How to Sample and Purge Groundwater Monitoring Wells Correctly

The quality of a groundwater dataset is determined well before a sample reaches the laboratory. Groundwater monitoring well sampling assesses the quality of underwater aquifers, but the results are only as reliable as the purging and sampling process used to collect them. Get the field method wrong and you risk under- or over-estimating contaminants, pH, salinity and dissolved oxygen – leading to flawed engineering decisions and compliance failures.

Groundwater monitoring is essential for protecting drinking water supplies and assessing industrial pollution. It helps detect and delineate contaminant plumes before they impact drinking water, and water samples from monitoring wells help track contamination and ensure compliance with regulations. Groundwater monitoring assists environmental and compliance teams in adhering to regulatory standards across mining, infrastructure and environmental projects.

This article focuses on monitoring wells used for environmental compliance, dewatering assessment and hydrogeological investigations on Australian mining and infrastructure projects. It covers the full purging and sampling workflow – from well development through to groundwater sample collection, QA/QC procedures, and how automated groundwater monitoring complements manual sampling. The guidance draws on Monitel’s experience designing and operating groundwater monitoring networks across Western Australia’s Pilbara, Goldfields and Southwest regions since the early 2000s.

Understanding Monitoring Wells and Groundwater Samples

A groundwater monitoring well is a purpose-built bore with a screened interval allowing water access from a defined aquifer horizon. Monitoring wells are typically installed 5–20 metres deep, though depths beyond 40 m are common in mining applications. Installation involves drilling and placing a slotted PVC pipe (commonly 50–100 mm nominal bore) surrounded by a sand filter pack, sealed with bentonite and cement, and finished at surface. Monitoring wells can be completed as flush-mount or above-ground types depending on site conditions and traffic.

The distinction between a well designed solely for water level measurement and one intended for groundwater sample collection matters. Sample wells require proper development, adequate diameter for pump access, and screen design that limits mixing across geological units. Proper well construction and monitoring framework are essential for reliable groundwater sampling.

Common applications include contamination assessments near fuel storage areas, seepage monitoring around tailings storage facilities, baseline monitoring near major cuttings and tunnels, and dewatering impact assessment around open pits and shafts. Monitoring wells require strict adherence to national guidelines for effective sampling, including WA Department of Water and Environmental Regulation guidance and the AS/NZS 5667 series.

Core Principles of Representative Groundwater Sampling

A representative sample is water that reflects in-situ aquifer conditions at the screened interval – not stagnant water that has been sitting in the casing, and not water altered by the sampling method itself. Stratification within the well bore, poor pump intake positioning, low recharge rates and inadequate well construction can all bias results.

The principles that underpin every reliable sampling method are:

• Minimise physical and chemical disturbance to the water column
• Avoid aeration that could alter dissolved oxygen, pH or oxidation reduction potential
• Avoid rapid drawdown that pulls water from unintended zones
• Manage purge volume relative to well yield and aquifer transmissivity
• Stabilise field parameters before collecting samples
• Maintain consistent methodology across sampling events for trend comparability

These principles directly inform the choice between low flow sampling, passive sampling methods and traditional well-volume purging discussed in the following sections.

Well Development Before Routine Groundwater Sampling

Well development is a one-off process carried out after drilling and installation, before any compliance groundwater sampling program begins. Well development is required to ensure representative water samples by removing drilling fluids, fines and residual bentonite from the filter pack, restoring hydraulic connection between the well and the formation.

Typical development methods include surge and bail, airlifting, and high-rate pumping followed by stepping down to lower discharge rates. Development continues until turbidity and field parameters (pH, conductivity, temperature) stabilise and sand or fines are minimal in the discharge water. This usually takes hours to days depending on well yield and formation type.

Poorly developed wells often show persistent turbidity during subsequent purging events, leading to analytical interferences – particularly elevated metals concentrations that do not reflect true aquifer conditions.

