Modelling of a water filtration pilot by a chemical tracer using the Modflow "Finite Differences"

Abstract

For the supply of drinking water to the population, we have set up a surface water treatment system: horizontal sand filtration. This process is based on the principle of bank filtration; it is a natural rustic treatment, based on slow biological filtration, which does not require any reagent. The objective of this work is to propose a tool to describe the groundwater flow on a complete water filtration system with the MODFLOW model in order to follow the evolution of the hydrodynamic parameters on the one hand and to determine the transit time of the water in the pilot. The calibration with a chemical tracer, fluorescein, allowing to optimise the defined parameters (hydraulic conductivities, recharge) of the aquifer in order to obtain calculated loads as close as possible to those measured in given observation points, and the reading and saving of the imposed hydraulic loads were carried out with the "results extractor" module. The variation of the slope of the 2nd compartment from 5% to 20% significantly reduces the transit time of the tracer from 6.3 h to 5.2 h in the device. It was also found that the transit time increased as the sand grain size decreased, but this did not indicate a malfunction of the device. The results obtained showed that it is possible to perform a hydrodynamic modelling of the system in order to reproduce the experiments performed on the physical device. The steady-state modelling shows that the curves calculated at each piezometer are close to the experimental curves, except for the variation in sand grain size (piezometer 3), probably due to the preferred path. The differences in concentration between the calculated and experimental values at piezometers 3 and 4 are due to a dilution and/or adsorption effect by the fine sand. The calibration of the model allows an appropriate sizing according to each problem