What is a leach field? A leach field (or drainage field) is an underground system of trenches and pipes used to release the wastewater from a septic tank into the surrounding soil. By ProMatcher Staff Updated: 03/11/2016 Get Free Quotes Today! Get Free Quotes Today! Find Local Pros Free Septic System Installation Cost Estimates Related Questions How Much Does It Cost to Put in a Septic Tank? What is an Engineered Septic System? What is a Septic Baffle? What is a Septic Alarm? What is a septic Lid?