Trace metal and nutrient distribution in an extremely low pH(2.5) river-estuarine system, the Ria of Huelva (South-West Spain)

Nutrient (nitrate, phosphate, silica) and dissolved metal (Al, Cu, As, Cd, Ni, Zn, Fe, U) distributions were studied in the mixing zones of the Tinto and Odiel rivers which drain the South Iberian pyrite belt. Phosphate distr ibution is strongly influenced by discharges from the fertiliser industry, especially in the Tinto mixing zone. The increase of silica content in this zone is related to a release of biogenic silica from diatoms. Nitrate conc entrations which are influenced by urban and industrial effluents showed an important maximum in the early stages of mixing in the Tinto las do the me tals). Compared to the Odiel river, the metal concentrations in the Tinto r iver reached higher values in relation to more intensive mining activities. Dissolved Fe, Mn, Al, Cu, Cd and Zn concentrations were correlated in the mixing zones of both rivers. This suggests that they have the same source a nd are subjected to the same controlling processes in the estuary. A maximu m concentration for these metals was observed in the early stage of mixing in the Tinto and reflects a decrease of redox in a low pH (< 3) environment . Downstream in the Odiel system, metals showed a slight removal. Dissolved uranium, present at a low level (0.05 mu mol l(-1)) in the rivers, is intr oduced by the phosphate fertiliser industry in the estuary and trapped in s edimentation areas. As a consequence, waters of the Gulf of Cadiz have a U content similar to that of the open seawater. (C) 1999 Elsevier Science B.V . All rights reserved.