water
a large part of the fresh water the world uses for drinking, irrigation, and industry comes from the water table. rainfall either runs off and is collected by the watershed and flows into rivers to the ocean. or it's absorbed into the ground and makes its way to the water table. the water table also has a system of underground streams and underground rivers that flow to the ocean. during periods of torrential rains these will swell. just like above ground rivers. but during periods of drought they run backwards. salt water from the ocean is drawn under the ground and infiltrates the water table. the next period of torrential rains washes out some of the salt but not all. it really doesn't take very many cycles for the brine to reach topsoil and seriously degrade its agricultural value. big deal. losing farmland won't kill us. oh wait.