How to Conduct a Groundwater Monitoring Well Purge

Groundwater monitoring well purging is a critical step in collecting representative groundwater samples. Before a sample is collected, stagnant water inside the well casing needs to be removed so the sample better represents groundwater conditions in the surrounding formation.

A properly performed well purging procedure helps reduce the influence of standing water that may have been affected by contact with the well casing, sediment, or the atmosphere. The specific method used depends on the well construction, groundwater conditions, sampling objectives, and applicable regulatory requirements.

This guide explains how groundwater monitoring well purging is typically conducted, the equipment involved, common purging methods, and the field parameters used to determine when a well is ready for sampling.

What Is Groundwater Well Purging?

Groundwater well purging is the process of removing water from a monitoring well before groundwater sampling.

Water that has remained inside the well may not accurately represent the groundwater flowing through the surrounding aquifer. Purging removes or displaces this water and allows groundwater from the formation to enter the well.

The objective is not simply to remove a specific volume of water. Modern sampling programs may use low-flow or passive sampling techniques that minimize the amount of water removed while still obtaining a representative sample.

well purging procedure should therefore be selected based on site conditions, well design, sampling requirements, and the project's approved sampling and analysis plan.

Why Is Well Purging Important?

Proper purging can help improve the representativeness and consistency of groundwater samples.

Several factors can affect water that has remained inside a monitoring well, including:

• Exposure to atmospheric oxygen

• Changes in temperature

• Sediment accumulation

• Interaction with the well casing and screen

• Chemical reactions within stagnant water

• Microbial activity

• Cross-contamination from equipment or previous sampling activities

If stagnant water is sampled without considering these factors, laboratory results may not accurately reflect groundwater conditions in the formation.

A properly planned well purging procedure helps establish more consistent sampling conditions and supports reliable groundwater monitoring data.

Equipment Needed for Well Purging

The equipment required will depend on the purging method and monitoring well design. Common equipment includes:

• Appropriate groundwater sampling pump

• Tubing compatible with the sampling equipment

• Water-level meter or electronic water-level probe

• Flow-control equipment

• Field parameter instruments

• Flow-through cell, where applicable

• Graduated container or flow meter

• Clean sample containers

• Personal protective equipment

• Decontamination supplies

• Purge-water collection containers

• Field documentation and sampling forms

All equipment should be appropriate for the contaminants being investigated and cleaned or decontaminated according to the project's sampling procedures.

Step-by-Step Well Purging Procedure

1. Review the Monitoring Well and Sampling Requirements

Before beginning the purge, review the well construction details and the project's sampling plan.

Important information may include:

• Well depth

• Screen interval

• Well diameter

• Groundwater elevation

• Well construction materials

• Expected groundwater contaminants

• Required sampling method

• Purge method

• Field stabilization criteria

• Purge-water handling requirements

Understanding the well design is important because the appropriate purging approach can vary considerably between wells.

2. Inspect the Well

Visually inspect the monitoring well and surrounding area before sampling.

Check for signs of:

• Damage

• Surface water intrusion

• Poor well-head conditions

• Sediment

• Unusual odors

• Spills or contamination near the well

• Damaged or missing protective components

  
  

Document any unusual conditions before beginning the sampling process.

3. Measure the Static Water Level

Measure the groundwater elevation before purging begins.

Use a properly decontaminated water-level meter or equivalent instrument and record the measurement according to the project's field procedures.

The static water level can help determine the volume of water present in the well and provides useful information about groundwater conditions.

4. Determine the Purging Method

The appropriate method depends on the monitoring objectives and well conditions.

Common approaches include:

Low-flow purging:

A pump is positioned within or near the screened interval and groundwater is removed at a controlled, low rate. Field parameters are monitored while purging continues until stabilization criteria are achieved.

Well-volume purging:

A predetermined volume of water is removed from the well before sampling. This approach has traditionally been used in some monitoring programs, although it may not always provide the same advantages as low-flow sampling.

Passive sampling:

Some monitoring programs use passive or minimal-disturbance techniques that collect groundwater without conventional purging. These methods require appropriate equipment and site-specific procedures.

The sampling plan and applicable regulatory requirements should determine which method is used.

5. Begin Purging at the Appropriate Flow Rate

If low-flow purging is being used, begin at a controlled flow rate designed to minimize drawdown and disturbance within the well.

The goal is to remove groundwater steadily without excessively lowering the water level or creating unnecessary turbulence.

Excessive pumping can potentially increase turbidity and disturb sediment around the well screen.

6. Monitor Field Parameters

During the well purging procedure, field parameters should be monitored at appropriate intervals.

Depending on the project, these may include:

• Temperature

• pH

• Specific conductivity

• Dissolved oxygen

• Oxidation-reduction potential

• Turbidity

Measurements are generally recorded throughout the purge so the sampler can determine whether groundwater conditions have stabilized.

7. Determine When Stabilization Has Been Achieved

For low-flow sampling, groundwater is generally considered ready for sampling when the required field parameters have stabilized according to the project's approved criteria.

Stabilization criteria can vary by project and regulatory program. The sampler should therefore follow the requirements established in the sampling and analysis plan rather than applying one universal set of values to every project.

If parameters have not stabilized, additional purging or investigation may be necessary.

