Monitoring Well Construction, Installation and Development

• To guide the construction, installation and development of groundwater monitoring wells

• Consultants or contractors hired to construct, install, and develop groundwater monitoring wells at confined feeding operations (CFOs)

• Agricultural Operation Practices Act

• Ensure representative groundwater samples
• Prevent contaminated surface water from reaching groundwater through the well space
• Prevent groundwater movement and transfer between aquifers
• Prevent improper construction or installation of groundwater monitoring wells. Improperly constructed or installed wells may need to be reclaimed or replaced, resulting in significant additional costs

1. Lithologic Log
   During drilling, geologic descriptions should be logged continuously and directly from drilling returns. The lithologic log should be recorded by a qualified individual using either the Canadian System of Soil Classification (NRC, 1988), the Unified Soil Classification System (ASTM, 2006) or the Wentworth scale (1922). There are significant differences in how the three systems describe texture or grain size. The system or combination of systems used must be specified on the drill or borehole logs, and must be used consistently for the site (see Appendix 2).
   
   The lithologic log should include a description of soil texture (clay, silt, sand, gravel) or lithology (for example sandstone, siltstone, claystone, coal), grain size, colour, moisture content, consistency, hardness, and secondary features such as staining, mottling, carbonates, gypsum, degree of cementation, degree of weathering and odour. The lithologic description should also include the presence or absence of visible fractures as indicated in core or split-spoon samples, or surface staining on rotary cutting returns.
2. Representative Samples
   Representative samples of sediments encountered during drilling must be collected for at least every 1.5 metres of drilling, or where changes in lithology are suspected or observed.
3. Decontamination of Equipment
   Decontamination of drilling and formation sampling equipment is a quality control measure that is included as part of standard drilling procedures. It is important for preventing cross contamination between sampling areas. The professional responsible for the drilling and sampling should evaluate the decontamination requirements for each site. Although decontamination is typically used where contamination is present, it may also be used in uncontaminated areas as a quality control and assurance measure.

1. Materials
   Monitoring wells are constructed of environmental quality threaded PVC pipe, 52 millimetre ID Schedule 40, with a machine slotted 3.0 metres long or 1.0 to 1.5 metres long, 0.020 or 0.010 slot screen. A 3.0 metre screen would typically be used for relatively shallow intervals. A 1.0 to 1.5 metre screen would typically be used for relatively deeper intervals. Longer screens may be required, depending on the borehole depth and stratigraphy of the site. The length of the screen selected depends on the thickness of the hydrogeologic unit, the vertical scale of the investigation, and specific requirements of the monitoring program. For typical water table monitoring wells, a screen 3.0 metres long is common. The screen must not extend to within less than two metres of the ground surface to allow for an adequate length of bentonite seal above the screen.
2. Joints
   The 52 millimetre PVC screens and all plain pipe sections must have threaded flush joints with
   O rings or Teflon tape at the joints. Solvent welded PVC pipes, screens and couplings should not be used to construct monitoring wells. PVC glue and primer can potentially introduce organic substances into the monitored environment. Screen bottoms must be closed with a heat welded seal or threaded end cap.
3. Size of Boreholes
   Groundwater monitoring well components are placed in each borehole. Boreholes must be at least 102 millimetres (approximately four inches) larger than the outside diameter of the casing and screen. For instance, when a 52 millimetre (two inch) well casing is used, the drill borehole diameter should be at least 154 millimetres (approximately six inches). This ensures adequate space around the well screen and casing, which allows for proper placement of completion materials such as filter sand and bentonite, and helps prevent materials from bridging between the casing and borehole walls.
4. Borehole Backfill
   The annular space between the screened section of the monitoring well and the borehole wall is backfilled with filter sand, typically 10/20 mesh (2.0 to 0.84 millimetres) fraction silica sand. Environmental silica sand is used because it does not contribute significant soluble ions to the groundwater. Sand of mixed lithologies may contain minerals that could affect trace element concentrations in monitored groundwater, and is not recommended. The filter sand is placed to approximately 0.3 metres above the top of the monitoring well screen to accommodate settling, and to prevent contact between the bentonite seal (described below) and the well screen. As the sand is placed in the borehole, the depth of filter sand pack is sounded with a weighted tape measure to ensure correct placement without bridging.
5. Borehole Sealing
   The annular space above the sand pack, refered to as the annulus, must be sealed to prevent surface water or shallow groundwater from moving through it. A primary, water tight bentonite seal, approximately 0.3 metres thick, is placed above the sand pack using bentonite pellets. Bentonite chips are not recommended for the primary seal. If bentonite chips are used for the primary seal, extra care must be taken to ensure that bridging does not occur if water is present in the borehole.
   
   To ensure hydraulic isolation between the ground surface and other potential water-bearing horizons, the entire remaining annulus must be backfilled to ground surface with hydrated bentonite chips. Cuttings should never be used to fill the annulus.
   
   While bentonite chips are being placed in the annulus, the depth of the bentonite chips must be sounded with a weighted tape measure to ensure correct placement of the seal, without bridging in the hole.
   
   The bentonite chips must be hydrated, by adding approximatively seven litres of fresh, potable water per 20 kilograms of chips after each bag is placed.The type and depth of backfill material should be noted on the well construction log.
   
   The top of the monitoring well must be capped with a snug fitting or threaded PVC cap. A small vent hole or slot should be drilled or cut below the bottom of the cap to allow air movement in and out of the monitoring well to allow water levels to fluctuate properly.

