Groundwater monitoring is the backbone of contaminated site management. Whether tracking a contaminant plume, verifying remediation effectiveness or demonstrating regulatory compliance, the data is only as good as the monitoring network that generates it. Poorly designed wells, improper installation and inconsistent sampling produce unreliable data that leads to wrong decisions and wasted money.

This guide covers the fundamentals of groundwater monitoring well design, installation and sampling for environmental professionals.

Why Monitor Groundwater?

Groundwater monitoring serves several critical purposes on contaminated sites:

  • Plume delineation - Defining the horizontal and vertical extent of dissolved contamination in groundwater
  • Migration tracking - Monitoring whether a plume is expanding, stable or shrinking over time
  • Remediation performance - Verifying that treatment systems are reducing contaminant concentrations as designed
  • Regulatory compliance - Meeting permit conditions that require periodic groundwater quality reporting
  • Risk assessment - Providing data for human health and ecological risk assessments
  • Natural attenuation - Demonstrating that natural processes are degrading contaminants without active intervention

Well Design

Well Construction Components

A standard monitoring well consists of:

  • Well screen - Slotted PVC or stainless steel pipe that allows groundwater to enter the well. Screen slot size is selected based on the formation grain size (typically 0.010 to 0.020 inch slots for most applications).
  • Well casing (riser) - Solid PVC or stainless steel pipe connecting the screen to the surface. Standard diameter: 2 inches (50mm) for monitoring wells, 4-6 inches for pumping wells.
  • Filter pack - Clean silica sand placed around the well screen to filter out fine-grained sediment and prevent the screen from clogging. Grain size selected to retain 85-95% of the formation material.
  • Annular seal - Bentonite clay or cement grout placed above the filter pack to prevent surface water from migrating down the borehole and contaminating the well.
  • Surface completion - Flush-mount vault (traffic areas) or above-ground protective casing (stick-up) with a locking cap to prevent tampering and unauthorized access.

Screen Placement

Screen placement determines what zone of the aquifer the well monitors. Critical considerations:

  • Water table wells - Screen spans the water table to capture floating product (LNAPL) or shallow dissolved contamination. Typically 10-15 feet (3-5m) of screen with the top at or slightly above the water table.
  • Deep wells - Screen placed at specific depths to monitor vertical contaminant distribution or deeper aquifer zones. Requires knowledge of site stratigraphy.
  • Multi-level installations - Nested wells or multi-port systems that monitor multiple depth intervals in a single borehole. Essential for understanding vertical contaminant migration.

Material Selection

  • PVC (Schedule 40) - Standard for most groundwater monitoring applications. Inexpensive, chemically resistant to most inorganic contaminants. Not suitable for sites with high concentrations of chlorinated solvents (can absorb/desorb VOCs).
  • Stainless steel (316 or 304) - Required for sites with high VOC concentrations or where PVC interaction is a concern. More expensive but chemically inert.
  • HDPE - Used in some applications but can sorb organic compounds. Less common for permanent monitoring wells.

Installation Methods

Hollow-Stem Auger (HSA)

The most common method for monitoring well installation in unconsolidated materials. A hollow auger is advanced to the target depth, and the well is constructed inside the auger as flights are removed. Allows continuous soil sampling during advancement.

Best for: Shallow to moderate depths (up to 50m), unconsolidated soils, standard monitoring wells.

Direct Push (Geoprobe)

Hydraulic or pneumatic systems drive small-diameter tooling into the ground. Faster and less expensive than HSA for shallow applications. Limited to smaller diameter wells and softer formations.

Best for: Rapid screening, temporary wells, shallow depth (up to 30m), soft formations.

Rotary Drilling

Used for deep wells or hard formations where augers cannot penetrate. Air rotary, mud rotary or sonic methods depending on geology and project requirements.

Best for: Deep wells, bedrock installations, difficult geology.

