Case study - Low-flow groundwater sampling on a nuclear site

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Courtesy of Geotech

Nuclear licensed site use of low-flow groundwater sampling shows: Reduced time, reduced quantity of purge water – both thus reducing cost - and reduced risk of operator contact especially with potentially radioactive-contaminated purge water thanks to a self-contained enclosed system. One of the biggest headaches on site is the purge water and the expense of getting rid of it – after it has itself been analysed for radioactivity. Low flow sampling is helping contain and reduce the problems.

On site comparison

During on-site comparison trials between bailing and low flow, operators from AMEC ran the sampling sequence for comparison in each borehole sequentially as:

1. Low-flow sampling

2. Low-flow with parameter stabilisation then sampling

3. Bailer sampling

Method was to do no purging with bailing but only with low-flow parameter stabilisation in the second part.

Note: When using the bailer the borehole has already been low-flow purged. There were more dissolved and suspended solids with the bailed samples than with low-flow samples but that did not affect the sample results other than visually. A recommendation of the comparison report is, another time, to run the sampling sequence with bailing done first to see if there are significant statistical differences.

Findings

Experience was of low-flow yields of 300-500ml/min for some 4 litres of sample taking 8-10 minutes. Bailing has 800-900ml in each bailer requiring five bailer quantities which for deeper wells took quite a time to extract – and longer than low flow. The team observed that had they been purging three well volumes with the bailer it would have taken a very long time.

By comparison with bailing, the low-flow took longer to set up the kit, but then sampling was much quicker especially with in-situ pumps in 2 of the 4 boreholes.

As this was a nuclear licensed site, using low-flow, greatly reduced the possibility and the risk of operators getting splashed with contaminated groundwater as it is contained in an enclosed system.

With the low-flow parameter stabilisation kit the maximum time taken to prove stabilisation was only 7-8 minutes at flow rates of 300-500ml/minute which produced minimum quantities of purged water. In order to take the sample required the purge-scan facility to be activated. The purge-scan facility automatically monitored selected water quality parameters at 20-second time intervals, then signalled achievement of stabilisation when three consecutive readings fell within 5 percent error limits.

An AMEC operator observed, “In terms of wastewater it was much quicker and you can rely on the parameter stabilisation kit to confirm you have brought in formation water.'

Geotech low-flow groundwater sampling kit

'The three well volumes directive is taken from the EA and British Standard guidance. Having the in-line (purge-scan) parameter measurement negates having to purge three well volumes. The three well volumes are to ensure formation water is drawn in but with in-line parameter measurement, we know and we can see we are achieving that.”

The purged groundwater from this nuclear site had to be contained and disposed of on-site in relation to its subsequent analysis, which even for uncontaminated groundwater, has a high on-site cost. The low-flow purge volumes to achieve parameter stabilisation were considerably smaller than if removing three well volumes before bailing.

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