Résumé
The rapid growth of cities and mines in developing countries has a major impact on the environment, through the discharge of untreated wastewater and mining waste. In the high-altitude tropical Andes, the Katari watershed combines a fast-developing urban area, upstream mining sites, and a downstream agricultural area, which are drained by the Katari River to Lake Titicaca. To assess the respective contribution of these areas to the watershed mercury (Hg) budget, Hg and monomethyl-Hg (MMHg) were measured in soil, sediment, surface water, groundwater, and the atmosphere (airborne fallout and total gaseous Hg). Further, two well-dated sediment cores collected in Lake Titicaca were used to reconstruct Hg accumulation rates over the last centuries. From instrumental river discharge, air and rainfall monitoring, the contribution of Hg originating from the atmosphere and the river was assessed. Results show that the mining area is a locally restricted main Hg hotspot. Although less contaminated, the urban area is the main source of MMHg for surface water through wastewater discharge, and the atmosphere the main source of total Hg through particulate and gaseous Hg emissions. Total Hg inventories in lake sediment from a bay at the outlet of the watershed show that at least one half of the Hg input originated from the river discharge, the rest being deposited from the atmosphere. The increasing development of the urban area without improvement of wastewater treatment implies that river discharge will be a source of Hg for the lake for many years.