Low-Flow Groundwater Sampling - A Practical Field Guide

Why Low-Flow Matters

Low-flow sampling isn’t just a regulatory checkbox - it produces better data. Traditional high-flow purging (3-5 well volumes) agitates the water column, increases turbidity, and can strip volatile compounds from solution before you even fill a vial. Low-flow minimizes these issues by drawing water from the formation at a rate that doesn’t disturb the well.

This matters regardless of what you’re sampling for:

• VOCs: Aggressive purging can volatilize benzene, TCE, vinyl chloride, and other compounds out of solution, biasing results low. A missed exceedance because of sampling technique is a real liability.
• Metals: High-flow purging mobilizes sediment and particulates, artificially inflating total metals concentrations. This leads to false positives - especially for iron, manganese, and aluminum - and wastes time and money chasing contamination that isn’t there.
• SVOCs/PAHs: While less volatile than VOCs, these compounds can adsorb to suspended particles mobilized by aggressive purging, complicating the interpretation of results.

Ohio-specific note: The Ohio EPA TGM Chapter 10 (Ground Water Sampling, Revision 3) is the primary reference for groundwater sampling under VAP and DERR programs. Low-flow purging and sampling is the recommended method. If you’re working under Ohio EPA oversight, your sampling procedures should be consistent with TGM Chapter 10 guidance.

Equipment Checklist

Before you head to the field, confirm you have everything. There’s nothing worse than driving to a remote site and realizing you forgot VOA vials.

Pumping equipment:

• Bladder pump or peristaltic pump with dedicated tubing
• Flow controller (for bladder pumps) or variable-speed controller (peristaltic)
• Compressed gas source (bladder pumps - typically nitrogen)
• Sufficient tubing for your deepest well (measure beforehand)
• Twine or safety cable to tie off the pump - if your tubing connection slips, you don’t want to lose the pump downhole. Tie the twine to the pump and secure it to the well casing independently of the tubing.

Field parameter instruments:

• Multiparameter meter (YSI Pro Plus or equivalent) with flow-through cell
• Calibration standards (see Calibration section below)
• Turbidity meter (if not built into your multiparameter)
• Electronic water level indicator

Sampling supplies - VOCs:

• 40 mL glass VOA vials with PTFE-lined septa and HCl preservative (minimum 3 per well - 2 for analysis, 1 for MS/MSD if required)
• Trip blanks (prepared by the lab - one per cooler)
• Equipment blanks / field blanks if using non-dedicated equipment

Sampling supplies - Metals:

• Unpreserved poly bottles for total metals (unfiltered)
• Preserved poly bottles (HNO3) for dissolved metals (filtered)
• 0.45 µm disposable syringe filters or in-line filters for field filtering
• 60 mL syringes for field filtering

Sampling supplies - SVOCs/PAHs:

• 1-liter amber glass bottles (check with lab for specific requirements)
• Preservative as specified by analytical method

General supplies:

• Cooler with ice (samples must be chilled to ≤6°C)
• Chain of custody forms
• Field notebook or electronic field forms
• Nitrile gloves (change between wells)
• Safety glasses
• Site-specific PPE per the Health and Safety Plan
• Alconox and DI water for decontamination (if using non-dedicated equipment)

Instrument Calibration

Calibrate your field instruments before every sampling event. Bad calibration = bad data, and there’s no fixing it after the fact.

pH

Calibrate with a minimum of two standards that bracket your expected range. Standard practice is a three-point calibration:

• pH 4 (red buffer)
• pH 7 (yellow buffer)
• pH 10 (blue buffer)

Rinse the probe with DI water between each standard. Accept the calibration only if each reading is within ±0.05 SU of the standard value.

Specific Conductance

Calibrate with a single standard close to the expected site conductivity. Common standards are 1,000 µS/cm or 1,413 µS/cm. If your site has high-TDS groundwater, use a higher standard. Accept within ±5% of the standard value.

Turbidity

Calibrate with a zero standard (DI water or manufacturer-supplied zero solution) and at least one turbidity standard. Typical standards are:

• 0 NTU (DI water)
• 10 NTU
• 100 NTU
• 1,000 NTU (if you expect highly turbid wells)

Use the standards appropriate for your expected range. Shake turbidity standards gently before use - particles settle.

