Space-time dynamics of dissolved organic carbon (DOC) concentration and water quality in the Turbolo River catchment (southern Italy)

This study investigates the spatial and temporal dynamics of dissolved organic carbon (DOC) and several other chemical-physical parameters concentrations in a Mediterranean headwater catchment (Turbolo River catchment, southern Italy) equipped with two multi-parameter sondes providing multiple-year (from 2019 to 2023) high-frequency measurements, complemented by discrete monitoring campaigns. The sondes were installed in two nested sections, a quasi-pristine upstream sub-catchment and a downstream outlet with anthropogenic water quality disturbances. Altogether, sixteen chemical-physical parameters were assessed: temperature, turbidity, electrical conductivity (EC), total dissolved solids (TDS), salinity, pH, ORP, ammonia nitrogen (N-NH₄⁺) and dissolved organic carbon (DOC) in continuous mode; alkalinity, dissolved inorganic carbon (DIC), free CO₂, not purgeable organic carbon (NPOC), total dissolved nitrogen (TDN), anionic and cationic content for discrete monitoring. In particular, DOC estimates were achieved by correcting the fluorescent dissolved organic matter -fDOM - values through an original procedure that did not require extensive laboratory measurements. Then, parameter dynamics at the seasonal and storm event scales were analyzed.

Results showed that all parameters have values consistent with those expected for fluvial water. Furthermore, the majority of the parameters generally recorded the highest values during the autumn season, showing then a decrease to spring lows and a new rise with the arrival of the driest months of the year. In particular, the seasonal scale analysis confirmed the climate control on DOC production, with increasing background concentrations in hot and dry summer months. On the other hand, the hydrological regulation proved crucial for DOC mobilization and export, with the top 10^th percentile of discharge associated with up to 79% of the total DOC yield. The analysis at the storm scale using flushing and hysteresis indices highlighted substantial differences between the two catchments. In the steeper upstream catchment, the limited capability of preserving hydrological connectivity over time with DOC sources determined the prevalence of transport as the limiting factor to DOC export. In the downstream catchment, transport- and source-limited processes were observed almost equally. The correlation between the hysteretic behaviour and antecedent precipitation was not linear since the process reverted to transport-limited for high accumulated rainfall values. The influence of storm events was also verified for other parameters, which were either positively (turbidity, N-NH₄⁺) or negatively (electrical conductivity, TDS and salinity) correlated with the streamflow variation.

Exploiting high-resolution measurements, the study provided insights into DOC and several other chemical-physical parameter dynamics in nested headwater catchments at multiple time scales.

 

Reference: Senatore et al., Water Resources Research, 2023, 59(11), e2022WR034397, https://doi.org/10.1029/2022WR034397