The U.S. Environmental Protection Agency published Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures (EPA/540/S-95/504) in 1996, and it is still the document EPA points practitioners to. In the decades since, low-flow sampling has become the default approach for site assessment and remedial performance monitoring.

Why not purge three to five casing volumes?

The traditional methodology is to purge a well with a bailer or a high-speed pump to remove three to five casing volumes, then collect the sample. That approach can work against sample quality by driving up turbidity. Otherwise immobile artifactual particles get entrained, which can overestimate certain analytes — metals and hydrophobic organic compounds in particular.

Filtering those particles out has proven to be an undesirable fix. Filtration can bias contaminant concentrations low by removing mobile, contaminant-associated particles along with the artifacts. Reducing sampling-induced turbidity in the first place, by purging and sampling at low flow, avoids the trade-off.

What “low flow” actually means

Low flow is about velocity, not volume. The term refers to the velocity with which water enters the pump intake and is imparted to formation pore water in the immediate vicinity of the well screen. There is no required flow rate and no required purge volume.

• Use an adjustable-rate pump to withdraw water from the screened interval at a rate that causes minimal drawdown.
• Measure drawdown in the well while pumping with a water level meter. Drawdown is the best available indication of the stress a given flow rate places on a given hydrogeologic setting.
• Typical rates run 0.1–0.5 L/min, but the right rate is site-specific.
• Target minimal drawdown — under 0.1 m — during purging, and adjust flow rate to hold it.
• Collect the sample once water level and field parameters have stabilized across three consecutive readings taken three to five minutes apart.

Parameter stabilization

Water quality parameters, not a fixed volume, tell you when you are drawing formation water. Monitor pH, specific conductance, dissolved oxygen (DO), oxidation-reduction potential (ORP), temperature and turbidity in line, continuously, through a flow-through cell.

Parameters generally stabilize in this order: pH, temperature and specific conductance first, then ORP, DO and turbidity. Performance criteria should account for water level drawdown, pumping rate and the specifications of the instrument doing the measuring.

Common stabilization criteria across three successive readings:

• pH — within ±0.1
• Specific conductance — within ±3%
• ORP — within ±10 mV
• DO or turbidity — within ±10%

Treat those as rough guidelines and defer to your project plan. A workable minimum subset, in lieu of all five parameters, is pH, conductivity and DO (or turbidity).

One practical warning: turbidity is a very conservative parameter. It is always the last to stabilize, and excessive purge times are almost always the result of turbidity criteria set too tightly. Natural turbidity in groundwater can exceed 10 NTU on its own.

Why regulators favor it

Many state agencies encourage low-flow sampling because it is designed to collect a sample that genuinely represents water in the screened section of the aquifer surrounding the monitoring well. It does not come from water mixed inside the casing by a bailer or an inertial sampler, and it is not an average of water drawn along a long screened interval.

Conclusion

A low-flow sample can generally be trusted to represent the contamination — or lack of it — in the aquifer, because the process that produced it minimizes stress on both the aquifer and the well. Low flow also strips out much of the operator-to-operator variability inherent in traditional bail-and-purge procedures. In summary, it reduces physical and chemical stress, reduces variability in procedure, improves your ability to judge stabilization by monitoring parameters continuously, and reduces the chance that changes in analyte concentration were induced by the sampling itself.

Equipment for a low-flow event

• Low-flow sampling — bladder pumps and controllers, 12 V submersible and peristaltic pumps, dedicated systems, and in-line filters. Match pump diameter to well casing.
• Water level meters — for the drawdown measurement the method depends on.
• Water quality meters — multiparameter handhelds with a flow-through cell give the continuous readings stabilization is judged on. Low-flow kits are available for the YSI ProDSS and ProQuatro.
• PFAS sampling — if the analyte list includes PFAS, wetted materials are constrained as well as flow rate.

Not sure how to size a pump for your casing and depth to water? Talk to a specialist before you mobilize.

Source: EPA — Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures (EPA/540/S-95/504).