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Conceptual illustration of sewer preferential pathways for vapor intrusion. A) Most residences (i.e., those without septic tanks), are connected to the sanitary (or combined storm and sanitary) sewer system. B) Some residences have foundation drain systems connected to land drain or storm sewer systems. Although prohibited by current building codes, some residences may have older or illegal foundation drains connected to the sanitary sewer system. Clean-outs and manholes are not shown.
