be available from
county zoning administrators, insurance agents, U.S. Army Corps of Engineers, DNR, MDH, and others.
If a watertight seal is installed in lieu of extending the casing more than 10 feet above the established
ground surface, a vent must not be installed.
STAT AUTH: MS s 103I.101; 103I.111; 103I.205; 103I.221; 103I.301; 103I.401;
103I.451; 103I.501; 103I.525; 103I.531; 103I.535; 103I.541; 103I.621; 144.05;
144.12; 144.383; 157.04; 157.08; 157.09; 157.13
HIST: 17 SR 2773; 33 SR 211
4725.4400 [Repealed, 17 SR 2773]
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End
of
Water-Supply Wells, Limestone, Flooding Section
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