Controls on Base Cation Fluxes in Watersheds: What We've Learned from Sr Isotope Studies at the WEBB sites

U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025-3591, USA, tdbullen@usgs.gov, 650-329-4577

Abstract

The USGS Water, Energy, and Biogeochemical Budgets (WEBB) forested watershed study sites comprise a spectrum of catchment morphologies influenced by a variety of climates. A key component of each WEBB study has been the determination of sources of solutes in streamflow, with the goal of understanding both nutrient fluxes and the linkages between the biogeochemistry and hydrology of each watershed. At each site, Sr isotopes have been used in conjunction with hydrology, mineralogy, and water chemistry to distinguish between atmospheric and mineral- weathering sources of base cations as well as to determine the variability of mineral weathering reactions along surface water, soil water and groundwater flowpaths. The results to date indicate that a common set of processes, which include differential poisoning of reactive mineral surfaces, cycling of nutrient cations within the forest ecosystem, and variable depletion of cation exchange pools in mineral soils, control base cation mobilization to streamflow. The relative importance of each process depends n the different biogeochemical and hydrologic characteristics of, and atmospheric inputs to each watershed. Clearly the main control on both base cation fluxes and variability of ⁸⁷Sr/⁸⁶Sr of streamflow at these watersheds is the development of contrasting weathering regimes along different flowpaths. In most cases, these weathering contrasts occur along different groundwater flowpaths, result from differences in redox conditions, water flux rates, microbial activity and relative concentrations of strong and weak acids, and do not require difference in mineral composition or abundance. Moreover, the general lack of Sr isotope equilibrium between soil waters and the soil cation exchange pool at these watersheds indicates that waters having only a short residence time in the unsaturated zone, such as snow melt and storm event waters, are relatively ineffective at mobilizing base cations from exchange sites. These conclusions underscore the importance of understanding watershed hydrology, and in particular the details of groundwater recharge and chemical evolution when using either the chemistry or ⁸⁷Sr/⁸⁶Sr of streamflow to assess the stability of forest ecosystem and soil base cation pools in response to environmental impacts such as acid rain and global climate change.

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