I pulled the drawings for the septic from the county. It shows the main line coming off the septic tank runs parallel to the front of our house.

I would have expected the lines for the leach field to come off at 90 degrees and shoot out, perpendicular to the house.

The drawing shows three lines coming off at a 45 degree angle, or so, and actually angling backwards towards the side of the property, with two of the shorter legs tied together at the end.

How could I verify precisely where the drain field is, and map it out? My first thought is Ground Penetrating Radar.

Any other technologies out there, or has anyone done this?

Thanks.

Depending on how old the property is, they may have run locating foil along the pipe runs to make it easier to find the field with standard locating equipment. Give 811 a call, they do locates for free so you don't dig up other utilities (like one of our neighbors did this weekend and killed our internet). The worst that happens is they can't find them, but it's cost you nothing but time.

Some people (like my brother, who's also a professional locator), believe in dowsing for stuff like this. Personally I think it's all hooie that takes advantage of gullible people and the ideomotor effect.

With all the extra holes drainage should improve! (EDIT: this comment was made in jest - sorry, I was born without a serious bone)

Kind of a "shitty" time of year.... A properly functioning drain field a lot of times will burn the grass above the ditch due to the gasses.
If the designer did his job correctly the drawing you have will be an as-built and should be pretty close. Question: Is the drawing you have stamped as an as-built? Or is it just a submitted design?

You can try to witch the lines.. I use to do this all the time with great results! Or you can buy a probe and poke around the yard feeling for the drain rock. Calling 811 isn't gonna do squat for locating your drain field, 811 is only for utility locates ie: underground power, communications, water, side sewer, gas and storm water.

Here in our park, our septic contractor hired out ground penetrating radar to located the tiles along with the ends for maintenance work.

I can check later today to see if the name is available.

Found this: https://www.ars.usda.gov/midwest-area/columbus-oh/soil-drainage-research/people/barry-allred/gpr-drainage-pipe-detection/

You can rent a camera w/ locator and run it from the outlet baffle into the D-Box. Locate and open the D-Box and then run the locate/camera down one of the pipes.

If the septic system is in Pierce County it may not have a d-box as they are not required by code there. If it is any other type of system than gravity it will not have a dbox. If it is a drip system no locator will find it. If it is gravity, then Massive Designs idea is how I would do it. If it is pressure distribution your lines can be as small as 1”, if it is drip the pipe is 1/2” with emitters embedded in the pipe 6-8” deep and very fragile. I would not recommend making extra holes with out knowing what type of system you have. Might wind up with effluent shooting out of the ground, or collapse a drain pipes in an old gravity.
What type of system does the as built show? If you only have 1 tank it will be a gravity.
White is the age of the system?
Also why are you trying to locate the drain field?
Certified septic guy here

It is in Pierce County, estimate 35 years old. Here is a picture. Red dots are rhododendron garden, green is the yard. The lines are a re-creation of the county drawing. Was hoping to put RV pad where the rhododendron garden is.

That era of systems typically do not have very accurate as built files with the county.
If you soil is 3-5 you could prob for it with a metal bar to find the drain rock around the pipe, if it is soil type 1 or 2 you might not be able to tell the difference. Also I would check what type of drain pipe you have first, they still used black corrugated pipe in those times and a metal bar will go straight through it easily and with out feeling it. Typically the trench width was 16” back then with 8” of rock below the pipe and 2” above. If you have access to look into the tank the depth of the outlet pipe will be the minimal depth of your drainfield pipe, it can be deeper however it will not be shallower.

Best way would be to run a locator camera and follow the sonde, you will probably have to dig a couple spots to open a hole to get to the camera in the two lines farthest from your drive way.

Other considerations are if you find a spot for your pad it would take up potential reserve area for when you current system fails. If sewer is available that’s a non issue. Your drain field should have the standard 10’ set back from the property line, my advice would be make a rock drive way along the fence line no more than 10’ wide. Also get rid of the Rhodes, you might not even get the camera to go the whole way with root structure in the pipe.