Total Organic Carbon Concentration and Export in a Human-Dominated Urban River: A Case Study in the Shenzhen River and Bay Basin

Abstract

In order to understand the organic carbon dynamics in urban rivers, the present study monitored the total organic carbon (TOC) concentration and export in the Shenzhen River and Bay basin. The results show that the average TOC concentrations ranged from 7.04 to 17.50 mg/L in the study area, which exhibited pronounced spatial and temporal variations due to urbanization level, rainfall–runoff, and effluent of wastewater treatment plants (WWTPs). The TOC concentrations of rainwater were averaged at 4.03 mg/L during 2011–2012, which was higher than that of some urban river basins in developed countries. As an average rainfall year, the total TOC export in 2012 was 11.2 × 106 kg/yr in the study basin, of which 37.5% was contributed by the effluent of WWTPs, 14.1% by wet deposition, and 48.4% by the surface non-point sources and endogenous pollution. The areal yield of TOC in the Shenzhen River and Bay basin was 23.73 × 103 kg/(km2.yr) in 2012, which was 2.86 times the Pearl River’s average value and 6.43 times the global average value. According to the predicted values of linear regression, the TOC concentration showed a gradual downward trend (R = 0.87, p < 0.001, n = 14) during the period 2006–2019, which also induced a decreasing TOC export (R = 0.23, p > 0.05, n = 14).