Low Flow Sampling

In 1996, the US EPA Office of Research and Development, developed and published a document entitled Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures (Publication EPA540/S-95/504). Since 1996, low-flow sampling has become the standard for obtaining representative water quality samples. In the UK, low-flow sampling (sometimes called low-flow purging) is a relatively recent technique. By using a submerged pumps such as a bladder or peristaltic pump, groundwater can be pumped to the surface at a rate of between 100ml and 1000ml per minute. In the past, the problem with sampling groundwater has been that much larger samples (often contaminated) were taken which often contained stagnant water located above and below the screen. By abstracting much smaller samples, there is less stress and hence movement of water in the acquifer. The quanity of water being measured at the surface will be less than that entering the acquifer.

Today, it is possible to abstract just a few litres of water instead of older techniques which often required 10 times more IDW (Investigative Dervied Waste) from contaminated wells. The bladder pump and controller are located at the surface and the delivery hose is lowered down to the screened section of the monitoring well. The pump controller delivers a continuous sample to a flow cell and multiparameter instrument at the surface. Water levels will also be monitored continuously to ensure that the pumping causes no stress or drawdown within the well. Representative groundwater samples can be taken once the Troll 9500 multiparameter sonde and flow cell indicate that the measured parameters and turbidity have stabilised and at are at an acceptable level to the user.

The main advantages of low-flow sampling over traditional methods are:

• Far greater accuracy, precision and quality of samples due to lower stress limits placed on acquifer during sampling.
• Totally traceable without the need for any paperwork; all data is stored on a ruggedised PDA along with calibration data. WIth user-defined thresholds, the software will inform the user of when the samples are constant and representative.
• Less stress on acquifer by reducing sample volumes to that of 5-10% of older techniques. Disposal of contaminated samples is less of an issue.
• Increased productivity and dramatic cost-benefit improvement.

When all the equipment is setup, the static water in the pump and delivery tubing are removed. The pumping and monitoring continues until all the selected parameters have stabilised. Stabilisation is usually confirmed by turbidity. ASTM Practice D6771 suggests the following criteria to define stabilization:

Sample Collection

Once the parameters have stabilised, a diverter valve is used to collect samples.

For more information on low-flow sampling please click here.