Understanding Monitoring Well Construction: Essential Supplies & Best Practices

Effective groundwater and environmental monitoring begins with properly constructed monitoring wells. Whether you’re an environmental consultant, geotechnical investigator, or field technician, understanding the essentials of monitoring well construction is crucial—not just for regulatory compliance, but for the quality and reliability of your results. In this guide, we’ll cover what a monitoring well is, essential supplies you’ll need beyond the basic PVC casing and screen, and best practices for installation and maintenance. Let’s demystify the process and help you achieve dependable outcomes for your next project.

What is a Monitoring Well? How is it Different from a Borehole?

A monitoring well is a purpose-built structure, typically installed in soil or rock, that enables groundwater sampling, water table level measurement, or contaminant transport studies. Unlike extraction wells (which remove water or contaminants) or traditional boreholes used for general subsurface exploration, monitoring wells are designed with the specific goal of observing and assessing subsurface conditions over time.

A borehole simply refers to the drilled or augered hole in the ground, which may be temporary or used for purposes other than monitoring (like mineral exploration). It’s only after casing, screening, and backfilling that a borehole becomes a functioning monitoring well. This distinction is key—while every monitoring well starts as a borehole, not every borehole becomes a monitoring well.

Core Components and Key Supplies

Beyond PVC: What Makes a Reliable Monitoring Well?

While the backbone of most monitoring wells is the PVC casing and well screen—critical for providing both access and filtration—several other supplies are indispensable:

• Filter Sand (or Gravel): Packed around the well screen, filter sand creates a permeable zone that supports the screen and prevents fine particles from entering the well. 
• Bentonite: This clay is essential for sealing and isolating the well from surface contamination and cross-formation flow.
• End caps and well covers: These protect the well from debris and unauthorised tampering.
• Clean backfill or grouting materials: Additional layers above the bentonite ensure the integrity of the well and surrounding soil structure.

Attention to these details can mean the difference between high-quality, actionable data and inconclusive, compromised sample results.

Understanding Filter Sand

So, what is filter sand? In the context of monitoring well installation, filter sand refers to a specific gradation of washed, inert sand that is placed in the annular space around the well screen. Its purpose is twofold: to hold the screen in alignment and to allow water to flow freely while filtering out finer particles and preventing clogging.

Selecting the correct sand size (commonly 2mm or 5mm, depending on lithology) is important to optimise flow and avoid silting issues. Improperly chosen or poorly graded filter sand can lead to reduced well yield or data inaccuracies over time. Quality, washed sand—such as sand 2mm—remains the industry standard for most applications.

The Critical Role of Bentonite

Bentonite, a naturally occurring clay, is most often used as a sealing material to prevent the downward migration of surface water and contaminants. In monitoring well construction, this typically means: 

• Placing bentonite chips or pellets above the filter sand pack: This creates a low-permeability seal. 
• Hydrating the bentonite: It expands to fill voids and form an effective barrier.
• Layering with additional backfill or grout: This delivers redundancy and structural stability.

The main uses of bentonite in well construction revolve around its superior sealing properties, ensuring that only the target aquifer or interval is accessed by the well. This is key to maintaining sample integrity and compliance with environmental guidelines.

If you need to source high-quality bentonite for your wells, you can buy bentonite in chip, pellet, or powder form to suit various field conditions.

Installation and Maintenance: Best Practices

Constructing an effective monitoring well relies not only on the selection of quality materials but also on proper installation methodology. Here are some essential tips:

• Use clean, dedicated equipment for drilling and installation to avoid introducing contaminants. Employ the right auger or drilling tool for your ground conditions.
• Align and centre the well screen precisely within the borehole to ensure complete and representative sampling.
• Place filter sand evenly, monitoring the volume and thickness to maintain contact with the screen throughout its length.
• Hydrate bentonite promptly and verify the seal. Some field staff prefer pelleting over chipping for deeper installations due to easier placement and hydration.
• Finish with an appropriate well cap and surface cover to prevent debris entry and unauthorised access. A secure monitoring well cap is recommended for long-term protection.

Regular inspection and maintenance are important. Periodically check for well integrity, verify cap security and ensure the annulus remains free of surface runoff or damage.

Avoiding Common Pitfalls

Even seasoned professionals can encounter setbacks in well construction. Watch out for these frequent issues:

• Incomplete hydration of bentonite can lead to ineffective seals and sample cross-contamination.
• Incorrect filter sand sizing can either permit fines migration (clogging the screen) or excessively restrict flow.
• Improper screen placement may result in drawing water from unintended strata, compromising your data.
• Omission of a protective surface cover risks debris infiltration and accidental well damage.

Summary

A well-constructed monitoring well is fundamental for reliable groundwater sampling, site assessment, and long-term environmental management. Apart from the PVC casing and well screen, both bentonite and filter sand play critical roles—sealing the well and ensuring data quality. By sourcing the correct materials, adhering to best practices during installation, and maintaining your wells regularly, you maximise the usefulness and reliability of your monitoring programme.

For a full range of monitoring well construction materials and best-in-class support, Site Investigation Supply is your go-to site investigation supply partner. Our team is here to assist you in selecting the right products so you can focus on trustworthy, actionable results in every subsurface investigation.