8. Collect the Groundwater Sample

Once the well meets the project's sampling criteria, collect the groundwater sample using the specified sampling equipment and technique.

Care should be taken to minimize:

• Turbulence

• Aeration

• Sediment disturbance

• Cross-contamination

• Unnecessary exposure of the sample to the atmosphere

Sample containers should be filled and preserved according to the laboratory and project requirements.

9. Manage Purge Water

Purge water may require appropriate collection, storage, treatment, discharge, or disposal depending on the site and the contaminants present.

Do not assume that purge water can be discharged onto the ground or into a stormwater system.

The project-specific environmental requirements should be reviewed before field activities begin.

10. Document the Procedure

Accurate field documentation is an essential part of groundwater monitoring.

Record information such as:

• Date and time

• Monitoring well identification

• Static water level

• Purge method

• Pump type

• Pump location

• Flow rate

• Purge duration

• Volume removed, where applicable

• Field parameter readings

• Water-level changes

• Sampling time

• Weather or unusual site conditions

• Equipment used

• Any deviations from the approved procedure

Detailed documentation makes it easier to evaluate the quality and consistency of groundwater data.

Low-Flow Purging vs. Traditional Well-Volume Purging

The two approaches differ in how groundwater is removed before sampling.

Factor	Low-Flow Purging	Well-Volume Purging
Water removal	Controlled and generally lower volume	Larger predetermined volume
Disturbance	Designed to minimize disturbance	Can cause greater disturbance
Field monitoring	Parameters monitored during purging	May rely more heavily on predetermined volumes
Drawdown	Typically minimized	Can be greater
Purge duration	Depends on stabilization	Often based on calculated volume
Site suitability	Common for many monitoring programs	May be specified for certain programs

Neither method should automatically be applied to every monitoring well. The appropriate approach depends on the sampling objectives, well construction, regulatory requirements, and site-specific conditions.

Common Mistakes During Well Purging

A poorly controlled well purging procedure can affect sample quality. Common problems include:

Purging Too Quickly

High pumping rates can cause excessive drawdown, increase turbidity, and disturb material around the well screen.

Relying Only on a Fixed Number of Well Volumes

Removing a predetermined number of well volumes does not necessarily demonstrate that groundwater conditions have stabilized.

Failing to Monitor Field Parameters

Without field measurements, it can be difficult to determine whether the water being sampled is representative of the groundwater formation.

Disturbing Sediment

Aggressive pumping or poorly positioned equipment can increase turbidity and introduce suspended material into the sample.

Inadequate Equipment Decontamination

Equipment that has not been properly cleaned can introduce contaminants or create cross-contamination between monitoring wells.

Poor Documentation

Missing information about purge rates, water levels, field parameters, or sampling conditions can make laboratory results more difficult to interpret.

How Long Does It Take to Purge a Monitoring Well?

There is no single purge time that applies to every monitoring well.

The time required depends on factors such as:

• Well diameter

• Screen length

• Aquifer characteristics

• Groundwater recharge rate

• Pump type

• Purge rate

• Drawdown

• Field parameter stabilization

• Sampling requirements

For low-flow sampling, the process may continue until the required field parameters meet the project's stabilization criteria rather than for a predetermined amount of time.

What Happens After Well Purging?

After the monitoring well has been properly purged and field conditions meet the sampling criteria, groundwater samples can be collected.

Samples are then typically:

• Placed into the appropriate laboratory containers.

• Preserved according to analytical requirements.

• Labelled and documented.

• Stored under the required conditions.

• Submitted to the laboratory using the appropriate chain-of-custody procedures.

The samples can then be analysed for the contaminants or groundwater characteristics specified in the monitoring program.

Frequently Asked Questions

What is a well purging procedure?

A well purging procedure is the controlled removal of water from a groundwater monitoring well before sampling. The purpose is to obtain groundwater that is representative of conditions in the surrounding formation.

How many well volumes should be purged?

There is no universal number that applies to every monitoring well. Traditional methods may specify a predetermined number of well volumes, while low-flow methods generally rely on controlled pumping and field parameter stabilization.

What parameters are monitored during well purging?

Common parameters include pH, temperature, specific conductivity, dissolved oxygen, oxidation-reduction potential, and turbidity. The exact requirements depend on the sampling program.

Can groundwater be sampled without purging?

Yes. Certain passive or minimal-disturbance sampling methods can collect groundwater without conventional well purging. However, the method must be appropriate for the well, contaminants, sampling objectives, and applicable requirements.

Why is low-flow purging used?

Low-flow purging is designed to minimize drawdown and disturbance while allowing groundwater representative of the formation to enter the well. It can also reduce the volume of purge water generated.

What should be done with purge water?

Purge water should be managed according to site-specific requirements and applicable environmental regulations. If contamination is suspected, appropriate collection and disposal procedures may be necessary.

Final Considerations

A reliable groundwater sampling program starts with a well-controlled well purging procedure. The objective is not simply to remove water from the well but to establish sampling conditions that provide representative and defensible groundwater data.

The appropriate purge method should always be selected based on the monitoring well design, groundwater conditions, contaminants of concern, sampling objectives, and applicable regulatory or project requirements. Consistent field measurements, proper equipment handling, and thorough documentation are equally important for maintaining data quality.