Distribution of mercury in the sediments of Amistad International Reservoir, Texas, USA

Mercury (Hg) is a widely studied pollutant of great concern in aquatic ecosystems, yet we know little about its distribution in large reservoirs, especially those in subtropical and tropical regions. Determining the spatial distribution of Hg in aquatic ecosystems is important for understanding how it cycles through systems. Most of the Hg investigations involving a spatial component have been done on larger scales (e.g. GILMOUR et al. 1998, DAVIS et al. 2008) or in boreal and temperate regions (e.g. GoRSKI et al. 2003). Amistad International Reservoir is a large subtropical reservoir located on the border between Texas, United States, and eoahuila, Mexico (29°27'N; 101 °03'W), in the Rio Grande/Río Bravo drainage basin. At conservation level (340.5 m a.s.l.) it covers approximately 263 km• The reservoir is oligotrophic with high alkalinity and pH, low dissolved organic carbon (DOe; IBWe 1997, TeEQ 2004), and low wetland area (authors, pers. observ.). These characteristics are not usually linked to elevated Hg levels in biota. VANMETRE et al. (1997) found low but increasing levels of total Hg over time in the sediments o f the reservoir (40-50 ppb at the surface) using 2 sediment cores and linked much of this increase to atmospheric sources. The only native sources of Hg in the basin are approximately 250 km upstream in the Big Bend region. This area, known as the Terlingua Mining District, was the third largest Hg mining area in the United States until the mines were closed in 1973 (GRAY et al. 2006). We present a preliminary description of the variation in sediment total Hg and some potential influences on the movement o f this H g through the system. Results to date suggest higher loading to the Rio Grande (RG) arm of the reservoir, while the Devils River (DR) arm has conditions that may facilitate the transformation of inorganic Hg into organic methyl mercury (meHg). Analysis offish tissue indicates that meHg i s accumulating in the large sport fish o f the system.