Photo Credit: GeotechENV
Low flow groundwater sampling is designed to collect representative water samples while minimizing disturbance inside a monitoring well. Instead of rapidly removing several well volumes, technicians pump groundwater at a controlled rate that limits drawdown, reduces sediment mobilization, and helps preserve the sample’s natural chemistry.
Following consistent field procedures, and selecting the right groundwater sampling pumps can improve sample quality while reducing purge-water generation and unnecessary field time.
What Is Low Flow Groundwater Sampling?
Low flow sampling uses a pump positioned within or near the screened interval of a monitoring well. Groundwater is removed slowly while water-level drawdown and field parameters are monitored.
The objective is to establish a steady flow of formation water into the well without significantly changing hydraulic conditions. The appropriate pumping rate depends on the well’s recovery rate, diameter, construction, and surrounding formation.
Low Flow Groundwater Sampling Procedure
1. Inspect the Well and Measure the Water Level
Begin by checking the well for damage, sediment buildup, or other conditions that could affect sampling. Measure and record the static water level before installing equipment.
Use clean or properly decontaminated equipment for each monitoring location to reduce the possibility of cross-contamination.
2. Position the Pump
Carefully lower the pump to the predetermined sampling depth, typically within the screened interval. Avoid repeatedly raising, lowering, or striking the pump against the well casing, as unnecessary movement can increase turbidity.
Several pump configurations can support field programs. The Typhoon Plastic Pump is a 12-volt submersible option designed for purging and low flow sampling at depths up to 50 feet. For shallow wells requiring higher initial pumping capacity, the Geosquirt Double Stage Pump can provide flow rates up to 2.5 gallons per minute.
3. Connect the Tubing and Control the Flow
Connect compatible tubing, such as 3/8-inch ID by 1/2-inch OD LDPE tubing, and begin pumping at the lowest practical setting.
A 3/8-inch barbed low flow control valve can be placed in the discharge line to further restrict flow when using a submersible pump. The valve is designed for 3/8-inch ID tubing and can reduce discharge to very low sampling rates. A low-flow pump AC-to-DC converter can also provide a convenient power option where suitable electrical service is available.
4. Monitor Drawdown and Field Parameters
Adjust the pumping rate until water-level drawdown remains minimal and stable. Route the discharge through a flow-through cell and monitor parameters such as:
-
Temperature
-
pH
-
Specific conductivity
-
Dissolved oxygen
-
Oxidation-reduction potential
-
Turbidity
Record measurements at regular intervals. Sampling should begin only after the required parameters stabilize according to the project work plan or regulatory protocol.
5. Collect and Filter the Samples
Once stabilization criteria are met, disconnect the flow-through cell if required and fill the appropriate laboratory containers directly from the discharge tubing. Avoid splashing, excessive agitation, and unnecessary exposure to air.
When dissolved metals analysis requires field filtration, a 0.45-micron single-sample filter can be installed according to the sampling plan.
Additional Pump Options for Low Flow Sampling
Different well conditions may require different equipment. The GeoSub stainless steel pump offers a durable submersible configuration, while the GeoSub 2 drop tube pump housing and lower intake support drop-tube system setups.
For shallow groundwater applications, the GeoPump II peristaltic pump can be paired with silicone tubing. A medium-hard equipment carrying case helps keep pumps, controllers, fittings, and accessories organized during transport.
Improve Sample Quality with a Controlled Process
Successful low-flow groundwater sampling depends on controlled pump placement, minimal drawdown, stable field readings, and careful sample handling. By matching the pump, tubing, filtration components, and flow-control equipment to site conditions, field teams can collect more representative samples while limiting turbidity and purge-water volume.