Although high levels of dissolved arsenic were detected in surface and ground waters of Nhecolândia, a subregion of the vast Pantanal wetlands in Brazil, the possible sources have not been clearly identified and the potential release from the wetland to the draining rivers has not been investigated. In this study we measured the dissolved As content in all the rivers and small streams that supply the southern Pantanal region, as well as in the two main rivers draining the wetland, i.e., the Cuiaba and Paraguay rivers and tributaries. In addition, Arsenic in surface waters, perched water-table, soils and sediments from 3 experimental sites located in the heart of Nhecolândia were compared. On the one hand, the results show the absence of As contamination in rivers that supply the Pantanal floodplain, as well as a lack of significant release from the floodplain to the main drains. The As contents in the rivers are less than 2 μg L-1 , with variations that depend on the lithology and on the geomorphology at the collection point (uplands or floodplain). On the other hand, they confirm the regional extension of As contamination in Nhecolândia's alkaline waters with some values above 3 mg L-1. Arsenic is mainly in the arsenate form, and increases with the evaporation process estimated from sodium ion concentrations. The pH of soil solution and surface water increases rapidly during evapo-concentration up to values above 9 or 10, preventing adsorption processes on oxides and clay minerals and promoting the retention of dissolved arsenic in solution. Solutions from organic soil horizons show higher As contents in relation to Na, attributed to the formation of ternary complex As-(Fe/Al)-OM. In this alkaline pH range, despite high levels of dissolved As, soil horizons and lake sediments in contact with these waters show As values that correspond to uncontaminated environments. Dissolved arsenic in the upper Paraguay River basin and Pantanal wetlands 1 Introduction Arsenic (As) is known to be a serious health risk, a toxic and carcinogenic chemical element. In addition to naturally occurring arsenic, its presence also depends on anthropogenic, mining, industrial or agricultural activities. It is present in the environment under four oxidation states (-3, 0, + 3, and + 5) and different organic and inorganic species. The amounts and relative proportion of oxidation states and chemical species of As in water are the result of a complex reactivity including oxidation / reduction, complexation, adsorption/desorption, precipitation and biological transformations (