ORP (Oxidation-Reduction Potential)

Calibrate with Zobell solution. The expected reading depends on temperature - at 25°C, Zobell solution should read approximately +228 mV. Accept within ±10 mV of the expected value. ORP probes degrade over time - if you can’t get a stable reading within range, the probe may need replacement.

Dissolved Oxygen

DO calibration is different from the other parameters because you typically don’t use a liquid standard. The standard method is water-saturated air calibration:

1. Place a wet sponge or damp paper towel in the calibration cup (do not submerge the probe in water)
2. Seal the cup loosely to allow air exchange but maintain high humidity
3. Wait for the reading to stabilize - this takes 5-10 minutes
4. The meter should read close to 100% saturation at the current barometric pressure and temperature
5. Accept the calibration

For the zero-DO point, most meters use a software-based zero or a sodium sulfite solution. Check your meter’s manual for the specific procedure.

Record all calibration data - pre-calibration readings, standard values, post-calibration readings, instrument serial number, date, and initials. If someone questions your data later, calibration records are your first line of defense.

Step-by-Step Procedure

1. Pre-Sampling Setup

Before you start purging, measure the static water level using an electronic water level indicator. Record it. This is your baseline - you’ll track drawdown against this throughout purging.

Lower your pump intake to the middle of the screened interval (or the middle of the saturated screen if the water table is within the screen). This positioning gives you the most representative formation water.

Tie off the pump with twine or a safety cable secured to the well casing, independent of the tubing connection. This prevents losing the pump downhole if a tubing fitting comes loose.

Connect your tubing to the flow-through cell on your multiparameter meter so field parameters are measured inline without atmospheric exposure.

2. Begin Purging

Start pumping at a low rate - 100 mL/min is a good starting point. Ohio VAP practitioners generally target 100-150 mL/min. EPA guidance defines low-flow as typically less than 500 mL/min, but lower rates produce better results in most formations.

Monitor the water level continuously for the first 5 minutes.

Target: drawdown ≤0.3 feet. If drawdown exceeds this, reduce your flow rate. If you can’t maintain 0.3 feet even at the lowest practical flow rate, that’s okay - document it and sample anyway, but note the drawdown in your field records.

3. Monitor Field Parameters

Once flow is established, begin recording field parameters every 3-5 minutes. You’re waiting for stabilization, which means three consecutive readings within these tolerances:

Parameter	Stabilization Criteria
pH	±0.1 SU
Temperature	±0.5°C
Specific Conductance	±3%
Dissolved Oxygen	±0.3 mg/L
ORP	±10 mV
Turbidity	±10% (or <10 NTU)

In practice, pH and temperature stabilize fastest. DO and turbidity take the longest. At most wells, you’ll hit stabilization within 15-30 minutes. Some tight formations take longer - be patient.

4. Collect Samples

Once parameters are stable, disconnect the flow-through cell and collect samples directly from the tubing discharge. Never sample from the flow-through cell - it’s dirty, water has been sitting in contact with the sensors, and you don’t trust it.

Collect in this order (most sensitive to least sensitive):

1. VOCs first - fill 40 mL VOA vials. Zero headspace - overfill slightly so a meniscus forms, then cap. No air bubbles. If you see a bubble, discard and refill. Let the water flow gently down the inside wall of the vial to minimize agitation. No filtering.

2. Dissolved metals (filtered) - draw a syringe of water from the tubing discharge, attach a 0.45 µm disposable filter, and push through into the preserved (HNO3) bottle. Field filtering must happen at the time of collection, not back at the lab. Label the bottle clearly as “dissolved” or “filtered.”

3. Total metals (unfiltered) - fill the sample bottle directly from the tubing discharge. Do not filter. Label clearly as “total” or “unfiltered.”

4. SVOCs/PAHs - fill amber glass bottles from the tubing discharge. These are less sensitive to atmospheric exposure than VOCs but still should be collected with minimal turbulence.

5. Other analytes (general chemistry, etc.) - collect last.

5. Post-Sampling

• Place all samples in the cooler on ice immediately
• Record the sampling time, field parameters at time of collection, total purge volume, and final water level
• Cap the well, secure the lock, and move to the next location
• Decontaminate non-dedicated equipment before the next well (Alconox wash, DI water rinse)
• Change gloves between every well

Metals Sampling - Total vs. Dissolved

The distinction between total and dissolved metals is one of the most important (and most frequently mishandled) aspects of groundwater sampling. Getting this wrong can lead to false positives, unnecessary remediation, or failed compliance.

