Push-Pull tests for the characterization of heterogeneity in alluvial aquifers

Abstract

In the scope of polluted site management, the heterogeneity of the subsoil is an important factor to take into account in the sizing of a remediation plan. This thesis aims at assessing the heterogeneity of an alluvial aquifer through the use of push-pull tests. Push pull tests are single-well tracer tests consisting in the injection of a tracing substance in the aquifer ("push"), followed by the extraction of the mixture of tracing substance and groundwater from the same well ("pull"). These tests are especially suited to polluted sites considering their ease of implementation and the small volume of potentially polluted water involved. Four tests were performed, during which a potassium chloride tracing solution was injected and then withdrawn from the aquifer. The differences between the tests was either the resting time that separated the injection and extraction phases, the injection rate or the injected volume. The influence of the resting period, of the injected volume and of the injection rate were studied. The mass recovery ratio was computed for each test and ranged between 70 and 80 %, ensuring a good representativity of the results. An analytical expression of a retardation coefficient was established, taking into account the natural flow in the aquifer. This expression is based on the recovery of the centre of mass of the tracer. Considering that the chosen tracer does not sorb on solid surface and that the dispersion effect is minimised in push-pull tests due to the flow reversal, the computed retardation highlights the importance of diffusion or immobile water effects. Indeed, the tracer recovery will be delayed as mass exchanges develop between high permeability zones and immobile water zones. Theses effects will be greater as the heterogeneity in the medium is important. The estimated retardation coefficients were comprised between 1 and 2.3 for the different tests but are expected to stabilise to a value of 2.4 as the elapsed time between the injection and extraction period increased. The stabilised value of the retardation is expected to correspond to the reaching of the equilibrium between the tracer concentration in high and in low permeability zones. The retardation coefficient is however highly dependant on the estimation of several parameters that are strongly correlated: the pore water velocity, the effective porosity and the dispersivity coefficient. Several propositions to make these estimations more robust have been presented.