“Impermeable substratum” means any subsurface material which is relatively impervious, such as hard pan, clay, slate-like materials, and other materials having a percolation rate of greater than 60 minutes per inch.

Source. (See Revision Notes #1 and #2 at chapter heading for Env-Wq 1000) #11184, eff 10-1-16; ss by #14486, eff 2-1-26, EXPIRES: 2-1-36 (formerly Env-Wq 1002.34)