Potential contribution of topography-driven regional groundwater flow to fractal stream chemistry: Residence time distribution analysis of Toth flow

Groundwater and surface water are interconnected. Toth's analysis of topography-driven groundwater flow, presumably exiting in lakes or streams, is one of the first illustrations of this connection. Recently, fractal behavior in time-series observations of stream chemistry, implying a power-law residence time distribution (PLRTD) has been attributed to heterogeneity in subsurface flow paths and mass exchange processes. We show through numerical simulations that topography-driven groundwater flow, i.e., Toth flow, and transport under homogeneous aquifer conditions results in PLRTDs and may therefore contribute to fractal behavior in surface water chemistry. For the first time, PLRTDs are explained with a purely physical basis. Heterogeneity, accounted for by a large dispersivity value, makes the PLRTD more pronounced and persistent. Late-time arrival of solutes from surrounding watersheds results in multi-modality in the RTD, but these late peaks also follow a PLRTD after arrival.