Groundwater Monitoring Well Sampling

SOP-F303 Revision 0.2
Author

Marc Los Huertos

EA Program, Pomona College

Published

2026-06-26

1 Purpose

This SOP describes procedures for collecting representative groundwater samples from monitoring wells using the low-flow (minimal drawdown) purging and sampling method. Low-flow sampling minimizes disturbance to the well and surrounding formation, reduces turbidity artifacts, and yields samples that are more representative of in-situ groundwater chemistry than traditional high-volume purge methods (Puls & Barcelona, 1996).

The procedures follow EPA low-flow guidance (Puls & Barcelona, 1996), EPA Region 4 groundwater sampling procedures (U.S. Environmental Protection Agency, Region 4, Science and Ecosystem Support Division, 2017), and Cal-EPA DTSC guidance on representative groundwater sampling.

2 Scope

  • Applies to: Sampling of existing groundwater monitoring wells with an inside diameter of 2 inches (5 cm) or greater, screened intervals of 3 m (10 ft) or less, and a water level above the top of the screen
  • Does not apply to: Well installation or development; direct-push (e.g., Geoprobe) sampling; spring or seep sampling; wells requiring high-volume purging for regulatory compliance; wells with water levels below the top of the screen (consult the PI for alternative procedures)
  • Location: All EA Program groundwater monitoring sites

3 Responsibilities

Role Responsibility
PI / Project Lead Approve sampling plan and well selection
Field Lead Coordinate logistics, maintain well access agreements
Trained Personnel Purge and sample wells per this SOP
New Students Complete training before participating in well sampling

4 Definitions

  • Dedicated pump: A pump permanently installed in a single well and not removed between sampling events; eliminates inter-well cross-contamination and decontamination burden; preferred for long- term monitoring programs.
  • Drawdown: The decline in water level within a well caused by pumping; in low-flow sampling, drawdown must remain at or below 10 cm to minimize vertical mixing within the screened interval and preserve the representativeness of the sample (Puls & Barcelona, 1996).
  • E-tape (electric tape, water level indicator): An electronic device consisting of a two-conductor cable with a sensing probe at the tip; when the probe contacts the water surface, an audible alarm sounds; used to measure depth to water from a reference point (typically the top of casing, TOC).
  • Flow-through cell: A sealed flow cell into which purge water is routed so that a multiparameter probe can measure field parameters continuously without exposure to the atmosphere; prevents degassing of CO2 and O2 that would bias pH, DO, and ORP readings.
  • High-volume purge: The traditional well purging approach in which 3–5 well volumes (or more) are removed before sampling; causes high drawdown, turbulence, and resuspension of fine particles; generally produces less representative samples than low-flow methods (Puls & Barcelona, 1996).
  • Low-flow (minimal-drawdown) sampling: A groundwater sampling technique in which the pump is set at the mid-screen interval and the flow rate is adjusted to maintain drawdown of 10 cm or less; sampling begins only after field parameters have stabilized (Puls & Barcelona, 1996; Standard Practice for Low-Flow Purging and Sampling for Wells and Devices Used for Ground-Water Quality Investigations, 2021).
  • ORP (oxidation-reduction potential): A measure of the electron-donating or -accepting capacity of a groundwater sample; expressed in millivolts; sensitive to exposure to atmospheric oxygen; must be measured in a flow-through cell without headspace.
  • Parameter stabilization: The criterion indicating that the water being pumped from the well is representative of the aquifer rather than stagnant casing water; defined as three consecutive readings within specified limits for pH, temperature, specific conductance, DO, ORP, and turbidity (see §7.3 criteria table).
  • Screen interval (screened interval): The perforated section of the well casing through which groundwater enters the well from the surrounding aquifer; the pump intake should be positioned at the center of this interval during low-flow sampling.
  • Standing water volume: The volume of water within the well casing above the bottom of the well screen; calculated as \(V = \pi r^2 h\), where \(r\) is the inside casing radius and \(h\) is the height of the water column; used to determine whether enough water exists for low-flow sampling.
  • TOC (top of casing): The surveyed reference elevation from which all depth-to-water measurements are taken; must be marked on the well casing with a permanent reference point.
  • Trip blank: A set of 40 mL VOC sample vials filled with laboratory-grade water in the lab, transported to the field in a sealed cooler alongside field samples, and returned to the lab unopened; detects contamination introduced during transport (APHA et al., 2012).
  • Well volume: The volume of water stored in the well casing above the screen; typically calculated from well construction records; one well volume is the minimum purge volume before parameter monitoring in high-volume purge methods.

