After some further kilometres, sodium begins to increase by ion exchange at the expense of calcium, producing a natural softening of the water. Eventually, the available calcium in the water is exhausted, but sodium continues to increase to a level greater than could be achieved purely by cation exchange. As chloride also begins to increase, this marks the point at which recharging water moving slowly down through the aquifer mixes with much older saline water present in the sediments (Figure ). The observed hydrochemical changes can thus be interpreted in terms of oxidation/reduction, ion exchange and mixing processes. .
