Fluvial geomorphology and hydrology in the dispersal and fate of pyrite mud particles released by the Aznalcóllar mine tailings spill

The Agrio-Guadiamar River reach affected by the Aznalcóllar mine tailings spill can be divided into three main sectors taking account of geomorphic characteristics and human-induced changes. Along the first 15 km, the valley presents the characteristics of a middle fluvial reach of moderate sinuosity and gradient; the floodplain declines gently towards the main channel and is built up mainly by lateral accretion deposits, which are mined by open pits. The second sector is located between 15 and 30 km downstream of the mine, and is characterised by higher channel sinuosity and a lower gradient; the floodplain is built on overbank deposits, separated from the main channel by natural levees, showing some low sinuosity flood channels. Finally, the lowest reach, with a similar low gradient, can be described as the proximal area of the Doñana marshlands, whose geomorphology has been modified by agricultural practices for the last 40 years. The spread of mining tailings deposited a mat of pyrite mud (fine milled heavy metal-rich ore) over the main channel and floodplain with a thickness of up to 50 cm in the first sector and then, progressively decreasing downstream. The pyrite mud was deposited over the natural sediments without significant mobilisation of the latter and without relevant geomorphic changes. The subsequent cleaning up of the pyrite mud with the help of heavy machinery resulted in the removal of most of the bushy and herbaceous vegetation as well as the reworking of bed sediments and the destruction of their natural armouring. Relatively small amounts of submerged pyrite mud that could not be removed from the main channel and that mixed with the shallow natural sediments during the cleaning works provides a long-term pollution element in the system. Pyrite mud remnants on floodplains are not being directly removed by fluvial activity in the mid-term because these sedimentation areas are susceptible to erosion only by the wandering of the main channel. However, these materials can be easily oxidised, therefore, plant uptake and underground water pollution should be monitored. On the other hand, the channel bed and banks are source areas of sediments and pyrite materials that can be easily eroded due to the destruction of the natural protection features (vegetation and armouring). Once eroded, the pyrite particles will be transported over longer distances than the coarser host natural alluvium, and deposited on floodplains or areas with low flow vel