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Groundwater sampling, purge or no purge that is the question

Abstract : There are uncertainties about to what extend the water in the well is representative of the water in the aquifer. So usually, for groundwater sampling a purging step is considered. This purging step used to consider a number of volumes purged (from 3 to 5 times the well’s volume) or more recently the stabilization of the physicochemical measures before the sampling itself. This purging step is nowadays done with flow rates less important that they used to be, due to the emergence of the low flow procedure (Puls and Barcelona, 1996) and the availability of new pumps. This purge step presents several constraints (Robin et Gillham 1987), among which the time (and thus cost) and the treatment/ elimination of the water purged. In France, a new standard (AFNOR, NF X 31- 615) will be published in 2017. It states that in some conditions the purge can be avoid. Moreover some questions remain on the spatial representativeness of the water sampled regarding the protocol used for the purge step (flow rate, position of the pump in the well screen interval). Considering also that in many wells there are very small differences in loads within the aquifer that can generate ambient fluxes in wells (Zinn and Konikow, 2007) which may have an impact on the results of the deployment of passive samplers compared to classic sampling. The representativeness of the sample taken is of paramount importance because it has a direct influence on the interpretation of the quality of the water and / or the pollution present. It is therefore necessary to know precisely which part of the aquifer is considered and how to proceed (purge or not). A project financed by the French Ministry of Environment between two public research bodies (BRGM and INERIS) has the objective to state the influence of the purge on the sampling representativeness. To answer those questions, three means will be used: literature review to know the position in other countries, on-site measurements campaigns for different hydrogeological context of the influence of some parameter (no purge, purge at different flow rate, at different positions in the well) and modelling of flows in and near the well for different scenarios (including the modelling of real cases) stating origin of the pumped water. This study will allow to evaluate the interest and the consequences of the purging processes during the realization of qualitative samples of water and will allow the production of guidelines for different cases.
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Alexandre Boisson, Valérie Guérin, Fabrice Quiot, Jean-Pierre Vergnes. Groundwater sampling, purge or no purge that is the question. 14. International Conference on Sustainable Use and Management of Soil, Sediment and Water Resources (AquaConSoil 2017), Jun 2017, Lyon, France. ⟨ineris-01863160⟩

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