The relationships between pH, colour, Fe, Mn, Cu, Zn, Cd and sulphates in surface water samples collected from the Tinto and Odiel Rivers (southwest Spain) have been studied. These rivers are adversely affected by the inputs of acid and heavy metals due to Acid Mine Drainage (AMD). Samples were collected from both rivers on a monthly sampling basis from 1995 to 2001. The analytical results showed that pH (mean pH 3.1) in the water samples of the Odiel River presented a greater correlation with heavy metals, colour and sulphates than in the Tinto River (mean pH 2.5). The tidal effect at the sampling point at the Odiel River appears to cause a greater variation of the pH values (2.5–4.4), resulting in the precipitation of iron hydroxide and the co-precipitation of other metals when the pH increased. This phenomenon was not observed in the samples collected from the more acidic Tinto River (pH 2.1–3.2). A similar relationship was observed for the heavy metals and the colour of the surface water of both rivers.

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