Beach erosion allows salt water to invade the fresh water aquifers you ignoramus..
That evasion doesn't answer the question. You obviously stink at the fundamentals of fluid mechanics. That goes along with how you stink at the fundamentals of all science.
When the ground water was fresh water, fresh water didn't previously come bubbling up out of the storm drains on sunny days with high tides.
Now that the ground water is salt water, it does come bubbling up out of the storm drains on sunny days with high tides.
Something has put more pressure behind that water. Simply substituting salt water for fresh would not do such a thing. So what could?
The sane and intelligent people can easily answer that question. Higher sea level is the driving force. The underground water is linked to the sea through porous rock, so it will rise to match sea levels. The sea levels are rising to levels never historically seen before, so water is rising out of the storm drains to match.
So, how does your theory explain it?