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Groundwater Sampling – Are we doing it right?

Wednesday, 29 November 2017 21:44:14 Europe/London

Groundwater Sampling – Are we doing it right?

The Future of Contaminated Land Risk Assessment

(Geological Society, Janet Watson Meeting, 6-7 November 2017)

Photo: Workshop in the Geological Society Council Chamber

I was recently invited to lead a groundwater sampling workshop at the Janet Watson meeting of the Geological Society on 7th November 2017.

This was a fascinating 2-day conference covering everything from conceptual models and risk assessment to treatment technologies, guidance and regulation. It also addressed the need for us all to rationalise and explain risk in a coherent and understandable way to a non-technical and risk-averse society. Amongst the eclectic mix of speakers and topics, I was excited to find I’d been allocated the Geological Society Council Chamber to deliver a one-hour workshop on groundwater sampling and to pose the question “Are we doing (groundwater sampling) right?”

Amongst the Icons.

Delivering a workshop underneath the gaze of some of the giants of British geology, and being in the splendidly grand Victorian Council Chamber for the first time, was a rare honour – and a little unnerving.

The chamber is curiously dominated by a painting recording a distinguished group of scientists examining the skull of Piltdown Man . The “discovery” of the skull in 1912 was hailed as the missing link between man and ape and used to demonstrate Britain (Sussex to be precise) as the undisputed birthplace of mankind. It was over 40 years later in 1953 that a forensic re-examination revealed the fossil as a fraudulent construction of a 500-year-old skull combined with the jaw of an Orang-utan! The lead perpetrator of the fraud, whilst strongly suspected, has never been conclusively identified. The whole shameful episode is a sobering reminder of the need for scientific rigour and the importance of maintaining high ethical standards in our day jobs.

A Noisy Workshop

Given the technical audience, and limited time, I decided not to do much talking and asked everyone to work in pairs to design a sampling strategy for a monitoring borehole at a contaminated site. The resulting noise in the packed-out room was tremendous, and I’m thankful for the enthusiasm with which everyone engaged. There was some good discussion and insightful exchanges as we shared and discussed together the best way to obtain a “representative” sample.

I’m not sure I changed anyone’s views on whether they or their organisation’s sampling processes were right or wrong, but here’s a few pointers I picked up from the session.

  • Most organisations have their own standard operating procedures for groundwater sampling, but few were aware which, if any, guidance underpinned these. 
  • Low Flow purge and sampling was the preferred methodology chosen by most of those in the room. However, it was clear that most do not fully appreciate the limitations of the Low Flow method, or the need to assess uncertainties (and fundamentally, the need for critical and considered interpretation of the resulting analytical results). 
  • Many seemed surprised that the simple concept of a well volume was significantly different in different guidance documents. 
  • There seemed some lack of awareness that if product layers are present (LNAPL or DNAPL) then the safest sampling strategy (in the absence of a pre-planned special procedure) is to not sample at all rather than mix product into groundwater (which would produce a very unrepresentative dissolved groundwater sample). 
  • And we ran out of time! 

Are we Doing it Right?

Analytical data generated from groundwater samples underpins all our risk assessment models. It is critically important to not only collect the best possible “representative” sample but to also understand its provenance. Our conceptual models depend on this for validation, to define the extent of contaminant distribution and to quantify the risk to water receptors. A considered interpretation of groundwater sampling results is even more critical where complex remediation strategies are being proposed.

The way we sample and the way we interpret data from groundwater sampling programmes from contaminated sites is not often questioned or subject to critical scrutiny. I worry that we have collectively fallen into the trap of ignoring the sometimes-inconvenient science underpinning the sampling methods we use in the interests of “getting the job done”.

It is tempting in the historic surroundings of the Council Chamber to draw a little from our history. It is exactly a consequence of others not questioning and looking more carefully at

the scientific evidence, why the well respected and influential perpetrator of the Piltdown Man fraud was successful, and why it took 40 years for their claims to be disproved.


For more on groundwater sampling see earlier blogs:

Links to In-Situ sampling equipment and CPD training courses

Follow links here for sampling products and training courses.

©Peter Dumble 2017 25 November 2017

Posted in Pete's Blog By News

Big Data and Barometric Compensation

Thursday, 2 November 2017 14:53:58 Europe/London

Is global barometric data accuracy good enough?

I was recently sent some information about a research group working on the design of a low cost data logger system for measuring water levels. This is conceptualised as a total pressure logger, programmed and managed using smartphone technology. 

Barometric Compensation

To carry out compensation for barometric presssure, the aim is to automatically download barometric data from a global weather simulation model providing data at 1 hour intervals for any point on the earth’s surface (e.g. see The data and statistical analyses available on these types of websites is hugely impressive, but can be expensive. For example, to access 30 years of historical data on the Meteoblue site costs between 84 and 120 Euros per location and is chargeable annually.

