Fully or Partially Purged – Does It Matter?

When taking a groundwater sample from a well or borehole, understanding just where the water sample originates within the groundwater body is critical, particularly when contaminants are present. Research by Lindsay McMillan highlights significant uncertainties in the interpretation of water quality data not only in long screen wells, but also from wells in which screens are less than 3 m in length and in which low-flow sampling is typically used. She is not the only researcher thinking this way.

Screen Volume – An Index for Purging?
The “screen volume” is a useful index against which to define purge volumes where the pump intake is placed within the well screen.  As such, it is worth thinking about how this can be used to inform good practice when using techniques such as low flow sampling.

The screen volume is defined as the volume of water within the screened interval of the well.

Consequently:

Q_SV = S.π.d² / 4000

where Q_SV is the screen volume in liters, S the screen length in meters, and d the internal diameter of the well screen in mm.  For example, a 3 m well screen in a 50 mm diameter monitoring well has a screen volume of approximately 6 liters.

Fully or Partially Purged?

Martin-Hayden et. al (2014)[i] demonstrate that between 3 to 5 times the screen volume needs to be purged from a well to be sure that the sample is taken after removing pre-purge water from the well and is derived from inflowing water from across the whole well screen. Such a sample is defined as being “fully purged”.  In the example above, 3 times the screen volume is 18 liters. This is the minimum volume that would need to be purged prior to sampling to be able to define the sample as fully purged.

Any sample taken where less water volume is removed is defined as “partially purged” and will contain a percentage of water arising from the well column prior to purging.

Low Flow Sampling and Screen Volume

When using “low flow” methodology to sample groundwater in wells we are encouraged to pump at rates of 0.5 liter per minute or less, with the pump intake positioned within the well screen[ii]. Crucially, we are asked to pump until stability of chemical parameters is achieved in 3 consecutive measurements at 5 minute intervals.  So it is theoretically possible to complete a low flow sampling exercise within 15 to 20 minutes, regardless of the purge volume, though most experienced practitioners would continue for longer.

But for how long?

By using the screen volume as an indicator it is relatively simple to compare a low flow sample to one which is fully purged. Using the same well example as above, it would take at least 36 minutes of low flow pumping to remove 3 times the well screen volume of 18 liters. Anything less than this would yield a partially purged sample.

McMillan, in her research (Reference i below), recommends that we should as a matter of course record the purge volume and relate this to screen volume. So far this recommendation has not appeared in any guidance documents – but perhaps it should.

A Word of Caution

This doesn’t mean that “partially purged” or even “no-purge” samples can’t be justifiably used, but is more a note of caution about how to interpret sample results. Partially purged and no-purge samples rely on natural mixing occurring between water within the well screen and adjacent strata to produce a “flow-weighted average” sample. However, full mixing may not occur resulting in uncertainty in the interpretation of sample origin and contaminant concentration.

For more articles by Peter Dumble

Peter Dumble is an Independent Hydrogeologist based near Tiverton in Devon.

[i] Martin-Hayden, J.M., Plummer, M., Britt, S.L., 2014. Controls of wellbore flow regimes on pump effluent composition. Ground Water Monit. Rem. 52 (1), 96–104. http://dx.doi.org/10.1111/gwat.12036.

[ii] US EPA, 2010. Low Stress (Low Flow) Purging and Sampling Procedure for the Collection of Groundwater Samples from Monitoring Wells, EPASOP- GW 001. http://www.epa.gov/region1/lab/qa/pdfs/EQASOP-GW001.pdf.