BSEE-DEC21-PG08.qxp_Layout 1 19/11/2021 10:41 Page 8
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The quality and availability of groundwater is an increasingly hot topic around the world, with tightening regulations and an increasing awareness of the impact of industrial activity. However, groundwater sampling from a borehole is complex and it can be difficult to ensure a representative sample. Mike White, an expert in groundwater sampling at QED Environmental, explains the problems associated with common sampling methods and outlines a potential solution.
C
ollecting a sample from a monitoring borehole that is truly representative of the quality of
groundwater is not as easy as it might seem.
The traditional purging methods, of hand bailing or high purge pumping, offer a good yield from a well, but can be time-consuming and, more importantly, problematic in terms of obtaining a representative sample. Guidelines are in place to remove stagnant water from the well, specifically that a 20-metre borehole with groundwater at 3-5 metres would typically require 3-5 well volumes prior to sampling in order to obtain formation water. Low-yield wells are typically evacuated and sampled upon recovery, usually within 24 hours.
However, little concern has been given to how purging protocols and devices used, such as bailers, affect the chemistry of ground water samples. It is just assumed that water that has passed through the well screen is a good sample.
This is a common mistake, because poor sampling techniques can mobilise solids, increase contaminants, such as NAPL microglobules, and change the chemical composition of the water with the introduction of O2 or CO2 in the upper parts of the well. What’s more, it is not unheard of for purging to remove all of the water from a well, which means it can be necessary to return to the site for another sample, which incurs extra time and cost.
In addition, there are four key issues we have identified with purging, in relation to sample chemistry and quality which, ultimately can introduce false results. Firstly, high purge volume can cause an underestimation of maximum contaminant concentrations due to dilution.
In a similar manner, high purging rates can cause overestimation due to contaminant mobilisation and increased sample turbidity. Thirdly, dewatering lower-yield wells affects DO and CO2 levels and can increase sample turbidity, both which can affect concentration of metals in a sample with either a low or high bias.
Finally, excessive drawdown can cause overestimation or “false positives” from soil gas or from mobilisation of soil-bound contaminants in the overlying formation or “smear zone”. Typically, from my own experience, it is clear that the quality of a sample also deteriorates over the course of a sampling routine. Our own tests demonstrate that it is not uncommon to go from 10 NTUs at the start of a test to 175 NTUs by the end of a twenty-minute sample time. What cannot be argued is that purging using a bailer or alternative method has the potential to cause agitation and, in turn, turbidity. Turbidity elevates metals and some organics bound to solids, such as PAHs, that can be removed via filtration, but this also is problematic. Sample filtration adds cost and time in the field or laboratory and there are regulatory programs, such as CCR, that require an unfiltered
ENVIRONMENTAL AWARENESS Towards better groundwater sampling
sample for metals. Filtration affects sample chemistry and it must be remembered that turbid samples that are filtered to remove solids are not the same as low turbidity samples.
The answer lies in using low-flow purging and sampling technology, such as QED Environmental’s Well Wizard sampling systems and Micropurge controllers, which can solve the problems encountered with traditional well purging methods, specifically the control of stress in formation water in and around the screen, which is often caused by high flow purging. A lower pumping rate minimises drawdown in-well mixing and formation stress, in turn isolating stagnant water above the screen. This lower pumping level reduces stress, and, in turn turbidity, which ultimately improves sample accuracy and reduces purge volume. Crucially, the samples represent naturally mobile contaminants, not stagnant water in the well or mobilised contaminants. Purge volume is based on stabilisation of water quality indicator parameters and not a minimum purge volume or purge time.
Most importantly, low flow sampling has a positive effect on data accuracy and precision. For example, analysis of one of our customer’s well purging data over a number of years demonstrated to us that wells purged and sampled with bailers result in high turbidity of circa ˃100 NTU. When the customer moved to a pump and bailer sampling methodology the result was varying turbidity levels of between 30-50 NTU. However, low flow sampling with dedicated bladder pumps methodology further reduced turbidity levels to zero.
A three-times well purge sample across 15 wells will typically require 2816 litres with an average purge volume of 190 litres. At an average pumping rate of 8-9 litres-per-minute the purging time per well would be 50 minutes – equating to 12.5 hours for the 15 wells.
Low purge sampling can also have a beneficial effect on the requirement for purge water handling and disposal. Variables from site to site can include site history, water flow path and potential downstream problems due to trace contaminants causing pollution.
8 BUILDING SERVICES & ENVIRONMENTAL ENGINEER DECEMBER 2021
This means that purged water needs to be containerised and disposed in the correct manner – all of which has a cost implication. What’s more, water may need to be stored on site in order to collect enough water for the sample. With low flow this equates to 20 litres per well, which can be contained in a bucket as opposed to requiring a 45-gallon barrel.
With low flow purging the time and cost benefits are immediately clear. For the same 15 well sample only 232 litres would be required with an average volume of only 12 litres. At an average pumping rate of 1.1 litres-per-minute the purging time per well is reduced to 13 minutes with a total purging time for 15 wells of 3.75 hours. Overall, this equates to an annual sampling cost of approximately £6,000 for low flow purging compared to circa £18,000. In conclusion, low-flow testing is a consistent, performance-based standard for purging, rather than an arbitrary rule of thumb. Most importantly, it can improve sample quality and reduce sampling costs, both directly in terms of reduced purge water handling and disposal, and indirectly by generating better data that result in better decisions.
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