Pollution in river tributaries restricts the water quality of ecological water replenishment in the Baiyangdian watershed, China

Abstract

Natural rivers often have complex water network structures, and the continuous water inflow from tributaries may have crucial impacts on the water quality of ecological water replenishment in the mainstream. This study selected two main inflow rivers of the largest lake in Hebei Province (Baiyangdian), the Fu River and Baigou River, to explore the influence of tributaries on the quality changes of ecological replenishment water in the mainstreams. In December 2020 and 2021, water samples were collected along the two river routes, and eutrophic parameters and heavy metals were determined. The results showed that the tributaries of the Fu River were all severely polluted. With the inflows of the tributaries, the comprehensive pollution index of eutrophication greatly increased along the replenished water route of the Fu River, and the replenished water in the lower reaches of the Fu River mainstream was mostly considered moderate to heavy pollution. Whereas, because the Baigou River's tributaries were only moderately polluted, the water quality in the Baigou River's replenished water was mostly better than moderate pollution. Due to the slight pollution of heavy metals in the tributaries, the replenished water in both the Fu and Baigou Rivers did not show any impact from heavy metal pollution. Correlation and principal component analysis indicated that the main sources of serious eutrophic pollution in the tributaries of the Fu and Baigou Rivers were related to domestic sewage, industrial wastewater, plant decay, and sediment release. This non-point source pollution then caused the decline in the quality of the replenished water in the mainstreams. This study exposed a long-standing but neglected problem in ecological water replenishment and provided a scientific foundation for conducting better water management to improve the inland water environment.

Keywords: Baiyangdian; Ecological water replenishment; Eutrophication; Heavy metal; Inflow river; Tributary.