Major ion chemistry and weathering processes of the Danjiangkou Reservoir, China

A total of 40 water samples were collected from five sites in the Danjiangkou Reservoir, an important drinking water source in China. Over the period November 2004–June 2006, eight field surveys were conducted and water temperature, pH, EC, major ions (Na+, K+, Ca2+, Mg2+, Cl- , SO2- 4 and HCO- 3 ) and Si were monitored to determine their temporal and spatial patterns across the reservoir. The controlling factors were analysed using the stoichiometry of the water chemicals and multivariate statistics including a correlation matrix and analysis of variance. The results revealed that waters in the reservoir are sub-alkaline and have a low solute load, and most water variables exhibit notable spatial and temporal variations. The major ion chemistry of the reservoir was controlled by carbonate weathering (of limestone) with the dominant ions Ca2+ and HCO- 3 contributing 63–81% and 73–81% to the major cation and anion budgets, respectively. Carbonate dissolution was produced by both sulfuric and carbonic acids; in particular carbonic acid and silicate contributed little to the reservoir waters, while SO2- 4 was important and originated primarily from anthropogenic inputs. This research will help water quality conservation in the Danjiankou Reservoir, China.