Remediation techniques for mitigating vapor intrusion from sewer systems to indoor air

Abstract

AbstractInvestigations at former dry cleaning sites in Denmark show that sewer systems often are a major vapor intrusion pathway for chlorinated solvents to indoor air. In more than 20 percent of the contaminated drycleaner sites in Central Denmark Region, sewer systems were determined to be a major vapor intrusion pathway. Sewer systems can be a major intrusion pathway if contaminated groundwater intrudes into the sewer and contamination is transported within the sewer pipe by water flow in either free phase or dissolved states. Additionally, the contamination can volatilize from the water phase or soil gas can intrude the sewer system directly. In the sewer, the gas phase can migrate in any direction by convective transport or diffusion.Indications of the sewer as a major intrusion pathway are: higher concentrations in the upper floors in buildings, higher concentrations in indoor air than expected from soil gas measurements, higher concentrations in bathrooms/kitchen than in living rooms, chlorinated solvents in the sewer system, and a pressure gradient from the sewer system to indoor air. Measurements to detect whether or not the sewer system is an intrusion pathway are simple. In Central Denmark Region, the concentrations of contaminants are routinely measured in the indoor air at all floors, the outdoor air, behind the water traps in the building, and in the manholes close to the building. The indoor and outdoor air concentration, as well as concentrations in manholes, are measured by passive sampling on sorbent samplers over a 14‐day period, and the measurements inside the sewer system are carried out by active sampling using carbon tubes (sorbent samplers). Furthermore, the pressure gradient over the building slab and between the indoor air and the sewer system are also measured. A simple test is depressurization of the sewer system. Using this technique, the pressure gradient between the sewer system and the indoor air is altered toward the sewer system—the contamination cannot enter the indoor air through the sewer system. If the sewer system is a major intrusion pathway, the effect of the test can be observed immediately in the indoor air.Remediation of a sewer transported contamination can be: prevention of the contaminants from intruding into the sewer system or prevention of the contaminated gas in the sewer system from intruding into the indoor air. Remediation techniques include the following: lining of the sewer piping to prevent the contamination from intruding into the sewer; sealing the sewer system in the building to prevent the contamination from the sewer system to intrude the indoor air; venting of manholes; and depressurizing the sewer system.