The Río Tinto has for a long time been contaminated by mining in the area, however the mining is not only the reason for the high contamination levels. The rocks in the basins substrate has a limited neutralization capacity, (Cáceras et al., 2013) as pH levels are low in the river and that under those conditions there is a positive charge on the particles. This makes it possible for some elements to leach out from the minerals out to the water. (Burden & Sims, 1999)

 

The sediment closest to the river has more fine-grained (clay and silt) material and less of the coarser grain sizes. In the clay fraction, minerals containing elements with a greater weight can be found. These minerals are characterized of being alumino-silicates. (Cáceras et al., 2013) Rodrígues-Tovar and Martín-Peinado (2014) classified the soils as Gleyic Fluvisol deposits, which is dominated by silty-clay textures and where gravel are absent.

 

Upstreams in the river there are coarser material then further downstreams. The upstream part of the river consist of gravel with among others, fragments of shales and sandstone. Sand that are coarse to very coarse are also present in the upstream river, the sand are very pure as it’s mainly made of quartzite. Boulders are more flattened than downstreams and are ordered in a massive and horizontal manner. The sand are ordered in a similar way, although layers of planar stratified gravels and smaller laminations of ripples also are present. (Cáceras et al., 2013)

 

A factor in Río Tinto that affects the sediments texture in both macroscopic and microscopic degrees is the biota. At the river margin the sediment have been deposited in laminated domal structures. These can be interpreted in two ways; the first as stromatolites and the second as stacked convex-upward laminae that are formed by microbial action and episodic precipitation on boulders or cobbles. As shown in the figure  other bedding features also exist in the sediments, these includes efflorescence popcorn precipitates (A), desiccation cracks (C ), and somewhat sticky, irregular folded deposits of schwertmannite and jarosite (E). (Fernández-Remolar et al., 2005)