Biofilm Accumulation and Substrate Type Influence Anomalous Transport in an Experimental Stream System

Understanding the transport and degradation of particles within fluvial systems requires accurate modeling of breakthrough curve tails, which can display non-Fickian dispersion. Anomalous tailing behavior is driven, in part, by exchange and retention at the sediment-water interface, and within the subsurface of the stream benthos. Underlying substrate size and biofilm accumulation on substrate surfaces have been shown to alter solute residence times in fluvial systems; however, the combined influence of these factors on solute tailing has yet to be explored. We use the Stochastic Mobile Immobile model, an implementation of a continuous time random walk, to determine the impact of substrate composition and biofilm colonization on reach-scale transport velocity (V) and dispersion (D), delivery to the subsurface (Λ), and retention within the subsurface, which is described by power law slope (β). During summer 2020 and 2021, we conducted a total of N=32 Rhodamine-WT releases in four experimental streams containing different substrata (sand, pea gravel, cobble, mix) at the Notre Dame Linked Experimental Ecosystem Facility (ND-LEEF) in Indiana (USA), each producing a breakthrough curve. To explore the effect of biofilm colonization, we conducted releases under shaded, early and late biofilm development, and senescent biofilm conditions. Regardless of the underlying substrate, we found significant correlations between increasing algal biomass (measured as chlorophyll-a) and linear decreases in reach-scale transport velocity (V; r = -0.73, p<0.05) and exchange rate into the subsurface (Λ; r = -0.46, p<0.05), along with increased retention within the subsurface (β; r = -0.56, p<0.05). Given the association of algal biomass and multiple solute transport parameters, it is critical to consider the interplay between the biotic and abiotic components of flowing systems when modeling solute transport and particle degradation dynamics.