Barometric Pressure Accuracy Claims

The research group suggests the accuracy of simulated barometric data is within ±2 mbar (equivalent to a water level range of ±2 cm of water). But can this be true? The data is simulated (modelled) and based on varying grid resolutions of between 4 and 30km in size. It is also dependent on historical data, meaning it is likely to be less reliable in parts of the world where weather stations are poorly distributed and where data is of poor quality. Meteoblue themselves are more straightforward with their claims, responding to my email by saying they have no verification data on the accuracy of barometric pressure records. It is certainly worth reading the disclaimers on their website:

Testing the Possibility

It is possible to download 2 weeks of weather data from the Meteoblue model without cost. So in the interests of testing the waters, I set up an In-Situ Rugged BaroTROLL Data Logger in a shady place in my garden at home in Devon. This was programmed to measure barometric pressure (and air temperature) at 1 hour intervals. This could then be compared to downloaded data from the Meteoblue site for the post code and grid reference for my home.

Before I go further, let’s be honest and recognise that this is not a scientifically controlled experiment. Neither is it likely to be comparable to anywhere else in the world or during different weather conditions, so please treat what follows as a random, non-statistical sample of one short term experiment.

Comparing Modelled to Actual Barometric Pressure

Figure 1 shows hourly data recorded between 16th and 23rd October, 2017. All records are based on GMT (i.e. UTC time standard). The black trace is the Meteoblue pressure data automatically reduced to sea level. The orange trace is barometric pressure data collected from the logger in my garden at 108 m above sea level. The red trace is the same data reduced to sea level pressure.

Data was gathered during a period of variable pressure conditions associated with Atlantic storms, including “Storm Brian” which passed through between 20th and 22nd October.

 First of all I should say – wow! In simple terms, the modelled data corrrelates remarkably well with the actual barometric record. It’s hard not to be impressed.


Figure 1: Modelled barometric pressure record compared to actual data 


Figure 2: Modelled temperature record compared to actual data

For reference, I’ve also plotted the modelled and actual temperature data (Figure 2). The overall trend is again remarkably good, but the extremes in temperature in the model are not present in the actual temperature record.

Magnitude of Barometric Pressure Error

The real question with these data is how reliable would water level measurements be if a total pressure logger were compensated using the modelled data?

 Figure 3 shows the difference in mbar between actual and modelled pressure. The overall range (max to min) is 11 mbar though the difference between the starting position and the extremes is between -5 and +6 mbars, which is more representative of the potential error during the test period.


Figure 3: The difference in mbar between Actual and Modelled Barometric Pressure

 Figure 4: Linear Regression Chart.


Figure 4 is a linear regression correlation chart for the two pressure records. Statistically this is exceptionally good, particularly at higher atmospheric pressures.

Most of the data lies within 1 to 2 mbar either side of the correlation line, though at lower pressures these difference errors rise to 6 or 7 mbar. It probably needs a real statistician to analyse this more precisely. The error margin is far better than I would have anticipated when starting out on this exercise.

 Is this the future?

Using simulated pressure data without local validation will inevitably introduce a level of uncertainty into the accuracy of water level measurements. In some parts of the world this may be acceptable and better than having no data at all on which to gauge the seasonality and sustainability of our fragile groundwater resources. 

But let’s be careful how we post this data for others to use. I urge anyone working on the development of the tools behind these smart technologies to clearly report the degree of uncertainty in their water level measurements. This needs to include not only the potential error in barometric pressure (±7 mbar or 7 cm of water in this case), but also all other instrument and measurement errors involved (though these should be much smaller and typically in the order of order of 1 to 2 cm). Without this information to guide interpretation there is a real danger of posting inaccurate and sometimes misleading data.

For more on this see earlier blogs

· Is Barometric Pressure Compensation Always Necessary?

· Groundwater Levels – being professional. Should we routinely be expressing uncertainty in our measurements?

· Making Groundwater Level Monitoring Affordable – the Pocket Dipper

· Monitoring groundwater levels in Sierra Leone

· Kilimanjaro – Feeling the Pressure (and the cold)

Links to In-Situ barometric and water level data loggers

· Water Level Meters

· Rugged TROLL Data Loggers

· Level TROLL Data Loggers 


©Peter Dumble 2017 26 October 2017 / 975 words

Posted in Pete's Blog By News

Monitoring a Remediation Process in Real-Time

Friday, 8 September 2017 17:50:35 Europe/London

Aqua Troll 600 Sonde

Aqua Troll 600 Sonde

High-resolution monitoring saves time (and money!) 

It’s not often I get excited by new products, but I was recently given access to some real-time monitoring data collected by the new In-Situ Aqua TROLL 600 multiparameter water quality sonde, which has really excited me with the possibilities of the instrument. I shall be presenting the story behind the data at the Contaminated Land Expo at London’s Excel Centre on the 27th and 28th of September 2017, so to get the complete picture come along to one of the presentations either at 1:30 pm on 27th or 12:00 pm on 28th in Seminar Theatre 13.

