Evaluation of sedimentation pond performance for a cleaner water production from an open pit mine at the edge of the Alto Tajo Natural Park

Controlling sediment produced by water erosion is the highest environmental challenge in steep, open pit sand mines located in erosive climates. Although restoration is impractical in active mine areas, sedimentation ponds are the Best Available Technique by the European Union to reduce sediment spillages from these environments. Few studies have described and supported such a solution with high resolution data. The María José mine is one of two active kaolin mines in the mining area of the Alto Tajo Natural Park, a highly erosive setting. It has a complex maintained pond system as a hydrologic and sedimentary control measure. In this landscape, mining can only be sustainable if clean water from production is ensured. The pond system retains sediment and water during a rain event, deposits the sediment, and eventually spills the water under controlled events. The María José spillages have been continuously monitored for suspended sediment concentration and water discharge at the mine outlet since 2012. During four years the efficiency of the pond system varied (96.7–99%), producing an average annual sediment yield of 5.6 Mg ha⁻¹ y⁻¹, a reduction of 98.4% compared with the sediment yield reported for similar local mines having no ponds. A quarter of this sediment was produced under controlled spillages allowing runoff management; the rest during large rainfall events when runoff decreased pond efficiency gives rise to an increase in suspended sediment concentration, as is typical of local natural rivers. Provided continuous maintenance, sediment ponds are shown to be a successful method to produce clean water. Successful pond efficiency implies that a mine produces water and sediment during baseflow conditions, allowing sufficient sediment volume to accumulate during rainstorms.