Hydrogeochemical variables regionalization : applying cluster analysis for a seasonal evolution model from an estuarine system affected by AMD

Abstract

During the last two decades spatial and temporal metal and nutrient variations have been extensively studied in estuarine waters (e.g. Apte et al., 1990; Zwolsman et al., 1997; Grande et al., 2000,2003a; Borrego et al., 2002; Sáinz et al., 2004, 2005). This study describes the spatial evolution of the hydrogeochemical parameters, which characterise a strongly affected estuary by Acid Mine Drainage (AMD). The studied estuarine system receives AMD from the Iberian Pyrite Belt (SW Spain) and, simultaneously, is affected by the presence of an industrial chemical complex. The estuary of the Tinto and Odiel rivers is located in the southwestern part of the Iberian Peninsula (Fig. 1) and is part of a group of estuarine systems developed in the northwestern coast of the Gulf of Cádiz, in the confluence of the Atlantic Ocean and the Mediterranean Sea. The Odiel River meets the Tinto River in its mouth, originating an estuarine system known as the Firth of Huelva; both rivers share the main access channel to the Firth of Huelva, called the Padre Santo Channel (Fig. 1). The serious contamination by AMD suffered by Tinto and Odiel rivers is due to the fact that the Iberian Pyrite Belt crosses their drainage basins (Fig. 1). This 230-km-long and an average of 50-km-wide geological formation is one of the biggest sulphide deposits in the world (Leistel et al., 1998). Its metallogenic richness has been the cause of its exploitation for over 5000 years (Davis et al., 2000). As a result of such a large mining activity, more than 100 abandoned mines and over more than 200 106 m3 of waste distributed in 70 mine dumps and 14 deposits have remained in the Tinto and Odiel drainage basins (Sáinz et al., 2004). Runoff from these sources is an everlasting polluting machinery carrying sulphates and heavy metals into these two rivers (Sáinz et al., 2004), and consequently to the estuary. Together with the sulphurs exploitation in the upper part of Tinto and Odiel river basins, an important industrial complex has also been developed along the estuary banks. Fertilizer and paper industries, as well as copper foundries, were here established since 1966. The industrial activity causes an important amount of polluting disposal into the estuary waters, adding relevant amounts of heavy metals and nutrients to the sediments and to the water (Grande et al., 2000). Therefore this system is one of the most polluted in Western Europe (Ruiz et al., 1998; Elbaz-Poulichet and Dupuy, 1999; Borrego et al., 2002). Grande et al. (2003b) proposed a hydrogeochemical zonation model for this estuary based on a single sampling campaign. On the other hand, the present study defines the estuary’s behavior along an entire hydrological year, based on the proximity Pearson ratios between variables by treating the given database with statistical tools, namely cluster analysis. This treatment allows to define and to understand the spatial evolution of each one of the defined areas by observing the mixing zone displacement.