Monitoring Well Installation - Design, Construction, and Completion

Overview

Monitoring wells are permanent or semi-permanent groundwater access points installed during environmental site investigations. They serve three primary purposes: measuring groundwater levels and flow direction, collecting representative groundwater samples for laboratory analysis, and monitoring changes in groundwater quality over time.

The quality of data you collect from a monitoring well depends entirely on how well it was designed and installed. A poorly constructed well can produce samples that do not represent actual groundwater conditions - introducing bias from surface water infiltration, cross-contamination between aquifer zones, or turbidity from inadequate filter pack design.

This guide covers the design and installation of conventional monitoring wells for environmental investigations. Well development (the process of removing fine materials and establishing hydraulic connection after installation) is covered separately in our well development guide.

Who Designs the Well

The consulting firm typically designs the well specifications based on the site’s hydrogeology, the regulatory program, and the investigation objectives. The driller installs the well according to those specifications. In Ohio, monitoring well design and installation should follow the Ohio EPA Technical Guidance Manual (TGM) Chapter 7. Under the VAP, following the TGM is required per OAC 3745-300-07(F)(6)(d)(viii)(b). For other programs, the TGM serves as guidance rather than a strict requirement, but deviations should be documented and justified.

Well Design

Diameter

2-inch diameter PVC (Schedule 40) is the standard for environmental monitoring wells. This accommodates most sampling equipment including submersible pumps, bladder pumps, and bailers while keeping the borehole size manageable.

1-inch wells are sometimes installed where space is limited or where direct push methods are used for shallow temporary monitoring points. They are more restrictive for sampling equipment - generally limited to bailers and narrow-diameter tubing pumps.

4-inch wells exist at older sites but are rarely installed for new environmental monitoring work. They were more common when larger-diameter pumps were standard equipment. The larger borehole disturbs more of the formation and requires more materials.

Screen Selection

The well screen allows groundwater to enter the well while keeping formation material and filter pack sand out. For environmental monitoring wells, PVC slotted screens are standard.

Slot size is selected based on the grain size of the surrounding formation material. The slot should retain at least 90% of the filter pack material.

0.010-inch slot is the most common choice for fine-grained formations - silts, fine sands, and clay-rich soils. This is the default for most Ohio monitoring wells given the state’s glacial till and lacustrine sediment geology.
0.020-inch slot is used for coarser formations - medium to coarse sands and gravels. The larger opening allows higher flow rates and easier development but is only appropriate where the formation material is coarse enough that sediment intrusion is not a concern.
Larger slot sizes (0.030-inch and above) are uncommon in environmental monitoring wells.

Prepack screens have the filter pack material factory-bonded to the outside of the screen. They are more expensive than standard slotted screens but are useful in direct push installations where placing a conventional filter pack in a small-diameter borehole is difficult. Your firm and driller will determine whether prepack screens are appropriate for a given installation.

Screen length is typically 5 to 10 feet. For water table monitoring, the screen is set to straddle the expected seasonal water table fluctuation range. For deeper plume monitoring, the screen is positioned at the target depth interval. Shorter screens provide more precise vertical sampling but may go dry during seasonal low water conditions.

Screen material: PVC is standard for most environmental monitoring. Stainless steel (304 or 316) may be required when monitoring for certain organic compounds where PVC could sorb or leach contaminants, or in highly corrosive groundwater environments.

Filter Pack

The filter pack is clean, uniform silica sand placed around the well screen in the annular space between the screen and the borehole wall. It serves two purposes: preventing fine formation material from entering the well, and providing a permeable zone around the screen that allows groundwater to flow freely into the well.

Material: #5 silica sand is commonly used in Ohio for environmental monitoring wells with 0.010-inch slot screens. The sand grain size should be matched to the slot size - the filter pack should be coarse enough to not pass through the screen slots but fine enough to filter out formation fines.

Placement: The filter pack extends from the bottom of the well to above the top of the screen. Ohio EPA TGM Chapter 7 recommends the bentonite seal be placed a minimum of 3 to 5 feet above the top of the screen to prevent bentonite from affecting groundwater chemistry at the intake. In practice, many Ohio firms use approximately 2 feet of sand above the screen before beginning the bentonite seal. The sand pack buffer above the screen prevents bentonite from settling into the screened interval during installation. Follow your project specifications and the applicable regulatory program requirements.

Pour the sand slowly and evenly down the annular space. Periodically measure the sand pack level with a weighted tape to confirm it is building at the expected rate. Bridging (sand locking up and not falling to the bottom) is the most common problem during sand pack placement, especially in deeper wells or tight annular spaces. A tamping rod can help break bridges.

Annular Seal

The annular seal prevents surface water from running down the outside of the casing into the screened interval, which would contaminate the well with non-representative water. It also prevents cross-contamination between different water-bearing zones.

A typical Ohio monitoring well seal consists of three layers above the filter pack:

Sand pack: From the bottom of the well to above the top of the screen (TGM recommends bentonite be placed 3-5 feet above the screen; many firms use a minimum of 2 feet of sand above the screen)
Bentonite seal: Bentonite chips (typically 3/8-inch or 1/4-inch) placed from the top of the sand pack to approximately 1 foot below the top of the riser. The bentonite hydrates (absorbs water and swells) to form a low-permeability seal. Add chips slowly and allow time for hydration - dumping too many chips at once can cause bridging above the water table where there is no water to hydrate them.
Sand collar: Approximately 6 inches of sand above the bentonite, below the surface completion. This provides a buffer between the bentonite and the surface seal.

