Evolution of and mechanisms controlling water chemistry in the Beijiang River Basin over the past four decades

This study investigated long-term hydrochemical data from the main channel (Beijiang) and three tributaries (Lianjiang, Wujiang, and Zhenjiang) in the middle and upper Beijiang River, the second-largest tributary of the Pearl River, from 1985 to 2023. The results showed that Ca²⁺ (>44.1 %) and HCO₃⁻ (>76.17 %) are the dominant cation and anion, respectively. These results indicate that weathering of carbonates and silicates was the primary natural factor influencing riverine chemistry in the basin. Total dissolved solids (TDS) fluctuated between 33.3 and 399 mg/L and pH between 6.5 and 9.2, with the highest TDS levels in Wujiang (191 mg/L), followed by Lianjiang (175 mg/L), Beijiang (149 mg/L), and Zhenjiang (84.7 mg/L). The TDS trend in Beijiang exhibited a significant decline before 2001, followed by a subsequent increase, while the average concentrations of acidic substances (Cl⁻, NO₃⁻, and SO₄²⁻) increased by >38 %, while pH value does not decrease in response to the elevated SO₄²⁻ due to dissolution of carbonate. Ion concentrations mostly decline with discharge rate, except for NO₃⁻, which showed transport limitation and enrichment with increasing discharge rate. The ion flux patterns were classified as discharge-dominated or concentration-dominated. The annual TDS flux of Beijiang was approximately 5.0 × 10⁶ tons and the ionic runoff modulus was 148.4 t·km⁻²·yr⁻¹, which was relatively high compared to that of other rivers in the world. The correlation between ions concentrations and socioeconomic factors suggests that socio-economic activities significantly influence water chemistry. Industrial relocation and large hydraulics project may lead to abrupt fluctuations and sustained increases in water chemistry parameters. The generalized additive models showed that natural factors explained 40.8–50.7 % of the deviance, while socioeconomic factors related to anthropogenic activities explained >79.5 % due to increasing SO₄²⁻, NO₃⁻ and Cl⁻ trends. Given the importance of the river to the Guangdong–Hong Kong–Macao Greater Bay Area, enhanced management and monitoring are needed to protect the regional water supply.