THE GEOCHEMICAL RESPONSE OF RIVERS TO STORM EVENTS AS INDICATORS OF WATERSHED PROCESSES

Hydrologic perturbations driven by rainfall events produce corresponding geochemical perturbations driven by chemical reactions occurring throughout a watershed. The goal of this research is to correlate geochemical and hydrologic perturbations to develop a predictive model of geochemical change in an entire watershed from measured fluctuations in river composition. Major and trace element concentrations were acquired from daily water sampling at the same location along an arid climate river in central Arizona with the intention of documenting changes in river composition induced during several anomalously large storm events. We have observed several perturbation patterns during discharge events. Increases in discharge often result in decreased concentrations of some elements (a rainwater/runoff dilution effect) or increased concentrations (a spiking effect), while other concentrations remain unchanged. Surprisingly, not all elements behave in the same way during all events. Major ions like Na and Cl respond to storms with a dilution effect followed by a prolonged increase in concentration. Other major ions like K, Mg and sulfate are similarly correlated with storm perturbations, however the magnitude and direction of concentration change is event specific. Nutrients such as nitrate and phosphate show a spiking effect in response to most discharge events. Be, Gd and Rb behave conservatively, mimicking conductivity during storm events. Metals such as Cu, Ni, Pb, Cs and Cr respond to large storms with a slight increase in concentration. Al, Fe, Ti, Ga, and Hf also spike with magnitudes that are event dependent. Rare earth elements, Y and Th concentrations slowly decrease with time, but exhibit simultaneous spikes associated with individual discharge events. Meanwhile, the responses of W, Re, U, Co, Cd and Mo are out of phase with the REE group and show somewhat more variable dependences with discharge. Unlike other cations, calcium can be diluted, spike or remain unchanged depending on the event. Element perturbation patterns are not simply a function of whether a storm event occurred, but represents a complex integration of surface and subsurface processes within the watershed that are triggered by storms.