Recommended Design Criteria for Ground Water Monitoring Wells...
"Wastewater Treatment Facilities"...
General...
The installation of ground water monitoring wells at wastewater treatment facilities may be essential when facilities such
as infiltration-percolation systems, stabilization and/or aeration ponds, wetlands, or employ land application techniques, have the potential for degrading the State's ground water quality. Discharge to ground water at these facilities is caused
by seepage or direct discharge of effluent through the soil profile. The necessity for monitoring wells and/or ground water discharge plans is based on a number of factors, including the soil type, depth to ground water, strength of the waste, mobility of the waste, and general quality of the ground water. If the potential for ground water degradation is high, monitoring wells are necessary to insure that these discharges are not degrading the State's ground water quality beyond
the ground water quality standards, especially when they are in close proximity to water supplies and other facilities
which can be impacted by ground water contamination.
Monitoring Well Drilling Methods...
1. Criteria for Drilling Method Selection :
The following factors should be considered when selecting an appropriate drilling method :
1. Hydrologic Information
2. Type of formation
3. Depth of drilling
4. Depth of desired screen setting
5. Types of pollutants expected
6. Location of drill site, i.e., accessibility
7. Design of monitoring well desired
8. Availability of drilling equipment
2. Overview of Various Drilling Methods :
a. Hollow-Stem Augering : Hollow-stem augering is one of the most desirable drilling methods for constructing
monitoring wells. No drilling fluids are used and disturbance to the geologic material penetrated is minimal. Auger rigs
are not generally used when consolidated rock must be penetrated and drilling depths are usually limited to less than 150 feet. In formations where the borehole will not stand open, the monitoring well can be constructed inside the hollow-stem augers prior to their removal from the borehole. The hollow-stem has the advantage of allowing continuous in situ geologic sample collection without removal of the augers.
b. Solid-Stem Augering : The use of solid-stem augering is most useful in fine grained, unconsolidated materials
that will not collapse when unsupported. The method is similar to hollow- stem except the augers must be removed from the borehole to allow insertion of the well casing and screen. In situ geologic samples are difficult to collect when using a solid-stem. In many cases it is necessary to rely on the cuttings, which come to the surface, for geologic sampling. This
is an undesirable method since the exact depth at which the cuttings come from is not known.
c. Cable-Tool Drilling : Cable-tool drilling is one of the oldest methods used in the water well industry. Even though the rate of penetration is slow, this method offers many advantages for monitoring well construction. With the cable-tool method, excellent formation samples can be collected and the presence of thin permeable zones can be detected. As drilling progresses, a casing is normally driven which provides temporary support for the borehole allowing construction of the monitoring well within the casing.
d. Air Rotary Drilling : In air rotary drilling, air is forced down the drill stem and back up the borehole to remove the cuttings. This type of drilling is particularly well suited for fractured rock formations. If ground water monitoring
for volatile organics is planned, the drilling air must be filtered to insure that oil from the air compressor is not introduced to the formation being monitored. Air rotary should not be used in highly contaminated environments because
the water and cuttings blown out of the hole are difficult to control and can pose a hazard to the drill crew and observers. Where volatile compounds are of interest, air rotary can volatilize these compounds causing water samples withdrawn from
the borehole to be unrepresentative of in situ conditions. The use of foam additives to aid cuttings removal can introduce organic contaminants into the monitoring well.
e. Mud Rotary Drilling : Mud rotary drilling is probably the most popular method used in the water well industry. However, mud rotary drilling does have some disadvantages for monitoring well construction. With mud rotary, a drilling
fluid (usually a bentonite water mix) is circulated down the drill stem and up the borehole to remove cuttings. The
drilling mud creates a wall on the sides of the borehole which must be removed from the screened area by development procedures. With small diameter wells, complete removal of drilling mud is not always achieved. The ion exchange potential
of most drilling muds is high and may effectively reduce the concentration of trace metals in water entering the well. In addition, the use of biodegradable organic drilling muds (i.e. Baroid, Revert, etc.) can introduce organic components to monitoring well samples.
Permits and Licensing...
The well driller shall be a licensed water well contractor in the State of South Dakota. The well driller must apply for
a variance if the monitoring wells will not meet the South Dakota Well Construction Standards, ARSD 74:02:04:23.
Decontamination of Drilling Equipment and Materials...
1. Condition of Drill Rig and Equipment : Prior to entering the work site the condition of the drill rig and equipment shall be such that it is not a potential source for monitoring well contamination. Leaking equipment seals or leaking tanks containing fluids should not be brought on-site.