For this blog, I thought I’d share a snapshot of the data record with you to give a flavour of what can be achieved by real-time, high-resolution monitoring. 

Background and Data Record

Five In-Situ Aqua TROLL 600 Multiparameter Sondes were positioned in monitoring boreholes 50 to 100 metres downgradient of another borehole used to inject a colloidal remediation fluid into a chlorinated solvent contamination source. The sondes were programmed to collect data every 15 minutes. The screen shot below shows the monitoring record from one of these locations over a period of 2 weeks following injection (each vertical line represents a 2 day period). Data was collected remotely and transmitted several times per day to  In-Situ’s web-based platform, HydroVu Data Services


HydroVu Data Record (with annotated notes)

Notes on Data Record

The main objective of monitoring was to try and identify the direction and speed of movement of the remediation fluid. Turbidity was chosen as an indicator to detect the presence of any (black) carbon-based reagent as it was diluted into the groundwater flow. 

The graphical record shows a signifcant rise in turbidity (by 8 NTU) three days after the end of the injection works. This continued for a further 6 days before water quality returned to background concentrations.

Other physical and chemical changes reinforced the interpretation that this change could only have arisen from the injection process.

  • A reduction in electrical conductivity by 200 uS/cm following the passage of the turbidity trace correlates with the addition of clean water following injection to help flush the remediation agent out of the borehole and into the groundwater system.
  • A rise and fall in pH may be associated with the passage of remediation agent (small rise in pH) and the passage of the flush water (lowering of pH).
  • A rise in water level (the graph is inverted) by 10 cm coincides with the addition of fluids. There was no rainfall during the monitoring period.
  • The temperature change is erratic, but shows changes which also correlate with the passage of injection fluids.

All the observed changes are relatively small compared to background and relatively short-lived. It is unlikely these changes would have been detected using conventional field sampling and analysis methods. 

In this case, high-resolution measurements identified changes in water quality which convincingly demonstrated the passage of the remediation fluid at the monitoring borehole.

This enabled a series of technical decisions to be taken very quickly to re-focus the remediation effort, which saved considerable time (and money) toward achieving effective remediation at the site.


©Peter Dumble 2017

 This study was conducted by Adam Hobson and his colleagues at S.S. Papadopulos & Associates in the United States.  Adam will be pesenting this study at two upcoming conferences: 1) the National Ground Water Association (NGWA) Conference on Fractured Rock and Groundwater on 3 October and 2) the NGWA Groundwater Summit on 4 December.

The links to the abstracts are:

Fractured Rock and Groundwater:

Groundwater Summit:



Posted in Pete's Blog By News

Electrical Conductivity Validation and Salinity A Cautionary Tale?

Thursday, 3 August 2017 21:15:09 Europe/London

Electrical Conductivity Validation and Salinity

A Cautionary Tale?

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Part 2: Inconsistent guidance

In Part 2 of his article on the guidance for Low Flow Sampling hydrogeologist Peter Dumble highlights some of the variances between official publications and some notable research.

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Posted in Pete's Blog By Peter Dumble

Low Flow Groundwater Sampling – a misunderstood methodology?

Monday, 5 June 2017 17:16:29 Europe/London

Part 1: Evolution of Technical Guidance

Peter Dumble looks at how the guidance for low flow purging and sampling protocols has developed in part one of a two part article on low flow groundwater sampling.

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groundwater trainingOver 1000 delegates, 23 years and still in demand.

It is 23 years since the first groundwater training courses started at Cranfield University in 1994.  Well over 1000 delegates have attended these courses, mainly from the UK and Ireland with the occasional overseas visitor. You may well be one of those.

It is useful to reflect on how these courses have evolved and why they are still needed and relevant for today, so here’s a quick history of the groundwater sampling training course.


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Posted in Pete's Blog By Peter Dumble

Robin Hazell 1927 to 2017

Friday, 31 March 2017 10:17:00 Europe/London

A life in Hydrogeology

Peter looks back at the life and career of influential hydrogeologist Robin Hazell who passed away in February. Robin Hazell was a long-time member of the IAH (International Association of Hydrogeologists) and noted as a mentor for successive generations of hydrogeologists.

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Is barometric compensation always necessary?

Friday, 3 March 2017 09:30:00 Europe/London

Rationalising uncertainty when using total pressure loggers for groundwater level measurements

Peter Dumble discusses whether it is justifiable, in some circumstances, to use unvented total pressure loggers for water level measurements without post-correcting the data for variation in atmospheric (barometric) pressure.

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Posted in Pete's Blog By Peter Dumble

Purge Volumes in Groundwater Sampling - Part 2

Monday, 30 January 2017 09:56:07 Europe/London

Differences between purge volume calculations

In his last blog post Peter discussed a range of purge volume methods.  In part 2 of this post he looks in detail at the numbers involved and analyses the difference between the methods.


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Posted in Pete's Blog By Peter Dumble
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