Trace metals in natural waters exist in a myriad of chemical and physical forms that influence their transport, bioavailability, and toxicity. The speciation of these metals is regulated by their interactions with metal-binding ligands comprised of natural organic and inorganic solutes, colloids, and particles. Despite their importance, information on the sources, nature, and stability of these ligands is lacking. In particular, the roles of the diverse biogeochemical and hydrologic processes that regulate the production and transport of strong-binding ligands along a hydrologic flow path are poorly understood.
Our goal is to provide information on changes in the nature and abundance of strong metal-binding ligands along representative hydrologic flow paths, and the fluxes of strong ligands into streams from both groundwater and hyporheic sources and processes. Specifically, we intend to determine: (1) the abundance and strength-spectrum of metal-binding ligands along key segments of hydrologic flow paths and associate ligand properties with hydrologic and geochemical conditions and biogeochemical processes; (2) the influences of hydrologic events (recharge, discharge) and biogeochemical processes (iron redox reactions, sulfate redox reactions, primary production) on temporal variability in ligand characteristics and abundance; and (3) the stability of strong ligands to chemical and biochemical transformations. We expect interface margins such as the hyporheic zone to be important sites of ligand production.
Our research is centered at Alequash Creek, within the site of the co-located NSF North Temperate Lake LTER and the USGS Trout Lake WEBB projects where the hydrologic system is especially well characterized and instrumented. Our approach is based on sampling and characterizing strong metal-binding ligands at key segments along the flow path while also quantifying transport. Ligands will be isolated on-site using filtration and ultrafiltration and characterized using detailed chemical analysis, spectroscopy, and related techniques. We expect strong ligands to be associated with organic carbon, iron oxide, and/or reduced sulfur-containing phases. We will emphasize the metals Cu, Pb, and Zn as both environmental contaminants and probes of ligand character. Assessment of binding strength using voltammetry is a key component of our approach, supported by biochemical response measurements. Using our study site as a model, our research will be structured to address two key areas: the importance of groundwater as a source of strong ligands to the riparian and hyporheic zones and the production of strong ligands within the hyporheic zone and transport into streams and lakes. These questions are central to understanding watershed linkages to streams and lakes and their influences on the abundance and bioavailability of metals in surface waters