When to Collect Each

Total metals (unfiltered): This is the default for comparison to most cleanup standards, including Ohio VAP UPUS values. Collect at every well, every event.

Dissolved metals (field-filtered through 0.45 µm): Collect alongside total metals. Dissolved results represent the mobile, bioavailable fraction of metals in groundwater. The difference between total and dissolved tells you how much of the total metals concentration is from suspended particulates (which may not represent actual groundwater contamination) versus dissolved-phase metals (which are mobile and relevant).

Field Filtering Protocol

• Use 0.45 µm disposable syringe filters or in-line filters
• Filter in the field at the time of collection - not back at the lab, and not hours later. Field filtering captures the dissolved fraction at the moment of sampling.
• Draw water into a clean syringe from the tubing discharge
• Attach the filter and push through into the preserved (HNO3) bottle
• Discard the first 10-20 mL of filtered water to rinse the filter
• Note: Ohio EPA generally prefers unfiltered (total) metals and only accepts filtered results under specific conditions - when wells are properly developed, low-flow sampling was used, parameters stabilized, and turbidity exceeds 10 NTU due to geology (not poor sampling technique)

Interpreting the Results

If total arsenic is 25 µg/L and dissolved arsenic is 2 µg/L, most of the arsenic is associated with particulates - likely from a turbid sample, not dissolved-phase contamination. The dissolved result is more representative of what’s actually moving through the aquifer.

If total and dissolved are similar, the metals are genuinely in the dissolved phase, and turbidity isn’t inflating the results.

Common Mistakes

Mistake #1: Forgetting the trip blank. A trip blank is a lab-prepared set of VOA vials filled with analyte-free water that travels with your sample cooler but is never opened in the field. It’s your control for detecting contamination during transport. One per cooler, and the lab should prepare it - not you.

Mistake #2: Air bubbles in VOA vials. Even a small bubble can cause significant VOC loss through volatilization. Check every vial before capping. If in doubt, redo it. Vials are cheap; remobilization is not.

Mistake #3: Sampling from the flow-through cell. If you collect your sample from the discharge side of the flow-through cell, you’re collecting water that’s been in contact with sensor surfaces and sitting in the cell. Always disconnect the cell and sample directly from the tubing before the cell. The flow-through cell is for monitoring parameters only - never for collecting samples.

Mistake #4: Not recording drawdown. Your field notes should include water level at every parameter reading interval. If someone questions your data quality later, drawdown records are your evidence that you actually performed low-flow sampling.

Mistake #5: Lab filtering dissolved metals instead of field filtering. If the bottle arrives at the lab unfiltered and they filter it there, you’ve lost the point - metals chemistry can change during transport (oxidation, precipitation, adsorption to bottle walls). Field filtering is not optional for dissolved metals. If you forgot to filter in the field, report the result as total metals, not dissolved.

Mistake #6: Not tying off the pump. Tubing connections can slip, especially with bladder pumps under pressure. A pump lost downhole means a fishing job or well replacement - easily a $2,000-5,000 mistake. Thirty seconds of tying a twine safety line prevents this.

When to Deviate from This Procedure

Low-flow is the default, but there are situations where it’s not practical:

• Wells that go dry during purging - switch to a minimal-purge or passive sampling approach. Document why.
• Emergency response sampling - speed may override data quality concerns. Document the deviation.
• Wells with dedicated passive diffusion samplers - these have their own protocol and don’t require purging at all.
• Very low yield formations - if you can’t sustain even 50 mL/min, consider volume-averaged purging with a peristaltic pump or passive sampling methods. Document the approach.

Always document any deviations from standard procedure in your field notes and explain the rationale in your report. Reviewers are much more forgiving of justified deviations than unexplained ones.

Related Resources

• Ohio VAP Groundwater Standards - VOCs - Standards to screen your results against
• Ohio VAP Groundwater Standards - Metals - Including total vs. dissolved guidance
• PFAS Sampling Best Practices - PFAS requires different equipment and procedures
• Ohio EPA TGM Chapter 10 - Ohio’s official groundwater sampling guidance