Spatial-temporal and multi-media variations of polycyclic aromatic hydrocarbons in a highly urbanized river from South China

- Monthly rainfall correlated with PAH levels in water and SPM but not in sediment.
- Mean sediment PAH concentrations decreased mainly due to the 4-ring PAH degradation.
- Water and SPM PAHs had similar sources which were different from that of sediment.
- Similarities of sediment PAHs vs. water/SPM PAHs decreased along the Maozhou River.
- Sediment PAHs were likely washed to lower reaches in the form of SPM by river water.

Comprehensive studies on polycyclic aromatic hydrocarbons (PAHs) within an urban river are urgently needed to carry out strategies to limit their contamination and dispersal. Here, we analyzed 16 PAH occurrences in water, suspended particulate matter (SPM), and sediment monthly for a year in the Maozhou River mainstream (Shenzhen, South China). Monthly rainfall positively correlated with both total PAH concentrations in filtered water (water PAHs) and SPM. Sediment PAH concentration increased from the river source to estuary. Compared to the earlier record, the sediment PAHs decreased at almost all sites due to the high-molecular-weight PAH (≥ 4 rings; especially the 4-ring PAH) degradation, except the estuary site that accumulated more low-molecular-weight PAHs (< 4 rings). Results suggest that the water and SPM PAHs had similar and recent sources (e.g., rainfall and storm runoff) and actively exchanged with each other. The sediment PAHs had relatively different and complicated sources (fossil fuel combustion: 44.0%; oil pollution: 28.4%; biomass burning: 27.6%), and showed a long-term accumulation effect and increasingly weaker source-sink relation with both water and SPM PAHs from river source to estuary. This study highlights a disconnection in the source and migration mechanism between the water body (including water and SPM) and sediment PAHs.

The GRAPHICAL ABSTRACT was from pictures we took of the Maozhou River when collecting samples.214