1. Elevation
   The elevation of the monitoring wells should be surveyed relative to a common datum or reference point. A local datum is suitable. This provides the relative elevation of the monitoring wells to each other. The difference between the groundwater elevations indicates flow direction. The top of the well casing is the most reliable point for the elevation survey, given that the height of the well casing above ground surface, commonly called stick up, is measured. Ground surface is typically not an accurate basis for the elevation survey.
   
   The relative elevations of the monitoring wells must be accurate to within 0.25 centimetres, as the difference in groundwater elevation across a site may only be a few centimetres. Consequently, some surveying methods, such as common global positioning system (GPS) techniques, are not suitable for establishing the relative elevation of monitoring wells.
2. Spatial Location
   The spatial location of the monitoring wells is also required. Although an accurate survey is preferable, the locations of the monitoring wells can be measured from a known reference point at each site. The location of the wells should be accurate to within a few metres, depending on the size of the facility and monitoring well spacing. Most GPS methods are suitable for determining monitoring well location. As with elevation, it is the relative location of the monitoring wells to each other and to the manure storage facilities and other site features that is required. Real world coordinates (referenced to established topographic maps) are not required, but can be helpful if the site needs to be mapped relative to larger scale features or receptors.

1. Borehole
   A borehole log must be prepared for each groundwater monitoring well, showing the lithologies encountered, depth drilled and completion details. (See section on drilling and logging boreholes).
   Borehole logs should include both a graphical and textural description of sediments encountered at each monitoring well location, completion details and key features.
2. Monitoring Well Completion
   The summary table of monitoring well completion information should include, at a minimum, the following information:

• Ground surface elevation (relative or geodetic)
• Top of well casing elevation (relative or geodetic)
• Height of casing stick up, above ground surface
• Total depth of monitoring well
• Diameter of borehole
• Screened interval and sand packed interval (depth below ground surface)
• Spatial location of monitoring wells
• Documentation of type, specifications and placement of all material (e.g. filter pack, casing, screen, bentonite seals)
• Development details (method used and volume of water removed)
• Groundwater level (depth below top of casing and ground level)

Appendix 1 – Additional Information

Typical borehole drilling methods
A truck mounted auger drilling rig is typically used for monitoring well installations, though various methods are available depending on site geology and characterization requirements. Auger rigs are well suited to drilling in fine to medium grained unconsolidated materials, or poorly consolidated bedrock.

Solid stem auger drilling can be used for shallow and deeper well completions in competent unconsolidated materials in which the borehole stays open and does not slough. Logging of the geologic material is accomplished by describing cuttings on the auger flights at regular intervals.

Hollow stem auger drilling is typically used if there are unstable borehole conditions, for example, saturated sand, in the shallow zone or completion zone or if substantial contamination is evident in the shallow zone. Hollow stem augers provide a temporary casing to support the borehole walls as drilling advances, minimizing the potential for shallower geologic materials to enter the borehole. Samples are commonly collected using a split-spoon sampler (coring tube), typically 45 to 60 centimetres in length, or other suitable coring devices.

Rotary drilling methods may be necessary for monitoring well installation when drilling into hard, competent bedrock sediments (for example sandstone, siltstone or shale). Rotary drills down the drill stem and up the annular space of the borehole to lubricate the drill bit and carry drill cuttings to the ground surface. Air rotary drilling uses compressed air to accomplish the same function. Air circulating rotary drilling is preferred over mud rotary. Air rotary drilling poses less risk of contaminating porous or fractured sediments in which a monitoring well is being installed.

In boreholes drilled with rotary drill methods, the screened zone should be cleaned and flushed with clean water to remove any drilling dust, debris (air rotary), or drilling fluid that may have coated the borehole walls (including pores, fractures, or other voids in the completion zone) before any of the components (filter sand, screen, etc.) of the monitoring wells are installed. This is necessary to ensure that good hydraulic connection in the monitoring zone is established and that water levels are able to fluctuate freely, and remove potential contamination.

Prevention of bridging
Centralizing devices may be installed in monitoring wells to overcome potential bridging problems. One centralizer should be installed at or below the bottom of the screen and unless it is expected that bridging problems with the filter sand may arise, a second centralizer should be installed above the top of the screen.

Centralizers also prevent the sides of the screen from contacting the borehole walls during installation, preventing smearing or clogging of the screen as it is lowered and installed through wet, clay rich materials. In the case of hollow stem drilling, the PVC screen and pipe are installed through the auger stem to the desired depth and centralizers are not required.

Appendix 2- Classification Systems of Sediment Size

Three classification systems are commonly used in Canada to describe and classify materials which make up unconsolidated and consolidated bedrock sediment. These classifications are the Unified Soil Classification System (ASTM),¹ the Canadian System of Soil Classification (NRC)² and the Wentworth scale.³ See Table 1, below, for a comparison of these three systems.

Table 1. Nomenclature and particle size used by common classification systems

Nomenclature (all units in mm)	NRC	ASTM	Wentworth
Boulder		>300	>256
Cobble		75 - 300	64 - 256
Very coarse gravel			32 - 64
Coarse gravel		19 - 75	16 - 32
Medium gravel			8 - 16
Fine gravel		4.75 - 19	4 - 8
Very fine gravel			2 - 4
Very coarse sand	1 - 2		1 - 2
Coarse sand	0.5 - 1	2 - 4.75	0.5 - 1
Medium sand	0.25 - 0.5	0.425 - 2	0.25 - 0.5
Fine sand	0.10 - 0.25	0.075 - 0.425	0.125 - 0.25
Very fine sand	0.05 - 0.10		0.0625 - 0.125
Silt	0.002 - 0.05	<0.075	0 0039 - 0.0625
Clay	<0.002		<0.0039
Colloid	<0.0002		<0.001