Sensitivity Analysis of a Reactive Transient Storage Model With Streambed Sorption Applied to Experimental Field Data

The transient storage model (TSM) has been widely used in simulations of stream solute transport and fate, with an increasing emphasis on reactive solute transport. In this study we perform sensitivity analyses of a reactive solute transport model (RSTM) that couples with a conservative TSM a formulation of sorption of a solute onto streambed sediments. The simulations were performed using the OTIS (One-dimensional transport with inflow and storage) model of solute transport for rivers and streams. The RSTM is analyzed to examine its effectiveness to reliably characterize stream and storage zone solute reactions from a previously reported data set. The field data are from a transport experiment with the addition of the conservative tracer chloride and the sorbing tracer strontium in Uvas Creek (Santa Clara County, California). Sensitivities of simulations to parameters within and among reaches, parameter coefficients of variation, and correlation coefficients are computed and analyzed. Our results indicate that 1) simulated values have the greatest sensitivity to parameters within the same reach, 2) simulated values are also sensitive to parameters in reaches immediately upstream and downstream (inter-reach sensitivity), and 3) simulated values have decreasing sensitivity to parameters in reaches farther downstream. Simulations of reactive solutes are shown to be equally as sensitive to model transport parameters and model reaction parameters; this being evidence of the control of physical transport on reactive solute dynamics. Similar to conservative transport analysis, reactive solute simulations also appear to be most sensitive to data collected during the rising and falling limb of the concentration breakthrough curve (CBC). In designing reactive stream tracer experiments, these findings have significance for appropriate sampling of the CBC.