A leach line also called a drain field is the underground component of a septic system that distributes treated wastewater into the soil. Perforated pipes laid in gravel-filled trenches allow effluent to slowly filter through layers of soil and gravel. Natural bacteria in the soil further treat the water before it reaches groundwater. Leach lines typically span 100 to 300 feet depending on soil type and household water usage. Proper maintenance ensures decades of reliable operation.