Why does overpumping of groundwater lead to salinization of coastal areas?

Hasn’t the groundwater level dropped?

Why can well drainage and well irrigation solve salinization?

I'm not a professional in this field, so I'll make some qualitative reasoning.

Groundwater is freshwater, even in coastal areas. It is freshwater with low salt content at most, and is definitely not as salty as seawater.

If groundwater is excessively pumped, seawater will take advantage of it, and the salt brought in by seawater will salinize the area.