How can rainfall affect the mineral content of ocean water?

The rainfall increases the mineral content of ocean water.

Explanation:

The minerals in the oceanic water are added by the rainfall through surface runoff. It is a phenomena in which the large volume free flowing water flows over the surface of the soil. This water is not absorbed by the soil instead it takes away along with it all surface materials like chemical fertilizers, soil, sediments, inorganic and organic minerals and deposit them to the water body (ocean, river or others).

Thus this can be said that rainfall increases the mineral content of the ocean water.  

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.