i don't know, but you're lucker than i am.
some government employees were told to clean out an old room and threw
away filed and approved building plans in an old storage room in
buffalo ny city hall back in the 1960s including some of mine from the
1910 era.
in your case hidden underground rocks and the number of years that have
gone by are other factors besides lost memories of former owners as to
where all the pipes are.
your thirsty tree probably has also enjoyed free fertilizer from your
system for many years.

Eigenvector wrote:
I recently requested an "As built" drawing from my county health department
for the septic system. What they faxed back to me was not exactly what I
was expecting to see. It was dated 1960 and listed the owners, builders,
and inspector.

I happen to have a degree in engineering and I was expecting to see an
engineering drawing or something official (maybe even on vellum) - what I
got was best described as "back of the napkin with a coffee ring as a bonus
decoration". First, is that realistic, I don't want to harp on it if in
fact that's what they're all like, and second how truthfully can I take the
measurements and layout of the "drawing"?

I'm looking at the drawing and it looks like they deliberately routed the
drain field so that it goes off in weird angles, makes several beelines for
the massive Douglas Fir in my backyard and otherwise doesn't efficiently
utilize the space provided for it. I was expecting a nice layed out route
involving 90 deg bends and long branches. What I see is the first line
making a large oblique angle into the middle of the yard then sending
branches back toward the tree, finally connecting back to itself after
circling the tree.

Was it common to design the field to incorporate trees as a means of
absorbing water?