Clay mineral and heavy metal distributions in the lower estuary of Huelva and adjacent Atlantic shelf, SW Spain

The Huelva estuary, on the south-western Spanish Atlantic coast, is an environment strongly polluted by acid mine drainage and industrial effluents. Clay mineralogy, heavy metal and particle-size distribution in estuarine and adjacent shelf sediments have been analyzed in order to identify the sources and transport pathways of the contaminated sediments. The estuarine sediments consist of detrital terrigenous minerals (illite, kaolinite, quartz, feldspars, dolomite and heavy minerals) derived from river catchments and coastal erosion, with biogenic components (calcite and aragonite) and minor authigenic minerals (pyrite and possibly gypsum). Mineral distribution pattern in the estuary-shelf system is controlled by grain-size sediment, physico-chemical conditions of waters and hydrodynamic factors. Important proportions of fine-grained sediments highly enriched in sulphide-associated heavy metals are supplied by the Tinto-Odiel river system. Most of these sediments are trapped when river waters reach the estuary because of flocculation processes during estuarine mixing, thus the estuary acts as a storage basin for metallic pollutants. In terms of public health, this estuary is well above recommended safety guidelines for most metals. Although the shelf sediments show metal concentration levels close to background values, eventually, a metallic plume emerges from the estuary to ocean, and consequently elevated metal concentrations can be locally detected on the inner shelf.