Designing a monitoring well involves a balance of technical requirements based on regulatory standards, the site’s hydrogeology, and the specific data objectives of the investigation (environmental or geotechnical). The design must ensure that the well provides a representative sample of the groundwater or an accurate measurement of the water level at the targeted depth.
I. Data Objectives: Environmental vs. Geotechnical
The intended use dictates the design choices:
- Environmental: The well must facilitate the collection of chemically unaltered groundwater samples. The materials used must be inert to the contaminants of concern (CoCs).
- Geotechnical: The well (often a piezometer) primarily needs to quickly and accurately measure the water pressure or level (piezometric surface).
- Combined Use: A standard monitoring well design often serves both purposes well.
II. Key Design Components and Material Selection
The standard monitoring well comprises four main components: the screen, the riser pipe, the filter pack, and the annular seals.
A. Screen and Riser Pipe Materials
Materials must be durable and chemically inert.
- Standard Material: PVC (Polyvinyl Chloride): The most common choice due to its low cost, durability, and chemical inertness to most common contaminants (e.g., petroleum hydrocarbons, metals).
- Alternative Materials: Stainless steel or Teflon are used for investigations involving specific contaminants that PVC might absorb (e.g., some pure-phase solvents like DNAPLs), though they are much more expensive.
- Avoid: Do not use galvanized steel or black iron in environmental wells, as they corrode and introduce metals that skew sample results.
B. Well Screen Design
The well screen allows water to enter the well while keeping soil out.
- Slot Size: Determined by the grain size of the surrounding aquifer material (from sieve analysis results). The slot size should retain 90% or more of the filter pack material. Common sizes are 0.010 or 0.020 inches.
- Length: Typically 5 to 10 feet long.
- Environmental: The screen should ideally straddle the water table interface or target the specific depth of the contaminant plume.
- Geotechnical: A short screen is sometimes preferred to measure a very precise hydrostatic pressure at a specific depth (a piezometer design).
C. Filter Pack (Gravel Pack)
The filter pack is placed around the well screen to prevent fine soil particles from clogging the screen slots.
- Material: Clean, washed, uniform silica sand (often Grade 20-40 or 10-20).
- Placement: The sand pack typically extends from the bottom of the borehole to approximately 1 to 2 feet above the top of the well screen.
D. Annular Seals and Grout
Seals are essential to prevent surface water infiltration and cross-contamination of different water-bearing zones.
- Bentonite Seal: A transition seal of bentonite chips or pellets is placed immediately above the filter pack (typically 1–3 feet thick). This seal is hydrated to swell and create an impermeable barrier.
- Grout (Cement-Bentonite Slurry): The remaining annulus (space between the borehole wall and the riser pipe) is filled to the surface with a cement-bentonite grout using a tremie pipe (bottom-up application).
III. Finalizing the Design
- Protective Casing: The well completion at the surface requires protection. This is typically a steel casing set in concrete (either above ground or flush with the ground in high-traffic areas).
- Documentation: The final design must be documented on a detailed well construction diagram and log, which is a regulatory requirement (e.g., submission of a Well Record in Ontario).
By carefully selecting materials and adhering to these design standards, the monitoring well becomes a reliable tool for both environmental characterization and geotechnical design.