Besides the usual stuff you can find on the web concerning septic tanks....
In our area we are all on top of a rock called Calichi, or soft limestone as a geologist friend tells me. Beneath this layer there is another layer of highly porous rock called "filtrante". It is a honey comb layer of crystlized limestone (i think) and it looks pretty neat.
In our area a septic system is vertical. I hole is dug down to the filtrante layer (12 to 20 feet). The top 4 or five feet is made into a standard septic tank. The overflow tube goes down through the bottom of the tank where the remander of the hole is filled with large rocks. The water then enters the filtrante layer and leaves the hole.
Now you know why we dont drink well water around here. On the same topic, in our area, the water table is 300 to 500 feet down.
P.S.
If a dominican can correct my spelling I would appreciate it.