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Metal contents in waters are of growing concern because of their potential toxicity above certain levels. In order to assess these problems, it is of primordial importance both to know their distributions between dissolved and particulate phases and their origin(s) in terms of "geochemical sources". It becomes therefore feasible 1- to evaluate their fate and bioavailability at estuaries, and 2- to workout solutions about these origins and their limitation. In order to evaluate the importance and temporal variability of potential metal sources in a small anthropized watershed, an hourly sampling was conducted at the estuary of a river over a short-time scale flood and compared with previous values obtained over the preceding year (Luck and Ben Othman, 1994). Trace elements and Pb isotopes were determined both on the dissolved and particulate phases (major, trace and Sr isotopes were also determined : see Ben Othman and Luck, 1996).
The Vène watershed is heavily anthropized (roads,vineyards,industrial harbour,industries). Water samples were collected in sept. 1994 during the first flood over 3 days on an hourly basis at the estuary in acid-cleaned PP bottles and filtered under class 100 clean air a few hours later. Trace concentrations were determined by ICP-MS.
Concentrations in the dissolved phase: Trace elements concentration ranges are similar to those observed over the year, and base levels are similar to other relatively undisturbed areas (Shiller and Boyle, 1987) for several elements (Zn, Cd; Pb ~ 150-10ppt). Concentration evolutions can be grouped in 3 classes :
1 - elements that are relatively undisturbed by the water discharge (V, Co, U, Tl, Zn);
2 - elements that have a tendancy to increase (Cu) with time;
3 - elements that have a tendancy to decrease (Mn, Ni);
a clear change of process can be identified on the basis of various elements around the middle of the big water discharge, probably corresponding to the initiation of karstic regime.
Concentrations in the particulate phase: Most trace elements show parallel variations with time : a decrease followed by a significant increase sometimes by 1-2 orders of magnitude (Co, V).
Isotopic compositions: Pb isotopes in the particulate show a range over the flood similar to that observed over the preceding year, inbetween gasoline, harbour and rock endmembers. However samples plot statistically towards slightly more radiogenic 206/204Pb values. The alignment is also better. The evolution of the isotopes is quite regularly shifting with time from the gasoline endmember towards more radiogenic values (rocks) Fig. 1).
Ben Othman, D. & Luck, J. M., (this volume) (1996).
Luck, J. M. & Ben Othman, D., Mineral. Mag. 58A, 541-542 (1994).
Shiller, A. M. & Boyle, E. A., Geochim. Cosmochim. Acta 51, 3273-3277 (1987).
Fig. 1.