The geochemical characteristics of the sediments in Río Tinto originates from the provenance of the source rocks. The mineral composition of the deposited sediments reflects the physical- and chemical conditions of the ambient water through time, and the presence of environmental changes can be seen in the stratigraphy as changes in chemical composition. (Cáceras et al., 2013) The unique chemical characteristics in Río Tinto makes the river bright red, which changes after the town Niebla. At this point the water becomes less acidic and the water is also influenced by the Atlantic Ocean. (Fernández-Remolar et al., 2005)

 

The sediments chemistry today has increasing levels of toxic elements, such as As, Cu, Pb and Zn, compared to the earlier deposited sediments. Another element that stands out are the levels of iron that are much higher than normal. Compared to the Odiel river that has the same source material the level of iron is 25 times higher in Río Tinto. These toxic elements are not unusual in soils from IPB that are contaminated from sulfide mining. This indicates that the contamination in the area has increased. However, there is evidence that toxic elements has been released by natural processes since the Miocene. This by oxidation of sulfides. (Cáceras et al., 2013)

 

Due to strong evaporation during the spring and summer, there is soluble Fe salts precipitated in the river beds and mining areas. When the rainfall during the autumn the redissolution of salts, due oxidation, is released into the river water. (Nieto et al., 2007) Groundwater formed from the rainfall percolates through the source rock that is of hydrothermal sulfide ores, where chemical - and microbiological oxidation forms acid sulfate solutions (Fernández-Remolar et al., 2005). This process creates by-products such as ferric and ferrous hydrated sulfates and oxyhydroxides. Minerals formed by these by-products may be ranging from jarosite, ferrihydrite/goethite and hematite. The pH level of the water is what determines if it’s hematite or goethite that are formed. (Fernández-Remolar et al., 2004) The salinity in the basin is one of the main factors to changing pH levels and with that releasing of metals. With a continuous influx of metals there will be increase of metals in the sediments constantly. (Cáceras et al., 2013)

 

In the Tinto River Basin the two different iron oxide minerals hematite and goethite are found. During time their proportion has changed against one another, in the youngest Quaternary sediments there is mainly goethite. However in the early Quaternary - Tertiary sediments the amount of hematite equals or exceeds the goethite. This means that there has been a transformation in conditions; from oxide-hydrated phases to dehydrated ferric oxides. (Fernández-Remolar et al., 2004)