Traditional Purging: Three to Five Well Volumes

The traditional approach to purging involves removing a fixed number of well volumes before sampling. Three to five well volumes are typically purged before sampling, depending on the monitoring plan and regulatory requirements. Purging removes stagnant groundwater from monitoring wells to ensure the sample reflects formation water. Well purging ensures samples are representative of aquifer water, and EPA guidelines recommend purging to obtain accurate groundwater samples.

To calculate a single well volume, use casing diameter and water column height. For a 100 mm bore with 40 m of standing water, one well volume is approximately 1,000 litres. Three volumes would require removing around 3,000 litres of purge water – a significant volume in remote field conditions.

Equipment for this method includes submersible purge pumps, high-rate peristaltic pumps or dedicated bladder pumps. Pumping systems can be temporary or dedicated for long-term use. The method remains suitable for initial baseline investigations, high-permeability aquifers, and legacy monitoring programs that must follow existing approval conditions. However, limitations include excessive purge water generation, potential mobilisation of fines, drawdown that dewaters the screen, and extended field time on deep or low yield wells.

Low Flow Purging and Sampling

Low flow purging is the process of pumping at rates typically between 0.1 and 0.5 L/min, with the pump intake set within the screened interval, aiming for minimal drawdown – often less than 0.1 to 0.3 m. Low-flow purging is preferred by regulators for groundwater sampling because it relies on stabilisation of field parameters rather than arbitrary well volumes to determine when formation water is being pumped.

Stabilisation criteria commonly adopted in Australian and international practice include:

• pH: ±0.1 units across three consecutive readings
• Electrical conductivity: ±3–5%
• Temperature: ±0.2–0.3 °C
• Dissolved oxygen: within ±10%
• Oxidation reduction potential (ORP): ±10 mV

Low-flow sampling minimizes disturbance and reduces turbidity, producing more accurate results for redox-sensitive species, volatile organic compounds and dissolved metals. In very low yield wells, flow rates may drop below 0.1 L/min. Low-flow purging minimizes disturbance and turbidity in groundwater, making it widely preferred for compliance programs on Australian mining and infrastructure projects. Research has demonstrated that contaminant concentrations and field parameters can stabilise with less than 10 litres of purge volume in shallow wells using low flow methods – a fraction of the volume required by traditional sampling methods.

Passive and No-Purge Groundwater Sampling Methods

Passive sampling eliminates the need to purge wells before sampling. These methods involve deploying devices at the target depth within the screened interval, where they equilibrate with formation water over a set period. All passive sampling is no-purge, though some no-purge techniques still use pumps (such as discrete interval samplers) to collect water without first removing the standing water column.

Common passive sampling devices include passive diffusion bags – which are common equilibrium samplers in groundwater monitoring, particularly for volatile organic compounds – mechanical grab samplers triggered from surface, and multi-level samplers for vertical profiling. Passive sampling systems can be deployed without purging the well, offering minimal disturbance and no purge water to manage.

No-purge sampling can reduce costs by 10–70% compared to low-flow methods, depending on site logistics and well network size. Passive sampling can reduce costs by 10–70% compared to purging, making it an attractive alternative for large long-term monitoring networks with frequent sampling events. However, acceptance of passive sampling methods in Western Australia depends on regulator and project-specific approvals, and validation studies comparing passive results to low flow results are often required before the method is adopted for compliance monitoring.

Field Stabilisation Parameters and In-Well Monitoring

Field teams measure parameters like pH and temperature on-site before analysis to determine when formation water is being pumped. The main stabilisation parameters are pH, electrical conductivity, temperature, dissolved oxygen, oxidation reduction potential and turbidity.

A flow-through cell connected to the pump discharge allows probes to measure these parameters in continuously refreshed water without exposure to atmosphere – avoiding the aeration and degassing that would otherwise alter dissolved oxygen and ORP readings. Measurements are typically recorded every 3–5 minutes alongside pump rate, water level and cumulative volume in a dedicated field sheet or digital form.

On new wells or where stabilisation behaviour is uncertain, Monitel often integrates multi-parameter sondes with data loggers during initial test campaigns to characterise how quickly each well reaches stable conditions. This information helps refine pumping rates and expected purge durations for future sampling rounds.

