Biogeochemistry of Isotopically-Distinct Sources of Pb as a Function of Hydrogeomorphology In a Coastal Wildlife Refuge.

²⁰⁶Pb/²⁰⁷Pb and ²⁰⁶Pb/²⁰⁸Pb isotope ratios are used to identify distribution and source of Pb in wildlife refuge soils and plants as a function of topographic position, and to characterize the biogeochemistry of lead as a function of soil mineralogy and hydrology to assess lead mobility and bioavailability to plants. Laguna Atascosa National Wildlife Refuge (LANWR) is a former Department of Defense owned WWII era target practice range, and copper-jacketed, lead cored bullets remain abundantly dispersed throughout the refuge, which may contribute anthropogenic Pb into the ecosystem. Geogenic sources of lead may include radiogenic lead daughter products from naturally occurring uranium and thorium in the several uranium-bearing formations in South Texas that have deposited sediment along the banks of former Rio Grande River traces within LANWR. Four topographic terraces are defined by plant community, hydrology and elevation in LANWR. Soil and plant samples are taken at three locations in each topographic terrace. Samples are analyzed for total lead concentration and for their respective isotopic composition using high-resolution inductively-coupled plasma mass spectrometry (HR-ICP-MS). The key objectives are to 1) identify isotopic composition of geogenic and anthropogenic sources of lead contamination in Laguna Atascosa National Wildlife Refuge by identifying end-member isotope ratios, 2) build a geographic model using several forms of geographic data including LIDAR, soil survey, and orthoimagery to distinguish topological terraces, 3) determine distribution of lead concentrations and lead isotopic ratios in four distinct soil fractions within each soil sample to trace anthropogenic and geogenic inputs of lead and 4) combine lead concentrations and compositions with geographic data including LIDAR, hydrologic, and soil surveys to build a GIS model to determine effect of mobility of lead on the distributions of geogenic versus anthropogenic end-member rich Pb isotopic compositions in soils and plants in each topological terrace.