Major ion chemistry of a representative river in South-central China: Runoff effects and controlling mechanisms.

The Gan River is a large tributary of the Yangtze River in Jiangxi Province, South-central China. Hydrochemical data for this river were analyzed for the period 1958-2016. Ca2+, Na+ + K+, HCO3-, and SO42- were dominant in river water, and pH and total dissolved solids (TDS) varied from 6.0 to 8.8 and 15.7 to 141 mg/L, respectively. The chemical composition of river water was different between the two periods 1958-1979 and 1980-2016. Monthly yields of all ions were positively correlated with river runoff. Monthly yields of SO42-, NO3-, and Cl- were more positively correlated with river runoff before 1980, indicating non-point sources, while multiple sources were indicated after 1980. Sea salt-sourced Cl- comprised less than 19% of the total Cl- in river water. Weathering of basin rocks with sulfuric acid reflected strengthening of anthropogenic activities after 1980. This was reflected by increases in Cl-/(Na+ + Cl-) and SO42-/(Na+ + Cl-) with gross domestic production, population, coal consumption, fertilizer use, and wastewater discharge. Although water quality in the Gan River makes the water acceptable for drinking according to the World Health Organization standards, increases in Cl- and NO3- concentrations after 1980 are of some concern.