A monovalent cation. The solubility of sodium salts is high and does not cause a RO scaling problem. Sodium, in seawater, is the prevalent cation. Sodium is the cation used to automatically balance a RO feed water analysis. Dietary sodium levels can range from 2000 mg/l for low-sodium diets to 3500 mg/l for average consumption levels. The US EPA has set a DWEL (Drinking Water Equivalent Limit) of 20 mg/l for potable water but is reevaluating the limit as to low. Daily consumption of 2 liters (0.53 gallons) of water with 100 mg/l of sodium would be only 200 mg. A relatively "hard water" with 10 grains per gallon (171.2 mg/L) of hardness (as calcium carbonate) results in only an additional 79 mg/L of sodium when softened.