The Tinto River crosses the mining area of Riotinto (Iberian Pyrite Belt, SW Spain), where it receives the highest contribution of contaminants (AMD). In this paper we apply a fuzzy computer tool, PreFuRGe, which allows qualitative interpretation of the data recorded in a database relating to the chemistry of water. Specifically, we aim to find information not likely to be detected by means of classical statistical techniques, and which can help in characterizing and interpreting the behavior of arsenic in a complex system. The conclusions present that the factors which most directly control the presence of total dissolved As are closely linked to the climate and are temperature and rainfall, and therefore pH. As (III) is also shown to be related to temperature and pH. In terms of temperature As (V) is found to operate in a way which is the opposite of As (III). In terms of pH the relationship is not as clear as for As (III). As for rain, the highest As (V) values are compatible with minimum or non-existent rainfall, while minimum values correspond to any value for rainfall, including very high. (C) 2009 Elsevier B.V. All rights reserved.