Grout slurry (cement-bentonite mixtures) is an alternative to bentonite chips and is specified in some state programs and for deeper wells. It provides a more continuous seal but requires tremie pipe placement to avoid bridging. Grout slurry is less commonly used for shallow environmental monitoring wells in Ohio but may be required in other states or for specific regulatory programs.

Important: Ohio EPA TGM recommends allowing at least 48 hours after grout or seal placement before developing the well. This allows the bentonite to fully hydrate and the seal to set properly. Developing too soon can damage the seal integrity.

Surface Completion

Surface completion protects the well from damage, prevents surface water entry, and provides a permanent reference point for water level measurements.

Flush mount completions are the standard for most environmental monitoring well installations. A flush-mount protective cover (typically traffic-rated cast iron or steel) is set in a small concrete pad at ground surface. The well casing is cut to sit just below the cover lid. Flush mounts are preferred because they are less visible, less likely to be damaged by vehicles or mowers, and do not interfere with property use.

Stickup completions use a protective steel casing (typically 4-inch steel bollard) extending 2-3 feet above ground surface, with a locking cap. Stickup wells are used primarily in wooded areas, remote locations, or situations where a flush mount could become buried under vegetation or soil over time and be difficult to relocate. They are also used where wells may be subject to flooding and surface water entry into a flush mount is a concern.

Both types should include:

A locking mechanism to prevent unauthorized access
A concrete surface pad (typically 2-3 feet square) sloped away from the well to direct surface runoff away
Clear well identification (well ID marked on the cover or protective casing)

Installation Process

Drilling

For most environmental monitoring wells in Ohio, the installation sequence involves two phases of drilling work, often during the same mobilization:

Soil investigation: Geoprobe or direct push borings to collect soil samples and characterize the subsurface. This provides the geologic information needed to design the well.
Well installation: Hollow-stem auger (HSA) drilling to advance the borehole and install the permanent monitoring well. HSA is the most common method for monitoring well installation because the hollow stem allows the well casing and screen to be lowered through the center of the augers after the borehole reaches the target depth.

The driller advances the HSA to the design depth. A split-spoon sampler may be used during drilling to collect soil samples and confirm the geologic conditions match the well design assumptions. If conditions differ significantly from what was expected (for example, encountering bedrock where unconsolidated sediment was anticipated), the well design may need to be adjusted in the field.

Setting the Well

Once the borehole reaches the target depth:

Lower the assembled well screen and riser through the center of the hollow-stem augers to the bottom of the borehole
Begin withdrawing the augers while simultaneously placing the filter pack sand around the screen
Continue withdrawing augers and adding sand pack to the design height above the screen (3-5 feet per TGM, or per project specifications)
Place bentonite chips and allow hydration as the remaining augers are withdrawn
Complete the surface seal

The key to a good installation is coordinating the auger withdrawal with material placement. If the augers are pulled too fast, the borehole can collapse before the filter pack and seal are in place. If they are pulled too slowly, the sand or bentonite can bridge inside the auger flights.

Documentation

As-Built Diagram

An as-built diagram is required for every monitoring well installation. It documents the actual construction details as installed (which may differ from the design specifications). The as-built should include:

Well identification number
Date of installation
Driller and consulting firm
Total depth of borehole and total depth of well
Depth to top and bottom of screen, screen length, slot size, and material
Depth and type of filter pack material
Depth and type of annular seal material
Surface completion type and description
Casing material, diameter, and stickup height above ground
Geologic log of materials encountered during drilling
Static water level measured after installation (before development)
Surveyed well location coordinates and top-of-casing elevation (when available)

Regulatory Filings

In Ohio, the Ohio Department of Natural Resources (ODNR) requires that well logs be submitted for all newly installed monitoring wells. The driller is typically responsible for filing the well log on behalf of the property owner.

After Installation

A newly installed monitoring well is not ready for sampling. The installation process introduces fine drilling materials, sand, and bentonite particles into the filter pack and screen area. The well must be developed to remove these materials and establish hydraulic communication with the formation before collecting representative groundwater samples.

Ohio EPA TGM recommends allowing the annular seal to set for at least 48 hours before beginning development. Development procedures - including surging, bailing, and pumping techniques - are covered in our well development guide.

After development, an additional stabilization period is needed before sampling. Ohio EPA TGM Chapter 8 (2009 revision) recommends a minimum of one week between development and the first groundwater sampling event for conventionally drilled wells. For shallow (<25 ft) small-diameter wells installed using direct push methods, Ohio EPA recommends at least 24 hours of post-development stabilization (TGM Chapter 10). Shorter time frames may be acceptable with documentation and justification.

Source

Ohio EPA Technical Guidance Manual (TGM) Chapter 7: Monitoring Well Design and Installation. ASTM D5092: Standard Practice for Design and Installation of Ground Water Monitoring Wells in Aquifers.