Chemical and spectroscopic characterization of humic substances from sediment and riparian soil of a highly polluted urban river (Suquía River, Córdoba, Argentina)

Abstract

The Suquía River, the largest urban river in Córdoba (Argentina), has been severely polluted for decades. Actions must be taken to restore its environmental quality by managing riparian zones for increased water-self purification. The current study aimed to characterize organic matter (OM) dynamics and humic substances (HS) spectrochemical properties along the lower-middle basin of the Suquía River. Riparian soil (0-20 cm) and sediment (0-10 cm) samples were collected from a reference location (S1) and four polluted sites (S2-S5) during a low-flow period. The contents of soil and sedimentary OM and HS fractions were analyzed by wet oxidation, as well as HS Fourier transform infrared (FT-IR) and ultraviolet-visible (UV-Vis) spectrochemical properties. The OM and HS fractions from riparian soil were high upstream of Córdoba City (S1 and S2, 50.2-50.4 g/kg OM) and within a 50 km downstream location (S5, 30.9 g/kg OM) owing to a surplus of fresh plant biomass-carbon (C) inputs. Highly heterogeneous sediment samples did not show any significant differences among sites (P > 0.05). The lowest values of the ratio of absorbances at 465 and 665 nm (E4/E6) (1.78) and the ∆ log K (0.15) coefficient (a measure of HS maturity degree) were obtained downstream of Córdoba City, for both riparian soil and sediment, indicating that HS were enriched by more condensed aromatic structures within highly degraded portions of the river. All samples exhibited similar IR spectra, implying overlapping recalcitrant-C structures at the functional group level, but with different absorbance intensity. Data from the current study constitute a baseline for understanding the chemical nature of HS from sediment and riparian soil along the Suquía River and can be used as a reference for future studies tracking OM compositional changes over time.