Part 2: Inconsistent guidance
You may wonder from Part 1 of this blog why I think Low Flow is a misunderstood groundwater sampling methodology.
My reasons are very simple in that the method is only scientifically justifiable in very specific circumstances. These have become obscured over time and translated differently within different guidance documents, particularly as they have crossed the Atlantic to the UK.
Here’s a flavour of some issues and what different guidance documents recommend.
Maximum screen length
BS ISO 5667-11 guidance, developed in the UK, is contradictory to earlier US guidance and its scientific origins. The dubious suggestion within BS ISO 5667 that “micro-purging” is “most suited to to open boreholes or piezometers with long screen lengths” is admittedly qualified (“where the formation has significant permeability”) and cautionary (“it is not directly comparable to whole screen purging and should be chosen with care and consideration of the objectives”). However, it is all too easy to take this guidance out of context (as it often is) to infer incorrectly that Low Flow Sampling can be used in any length of screened borehole.
All other US and UK guidance are non-specific on screen lengths, and these also leave open possibilities for using the method inappropriately.
Sampling where Free Phase LNAPL or DNAPL is present
Apart from the (scientifically based) CLA:IRE guidance, the two UK guidance documents which mention DNAPL and LNAPL both recommend the use of low flow sampling and contradict earlier US guidance documents and the scientific advice of Puls and Barcelona not to do so. The main reason not to use low flow (or any other pumping method) is to avoid artifically mixing free product into groundwater. There is an argument that if pumps can be reliably installed below product layers and remain uncontaminated, then a dissolved phase sample may be reliable obtained – but this should always be managed as a special procedure and not used routinely.
How is water quality stability measured and when is stability reached?
I would hazard a guess and say that most practitioners carrying out low flow sampling spend less than 30 minutes to purge and take a groundwater sample. There are some (rare) practitioners who insist on low flow purging for a minimum of 1 hour. I know of many others who spend less than 10 minutes purging despite guidance above suggesting the absolute minimum time to obtain 3 consecutive stability readings at 3 to 5 minute intervals would be 9 to 15 minutes PLUS any time prior to this required to stabilise the water level. But I know of no-one contemplating purging for up to 2 hours suggested by the USEPA 2010 guidance.
I believe we should routinely record the number of “screen volumes” purged as an indicator of the overall purge status of the well screen (i.e. fully or partially purged). This will have the inevitable consequence of increasing the overall time spent on-site – but will bring in the benefit of drawing attention to the length and diameter of the screened interval which is a crucial factor in understanding and interpreting the origin of a groundwater sample.
- Puls & Barcelona, 1996. Low-flow (minimal drawdown) groundwater sampling procedures. USEPA.
- USEPA (Region 1) 1996 (revised 2010). Low stress (low flow) purging and sampling procedure for the collection of groundwater sample from monitoring wells.
- ASTM D6771, 2002 (revised 2012). Standard practice for low-flow purging and sampling for wells and devices used for groundwater quality investigations.
- Environment Agency (England and Wales) 2003. Guidance on monitoring of landfill leachate, groundwater and surface water. 283pp.
- CL:AIRE Technical Bulletin TB3, 2008. Principles and practice for the collection of representative groundwater samples.
- BS ISO 5667-11, 2009. Water quality – Sampling – Part 11: Guidance on sampling of groundwaters.
- NIGLQ (Nuclear Industry Group for Land Quality), 2015. Nuclear industry code of practice for routine water quality monitoring. Version 1, February 2015.
- Association of Geotechnical Specialists, 2010. AGS Guide to Environmental Sampling.
- BS 10175, 2011. Investigation of potentially contaminated sites – Code of practice.
- 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.
- McMillan, L.A., et al, 2014. Influence of vertical flows in wells on groundwater sampling. J. Contam. Hydrol,169, 50-61. http://dx.doi.org/10.1016/j.jconhyd.2014.05.005
- USEPA (Region 4) 2010. SESD Operating Procedure. Groundwater Sampling.
©Peter Dumble 2017