A Colorado Licensed Professional Engineer must design a system if any of the following occur: (a) the following soil types are identified within 4 feet below the depth of the proposed soil treatment area (STA): 0, 3A, 4, 4A, 5, R-O, R-1, R-2; treatment levels TL2, TL2N, TL3, and TL3N as specified in tables 10 and 11 of Regulation O-17 (b) groundwater, bedrock, or other limiting layer is present within 4 feet below the depth of the proposed STA, (c) the ground slope at the area of the proposed STA is greater than 20%, or (d) the proposed system is a commercial system, (e) the proposed system utilizes pressure distribution.