Solute transfer in the unsaturated zone-groundwater continuum of a headwater catchment

This study deals with solute transfer in the vertical continuum between the unsaturated zone and shallow groundwater of a weathered granite aquifer in the Kerbernez headwater catchment of western France. The objectives are (i) to determine the mechanisms responsible for solute transfer in the unsaturated and water-table fluctuation zones of the aquifer, and (ii) to analyse the implications of these results on solute transfer times at the catchment scale. An experimental site located in the plateau area of the catchment was equipped with 6 tensiometers, 18 ceramic cups at depths from 0.25 to 2.5 m and 7 piezometers from 3 to 20 m. Measurements of hydraulic head and water sampling were,carried out over a period of 2.5 years in the unsaturated zone (0-2 m), the water table fluctuation zone (2-9 m) and the permanently saturated zone (> 9 m). Two tracer experiments were carried out by applying two pulses of water, one enriched with deuterium and the other with bromide. Natural chloride concentrations, as well as deuterium and bromide concentrations, were analysed from solution samples. From the artificial tracer concentrations, two porosity compartments can be identified and partly quantified: (1) the slow-mobile porosity (36% of the bulk volume), accounting for the slow piston-flow transfer (2-3 m per year), and (2) the rapid-mobile porosity, which transfers small quantities of bromide at a rate of 19 cm h(-1) down to the water table. Natural chloride concentrations are characterised by a high temporal variability in the water-table fluctuation zone, whereas the concentrations remain steady in time in the permanently saturated zone (42 mg l(-1) at 20 m depth). The temporal variability is related to the water-table fluctuations and follows the same pattern each hydrological year, i.e. low concentrations during rising water-table followed by a progressive increase in concentrations during the periods of high piezometric level and water-table recession. This pattern is explained in terms of the two mobile porosity compartments and groundwater hydraulics. Based on these findings, we propose a conceptual model of solute transfer along the hillslope of a headwater catchment. We conclude that mixing in the water-table fluctuation zone could occur at two spatial scales. Firstly, at the pore scale, with mixing of waters in slow mobile and rapid mobile porosity, and secondly, at the scale of the hillslope. The mixing at this latter scale would appear to result from differences of flow path geometry and velocity between the unsaturated zone and the groundwater. (c) 2006 Elsevier B.V. All rights reserved.