Using screen volumes to guide low flow purging sampling in DNAPL source zones
A new paper published in the Journal of Contaminant Hydrology (McMillan et al 2018) has identified the need to be cautious when using
indicator water quality parameters to indicate the stability of groundwater chemistry during purging.This work was produced as part of an In-Situ sponsored PhD studentship at the University of Birmingham and is the second peer-reviewed paper on sampling issues arising from this research. The paper was a collaborative international effort teaming up with researchers in North America at Geosyntec Consultants who had formerly worked on the SABRe (Source Area BioREmediation) research site at an industrial facility in the UK that hosted the field research conducted (https://www.claire.co.uk/projects-and-initiatives/34-sabre/102-sabre ). My contribution to this paper was in the role as a mentor to the PhD programme and in providing comments on the sampling issues raised by this particular line of research.
Low Flow Sampling from Contaminated Groundwater
Low flow sampling as a methodology has been widely accepted and is being increasingly adopted in the UK for contaminated groundwater investigations and monitoring. This paper questions if the volume of water removed prior to sampling is sufficient when relying solely on reaching chemical stability with indicator parameters (typically Temperature, EC, DO, pH and ORP).
Many practitioners aim to purge for as short a time as possible and I have seen purge times which have been as little as 10 minutes. Rarely do purge times extend beyond 30 minutes in routine monitoring operations before a sample is taken, regardless of screen length or diameter resulting in purge volumes amounting to fractions of 1 screen volume.
This research has shown that in order to reach stability with complex DNAPL constituents, reliance on indicator parmeters may not be sufficient. The paper goes on to recommended the use of minimum purge volumes, expressed as a number of screen volumes (see links below to earlier blogs on screen volumes).
Recommendations for New Guidance
By comparing a digital borehole model to field data collected from multi-level and short screened (3m) monitoring wells researchers concluded that:
“Pumped samples using low flow rates should ideally only be taken after purging at least 1 screen volume (SV) to overcome much of the expected initial sample quality variability.
The paper then adds:
“However, the potential for concentration temporal variability should be initially, and occasionally, examined by sampling over 2-3 SV purged or more. Purged volumes should be recorded to evaluate potential concentration sensitivity to volume variation between sampling events.”
I have for some time advocated the use of screen volumes as an indicator of the overall purge status of the well screen (i.e. fully or partially purged) – see earlier blogs listed below. Whilst this will in some cases increase the overall time spent on-site and the volume of purge water, this approach will, in my view, increase the confidence of researchers, regulators and practitioners in their understanding and interpretion of the origin of contamination in a groundwater sample.
References – Free to Download
The current (2018) paper and the earlier (2014) paper by McMillan et al are free to download using the links below.
McMillan, L.A., Rivett, M.O., Wealthall, G.P., Zeeb, P., Dumble, P., 2018. Monitoring well utility in a heterogeneous DNAPL source zone: insights from proximal multilevel sampler wells and sampling capture-zone modelling. Journal of Contaminant Hydrology, 210, 15-30..
McMillan, L.A., Rivett, M.O., Tellam, J.H., Dumble, P., Sharp, H., 2014. Influence of vertical flows in wells on groundwater sampling. Journal of Contaminant Hydrology, 169, 50-61.
Related SABRe project research of interest
Rivett, M.O., Dearden, R.A., Wealthall, G.P., 2014. Architecture, persistence and dissolution of a 20 to 45-year-old trichloroethene DNAPL source zone. Journal of Contaminant Hydrology, 170, 95-115.
Low Flow Groundwater Sampling – a misunderstood methodology? Part One
Low Flow Groundwater Sampling – a misunderstood methodology? Part Two
Purge Volumes in Groundwater Sampling - Part 1
Purge Volumes in Groundwater Sampling - Part 2
Screen Volume and Low Flow Groundwater Sampling
Importance of Screen Length and Diameter
Low flow Sample Origin
Links to groundwater sampling devices:
Links to training and courses:
REDR Drilling Course (8-11 May 2018)
CPD Endorsed Courses (None currently scheduled)
©Peter Dumble 2018