HYDROLOGICAL EXCHANGE AND SEDIMENT CHARACTERISTICS IN A RIVERBANK - RELATIONSHIP BETWEEN HEAVY-METALS AND INVERTEBRATE COMMUNITY STRUCTURE

The first metre of bed sediments of the Rhone River functions as a fil ter for fluxes of heavy metals and epigean organisms between surface a nd interstitial environments. To study the efficacy of this bank filtr ation, three sampling stations were established at increasing distance s from pumping wells, resulting in a gradient of hydraulic characteris tics. Station A, a permanent downwelling area with very high hydraulic gradients, low hydraulic conductivity, low oxygen content, and rather high metal concentrations contained a fauna exclusively composed of e pigean organisms. At station B, intermediate hydraulic gradients, high hydraulic conductivity near the surface, and variable direction of wa ter exchanges resulted in well-oxygenated pore water and a diversified fauna with hypogean and epigean species. Finally, at station C, low h ydraulic gradient, low hydraulic conductivity, low water exchange, and high sediment metal concentrations resulted in low pore-water oxygen concentrations and low interstitial fauna density and diversity. At th is site, low oxygen content and low biodiversity were related to the c logging of shallow sediments and low filtration efficiency. Thus, bank filtration efficiency, a property that depends mainly on natural or h uman-induced hydraulic gradients and sediment granulometry, determines pore-water chemistry, metal distribution, and faunal composition in t he shallow interstitial environments of the Rhone River.