Spatial and temporal analysis of water chemistry records (1958-2000) in the Huanghe (Yellow River) basin

[1] The chemistry of major dissolved ions (Ca2+, Mg2+, Na+ + K+, HCO3-, SO42-, Cl-) and dissolved silica (SiO2) in the river water of the Huanghe ( Yellow River), China, was studied from historical records at 100 stations in the drainage basin for the period 1958 - 2000 ( not all the parameters were continuously monitored during the entire period). This river system ( 750,000 km 2) presents an exceptional temporal and spatial water chemistry variability compared to other major rivers. The total dissolved solid ( TDS) concentration of the Huanghe varied over 2 to 3 orders of magnitude throughout the basin, with a median TDS concentration of 452 mg/L, which is about 4 times the world spatial median value (WSM). In particular, the concentrations of Na+ + K+, SO42-, and Cl- were 10 - 20 times higher than in other major world rivers. Similar to the Changjiang ( Yangtze River) and many other Himalayan rivers, the TDS at a given station is seasonally variable and inversely related to river runoff with a variation factor less than 2.0, despite a water dilution of fourfold to fivefold in the summer flood season. In addition to chemical weathering of sedimentary rocks, evaporation and fractional crystallization were found to be the major natural process controlling the major element chemistry of the Huanghe, owing to the abundance of loess and clastic rocks under arid and semiarid climates. A persistent increasing trend from year to year has been observed in the concentrations of TDS and all the salts except for HCO3- at all main-channel stations except for the uppermost Lanzhou. The rate of increase in the TDS concentration was the highest in the middle reaches (10.52 mg/L yr(-1) at Toudaoguai) and remained to be unusually high in the lower reaches (5.5 mg/L yr(-1) at Stations Luokou and Lijin). The increasing trend coincided with a significant decrease in water discharge at most of the main-channel stations in the past 40 years, and is attributed to increasing regulations of reservoirs and intensive water withdrawal for irrigation and diversions, as well as many other anthropogenic processes. Despite the increasing trend in TDS concentrations, the seaward TDS flux to the sea by the Huanghe has decreased dramatically from more than 20 x 10(9) kg/yr in the 1960s to less than 10 x 10(9) kg/yr at present, due to the sharp decrease in water discharge ( sometimes completely dried up) in the lower reaches of the river. The significant decreases in sea-ward fluxes of water and dissolved salts are expected to have profound physical, ecological, and socioeconomic impacts on the lower reaches of the river, the coastal areas and the Bohai Sea.