Concentrations and exports of solutes from surface runoff in Houzhai Karst Basin, southwest China

Abstract

Chemical concentrations and export rates from surface water were analyzed in Houzhai Karst Basin (HKB, area 80.65km2 in total), southwest China, for the past two decades from 1986 to 2007. Surface water chemistry was dominated by ions of calcium (Ca2+), magnesium (Mg2+), bicarbonate (HCO3−) and sulfate (SO42−), which together made up 96% (by weight) of the total dissolved solids measured in this study. Seasonal variability of chemical export rates was positively correlated with surface runoff discharge, with the concentrations generally diluting at high discharge. From 1986 to 2007, as a result of the responses of karst weathering to climatic factors, the concentrations of Ca2+, Mg2+, HCO3−, and SO42− slowly increased in surface water. By contrast, changes in the concentrations of sodium (Na+), potassium (K+), and chloride (Cl−) showed a slowly decreasing pattern. We also found that the annual dissolved inorganic carbon (DIC) flux ranged from 10.1 to 34.1gCm−2yr−1, with an average value of 24.2gCm−2yr−1. Rainfall had the most important influence on DIC flux, although water pH may greatly affect the proportions of the three fractions (CO2, HCO3−, CO32−) of DIC. High water temperature in the wet season may reduce the solubility of CO2 in water and consequently affect DIC export rate, but this effect is much smaller than that of rainfall. Based on the chemical mass balance in surface water, our results indicate that high concentrations of SO42− by the large gypsum (CaSO4·2H2O) dissolution offset some anions such as HCO3− in karst water, which can partly explain why carbon uptake is often overestimated by the empirical model using concentrations of HCO3− in this karst water. It is concluded that the fluvial flux of DIC from karst water is important to the regional or national carbon budgets.