Spatial-seasonal distribution and ecological risk assessment of multiple herbicides (chloroacetamides, triazines, chlorophenoxy acids, bentazone) and their transformation products in the Yangtze River, China.

Herbicides such as chloroacetamides, chlorophenoxy acids, triazines, and bentazone are widely used in agriculture, contaminating aquatic ecosystems. However, their distribution and ecological risks in the Yangtze River are not clear. Thus, we analyzed 144 surface water samples (72 each in dry and wet season, respectively) collected from 72 sites along the Yangtze River mainstream, targeting 37 compounds, including these herbicides and their transformation products. Most compounds (31/37) were detected in > 90% of the samples. The dominant compounds were acetochlor oxanilic acid (median: 30.5 ng/L), atrazine (15.4), acetochlor ethanesulfonic acid (14.7), acetochlor (5.53), 2-methyl-4-chlorophenoxyacetic acid (5.27), followed by diaminochlorotriazine (4.72), bentazone (4.47), hydroxypropazine (3.93), prometryn (3.50), metolachlor (3.14), deethylatrazine (3.11), and others. The concentrations of the target compounds significantly increased from upstream to downstream; for example, the cumulative concentration in Shanghai (median: 660 ng/L) was approximately 300 times higher than that in Qinghai (2.18 ng/L). Significant seasonal variation was observed, with higher cumulative concentration in June (median: 198 ng/L) than December (63.9 ng/L; p < 0.05). The ecological risk assessment indicated that pretilachlor, acetochlor, prometryn, hydroxypropazine, hydroxyterbuthylazine, and butachlor oxanilic acid posed high risks (risk quotient of > 1) to aquatic organisms in some sampling sites. This study is the first to report the occurrence of some transformation products of the herbicides (e.g., hydroxysimazine, 4-chlorophenoxyacetic acid, and hydroxylated chlorophenoxy acids) in river water. Our findings highlighted the widespread occurrence of the herbicides in the Yangtze River, and targeted pollution mitigation for pretilachlor, acetochlor, prometryn and their transformation products is urgently needed, especially in the downstream reaches including Jiangsu and Shanghai, to protect the aquatic organisms.