Standard Operating Procedures for Purging and Sampling Groundwater Monitoring Wells

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Standard Operating Procedures for Purging and Sampling Groundwater Monitoring Wells

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Purging a Well

Purging a monitoring well is a critical field procedure performed before collecting groundwater samples for environmental analysis. The goal is to remove stagnant water that has been sitting in the well casing (which is not representative of the actual aquifer water) and draw in fresh formation water from the surrounding geological zone.

The procedure is guided by regulations and standard practices, such as those from the EPA or provincial environmental agencies (e.g., Ontario Ministry of the Environment guidance).

There are two main methods: the traditional three to five well volumes method and the more modern low-flow sampling method.

Method 1: Traditional Three to Five Well Volumes Purge

This method involves calculating the volume of stagnant water in the well and removing a specific multiple of that volume until the well is adequately purged.

  • Measure Static Water Level: Use a water level meter to determine the depth to water (DTW) from the top of the well casing (TOC).
  • Calculate Stagnant Well Volume:
    1. Measure the total depth (TD) of the well from TOC.
    2. Calculate the height of the water column (Hwater = TD − DTW).
    3. Use the well’s diameter to calculate the volume per foot.
    4. Vgallons ≈ 0.163 gallons/foot × (well diameter in inches)2 × Hwater.
  • Purge the Water: Use a pump (peristaltic, submersible, or bailer) to remove the calculated volume of water (typically three to five times the stagnant volume).
  • Check for Recharge: In low-yield wells, if the well is pumped dry before the required volume is removed, allow it to recharge and empty it again. Purging is complete when the well has been evacuated three times (or the required volume is removed).

Method 2: Low-Flow Sampling (Preferred Method)

Low-flow purging is generally preferred by regulators as it minimizes disturbance to the formation, reduces turbidity (sediment in the water), and provides more representative samples.

  • Set Up Pump: Use a variable speed pump (peristaltic, bladder, or submersible) set to a low flow rate (typically 0.1 to 0.5 Liters per minute). The pump intake should be placed within the screened interval.
  • Monitor Water Level: Continuously monitor the water level in the well. The goal is to stabilize the water level, which confirms you are drawing water from the formation itself and not just the casing.
  • Monitor Field Parameters: Use a flow-through cell and multi-parameter field meter to continuously monitor water parameters (pH, temperature, conductivity, dissolved oxygen (DO), turbidity, and oxidation-reduction potential (ORP)).
  • Purging is Complete When Parameters Stabilize:
    1. Continue purging until the field parameters are stable for three consecutive readings (e.g., ± 0.1 for pH, ±10% for conductivity).
    2. Turbidity should ideally be below 10 Nephelometric Turbidity Units (NTU) for most analyses.

Groundwater Sample Collection

Collecting a groundwater sample from a monitoring well must be done using meticulous procedures to ensure the sample is representative of the aquifer and free of cross-contamination.

Here are the step-by-step procedures for collecting a groundwater sample after the well has been properly purged:

1. Preparation

  • Ensure Well is Purged: Do not sample until the well has been properly purged (either via the three-volume method or low-flow stabilization method) and field parameters are stable.
  • Don PPE: Wear clean, disposable nitrile or latex gloves. Change gloves between each sample location.
  • Prepare Containers: Lay out the pre-cleaned, lab-provided containers for the required analysis. Ensure containers are suitable for the Contaminants of Concern (CoCs) and contain the correct preservatives.

2. Sample Collection Methods

The method used for sampling depends on the equipment used for purging.

A. Low-Flow Sampling Method (Preferred)

If you used a low-flow pump for purging (peristaltic, bladder, or submersible), you can sample directly from the pump tubing.

  • Maintain Flow Rate: Keep the flow rate low (e.g., 100 mL/min to 500 mL/min) to prevent aeration or agitation of the water.
  • Fill Containers Directly: Fill the sample containers directly from the discharge tubing.

B. Bailer Method (Acceptable, but less preferred)

A bailer is an inert, small-diameter tube used to manually retrieve samples.

  • Lower Bailer Slowly: Lower the bailer into the well gently to minimize disturbance of the water column and resuspension of sediment (turbidity).
  • Retrieve Sample: Retrieve the bailer and pour the water gently into the sample containers.

3. Order of Sample Collection

Samples for different analyses must be collected in a specific order to minimize the loss of volatile compounds and chemical alteration:

  • Volatile Organic Compounds (VOCs/BTEX): Collect these first. Fill the 40mL VOA vials until a convex meniscus forms, then cap immediately to ensure zero headspace (no air bubbles) in the vial. Invert and tap gently to check for air bubbles; if present, repeat the process.
  • Petroleum Hydrocarbons (PHC F1/F2): Fill these vials next, again ensuring minimal headspace.
  • Metals (Dissolved and Total):
  • Dissolved Metals: Filter the water through a 0.45-micron filter attached to the tubing before putting it in the bottle and adding acid preservative.
  • Total Metals: Collect the unfiltered water directly into the bottle and add acid preservative.
  • General Chemistry/Nutrients/Semi-Volatiles (PAHs/PCBs): Collect these last.

4. Preservation and Storage

  • Add Preservatives (If Applicable): For metals and nutrient samples, ensure the lab-provided acid or base preservatives are added immediately if they aren’t pre-charged into the bottle.
  • Labeling: Immediately label each sample container clearly with project ID, well ID, date/time, analysis, and preservative used.
  • Cooling and Documentation: Place samples into a cooler with wet ice to chill them to 4 degrees Celsius. Complete the Chain of Custody (CoC) form and prepare samples for transport to an accredited laboratory.
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