HYDROGRAPH SEPARATION - A COMPARISON OF GEOCHEMICAL AND ISOTOPIC TRACERS

A chemical hydrograph separation can be used to study runoff processes on a watershed scale provided the soil kinetics of the geochemical tracer are well documented. To separate streamflow into surface (SF) and subsurface flow (SSF), the supply of the tracer from the soil should be uniform throughout the soil profile and independent of the residence time of the soil/groundwater. This paper examines the use of two weathering products, dissolved silica and magnesium, for separating spring runoff into SF and SSF in small headwater streams on the Canadian Shield. Field observations and leaching experiments in the laboratory suggested that both tracers were released very rapidly from the podzolic soils. Silica allowed a well-defined separation of streamflow into SF and SSF since it is absent in melt/rain water and its concentration varies little throughout the soil profile. Magnesium was used in those streams with wetlands when silica was not behaving conservatively. Both geochemical tracers suggested higher contributions of SSF to total spring runoff ( > 90%) than estimates (almost-equal-to 72%) based on the stable isotope deuterium. This difference may result from laterally flowing soil-water that did not mix with the phreatic/groundwater reservoir.