The Role of Anomalous Transport in Long‐Term, Stream Water Chemistry Variability

• Published in: Geophysical research letters Vol. 50; no. 14
• Main Authors: Dentz, Marco, Kirchner, James W., Zehe, Erwin, Berkowitz, Brian
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.07.2023; Wiley
• Subjects: Catchment area; catchment hydrology; Catchments; Chemical transport; Chemicals; continuous time random walk; Dissolved chemicals; Distribution; Hydrology; Mathematical analysis; non‐Fickian transport; Rain; Rainfall; Random walk; Rivers; Runoff; Scaling; Spectral analysis; Spectrum analysis; Time measurement; Tracers; Transport; Travel time; travel time distribution; Water chemistry; Water quality
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL104207

We investigate the occurrence of anomalous (non‐Fickian) transport in an hydrological catchment system at kilometer scales and over a 36‐year period. Using spectral analysis, we examine the fluctuation scaling of long‐term time series measurements of a natural passive tracer (chloride), for rainfall and runoff. The scaling behavior can be described by a continuous time random walk (CTRW) based on a power‐law distribution of transition times, which indicates two distinct power‐law regimes in the distribution of overall travel times in the catchment. The CTRW provides a framework for assessing anomalous transport in catchments and its implications for water quality fluctuations. Plain Language Summary Rain falling on an hydrological catchment, and chemicals dissolved in the rain, can follow circuitous pathways below the ground surface until they reach a stream outlet that drains the catchment. Dissolved chemicals can diffuse into lower conductivity regions within the subsurface, and chemicals can also be transported in relatively fast pathways. We investigate a unique data set that monitors chemical transport over kilometer scales, and over a long, 36‐year duration. We develop a mathematical framework to describe the transport and retention of chemical tracers in a catchment, and their arrival times to a draining outlet. Solutions of the equations exhibit characteristic features of tracer concentration variations, and offer a means to characterize and quantity catchment response to chemical inputs. Key Points An hydrological catchment system at kilometer scales is shown to exhibit anomalous (non‐Fickian) transport over a 36‐year period A continuous time random walk suggests two distinct power‐law regimes in the distribution of overall catchment travel times In the catchments considered here, preferential flow appears to occur at all length and time scales