In the Touch catchment (South-West of France), samples of sediment and water were collected to evaluate the mobility and bioavailability of some potentially toxic elements (Cr, Co, Ni, Cu, Zn, As, Pb and Cd) for ecosystems. In addition to these trace elements, certain compounds of the dissolved phase (pH, major ions, and dissolved organic carbon) were also studied. The results show two groups of trace elements in the Touch aquatic environment. We distinguish the elements associated with more than 50% in the residual phase (Cr, Zn, Ni and Cu) and those whose non residual fractions are dominant more than 60% (As, Cd, Co and Pb). In the non-residual phases, the trace elements have a significant association with the iron oxides, with the exception of Cd which is controlled to about 50% by the carbonate phases. The first two phases of the sequential extraction (water soluble and exchangeable) are insignificant in the pollutants controlling. Arsenic despite its very high proportion of non-residual phase (about 90%) is relatively less mobile than certain elements such as Cd, Co and Pb that have non residual proportions less important. Trace elements more stable in relation to physico-chemical conditions prevailing in the Touch aquatic environments are Cr, Zn and As, and most mobile are Cd, Co and Pb. The availability of these elements is particularly sensitive to the Physico–chimical conditions of this environment. Their presence in solution increases with the concentrations of inorganic ligands such as chlorides and organic ligands (DOC), while it decreases with increasing pH and alkalinity (in particular Cd and Pb because of their relative abundance in the carbonate phases).

The Gascogne region, located in the South West of France, is cultivated at more than 75% of its area. To evaluate the impact of anthropogenic activities on the river quality in this region, 75 samples of river waters were collected. Major elements, DOC, silica and pH were analyzed in the dissolved fraction of these samples. The results show that the geochemical composition of the Gascogne rivers, generally reflects regional lithological characteristics. However, at some locations, very highly cultivated, these lithological contributions are accentuated by the agricultural activities. Only Cl-, Na+, DOC and to a lesser degree SO4- are influenced by the urban activities, in particular around major cities. The Gascogne rivers are characterized by an alkaline pH (pH-7.47, pH-8.11). The most abundant ions in these surface water are HCO3- and Ca2 +. These two elements represent about 70% of Total Dissolved Salts (TDS) and have concentrations going respectively from 0.61 to 6.55 meq.l-1 and from 0.74 to 6.25 meq.l-1. The Na+, Cl- and Mg2+ contents, respectively vary from 0.14 to 3.24 meq.l-1, from 0.17 to 5.23 meq.l-1 and from 0.14 to 2.62 meq.l-1. The less abundant elements (K+, NO3- and SO4-) have values less than 1.5 meq.l-1. The DOC concentrations are between 1.04 mg.l-1 and 7.96 mg.l-1. Those of the silica range from 1.29 to 5.87 mg.l-1, with 2.87 mg.l-1 like average.