Using Multiple Tracers to Verify Model Simulations of Solute Transport at the Five USGS Water, Energy, and Biogeochemical Budget (WEBB) Sites

The USGS WEBB sites are pristine forested watersheds, each with unique climate, topography, geology, and soils. The dominant flow paths and average residence times in specific hydrologic 'compartments' are also unique to each site. This poster presents the development of a model that simulates the transport of a variety of conservative tracers through watersheds. The model includes eleven hydrologic compartments: atmosphere, snowpack, vegetation; soil, shallow and deep macropore, unsaturated and saturated zones of the hillslope; riparian zone, stream/hyporheic zone, and lakes. The core hydrology model is a modified version of TOPMODEL. Data from the National Atmospheric Deposition Program provides the precipitation chemistry input into the model. Modeled terrestrial sources are assumed constant for all solutes except dissolved organic carbon (DOC). A first-order production rate for DOC in the soils is incorporated. The observed fluxes of solutes through and out of each watershed are derived from solute concentrations measured in the hillslopes and streams of each WEBB watershed since 1991. Solute concentrations in a given compartment at a given time depend on mixing, sorption/desorption, and evaporation. Geochemical computations are made using PHREEQC. By simulating the transport of conservative tracers through the landscapes, predicted streamflow generation mechanisms can be evaluated by comparing simulated and observed solute concentrations in the stream. Isotopic compositions of oxygen and hydrogen are used to estimate the extent of evaporation of open water bodies, soil water, and the mixing of waters in the saturated zone. Other compartment-specific tracers/indexes of solute sources, flow paths, and processes include: chloride (precipitation), charge-balance alkalinity (mixing of shallow and deep flow paths), dissolved organic carbon (soils), potassium and silica (weathering and biology), sulfate (sorbtion/desorbtion in soils), and sulfur-34 (terrestrial versus atmospheric sources of sulfur).