There are many areas in Northern California which have problems with salt water intrusion. I happened to live in one such area called the Inglenook Fen. We had sw intrusion up to a point about 100 yards from our rather shallow well. Our house was sinking but it sank at an uneven rate with the salt water side sinking faster than the other and it became rather obvious after 6-7 years of living there..One could tell rather easily by simple observation that the house was "crooked".
Only an observation....