Distribution characteristics and health risks of phthalates in the middle and lower reaches of the Hanjiang River under the influence of water and sediment changes during cascade dam construction

<p indent="0mm">River sediment is an important adhesion carrier of phthalates (PAEs). The superposition of cascade dams has changed the spatial heterogeneity of river runoff, sediment dynamics, and water temperature, thereby influencing the transport behavior of PAEs and leading to a series of ecological and environmental impacts. In this paper, cascade reservoirs in the middle and lower reaches of the Hanjiang River were selected to investigate the changes in water and sediment characteristics, explore variations in PAE composition and concentration with changes in sediment concentration and particle size distribution, and assess their associated health risks. The results showed that after the construction and operation of the cascade reservoirs, sediment concentration in the middle and lower reaches of the Hanjiang River decreased. The sediment mainly originated from the recharge along the river, leading to a further decline in sediment content. The average sediment concentration progressively increased from one dam to another, whereas the median particle size of sediments (<italic>d</italic>₅₀) decreased step by step through the reservoirs. The average ΣPAE concentration in each reservoir was between 0.316 and <sc>0.747 μg/L</sc> during the flood season and between 1.860 and <sc>3.139 μg/L</sc> during the non-flood season. Dibutyl phthalate and bis(2-ethylhexyl) phthalate were the main components. Compared with previous concentration levels, ΣPAEs and their monomer concentrations have increased, likely owing to pollutant inflow from the surrounding areas, reduced water flow, and a decline in water self-purification ability. The concentration of PAEs during the nonflood season was considerably higher than that during the flood season, indicating that PAE pollution mainly originated from point sources. Along the flow direction of the cascade reservoirs, the concentration of PAEs in the water increased from one dam to another, following a trend consistent with changes in sediment content and particle size refinement during the flood season. The ΣPAE concentration showed a significant negative correlation with the <italic>d</italic>₅₀ of sediment, with a Spearman correlation coefficient of −0.657 (<italic>p</italic>&lt;0.1). During the nonflood season, the monomer concentration of PAEs gradually decreased from one reservoir to another, indicating an overall trend opposite to the changes in sediment content and particle size along the way. However, the correlation between sediment content and particle size was not significant, possibly owing to the gradual settlement of PAEs with sediment under the condition of low flow velocity in nonflood season. The results of the health risk assessment showed that the noncarcinogenic risk index of PAEs in the middle and lower reaches of the Hanjiang River was less than 1, indicating no substantial noncarcinogenic risk to human health. However, potential risks in certain local areas should be given attention.