Metal Partitioning and pH-Buffering During Mixing Processes in an Estuary Strongly Affected by Acid Mine Drainage; The Ria of Huelva Estuary (SW Spain)
Basallote, M Dolores; Cánovas, Carlos R; Pérez-López, Rafael; Borrego-Rodríguez, Pedro; Macías, Francisco; Olías, Manuel; Nieto, José Miguel
University of Huelva, Spain
The Odiel and Tinto rivers drain the Iberian Pyrite Belt (IPB), which is known as one of the largest polymetallic massive sulfide deposits in the world. Both rivers, with acidic pH values (2-4) and high concentrations of metals (and metalloids), flow into The Ria of Huelva Estuary (SW Spain). This estuary is affected by a chronic metal pollution by mining activities and has been considered to be the main metal contributor to the Gulf of Cádiz. For this reason, this estuary has been the focus of intense research (e.g. Davids et al. 2000; Elbaz-Poulichet, 2001, and references therein). However, a detailed characterization of metal behavior and fluxes along the estuary upon different hydrological regimes (i.e. dry season, flood events, accidental mine spills, etc.) has not been properly addressed. In order to bridge this gap, systematic field water samplings upon diverse climate conditions (i.e. high and low river discharges) were carried out comprising the sharp pH gradient (3-8) observed along the estuary. Dissolved and total (dissolved plus particulate) element concentrations were analyzed by ICP-MS to determine the role of the dissolved and particulate fraction in the metal transport along the estuary.
Although leachates from other industrial sources may also affect the metal concentration in the estuary, pollutant levels along the estuary are controlled by the ratio of acidic riverwater (pH from 2 to 4) and seawater (pH 8) mixing. During the estuarine mixing, a different behavior among elements is observed. The most outstanding fact is the intense precipitation of Fe and Al in the fluvial domain of the estuary. Hence, two buffers at pH ranges between 3-4 and 4.5-5.5 were observed related to Fe and Al precipitation, respectively. Other trace elements such as Pb, As, and REE suffer sorption processes onto Fe and Al minerals precipitated in the estuary and incorporated into the particulate matter. Conversely, other elements less particle-reactive such as Cd, Co, Mn, Ni, Zn or Cu, have a quasi-conservative behavior and are transported mainly in the dissolved fraction to the ocean. However, in some cases, the metal transport is predominant in the particulate matter, reaching even the littoral waters. The information provided by this study is essential to trace the metal fate from rivers affected by AMD to the ocean, particularly to the Gulf of Cádiz and the Mediterranean Sea, enriched in metals compared to other seawaters worldwide.