Distribution and Composition of Suspended Particulate Matter in Guadiana Estuary (Southwestern Iberian Peninsula)

The construction of the Alqueva Dam may have a significant impact on the suspended sediment concentration (SSC) of the Guadiana River, its estuary and coastal zone. Suspended particulate matter (SPM) was collected along the Guadiana Estuary at distinct river flow and tidal conditions. 84 samples of SPM were collected during low and high spring tides, in November 2000 and February 2001, respectively. During November 2000, in low-runoff conditions, the mean SSC value was 32.45 mg/l and the highest values were found at 20-30 km, upstream of the river mouth. However, after short periods of heavy rain, the SSC increased in the water column and the highest values (501.6 mg/l) were found near the river mouth. In what concerns the mineralogical composition, the main minerals in SPM are phyllosilicates, quartz, plagioclases. Illite, kaolinite, chlorite, smectite and irregular illite-smectite mixed-layers are the main clay minerals. In low-runoff, the increment of quartz and illite concentrations seems to be associated to the entrance of saline water in the estuary and, consequently, to flood-tide flows. Kaolinite relatively to illite, exhibits an opposite behavior, indicating that its source is largely continental. The total contents of phyllosilicates and, particularly, chlorite enrichment during the flood-situation seems to be associated to low-tide flows being exported to the adjacent shelf. This study confirms the importance of energetic events, in transferring particles to deeper domains, while during low runoff the main trend is the import from the adjacent ocean.