Stop at the informational sign at the posted coords and count the number of yellow Infiltration Chambers depicted in the drawing. (Not the ones in the photo.) Let this number = "A".
Start with the posted coordinates:
Add (4 x A) - 6 to the North coordinate's decimal number.
Add (3 x A) - 1to the West coordinate's decimal number.
checksum:
Add all 7 digits of North = 31
Add all 8 digits of West = 30
Once you get to ground zero, walk 20 feet south :)
Congratulations to Noncentric on their First To Find!