2. Procedures to be Used for Cleaning Equipment : All steam cleaning must be done on-site or at an approved off-site location. Any equipment that will not be used at the site should be removed prior to entering the site. The drill rig must
be steam cleaned with water that is from a potable source. All drill rods, bits, casing, samplers, pipe wrenches, etc., should be laid on supports and cleaned until all visible signs of grease, oil, mud, etc. are removed. Cleaned equipment should not be handled with soiled gloves. Surgeon's gloves or new clean cotton work gloves should be used. The use of new painted drill bits and tools is not recommended since paint chips could be introduced into the borehole. All water tanks, pumps, mud pans, hoses, including hoses and tanks used to transfer water from the potable source must be cleaned. Fittings
on the drilling equipment may be lubricated with non-petroleum based substances, such as vegetable oil. Every precaution
must be taken to prevent contamination of the well with oil, grease or any other substance. Lubricants must not be used on drilling and sampling tools or fittings.
3. Decontamination of Materials : At a minimum, casing and well screen should be washed with detergent and rinsed thoroughly with clean water. To ensure that these materials are protected from contamination prior to placement in the borehole, materials should be covered (with plastic sheeting or another material) and kept off the ground.
4. Decontamination of Well Development Equipment : All pumps used in well development must be steam cleaned prior to developing each monitoring well. Pumps which leak or may otherwise cause contamination must not be used. Electrical tape should not be used to attach wires to the discharge pipe of submersible pumps. Wire should be attached using stainless
steel or plastic clips. Air compressors used for development must be equipped with operable oil traps and a filter.
Nitrogen gas if used for development must be regulated and passed through an oil trap and filter before it enters the
well.
Monitoring Well Design Components...
1. Location and Number : A minimum of three (3) monitoring wells must be installed to determine ground water flow direction. One well should be located in an area upgradient of the facility with two located downgradient. However, the number of monitoring wells will also depend on the size of the facility. The downgradient wells should not be spaced more than 500 feet apart and should not be located more than 200 feet outside of the site perimeter.
2. Diameter : Monitoring wells can be constructed of casing and screen materials that are a minimum of two inches
inside diameter. The borehole into which the monitoring well is to be installed must be a minimum of four inches greater
in diameter than the casing (i.e. a two inch well must be installed in a six inch borehole).
3. Casing and Screen Material : Well casing and screen can be made of stainless steel, teflon, or polyvinyl chloride (PVC). If PVC is selected for well construction only those materials that meet or exceed the National Sanitation Foundation (NSF) Standard 14 must be used. Rigid PVC materials used for casing and well screen which meet or exceed NSF Standard 14
are essentially free of plasticizers. All casing and screen must be supplied with threaded flush joints or threaded
couplers, PVC casing and screen must not have glue joints. The well casing should extend two feet above the ground
surface when the well is completed.
4. Screen Length, Depth of Placement and Gravel Packing : The monitoring well(s) must be drilled to a depth that will monitor the first water encountered. The well screen should extend 5-15 feet into the water table with sufficient screen above the water table to detect any substances less dense than water. The well screen length should be based on water table fluctuations and other site conditions (such as confining layers). The well screen must be installed into a stable borehole. If the borehole tends to cave in or "blowout" steps must be taken to stabilize the hole prior to screen installation. The screen gravel-pack must be composed of prewashed, uniform (graded) quartz sand. The size of the gravel-pack chosen must be compatible with the slot size of the well screen. For natural-packed wells (no gravel-pack), where relatively homogeneous, coarse materials predominate, the screen slot size must be compatible with the formation. Placement of the gravel-pack
should be done carefully to avoid bridging in the borehole and to allow uniform settling around the screen. A tremie pipe
can be used to guide the sand to the bottom of the hole and around the screen. If the well is shallow the sand can be
poured from the surface. The gravel pack must extend a minimum of 1-2 feet above the top of the screen. Field measurements should be taken to confirm the gravel-pack has reached this level prior to grouting.
5. Sealing Materials and Procedures : The seal used directly above the gravel-pack should be of bentonite in the pelletized form. Enough pelletized bentonite should be added so that a seal two feet thick is formed directly above the gravel-pack. If no water is present in the borehole where the pellets were installed, water from a potable source must be added to fully expand the pellets. If it is not possible to install the bentonite seal as described above, an alternate procedure of installing the seal via tremie pipe may be used. The seal must be composed of pure bentonite and mixed with potable water. The bentonite mixture should be thick enough to provide an adequate seal while remaining pliable enough to
be pumped. The annular space remaining above the seal must be grouted. The grout mixture should be composed of Portland cement and powdered bentonite. The use of Hi Early Type III cement in the mixture is prohibited due to the high heat of hydration. The grout mixture proportions must be six gallons of water to one 94 pound sack of cement. Two pounds of
powdered bentonite should be added per sack of cement to control grout shrinkage. The grout must be thoroughly mixed
until a consistent slurry is reached throughout. Under NO circumstances should the grout be added to the well annulus
dry, it must be premixed and injected as described below. Grout must be injected via tremie pipe immediately above the bentonite seal that was placed above the well screen. All hoses, pipes, tubes, water swivels, drill rods or passageways through which the grout is to be pumped should have an inside diameter of at least 0.5 inches. Grout injection must
continue until clean grout reaches the ground surface. The well must not be disturbed for at last 48 hours after grouting
to allow the grout time to set up.
