Investigating the relationships between chemical element concentrations and discharge to improve our understanding of their transport patterns in rural catchments under subtropical climate conditions

摘要

Solute and particulate elemental concentrations (C) exhibit different responses to changes in discharge (Q), and those relationships are not well understood in subtropical agricultural environments. The objective is to describe the transport processes of different chemical elements during a set of contrasted rainfall events (2011 -2015) that occurred in a small rural catchment under subtropical climate. The study was carried out in the Lajeado Ferreira Creek catchment (1.23 km~2), southern Brazil. To this end, the concentrations in dissolved organic carbon (DOC), Cl~-, NO_3~-, SO_4~-, ten chemical elements (in either dissolved or paniculate forms) and suspended sediment concentrations (SSC) were determined. Metric indices were then calculated to characterize their transport patterns: (ⅰ) the best fit slope between log-C and log-Q(β), (ⅱ) the coefficient of variation of C and Q, (ⅲ) shape of the hysteresis loop and hysteresis index, and (ⅳ) the flushing index. All paniculate elements along with the dissolved inorganic phosphorus (PO_4~(-3)) were shown to be controlled by the sediment dynamics. Geogenic elements (Fe~(2+), Zn~(2+), Cu~(2+), Mn~(2+), Si~(4+)) showed a dilution effect with increasing Q values, likely because they were mainly transported with subsurface and base flow. Dissolved elements that are mainly supplied with fertilizers (Na~+ and cl~-) as well as DOC showed a dilution effect, although they were mainly transported by surface runoff. Finally, a chemostatic behavior was found for those chemical elements (Mg~(2+), K~+, Ca~(2+), NO_3 and SO_4 ) that are supplied by more than one flow pathways. The results demonstrate that under subtropical climate conditions, the transport of essential nutrients including PO_4~(-3) and metals (in particulate form), are mainly transported with surface runoff. Accordingly, runoff control on cultivated hillslopes should be improved to reduce the potential contaminant supply to the river and to reduce the potentially deleterious impacts that they may cause in downstream regions.