Laboratory Study on Fate and Transport of Water-Borne eDNA: A Tale of Tempered Tails and Parafluvial Processes

Environmental DNA (eDNA) has emerged as a powerful marker for assessing species presence in flowing waters; however, estimating the spatial and temporal origins of these signals remains a challenge. This study presents a set of laboratory experiments examining the role of bottom substrates and hyporheic exchange on eDNA fate and transport. A conceptual model is proposed wherein eDNA transport is governed by competition between adsorption/desorption processes and hydrodynamic forces which remobilize eDNA across the water column and sediment storage zones. Retention experiments were conducted for plane and gravel beds, while breakthrough experiments were performed across both bed types and two hydraulic regimes. To quantify eDNA settling rates and validate the conceptual model, an independent experiment under quiescent conditions was performed. Breakthrough curves were analyzed using a continuous time random walk (CTRW) model. Enhanced exchange and retention rates were observed for eDNA in the gravel bed, showing that surface-subsurface interactions and turbulence influenced downstream transport distances. Effective velocities were significantly lower for eDNA than for a conservative tracer across both bed types at low flows. These results underscore that parafluvial processes, including hyporheic exchange and storage in sediments, play a crucial role in eDNA transport dynamics within slow-moving lotic environments.