Sodium in natural bodies of water comes from the weathering process of igneous rocks and evaporated minerals. The amount of sodium varies greatly in different conditions. In most rivers, the concentration of sodium falls between single-digit mg/L to double-digit mg/L. However, the concentration in salt water can reach up to more than 100000 mg/L. In high salt concentration water, sodium is the dominant positive ion. In seawater, sodium accounts for 81% of the total positive ion weight. Sodium in water with a concentration below 1000 mg/L exists in a free state while in higher salt concentration water there may have other ions and complex compounds such like NaCO-, NaHCO3 and NaSO4-. Moreover, almost all of the negative ions can form ion-pairs with sodium in seawater.