T119 Sources, Transport, and Fate of Trace and Toxic Elements in the Environment (Posters)

Monday, 19 October 2009
9:00 AM - 6:00 PM
Hall A

DISTRIBUTION AND REACTIVITY OF OXYANIONS (AS, V, SB, MO) IN THE SURFACE FRESHWATER REACHES OF THE GIRONDE ESTUARY (FRANCE)

Paper number: T119-22
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The behavior of oxyanions (As, V, Sb and Mo) along the surface freshwater reaches of the Gironde Estuary (south-west France) was studied at different time scales. For this, we measured particulate and dissolved (<0.2 µm and <0.02 µm) As, V, Sb and Mo concentrations and environmental parameters (temperature, conductivity, dissolved oxygen, pH, SPM concentration, current velocity, water depth) during i) a long-term monitoring (1999-2006) at the main entry of the estuary (upstream tidal limit) and ii) a high frequency sampling (every 30 min) during two consecutive tides at a fix station upstream the maximum turbidity zone (MTZ).

Dissolved oxyanion concentrations at the entry of the estuary showed clear seasonal, discharge-independent variations with maximum values during summer. Such temperature-related variations were attributed to the degradation of particulate As, V, Sb and Mo carrier phases in bottom sediment and/or adjacent aquifers, rather than release from suspended particulate matter. Along the surface freshwater reaches of the estuary, contrasting behaviors were observed: Whereas dissolved Sb concentrations seemed rather constant, dissolved As and V clearly showed tidal cycles, i.e. partly depended on physical process (e.g. tidal mixing and oscillation). Maximum concentrations at high tide may reflect metal distributions in the downstream MTZ i.e. equilibration of river-borne As and V with estuarine water and SPM in the MTZ. The dissolved Mo signal suggested tidal variations as observed for As and V overlapped by strong peaks coinciding with maximum current velocities. These peaks were attributed to remobilization of soft mud/sediment pore water rich in dissolved Mo due to release from organic matter. Such information on temporal variations in trace element behavior at different scales (from season to tide) supports the development of suitable and efficient water quality monitoring strategies as a tool for sustainable water resources management.