Groundwater Sample Collection and Handling

Once field parameters have stabilised, sample collection follows a specific order designed to minimize cross-parameter interference:

1. Volatile organic compounds and volatile compounds (headspace-free fill, no aeration)
2. Dissolved metals (field-filtered through 0.45 µm)
3. Total metals
4. Major ions and general chemistry
5. Petroleum hydrocarbons
6. Microbiology (if required)

Common sampling devices include low-flow bladder pumps, peristaltic pumps (suitable for shallow applications but limited by depth and not ideal for VOC work), bailers, and discrete samplers. Dedicated tubing is preferable for long-term monitoring wells to reduce contamination risk.

Samples must be preserved and transported to maintain data integrity during groundwater analysis. Each sampling container must be the correct type for the requested analysis, with appropriate preservatives applied (e.g. nitric acid for metals to pH <2). Samples are placed in a cooler on ice to below 4 °C immediately after collection. Maximum holding times prior to laboratory receipt vary by analyte. Every bottle must be labelled with well ID, date, time, depth, sampler name, preservative type and requested analysis to maintain full traceability.

Preventing Contamination and Cross-Contamination

Groundwater samples must be free of cross-contamination for accuracy. Contamination control ensures that results reflect subsurface conditions rather than field artefacts introduced by equipment, handling or site conditions.

For non-dedicated equipment, decontamination between wells follows a standard procedure: wash with non-ionic detergent, rinse with potable water, then rinse with deionised water. More stringent protocols apply for trace metals and organics work. Dedicated tubing or dedicated pumps for long-term monitoring wells are a concern worth addressing early in program design, as they substantially reduce cross-contamination risk.

Wellhead hygiene includes removing surface debris, checking for damaged caps, ensuring no surface water can enter during sampling, and keeping sampling gear off the ground. Personnel should wear suitable gloves and change them between wells and before handling sample bottles – particularly where petroleum hydrocarbons or PFAS are target analytes.

QA/QC Procedures and Chain of Custody

Groundwater monitoring data must withstand regulatory and third-party scrutiny. Quality Assurance/Quality Control procedures ensure sample representativeness, and documented chain of custody is non-negotiable.

Quality assurance samples, such as duplicates and field blanks, are critical for data integrity in groundwater monitoring. Typical field QA/QC samples include:

• Field blanks (equipment rinsate blanks and trip blanks, especially for VOC work)
• Field duplicates collected from the same well to assess precision
• Matrix spikes as requested by the laboratory or project QA plan

QA/QC frequencies are commonly set at 1 in 10 or 1 in 20 samples in project-specific sampling and analysis plans. NATA-certified labs perform QA/QC procedures on groundwater samples received, providing data qualifiers and detection limit reporting.

Chain of custody documentation must include unique sample IDs, sample matrix, requested analyses, preservatives, sampler name, date and time of collection, and sign-off at every handover point. Monitel’s groundwater sampling programs are designed so QA/QC data can be readily interrogated alongside routine results through digital reporting platforms.

Coordinating Groundwater Level Monitoring with Sampling Events

Depth-to-water and static water level measurements are recorded before purging, during low flow sampling and after recovery. Manual water level gauging – measured to the nearest millimetre from top-of-casing and referenced to mAHD where surveyed – is combined with collecting groundwater samples to support hydrogeological interpretation.

Drawdown during purging and recovery times reveal aquifer response, helping to refine pumping rates for future rounds. Consistent water level data across campaigns is critical for distinguishing seasonal variation from changes driven by dewatering or abstraction.

Automated Groundwater Monitoring Between Sampling Rounds

Automated groundwater monitoring uses submersible pressure transducers, vibrating wire piezometers and data loggers installed in monitoring wells. While manual groundwater sampling may occur quarterly or monthly, automated systems record water level (and sometimes temperature and conductivity) at 15-minute to hourly intervals. Automated groundwater monitoring can reduce safety risks and improve data collection frequency compared to manual-only programs.

