Preliminary single-day spatial survey of water quality, nutrients, and dissolved organic matter in the estuarine reach of the Licun River receiving reclaimed water

To provide a preliminary snapshot of water quality conditions in an urban river receiving reclaimed water, a single spatial survey was conducted in October along the reclaimed water-receiving estuarine reach of the Licun River in Qingdao. Sampling sites were located upstream of the wastewater treatment plant (WWTP) outfall, in the downstream receiving reach, and at the estuary. Conventional physicochemical parameters, nitrogen and phosphorus nutrients, total organic carbon (TOC), and dissolved organic matter (DOM) fluorescence features were analyzed, and 3D-EEM-based fluorescence component analysis was used to describe broad DOM compositional differences. The results from this one-day survey showed that downstream sites receiving reclaimed water were associated with higher TN, electrical conductivity, and TOC than upstream sites, suggesting that WWTP effluent was an important contributor to spatial water quality differences during the sampling event. NO --N was the dominant nitrogen form at most riverine sites, whereas the estuarine site showed higher NH +-N together with markedly elevated conductivity and turbidity, indicating that this site represented a distinct hydrochemical setting during sampling and may have been influenced by estuarine mixing and/or local sediment disturbance. Most DOM fluorescence fractions changed only modestly among the riverine sites, whereas the signal labeled C5 showed the clearest longitudinal variation. Overall, the data provide a preliminary, single-day picture of spatial heterogeneity in water quality and DOM characteristics in the Licun River receiving reclaimed water. Longer-term seasonal and tidal-cycle monitoring is needed before temporal trends or process-level controls can be resolved.