7 Del. Admin. Code § 7302-9.0
All licensed water well contractors and pump installer contractors shall mark each well drilling rig, well driving rig, water tank truck, and well and pump service vehicle used by the contractor or his employee(s) with legible and plainly visible identification markings. The identification markings shall include the name of the company, corporation, or contractor and the water well or pump installer contractor's valid license number. The identification marking(s) shall be displayed on each side of the vehicle(s) and equipment in letters and numerals at least 11/2 inches high. The letters and numerals shall be in a color sufficiently different from the color of the vehicle or equipment so that identification markings are plainly legible. Weatherproof decals or magnetic signs meeting the requirements of these regulations are acceptable.
7 Del. Admin. Code § 7302-9.0