Monitoring Well Development - Methods, Criteria, and Documentation

Overview

Well development is the process of removing fine-grained material (drilling mud, silt, clay, and sand fines) from the filter pack and surrounding formation after a monitoring well is installed. The goal is to establish a good hydraulic connection between the well and the aquifer so that groundwater samples are representative of formation water, not water influenced by installation artifacts.

A well that is not adequately developed will produce turbid samples with artificially elevated metals concentrations, and may not yield enough water for proper purging and sampling. Development is not optional - it is a required step between installation and sampling.

When to Develop

Wait after installation. Ohio EPA TGM recommends allowing at least 48 hours between grout or seal placement and the start of development. This gives the bentonite seal time to hydrate and set. Developing too soon can damage the seal, allowing surface water or cross-contamination between zones.

In practice, many firms schedule development for a separate mobilization 2-7 days after installation. This also allows the borehole to stabilize and any drilling-induced turbidity to begin settling.

Who develops the well. Sometimes the drilling crew develops the well immediately after installation (after the waiting period). Other times the consulting firm’s field staff returns to develop the well themselves. This depends on the project, the budget, and the regulatory program. For projects where the consultant needs to closely control and document the development process (common under VAP), the consultant typically handles development.

Development Methods

Surging with a Bailer

The simplest and most common method for developing small-diameter (1-inch and 2-inch) environmental monitoring wells. The bailer is raised and lowered repeatedly in the water column, creating a surging action that pulls fine material through the screen and into the well. The turbid water is then bailed out and discarded.

Procedure:

Measure and record the static water level and total well depth before starting
Lower the bailer to just above the bottom of the well
Raise and lower the bailer through the screened interval with moderate speed. You want enough agitation to mobilize fines but not so violent that you damage the screen or pack sediment into the formation.
After several surging cycles, pull the bailer up slowly to remove turbid water
Empty the bailer into a purge water container. Observe the color and clarity.
Collect field parameter readings after each well volume removed - at minimum turbidity, pH, and specific conductance. Temperature, ORP, and dissolved oxygen are also useful. Record the readings on your development log. These readings are how you demonstrate the TGM stabilization criteria are met.
Repeat: surge, bail, surge, bail. Track the cumulative volume removed.
Continue until development completion criteria are met (see below)

Advantages: Simple, inexpensive, no power equipment needed, works in all well diameters and yield conditions. The bailer itself is disposable, so there is no cross-contamination risk between wells.

Disadvantages: Slow, especially in wells with long water columns. Labor-intensive. Turbidity can be difficult to reduce to low levels by bailing alone because the surging action continuously remobilizes settled fines. This is the method where you are most likely to spend extended time waiting for turbidity to come down.

Pumping

A submersible pump or centrifugal pump is lowered into the well and used to remove water at a rate high enough to pull fines through the screen and out of the well. Pumping generally clears turbidity faster than bailing because it creates a continuous outward flow that moves fines away from the well rather than agitating them back and forth.

Procedure:

Measure and record the static water level and total well depth
Lower the pump to just above the top of the screen
Start pumping at a low rate and gradually increase
Monitor the water level in the well - do not pump the well dry if possible
Periodically vary the pumping rate (surge pumping) to alternately stress and relax the formation, which helps pull fines from deeper in the filter pack
Collect field parameter readings after each well volume removed - turbidity, pH, specific conductance, temperature, ORP, dissolved oxygen. Record each reading with the cumulative volume on your development log.
Continue until development criteria are met

Advantages: Faster than bailing, especially for wells with good yield. Creates a more effective outward flow gradient that pulls fines away from the well. Can be combined with surging for maximum effectiveness.

Disadvantages: More expensive (pump rental and power equipment). Risk of pump damage from sediment-laden water, especially in wells with high turbidity or fine sand. Requires sufficient well yield to sustain pumping - in low-yield wells, the pump may run dry and need to cycle on and off.

Surge Block

A surge block (or surge plunger) is a tight-fitting disk attached to a rod that is raised and lowered inside the well casing, creating alternating positive and negative pressure pulses through the screen. This is one of the most effective methods for loosening and mobilizing fines from the filter pack.

Procedure:

Lower the surge block to the screened interval
Work the block up and down through the screen zone with steady, moderate strokes
After a surging period, remove the block and bail or pump out the turbid water
Repeat until water clears

Advantages: Very effective at loosening compacted fines. Creates strong bi-directional flow through the screen that cleans both the filter pack and the near-well formation.