Well Development

After installation, wells must be developed to remove drilling fluids, fine sediment and disturbed formation material from around the screen. Development methods include:

  • Surging with a bailer or surge block
  • Pumping at increasing flow rates
  • Air lifting (not recommended for VOC monitoring wells)

Development is complete when turbidity drops below 10 NTU and pH, conductivity and temperature stabilize. Allow 48 hours to 2 weeks for the well to equilibrate before the first sampling event.

Groundwater Sampling

Low-Flow (Micropurge) Sampling

The current standard of practice for most monitoring programs. A dedicated or portable pump is placed at the screen interval and operated at low flow rates (0.1 to 0.5 L/min) to minimize drawdown and turbidity. Indicator parameters (pH, temperature, conductivity, dissolved oxygen, ORP, turbidity) are monitored until stable before sample collection.

Advantages: Produces representative samples of formation water, minimizes artificial turbidity, reduces purge volume and waste disposal costs.

Three-Volume Purge

The traditional method: remove three well volumes of standing water before sampling. Still used but increasingly replaced by low-flow sampling because it removes formation water along with stagnant casing water, can increase turbidity and generates large volumes of purge water requiring disposal.

No-Purge Sampling

Passive sampling devices (diffusion samplers, passive bailers, snap samplers) that equilibrate with formation water in the screened interval. No pumping required. Increasingly accepted by regulators for long-term monitoring programs where baseline data from conventional methods exists.

Sample Handling

  • Containers: Laboratory-supplied, pre-preserved containers appropriate for each analyte group (VOCs in 40mL glass vials with zero headspace, metals in acid-preserved HDPE bottles, etc.)
  • Preservation: Chemical preservatives and temperature (4°C cooler with ice) per method requirements
  • Hold times: Strict maximum times between sample collection and laboratory analysis (e.g., 14 days for VOCs, 180 days for metals)
  • Chain of custody: Documentation tracking every sample from collection through analysis. Essential for regulatory and legal defensibility.

Quality Assurance / Quality Control

QA/QC is non-negotiable in groundwater monitoring. Standard field QC samples include:

  • Trip blanks - VOC-grade water carried with the sample containers to detect contamination during transport. Required for every cooler containing VOC samples.
  • Equipment blanks - Analyte-free water passed through non-dedicated sampling equipment after decontamination. Verifies decontamination effectiveness.
  • Field duplicates - Second sample collected from the same well at the same time. Measures sampling precision. Typically one duplicate per 10-20 samples.
  • Matrix spike / matrix spike duplicates - Laboratory QC samples that measure method accuracy and precision in the site-specific matrix.

Data Interpretation

Groundwater monitoring data should be evaluated in context:

  • Spatial trends - Contour maps showing concentration distribution across the site. Is the plume expanding, stable or shrinking?
  • Temporal trends - Time-series plots for each well showing concentration changes over multiple sampling events. Statistical trend analysis (Mann-Kendall) provides objective assessment.
  • Comparison to standards - Results compared to applicable regulatory criteria (drinking water standards, site-specific risk-based criteria, background concentrations).
  • Hydrogeological context - Groundwater flow direction and velocity, seasonal water table fluctuations, potential receptors (wells, surface water bodies).

Common Mistakes

  1. Insufficient well network - Too few wells or wells in the wrong locations. The plume extends beyond the monitoring network and goes undetected.
  2. Wrong screen interval - Screening across multiple water-bearing zones dilutes contaminant concentrations and masks the true extent of impact.
  3. Poor well development - Produces persistently turbid samples that inflate metals results and complicate data interpretation.
  4. Inconsistent sampling procedures - Different field staff using different methods at different times produces incomparable data sets.
  5. Ignoring well condition - Damaged, silted or biofouled wells produce unrepresentative samples. Annual well condition inspections should be part of every monitoring program.

The Value of Good Data

Groundwater monitoring programs can run for years or decades. The difference between a well-designed program that produces defensible data and a poorly designed one that produces questionable data is the difference between informed decision-making and expensive guesswork. Every dollar spent on proper well design, installation and sampling saves multiples in avoided re-work, regulatory disputes and remediation missteps.

Get the monitoring right and the site management decisions follow naturally. Get it wrong and everything downstream is compromised.