Chemical evolution of runoff in Eastern Mediterranean mountainous karstic terrains

Abstract

The Israeli central mountain rim was chosen to represent the transformation of rain chemical composition to runoff composition in a carbonate landscape under a Mediterranean climate. It was found that at the mostly dolomitic slopes of the Samaria Mountains the main processes shaping runoff chemistry are dissolution of dry fallout (dust) and salts that were precipitated during evaporation of water from previous small-scale runoff events. Calcium and sulfate concentrations in the runoff are defined mainly by the dissolution of dust minerals. The main input of sodium and chloride into runoff composition comes from both rain and dissolution of halite that remained from the evaporation of the previous runoff, whereas the main input of magnesium is connected to dissolution of dolomite outcrops. A combination of forward and inverse modeling by using PHREEQC software was found to be effective for studying runoff chemistry formation. Numerical simulations of runoff sample evaporation have shown that the main minerals precipitating during dry periods are calcite, anhydrite/gypsum, glauberite and halite. The high Na to Cl equivalent ratios (about 1.3 at average) may reflect two-phase dissolution of sulfates and halite in the initial stage that whereas and of glauberite in the following stage. Rainwater transformation to runoff inverse modeling was found to be consistent with the results of forward modeling, confirming these runoff chemical processes.