This continuous dataset provides context for laboratory results – identifying whether a sample was collected during peak drawdown, recovery, a recharge event or tidal influence. Monitel integrates automated telemetry and cloud dashboards across groundwater monitoring networks, with automated alerts on water level thresholds configured to trigger investigation or additional groundwater sampling outside the routine schedule. For details on bore instrumentation and automated reporting, refer to Monitel’s wells and bores and monitoring platform and reporting services.

Data Management, Reporting and Regulatory Compliance

Defensible groundwater sampling is only useful if data are stored, validated and reported in a way that satisfies approval conditions. Typical data flows run from field sheets or digital apps through laboratory electronic data deliverables, into a central database or monitoring platform, and out to dashboards, maps and compliance reports.

Common checks include validation of lab qualifiers, comparison against historical trends, review of QA/QC results (blanks, duplicates), and flagging outliers that may indicate sampling artefacts. Many WA approvals require regular groundwater monitoring reports – quarterly or annually – summarising trends, exceedances against trigger values and interpretation of impacts.

Monitel’s monitoring platform can combine groundwater level, quality and structural or vibration data into a single reporting environment where projects require multi-disciplinary monitoring.

Choosing Appropriate Purging and Sampling Methods for Your Site

Selecting the right method is a site-specific decision. Key factors include:

• Well yield and aquifer transmissivity
• Target analytes (e.g. volatile compounds versus metals)
• Regulatory requirements and existing approval conditions
• Frequency of monitoring and available field time
• Safety constraints and purge water management

Method changes during a long-term program should be managed with side-by-side sampling and statistical comparison to maintain trend integrity. The chosen method should be documented in a site-specific Sampling and Analysis Plan or Groundwater Monitoring Plan, referencing relevant standards and approvals. Monitel can support clients and their hydrogeological consultants in method selection and in demonstrating that an alternative approach still produces reliable, comparable datasets obtained from the same well network.

Monitel’s Approach to Groundwater Monitoring Wells and Sampling

We provide end-to-end support for groundwater monitoring programs: from monitoring well and bore instrumentation through to automated groundwater monitoring systems, manual sampling campaigns, data acquisition and web-based reporting.

Our approach is vendor-agnostic in instrumentation selection, focusing on robust sensors, reliable telemetry and defensible data rather than specific hardware brands. We have delivered groundwater monitoring solutions on WA mine sites, transport corridors, major excavations and industrial facilities – often integrating groundwater data with geotechnical and structural monitoring within a single platform.

Common Pitfalls and Practical Tips in Groundwater Well Sampling

Recurring issues we see on projects include:

• Inconsistent purge rates between campaigns, undermining data comparability
• Moving pump intake depth between sampling events
• Incomplete equipment decontamination, particularly for trace metals and organics
• Missing or incomplete field notes and QA/QC samples
• Attempting standard well-volume purging on low yield wells, dewatering the screen

When sampling for volatile organic compounds, avoid aeration at all costs – fill bottles with zero headspace, use inert tubing, and sample immediately after stabilisation. In low yield wells, reduce flow and rely on stabilisation criteria rather than forcing a fixed volume that the well cannot sustain.

Standardised field forms, pre-mobilisation equipment checklists and calibrated instruments prepared prior to every event are the simplest ways to maintain consistency. Safety considerations specific to Australian conditions – heat exposure, working around open boreholes, gas-prone environments and remote area logistics – must be addressed in every sampling procedure.

The Next Steps

Representative groundwater samples depend on correct purging, appropriate flow rates, stabilisation of key parameters, contamination prevention and robust QA/QC. The sampling method must suit the site, the analytes and the regulatory framework – and it must be applied consistently across every event to produce defensible, comparable data.

Automated groundwater level monitoring in wells complements periodic sampling by providing continuous context and early warning of changes between manual events. Together, these elements form a complete groundwater monitoring program capable of supporting long-term compliance and informed engineering decisions.

If you need to establish new monitoring bores, review existing purging and sampling methods, or integrate automated data acquisition into your groundwater network, contact Monitel. Our team works with engineers, hydrogeologists and environmental managers to develop defensible, long-term groundwater monitoring programs tailored to Western Australian project conditions.