Measuring catchment-scale chemical retardation using spectral analysis of reactive and passive chemical tracer time series

Catchment-scale chemical transport is jointly controlled by hydrological and chemical processes. Water may take a complex set of flowpaths underground toward the stream, carrying soluble substances with it. Some chemical constituents are non-reactive; these act as passive tracers, moving with the water. Other constituents react with the porous medium; concentrations of these reactive tracers reveal how porewaters chemically interact with the subsurface. Thus, passive and reactive chemical tracers are indispensable tools for understanding hydrological and chemical processes at whole-catchment scale. Transport of reactive chemical species can be quantified by the retardation factor, which measures the transport velocity of a reactive solute relative to the fluid that carries it. Retardation factors are conventionally determined by batch or column experiments involving small volumes of porous media. However, the transport media in typical catchments are highly heterogeneous, so retardation factors measured on small samples cannot be straightforwardly scaled up to model chemical transport at the catchment scale. Here, a novel method for determining whole-catchment retardation factors is presented which compares the power spectra of atmospherically derived passive and reactive tracers in rainfall and stream water. Using this technique, whole-catchment retardation factors of 2.4–2.9 were determined for sodium in four small catchments at Plynlimon, Wales. Because the effective retardation factor of a catchment depends on the degree of bypassing by preferential flow, our method for quantifying whole-catchment chemical retardation is of particular use for studying flowpaths and flow mechanisms at catchment scale.