This is an image of an estuary near San Diego, California.
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Cation Exchange in Groundwater
Groundwater often contains dissolved minerals, such as dissolved calcium, or magnesium, which come from the weathering.
As this water percolates through an aquifer, it may be modified and changed. In such an aquifer, calcium or magnesium in the water may be exchanged for sodium.
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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