WATER & WASTE TREATMENT


TOWARDS BETTER GROUNDWATER SAMPLING


Mike White, an expert in groundwater sampling at QED Environmental, explains the problems associated with common sampling methods and outlines a potential solution


awareness of the impact of industrial activity. However, groundwater sampling from a borehole is complex and it’s often difficult to ensure a representative sample. The traditional purging methods, of hand


T


bailing or high purge pumping, offer a good yield from a well, but can be time-consuming and problematic in terms of obtaining a representative sample. Guidelines are in place to remove stagnant


water from the well, specifically that a 20m bore hole with groundwater at 3-5m 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 24h. 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, purging may 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 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 also 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 of which can affect concentration of metals in a sample with either a low or high bias.


30 SEPTEMBER 2021 | PROCESS & CONTROL


he quality and availability of groundwater is a hot topic around the world, with tightening regulations and an increasing


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, it is clear that the quality of a


Low-flow testing 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 results in better decisions


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. This elevates metals and some organics bound to solids, that can be removed via filtration, but this is also problematic. Sample filtration adds cost and time in the


field or laboratory and there are regulatory programs, such as CCR, that require an unfiltered 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, 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 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. Low purge sampling can also have a


beneficial effect on the need for purge water handling and disposal. Water may have to be stored on site in order to collect enough water for the sample. With low flow this equates to 20l/well, which can be contained in a bucket, as opposed to requiring a 45-gallon barrel. A three-times well purge sample across 15


wells will typically require 2816l with an average purge volume of 190l. At an average pumping rate of 8-9l/min the purging time per well would be 50 minutes – equating to 12.5 hours for the 15 wells. With low flow purging, for the same 15 well


sample only 232l would be required with an average volume of only 12l. At an average pumping rate of 1.1l/min the purging time per well is reduced to 13 minutes with a total purging time of 3.75 hours. This equates to an annual sampling cost of around £6,000 for low flow purging, compared to circa £18,000.


QED Environmental Systems www.qedenv.com


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70