This is an image of an estuary near San Diego, California.
Click on image for full size
Cation Exchange in Groundwater
Groundwater often contains dissolved minerals in the form of ions, such as dissolved calcium, or magnesium, which come from the weathering of surface rocks as well as dissolved organic compounds such as detritus, animal waste, or human contaminants.
As this water percolates through an aquifer, it may be modified and changed. Material comprising the aquifer may contain material which has a strong natural tendency to exchange one ion for another. In such an aquifer, calcium or magnesium in the water may be exchanged for sodium.
After awhile excess accumulation of one particular ion in the soil of the aquifer may force a chemical change in the rock which form the aquifer leading to metamorphism in that rock.
Eventually dissolved minerals are carried to the sea where these ions form salts. Thus sea water, the repository of dissolved material, especially carbonates, tends to be salty.
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