First, an inspector comes out and they determine where the leach field and the tanks can go. For us, here in Placer County, just north of Sacramento, the tanks must be 100 feet from a well, 50 feet from a creek or pond. The leach field must be 20 feet from any retaining wall, 100 feet? from any water source and must be aligned with the lay or slope of the land. The length of the leach lines required are determined by the number of bedrooms you will have. Here is a wonderful website with everything you ever wanted to know about Septic Systems.

Then they must dig a very large hole, one that will accommodate two 1500 gallon concrete tanks. The dirt was extremely sandy at one level, then you can see where the clay soil is beneath it. Most likely the sandy soil was left from centuries ago when the creek was once this high. Also, another theory was that sand in the creek bed was blown up onto this area by the Gold Miners. I wanted so badly to grab a bucket of the black speckled sand, as it was sure to be full of gold dust! But time was against me. [sigh] They make sure the entire bottom of the hole is level using a laser and a tall poll that responds back to the laser sitting outside of the hole. They also remove any large rocks.