5 Safety and Hazards

WarningHazards
  • Physical: Heavy well caps and locks; lifting pumps and equipment; uneven terrain around well pads; potential for back strain when bending over flush-mount wells
  • Chemical: Groundwater may contain VOCs, metals, or other contaminants. Purge water may require containerization as hazardous waste. Preservative acids in pre-preserved bottles are corrosive.
  • Biological: Standing water in well vaults may harbor mosquitoes or other vectors
  • Confined spaces: Flush-mount well vaults may be classified as confined spaces. Do not enter vaults.

5.1 Required PPE

5.2 Emergency Procedures

Refer to SOP-F001 Field Safety and SOP-L003 Hazardous Materials Handling.

6 Equipment and Materials

6.1 Sampling Equipment

Item Notes
Low-flow peristaltic pump or bladder pump Programmable flow rate; 0.1–0.5 L/min
Pump tubing (dedicated or new) PTFE or polyethylene; appropriate ID for pump
Flow-through cell With ports for multiparameter probe
Multiparameter probe pH, temperature, SpC, DO, ORP, turbidity; calibrated per instrument SOP
E-tape (water level indicator) Metric and/or English gradations
Pre-labeled sample bottles Per analyte; type and preservative per project QA plan (APHA et al., 2012)
Deionized water For equipment blanks and rinsing
Well keys For locked well caps

6.2 Field Documentation

Item Notes
Well construction logs Casing depth, screen interval, diameter
Groundwater purge log (Appendix A) For recording stabilization parameters
Chain of custody (COC) forms Required for lab submission
Field notebook
Camera Photo-document well condition
GPS unit or smartphone For confirming well locations

7 Procedure

7.1 Pre-Sampling Preparation

  1. Review well construction logs for each well: casing material, diameter, total depth, screen interval, and previous water level data.
  2. Calculate the volume of standing water in the well casing above the screen using \(V = \pi r^2 h\).
  3. Calibrate the multiparameter probe per the manufacturer’s instructions. Record calibration data on the purge log.
  4. Assemble pump, tubing, and flow-through cell. For non-dedicated equipment, decontaminate with deionized water before use at each well.

7.2 Water Level Measurement

  1. Remove the well cap. Note the condition of the well (damage, standing water in vault, obstructions).
  2. Wipe the e-tape sensor with a clean paper towel.
  3. Lower the e-tape slowly until the alarm sounds at the water surface.
  4. Record depth to water from the TOC to the nearest 0.3 cm (0.01 ft).
  5. Measure well depth from the same reference point (after sampling, to avoid resuspending settled solids).
  6. Calculate the water column height and standing water volume.
TipTip

Always measure water level before inserting the pump. Lowering the pump displaces water and changes the apparent water level.

7.3 Pump Placement and Low-Flow Purging

  1. Slowly lower the pump intake to the center of the screened interval. Avoid dropping the pump or allowing it to contact the bottom of the well.
  2. Start the pump at the lowest speed setting and slowly increase until discharge begins. Target a flow rate of 0.1–0.5 L/min (Puls & Barcelona, 1996; Standard Practice for Low-Flow Purging and Sampling for Wells and Devices Used for Ground-Water Quality Investigations, 2021).
  3. Check the water level. Drawdown must not exceed 10 cm. If drawdown exceeds 10 cm, reduce the pumping rate.
  4. Route discharge through the flow-through cell for continuous monitoring.
  5. Record field parameters and cumulative purge volume on the purge log at 3–5 minute intervals.
  6. Continue purging until field parameters have stabilized (Puls & Barcelona, 1996):
Parameter Stabilization Criterion
pH ±0.1 units over three consecutive readings
Temperature ±0.2 °C over three consecutive readings
Specific conductance ±3% over three consecutive readings
Dissolved oxygen ±0.3 mg/L over three consecutive readings
ORP ±10 mV over three consecutive readings
Turbidity ±10% over three readings (or < 10 NTU)
  1. Once all parameters are stable, proceed immediately to sample collection. Do not turn off the pump between purging and sampling.
ImportantIf the Well Purges Dry

If the well dewaters before stabilization, stop pumping and allow the well to recover. Sample as soon as adequate volume has recovered. Record the dewatering event and recovery time on the purge log. Note that samples from wells purged to dryness may not be representative.

