Background Social and economic development in the south-west of the Iberian Peninsula - mainly Huelva, Spain - has been strongly associated with intensive mining in the Iberian Pyrite Belt (IPB). This belt of land - approximately 250 km long and 30-50 km wide – is a notable source of volcanic and sediment-hosted deposits of sulphide. Its original reserves were estimated at 1700 Mt (Sáez et al., 1999). However, at present, mining has ceased and the only legacy for the population of the region is the existence of more than 100 abandoned mines and over 2x108 m3 of waste, which is scattered across the area in the form of mine tailings, tailing dams, open pits, etc. These remnants of the area’s mining past have a very severe impact on the landscape and present a significant environmental risk, especially from the leaching of minerals into the natural environment. The superficial oxidation of pyrite – an iron sulphide and the most common of the sulphide minerals - results in a mine-water discharge characterised by its elevated acidity and high concentrations of sulphates, metals and metalloids. In a phenomenon known as Acid Mine Drainage (AMD), these acid leachates with high contents of metals flow into the local Tinto and Odiel rivers, through the Marismas del Odiel Nature Reserve and into an important coastal wetland - the Ría of Huelva Estuary. The effluents from the Odiel River Basin are some of the most contaminated in the world. The EU Water Framework Directive (2000/60/CE) fixed 2015 as the deadline for achieving the good ecologic and chemical status of European waters – although this was extended to 2027 for highly contaminated waters. However, traditional treatments of waters contaminated by acid leachate from mining waste are very expensive. This is one of the reasons why many such mining activities in Spain and elsewhere in Europe are no longer economically feasible. Objectives The LIFE ETAD project aims to remediate waters contaminated by mining activities through passive treatment that relies on natural water flow and biogeochemical reactions. It hopes to demonstrate a pilot full-scale application of passive treatment technology to clean acid waters from mining leachate to the standards required for irrigation water. The project will use dispersed alkaline substrate (DAS) as the passive treatment. This consists of an inert, coarse, high-surface matrix, such as wood chips, mixed with a fine-grained alkaline material, such as limestone sand. Passing the water through this treatment will remove metals such as iron, zinc, nickel, cadmium, cobalt and aluminium. Only natural energy sources will be needed to power the system - gravity, microbial metabolic energy and photosynthesis. The project expects to show the technical and economic feasibility of this process as a means of cleaning water contaminated with acid leachate from mines. The project thus hopes to contribute to the faster achievement of the objectives of the National Hydrological Plan for the Tinto, Odiel and Piedras Rivers and the EU Water Framework Directive. It also hopes to enable the development of vineyards and citrus plantations as alternative economic activities in the affected areas. Through the validation of this water treatment process at these highly polluted sites in southern Spain, the project also expects to demonstrate its potential use in other locations and possibly the renewed economic feasibility of mining activities to meet growing demand for minerals from countries such as China. Expected results: An economically and technically feasible passive system of treating contaminated wastewater from mining sites, demonstrated at full-scale, and achieving: 90-100% removal of acidity and metals from the contaminated water; pH values of the treated water within the range 6.0-8.0; Contaminants reduced to close to the detection limit; Water clean enough for irrigation, with the potential for further output solutions to provide water that can be used for human consumption.