The mining district of Tharsis is located on the Iberian Pyrite Belt (IPF), which is rich in massive polymetallic sulphides. It has been exploited for almost 5,000 years, although large-scale exploitation began in 1,866 and continued until 2,001, when it completely ceased. Sulfides on contact with the atmosphere generate an acid leachate known as acid mine drainage (AMD) containing high concentrations of toxic metals such as Fe, Zn, Cu, etc. As a result of mining, 5 large open pits have been generated, 4 of them flooded with acidic water, and huge areas of dumps, where numerous acid leachates are generated. The largest permanent discharge of acid water from the Tharsis area, and probably also from the entire Iberian Pyrite Belt, is located at the base of a 66 Ha dump and up to 75 meters high. Its flow varies between 2.2 and 6.3 L/s (percentiles 10 and 90), with a maximum of 12.6 L/s in a period of heavy rains. It has average pH values of 2.4, electric conductivity of 21.9 mS/cm, Eh of 577 mv and a thermal character because the oxidation reaction of pyrite is exothermic, with an average temperature of 25ºC. Likewise, it has very high concentrations of SO4 (31 g/L), Fe (4.6 g/L), Al (1.4 g/L), Zn (0.5 g/L) and many other toxic elements. Therefore, the polluting load of this spring is very high and represents an important source of acidity to the Alcolea reservoir (246 hm3 capacity), which is currently under construction. The water quality of this reservoir is uncertain due to the numerous acidic contributions it will receive. In this paper we analyze the results of the periodic control of this acid upwelling between February 2,017 and March 2,018, with a periodicity between fortnightly and monthly, especially its response to rainfalls and the evolution of the physicochemical parameters, the concentrations of metals and of the pollutant load.