Fractal stream chemistry and its implications for contaminant transport incatchments

The time;it takes for rainfall to travel through a catchment and reach the stream is a fundamental hydraulic parameter that controls the retention of soluble contaminants and thus the downstream consequences of pollution epis odes(1,2). Catchments with short flushing times will deliver brief, intense contaminant pulses to downstream waters, whereas catchments with longer fl ushing times will deliver less intense but more sustained contaminant fluxe s. Here we analyse detailed time series of chloride, a natural tracer, in b oth rainfall and runoff from headwater catchments at Plynlimon, Wales. We s how that, although the chloride concentrations in rainfall have a white noi se spectrum, the chloride concentrations in streamflow exhibit fractal 1/f scaling over three orders of magnitude. The fractal fluctuations in tracer concentrations indicate that these catchments do not have characteristic fl ushing times. Instead, their travel times follow an approximate power-law d istribution implying that they will retain a long chemical memory of past i nputs. Contaminants will initially be flushed rapidly, but then low-level c ontamination will be delivered to streams for a surprisingly long time.