The proposed work is an investigation of mercury (Hg) in river systems that have experienced large anthropogenic inputs of this toxic substance (e.g., by upstream mining). This interdisciplinary project will identify critical locations in river systems where the risk of Hg input to food webs increases and elucidate the processes by which this occurs. The study will focus on the longitudinal (downstream) transport and biogeochemical processing of sediment-adsorbed Hg derived from hydraulic gold mining in the Sierra Nevada and mercury mining in the Coast Ranges within and through the Yuba-Feather-Sacramento River system of Northern California, USA. It will document the primary sources (Coast Range v. Sierra Nevada) of Hg contamination to lowland ecosystems in the Sacramento Valley and Bay-Delta and the relative contribution and risks of each. We will: 1) mathematically model flood inundation in river corridors to identify areas of high potential of oxidation/reduction; 2) identify preferential zones of sedimentation through numerical modeling of event-based washload transport; 3) identify distinct contamination sources to lowlands by conducting Hg stable isotopic analysis of sediment; and 4) investigate Hg speciation in conjunction with changes in Hg species isotopic signatures, associated with redox conditions and sediment source.
In river basins beset by Hg contamination from atmospheric deposition or anthropogenic inputs from mining, there is great uncertainty about the interplay among sediment transport, deposition, and remobilization and the evolution of Hg reactivity in response to inundation regimes and ambient chemistry. These factors have important implications for bioavailability of sediment-adsorbed Hg to food webs, as it travels to sensitive downstream ecosystems where methylation potential is high. This research will address this problem and provide new understanding into the physical and biogeochemical processes that threaten sensitive lowland ecosystems. The importance of sediment deposits originating from legacy mines, especially in the Bay-Delta watershed, has recently increased due to realizations that they may contain high levels of Hg that may be available to food webs. These factors are being investigated by various government agencies and nonprofit groups responsible for river basin management. The research team has developed close contacts and relations with personnel from interested parties in the US with whom findings will be shared.