6. Well Development : All monitoring wells must be developed to produce representative formation water.
Representative formation water is assumed to have been obtained when, pH, temperature and conductivity readings
are stable and the water is turbid free, and the minimum periods of development specified below have been reached.
Various methods of well development are described below :
a. Submersible Pumps : Submersible pumps include centrifugal or positive displacement type pumps which are operated under submergence. If a submersible pump is utilized for well development, it must be of a type and capacity such that it
can pump water from the well continuously for a period of at least five minutes without shutting off. Back pressure or
other methods may be utilized to accomplish the desired pumping rate. The pump must be capable of being turned off and on instantaneously to create a surge in the well. When using a submersible pump it is recommended the well be developed for a minimum of four hours.
b. Bladder Pumps : A bladder or diaphragm pump is the type of pump which operates under the cycling of compressed air. The compressed air cycling inflates and deflates a diaphragm which creates a pumping action. Bladder pumps capable of well development must pump a minimum of 2 gallons per minute (GPM) continuously when installed in the well. Development with a bladder pump should be done for a minimum of eight hours.
c. Jet Pumps : A jet pump utilizes the Venturi principle to create sub-atmospheric pressure which allows a
suction pump to be utilized below a depth at which suction alone would not normally lift water. Jet pumps capable
of well development must pump a minimum of 3 GPM continuously when installed in the well. The recommended development
time with a jet pump is four hours.
d. Suction Pumps : Suction pumps cannot be used in wells which are deeper than 25 feet. Suction pumps used to develop wells less than 25 feet deep must be capable of pumping at least 5 GPM continuously without pumping the well dry in less
than five minutes. The recommended minimum development time with a suction pump is four hours.
e. Bailers : Bailers used in well development must be constructed of teflon, stainless steel or PVC. Bailers must
have a lower check valve and be lowered into the well with non water absorbing cable. Wells developed using a bailer
must be bailed until a turbid free discharge is attained. If the well has insufficient recharge to permit continuous
bailing, then the well should be bailed dry and allowed to fully recharge. This cycle should be repeated five times
or until a turbid free discharge is attained.
f. Compressed Air : Compressed air supplied by an engine-driven compressor equipped with an oil trap may be used ; provided the source of compressed air is capable of evacuating 50 percent of the water column from the well once every minute. The recommended well development time for compressed air is four hours with cycling at two minute intervals.
7. Security : All monitoring wells must be fitted with an outer protective casing. This protective casing must be constructed of steel and be supplied with a steel locking cap. The steel casing must be securely set in cement and should extend a minimum of two inches above the top of the monitoring well casing. As additional protection, it is recommended
that three (3) posts be installed around each monitoring well. The posts should be constructed of five- inch steel casing
and extend a minimum of two feet into the ground with three feet above ground. Each post should be filled and grouted in place with concrete.
Geologic Sampling of Formation...
Each monitoring well boring must be sampled via split spoon sampler at five foot intervals or when a change in lithology
is encountered. The split spoon sampling must be performed in accordance with ASTM method D-1586. Samples must be placed
in one pint glass jars and labeled as to location, depth, blow counts, and soil classification. A detailed geologic log
must be generated from these samples and submitted to the Department.
Sampling of Monitoring Wells...
Parameters to be sampled for at a facility should be based on the character of the effluent being discharged but should
at a minimum include the following :
1. Temperature (degrees C)
2. pH, standard units
3. Conductivity (adjusted to 25 degrees C)
4. Nitrate, mg/l as N
5. Ammonia, mg/l as N
6. Fecal Coliform, N/100 ml
7. Nitrites, mg/l
8. Chlorides,mg/l
9. Total Dissolved Solids, mg/l
10. Sulfate, mg/l
11. Ground water elevations, measured from top of well casing
As for proper procedures for collection of monitoring well samples, the Ground Water Quality Program of the Department of Water and Natural Resources has a guidance document available which outlines the proper procedures for sampling a monitoring well.
Deviations From Design Criteria...
The Department may consider and allow deviations where adequate documentation is provided to prove the need for such deviation.
