How deep should a monitoring well be?

Thomas Kwader explains that probably the most important factor in determining the depth of a monitoring well is the relative ground water level at the time of installation.

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One of the more challenging aspects of defining the depth for a monitoring well is taking into consideration varying scenarios for shallow water table aquifers. When assessing water table wells, the screen length should typically be short (about 5 to 15 feet in length) to collect a representative water sample from a specific hydrogeologic zone. This is crucial because water quality can differ significantly based on depth, influenced by factors such as proximity to the source, the particular gravity of the tested compounds, the geological composition at different borehole layers, and the vertical water levels or head gradient by depth.

In numerous instances, well construction regulations may require that the top of the screen be set above the average high water table to ensure the capture of any lightweight floating compounds, such as motor fuels or lubricants. Natural fluctuations in the water table may exceed the screen's length, posing a challenge to designing a well that meets all regulatory criteria.

Monitoring wells near brackish or saltwater conditions experience marked variations in water quality with depth due to 'freshwater lenses' formed from rainfall floating on denser saltwater. This can cause water quality from a single sample to fluctuate based on rainfall or tidal changes.

Setting the depth of a monitoring well accurately is crucial, particularly considering the groundwater levels at the time of installation. Many wells installed during unusually wet periods have short screens positioned slightly more than halfway below the water level. When normal or drier conditions return, these wells may lack enough water for sampling or measurement, leading to frustration for clients and regulatory bodies. However, this necessitates deeper well installations by drillers.

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Sealing the Upper Annular Space

The space between the monitoring well casing and the wall of the well borehole, known as the "annular space," must be sealed effectively to prevent it from becoming a conduit for poor-quality water, pollutants, or contaminants. Since monitoring wells are designed to capture water from specific intervals, the annular seal isolates the well intake to just one water-bearing unit. It also maintains the structural integrity of the casing and protects it from chemical corrosion. Effective sealing is critical given proximity to contaminated areas.

General guidelines for sealing methods and monitoring wells' requirements are found in Section 9, Section 13, and Appendix B of the Water Well Standards. Specific requirements for monitoring wells include the following:

Minimum Depth of Annular Seal

• 1. Water quality monitoring wells and those in contaminated areas. The annular space should be sealed from the top of the filter pack or monitoring zone to the ground surface, preventing the filter pack from extending into another water-bearing unit or across confining layers unless approved. The seal should be at least 20 feet long, with shorter seals permissible for shallow zones with regulatory approval.
• 2. Other Monitoring Wells. Similarly, the upper annular seal must extend from the ground surface to a minimum of 20 feet, following the same provisions for exceptions.
• 3. Sealing Off Strata. Additional sealing material below the upper annular seal is needed to prevent contaminants from moving to quality water zones, as specified in Section 13.
• 4. Shallow Water Level Observation Wells. Those less than 15 feet deep, used in agricultural settings, are exempt from seal requirements unless regulated.
• 5. Areas of Freezing. The annular seal top can be below ground in freezing-prone areas but no deeper than 4 feet, often incorporated into a vault structure.
• 6. Vaults. With regulatory approval, the seal's top and well casing can be below ground, but not more than 4 feet, extending to a structurally sound subsurface vault.

Sealing Conditions

• 1. Temporary Conductor Casing. This should be removed during casing and seal placement per Section 9 standards. If removal isn't possible, seal material must bridge between casings adequately.
• 2. Permanent Conductor Casing. Requires a borehole at least 4 inches larger than the casing's diameter, with matching seal material depths in Subsection A.

Radial Thickness of Seal

A minimum of two inches of seal material is required between the borehole wall and casings in the interval to be sealed, except as specified in Section 9. Additional spacing may be necessary for proper installation.

Sealing Material

Acceptable materials include neat cement, sand-cement, or bentonite clay, selected based on structural and sealing properties, and the chemical environment. Used drilling mud or cuttings aren't permissible.

• 1. Water. Should be of drinking quality, free of petroleum and suspended matter, suitable for ensuring proper sealing material consistency.
• 2. Cement-Based Sealing Materials. Must adhere to Section 9 standards and avoid additives that might interfere with water quality.
  • a. Additives. Avoid those altering cement setting times that could impact water quality assessments.
  • b. Cooling Water. Exercise caution when using to protect plastic casing during cement setting, ensuring no water introduced affects water quality.
• 3. Bentonite-Based Sealing Materials. Must meet Section 9 standards for proper hydration and placement.

Transition Seal

A bentonite-based seal up to 5 feet long separates the filter pack from cement seal materials, preventing cement infiltration. Fine sand, less than 2 feet long, can substitute to block bentonite infiltration. Fine bentonite forms are installed dry above water; coarse forms like pellets are used below the water level to avoid pipe clogging. Hydration stabilizes the seal typically within 0.5 to 1 hour, ensuring no bridging occurs.

Placement of Annular Seal Material

Remove loose debris before placement, using pressure grouting, tremie pipes, or equivalent up to 30 feet deep. Casing spacers ensure proper space for seal material distribution and structural integrity, constructed of corrosion-resistant materials, with placement standards specified in Section 9.

Additional guidance and standards for annular seal placement are detailed in Section 9, Section 13, and Appendix B of the Water Well Standards.

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