A Virginia contractor is installing a septic system and encounters groundwater at 8 feet depth. The USBC requires a minimum separation of 2 feet between the bottom of the drain field and groundwater. If the drain field requires 18 inches of depth, what is the maximum depth for the top of the drain field?
Correct Answer
C) 4.5 feet
Groundwater at 8 feet minus 2 feet separation minus 1.5 feet drain field depth = 4.5 feet maximum depth for top of drain field.
Why This Is the Correct Answer
Work backward from the groundwater level: Groundwater is at 8.0 ft. The bottom of the drain field must be at least 2.0 ft above groundwater, so the bottom of the drain field can be no deeper than 8.0 − 2.0 = 6.0 ft. The drain field itself is 18 inches (1.5 ft) deep, so the top of the drain field is at 6.0 − 1.5 = 4.5 ft maximum depth.
Why the Other Options Are Wrong
Option A: 6.0 feet
6.0 feet is the maximum allowable depth for the BOTTOM of the drain field (groundwater depth minus the 2-foot separation buffer). The question asks for the top of the drain field, which must be 1.5 feet shallower than the bottom.
Option B: 5.0 feet
5.0 feet does not correspond to any step in the correct calculation. It may result from subtracting only 3 feet total from the 8-foot groundwater depth without accounting for the correct field depth of 1.5 feet.
Option D: 5.5 feet
5.5 feet results from subtracting only 1 foot (half the drain field depth) instead of the full 1.5 feet from the 6.0-foot bottom limit. This represents an arithmetic error in the final subtraction step.
Memory Technique
Use three steps: (1) Bottom limit = Groundwater − Separation = 8 − 2 = 6 ft. (2) Convert field depth: 18 in = 1.5 ft. (3) Top limit = Bottom limit − Field depth = 6 − 1.5 = 4.5 ft. Write the three-step chain on scratch paper before solving.
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