Thats .22ac of lawn, so no way am I going to spend $50k+ to remove the irrigation system, 3-4" of top soil and hauled away and then truck in and spread over 800 hundred yards of gravel. 

After removing the tank, they pointed out that there are 8 holes on the tank and sent me a proposal for soil remediation for total cost of $8900.

And hold all properties underneath it?

The neighbor of that rental property actually filled the pool with soil.

My initial thought was that I wouldn't have the same tenant living underneath me all the time, but that I could increase my monthly cash flow with a reasonable amount of money invested. 

We have cracking issues on a duplex in CA due to bad soil/earthquakes - the cracks always reappear.

(REMEMBER THIS ALWAYS) You have greatness in you,(YOU ARE ALREADY CREATE LIKE THIS< WITH GREATNESS)  just like a seed. ( a seed) doesn't say I am just a seed, a seed has potential inside, it just needs the correct environment and timing and condition to become GREAT and ABLE to create fruit, It needs, Soil, Water, and correct temperature to grow (and time) .

He is allergic to cat dander and the house has so much inside, in the attic, in the yard, garage and underneath the house he got instant sinus problems and severe rashes on his entire body .

Check the gutters, downspouts and grade of soil/cement around the house and make sure it's draining properly as a lot of basement moisture issues can be fixed by backfilling and extending your drainage further from the building.

Or is there any way we can find out what's going on underneath?