Effects of benthic and hyporheic reactive transport on breakthrough curves

In streams and rivers, the benthic and hyporheic regions harbor the microbes that process many stream-borne constituents, including O2, nutrients, C, and contaminants. The full distribution of transport time scales in these highly reactive regions must be understood because solute delivery and extended storage in these metabolically active zones control the opportunity for biogeochemical processing. The most commonly used transport models cannot capture these effects. We present a stochastic model for conservative and reactive solute transport in rivers based on continuous-time random-walk theory, which is capable of distinguishing and capturing processes not described by classical approaches. The model includes surface and subsurface storage zones with arbitrary residence-time distributions. We used this model to evaluate the effects of sorption and biological uptake on downstream solute transport. Linear or mildly nonlinear sorption in storage delays downstream transport without changing the fundamental shape of the breakthrough curves (BTCs). Highly nonlinear sorption isotherms can induce power-law tailing in stream BTCs. Model simulations show that sorption of commonly used solute tracers is not sufficient to explain the power-law tailing that has been observed in field tracer-injection studies, and instead, such tailing most probably reflects broad distributions of hyporheic exchange time scales. First-order biological uptake causes an exponential decline in in-stream tracer concentrations at the time scale of the uptake kinetics, thereby tempering power-law BTCs. The model can be used to calculate reach-scale reaction-rate coefficients in surface and subsurface storage from observed BTCs of co-injected conservative and reactive solutes, providing new capability to determine reaction-rate coefficients in storage zones with broad residence-time distributions.