COMPARISON OF HYDROGRAPH DECONVOLUTIONS USING RESIDUAL ALKALINITY, CHLORIDE, AND OXYGEN-18 AS HYDROCHEMICAL TRACERS

Hydrograph deconvolution using geochemical tracers is currently widely used for determining the hydrologic mechanisms occurring in watershed s. However, few chemical parameters can be used as tracers because the ir involvement in biogeochemical processes prevents them from behaving in a conservative way. The aim of this study was to combine several g eochemically controlled parameters into a single tracer. Residual alka linity is a combination of several controlled parameters and is conser vative in a wide range of natural environments. It was used in this st udy for quantifying the contributions of surface runoff and of groundw ater flow during a flood in a Mediterranean watershed underlain by sed imentary rock. A preliminary geochemical study revealed that interacti ons with calcite, dolomite, and the clay-humus complex controlled calc ium and magnesium concentrations as well as carbonate alkalinity (Alk( c)), which prevented using them as tracers. Nevertheless, although res idual alkalinity (Alk(residual)) is a combination of these three param eters (Alk(residual) = Alk(c) - 2[Ca2+](T) - 2[Mg2+](T)), it provided results that were highly comparable to those obtained using chloride a nd delta(18)O. Contrary to the most cases in the literature, the contr ibution of direct runoff was dominant (about 80% at peak discharge). A ccuracy estimates, which took into account analytical errors, temporal variations in the isotopic signature of rainfall, and the spatial var iability of chemical elements, supported this result and confirmed tha t residual alkalinity is a useful concept in hydrology.