Low-flow groundwater sampling on a nuclear site

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.

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.