Disadvantages: Can be too aggressive in poorly constructed wells - may damage screens or displace the filter pack. Not commonly used for small-diameter environmental wells. More common in larger water supply well development.

Combining Methods

In practice, development often uses a combination of methods. A common approach is to begin with surging using a bailer to loosen fines, then switch to pumping to clear the turbid water efficiently. This combines the agitation effectiveness of surging with the removal efficiency of pumping.

When Is Development Complete

Development is complete when the well produces water that is reasonably clear and representative of formation conditions. Ohio EPA TGM Chapter 8 provides specific criteria.

Primary Criteria (TGM Chapter 8)

Development should continue until all of the following are met:

Turbidity is 10 NTU or less
pH and specific conductance have stabilized over at least three successive well volumes
Water can enter the well as readily as hydraulic conditions allow

Other parameters such as temperature, oxidation-reduction potential, and dissolved oxygen may also be useful indicators of stabilization.

When 10 NTU Cannot Be Achieved

In fine-grained formations (common in Ohio’s glacial soils), reaching 10 NTU may be difficult or impossible. The TGM allows development to stop when all of the following are met:

Multiple development methods have been tried
Proper well construction has been verified
Turbidity has stabilized within 10% over three successive well volumes
Conductivity and pH have stabilized over at least three successive well volumes

This is an important distinction. The TGM does not require you to keep developing indefinitely - it requires you to demonstrate that you made a reasonable effort and that conditions have stabilized, even if absolute turbidity remains above 10 NTU. Document the stabilization trend in your development log.

Practical Notes on Turbidity

Turbidity is typically the last parameter to stabilize during development, especially in Ohio’s fine-grained glacial soils. In practice, reaching low turbidity can be the most time-consuming part of development.

The TGM notes that rigorous development techniques applied to wells in predominantly fine-grained formations can actually increase turbidity rather than reduce it. Development in silt and clay should use gentle action.

What to do when turbidity will not come down:

If you are bailing, consider switching to pumping if the well yield supports it
If pumping, try alternating between high and low pump rates (surge pumping)
Document the volume of water removed and the turbidity trend over time
Track pH and conductivity alongside turbidity to demonstrate the stabilization trend the TGM requires

Volume Removed

While the TGM does not specify a minimum volume for development, tracking the total volume removed provides documentation that a reasonable effort was made. Many practitioners target a minimum of 3-5 well volumes removed during development, though some wells require far more.

Ohio-Specific Requirements

VAP

Under the VAP, groundwater investigation activities must follow the Ohio EPA TGM per OAC 3745-300-07(F)(6)(d)(viii)(b). This means well development must meet the TGM Chapter 8 criteria described above.

BUSTR

BUSTR sites follow the BUSTR Technical Guidance Manual, which references the Ohio EPA TGM for monitoring well procedures. The same development criteria apply.

Waiting Period Before Sampling

Ohio EPA TGM recommends a minimum of one week between well development and the first groundwater sampling event. This was added in the 2009 revision to TGM Chapter 8 and applies to conventionally drilled monitoring wells.

For shallow (<25 ft), small-diameter (<2 inch) monitoring wells installed using direct push methods, Ohio EPA recommends a post-development stabilization period of at least 24 hours prior to sampling (TGM Chapter 10).

Shorter time frames may be acceptable with documentation and justification, but the key point remains: do not install a well, develop it, and sample it all in the same day. There must be a documented stabilization period between development and sampling.

Documentation

Record the following for every well development event:

Well identification number
Date and time of development (start and finish)
Developer name and firm
Static water level before development
Development method(s) used
Volume of water removed (track by counting bailer trips or measuring pump output)
Field parameter readings at intervals during development (turbidity at minimum, plus pH, specific conductance, temperature if your protocol requires them)
Final field parameter readings
Visual description of water clarity at start and end
Any problems encountered (well went dry, excessive sediment, equipment issues)
Time well was allowed to stabilize after installation and before development

This documentation becomes part of the site’s data package and supports the representativeness of subsequent groundwater sampling results.

Purge Water Disposal

Water produced during development may contain elevated levels of the contaminants under investigation. Handle and dispose of development water according to your project’s waste management plan. In most cases, development water is drummed on-site and characterized before disposal. Do not discharge development water to the ground surface, storm drains, or surface water without authorization.

Source

Ohio EPA Technical Guidance Manual (TGM) Chapter 8: Monitoring Well Development, Maintenance, and Redevelopment. Ohio EPA TGM Chapter 10: Ground Water Sampling. ASTM D5521: Standard Guide for Development of Ground-Water Monitoring Wells in Granular Aquifers.