7.4 Sample Collection

  1. Disconnect the flow-through cell from the discharge line. Collect samples directly from the pump tubing (U.S. Environmental Protection Agency, Region 4, Science and Ecosystem Support Division, 2017).
  2. Maintain the same low flow rate used during purging.
  3. Collect samples in this order to minimize bias:
    1. VOCs (if applicable) – 40 mL glass vials, zero headspace
    2. Dissolved gases (if applicable)
    3. Total metals and other unfiltered analytes
    4. Filtered (dissolved) analytes – attach a 0.45 µm in-line filter
    5. General chemistry (pH, conductivity, alkalinity, nutrients)
    6. Bacteria (use sterile bottles)
  4. Fill bottles slowly to minimize aeration and turbulence.
  5. Cap immediately, label, and place in ice.

7.5 Post-Sampling Procedures

  1. Measure and record the final depth to water.
  2. Measure the total well depth.
  3. Remove the pump and tubing. Replace the well cap and lock.
  4. Containerize all purge water per the site waste management plan.
  5. Decontaminate non-dedicated equipment and collect an equipment blank.
  6. Complete the chain of custody form.

7.6 Decontamination Between Wells

  1. Flush the pump and tubing with ≥ 1 L of laboratory-grade detergent solution (Alconox or equivalent).
  2. Flush with ≥ 3 L of deionized water.
  3. Change nitrile gloves between wells.
  4. Collect one equipment blank per sampling event (APHA et al., 2012).

8 Sample Preservation and Holding Times

Follow preservation requirements specified by your analytical laboratory (APHA et al., 2012). Common groundwater analytes:

Analyte Container Preservative Holding Time
pH Poly, 250 mL None; measure in field 15 minutes
Specific conductance Poly, 250 mL Refrigerate 4 °C 28 days
Alkalinity Poly, 250 mL Refrigerate 4 °C 14 days
Nitrate + nitrite Poly, 250 mL H2SO4 to pH < 2; 4 °C 28 days
Total metals Poly, 500 mL HNO3 to pH < 2; 4 °C 6 months
Dissolved metals Poly, 500 mL Filter 0.45 µm; HNO3 to pH < 2; 4 °C 6 months
VOCs Glass, 40 mL (×3) HCl to pH < 2; 4 °C 14 days

9 Quality Control

  • Field blank: One per event; deionized water at the well site (APHA et al., 2012).
  • Equipment blank: One per event from the final deionized rinse.
  • Field duplicates: One per 10 wells sampled.
  • Trip blanks (VOCs only): One per cooler.

10 References

APHA, AWWA, & WEF. (2012). Standard methods for the examination of water and wastewater (22nd ed., p. 1360). American Public Health Association.
Puls, R. W., & Barcelona, M. J. (1996). Low-flow (minimal drawdown) ground-water sampling procedures (EPA/540/S-95/504). U.S. Environmental Protection Agency, Office of Research; Development. https://www.epa.gov/sites/default/files/2015-06/documents/lwflw2a.pdf
Standard Practice for Low-Flow Purging and Sampling for Wells and Devices Used for Ground-Water Quality Investigations, Pub. L. Nos. ASTM D6771-21 (2021). https://doi.org/10.1520/D6771-21
U.S. Environmental Protection Agency, Region 4, Science and Ecosystem Support Division. (2017). Groundwater sampling operating procedure (SESDGUID-301-R4). U.S. EPA Region 4 SESD. https://www.epa.gov/sites/default/files/2017-07/documents/groundwater_sampling301_af.r4.pdf

12 Training Record

Before sampling groundwater monitoring wells independently, personnel must:

  1. Read this SOP, SOP-F001, and SOP-F300 in full
  2. Review well construction logs for the wells to be sampled
  3. Demonstrate proper water level measurement technique
  4. Observe a trained operator complete a full low-flow purge and sample collection at one well
  5. Perform a supervised purge and sampling event achieving stabilization within the criteria in §7.3
  6. Demonstrate proper decontamination and chain of custody procedures
  7. Be signed off by the PI or field lead

13 Appendix A: Groundwater Purge Log

Print on waterproof paper and complete one form per well.

Site: ______________ Well ID: __________ Date: __________ Sampler: ______________

Well Construction: Casing diameter: ____ in | Total depth: ____ ft | Screen: ____ to ____ ft below TOC

Pre-sampling water level: ____ ft from TOC Calculated standing water volume: ____ L

Time Elapsed (min) Cum. Vol. (L) Flow (L/min) Drawdown (cm) pH Temp (°C) SpC (µS/cm) DO (mg/L) ORP (mV) Turbidity (NTU)

Stabilization achieved? Y / N Time: __________

Post-sampling water level: ____ ft from TOC Total well depth (post-sampling): ____ ft from TOC