Fractal stream chemistry and its implications for contaminant transport in catchments

Abstract

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 episodes. Catchments with short flushing times will deliver brief, intense contaminant pulses to downstream waters, whereas catchments with longer flushing times will deliver less intense but more sustained contaminant fluxes. Here we analyse detailed time series of chloride, a natural tracer, in both rainfall and runoff from headwater catchments at Plynlimon, Wales. We show that, although the chloride concentrations in rainfall have a white noise 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 flushing times. Instead, their travel times follow an approximate power-law distribution implying that they will retain a long chemical memory of past inputs. Contaminants will initially be flushed rapidly, but then low-level contamination will be delivered to streams for a surprisingly long time.

The phosphorus transfer continuum: Linking source to impact with an interdisciplinary and multi-scaled approach

“…It embraces (i) differences in the relative importance of the controls on P transfer at different scales from point to plot, field, hillslope and catchment Harris and Heathwaite, 2005); (ii) temporal variations and dynamics ; and (iii) differences in the relative contribution from different P fractions according to the pathway of transfer and the environmental controls on movement. Recent analyses of the movement of contaminants in catchments indicate that the flow paths may be fractal in nature and that there are some paradoxical properties of delivery mechanisms to streams (Kirchner, 2003;Kirchner et al, 2000Kirchner et al, , 2004. Most of the data have not been collected at the correct scale and lack the resolution to clarify many of these issues (Harris and Heathwaite, 2005;Kirchner et al, 2004).…”

“…This new approach will take the discipline forward, with complexity based approaches arising from high resolution data collection. This is certainly the case for water quality in Table 2 Different methodologies associated with disciplines and scales for studying the phosphorus transfer continuum general (Kirchner et al, 2000(Kirchner et al, , 2004, although for P per se, the degree of progress will be constrained by our development of electrodes and appropriate technologies (Le Goff et al, 2004). One particularly novel and exciting development is described in Scholefield et al (2005) who demonstrated concerted diurnal patterns in riverine nutrient concentrations and physical conditions.…”

Controls on snowmelt water mean transit times in northern boreal catchments

“…The lumped parameter convolution approach uses a weighting or kernel function, g , to describe the transport of a conservative tracer through a catchment. Using this approach, an output signal (streamflow in this study) at any time, out t , consists of the input signal (combinations of snowmelt streamflow in this study) of the tracer, in t , applied uniformly over the catchment in the past (t ), which becomes lagged according to its transit-time distribution, g (Barnes and Bonell, 1996;Kirchner et al, 2000):…”

Fractal patterns in riverbed morphology produce fractal scaling of water storage times

“…Travel times in rivers and watersheds are very broadly distributed [Kirchner et al, 2000;Haggerty et al, 2002;Stonedahl et al, 2012;Aubeneau et al, 2014], but the mechanisms that produce travel time distributions over many orders of magnitude are not known precisely [Boano et al, 2014]. The exchange of water between surface and subsurface flows, generally termed hyporheic exchange, plays a critical role in structuring fluvial ecosystems [Boulton et al, 1998;Aubeneau et al, 2015].…”