ALTERNATIVE EXPOSURE PATHWAY OF VOC VAPORS
Legacy Sewers, Mains, Laterals and Plumbing Systems meet VOC plumes
There are hundreds of thousands of shallow VOC groundwater and vapor plumes in urban areas in North America. Urban sewer systems installed between 50-100 years ago are well past their design life. These legacy sewer mains and associated laterals and components, exposed to earth subsidence, corrosive substances, pipe settling, biological intrusion and pipe material failure, develop cracks, separations or damage over time. Video camera inspections of sewers show the relatively common breaches in or between the concrete, clay or transite (asbestos containing) pipes or corrosion in cast iron pipes. Tree and plant roots grow into the sewer system and commonly damage sewer pipe integ- rity. Pipe connections, junctions, man- holes, etc. likewise develop structural damage which results in leaks of sewage wastes out of the pipes/structures, but also allows the inflow of groundwater and vapors into the pipes (Jacobs et al, 2014).
Legacy sewer system pipes experi- ence a baseline of infiltration and inflow (I&I) of groundwater through the whole year, but they experience significant increases of flow from this source during the rainy season. Figure 3 to the right shows that variations in wastewater flow are noted after rain events. I&I during significant rain events within a wastewater pipeline system in a north- ern California collection system was shown to contribute 8 to 33 times the amount of daily sewer flow shortly after a strong storm (SASM, 2010). The ‘8 to 33’ times were measurements at various sewer reaches during the same storm event. The collection system, including the trunk lines and the sewer laterals, was originally installed starting 6 or more decades ago. The conceptualized diagram showing the wastewater flow components of rainfall-dependent infil- tration/inflow (RDI/I) into sewer pipes (Figure 4) illustrates the lag between the timing of rainfall and its infiltration into the sewer pipes. Unlike storm induced wastewater flow increases, diurnal base wastewater flow (BWF) shows increased flow only in the early morning hours and during the dinner through evening hours. Groundwater infiltration (GWI) into sewer pipes (in the dry weather) is relatively constant during a 24-hour
30 TPG •
Jul.Aug.Sep 2017
period in an area with no tidal influ- ences. I&I leakage in sewer sections in northern California is frequently con- firmed with smoke testing, flow meters and video inspections. These tests and inspections consistently indicate that breaches of unpressurized sewer lines are common and that the failed sewer lines provide opportunities for vapors, as well as groundwater, to enter and leave the sewer system.
Leakage in and out of the sewer system (the pipes connecting drains inside buildings to the wastewater plant) is not limited to subsurface fixtures (pipes, connections, valves, pumps, etc.). Within buildings, examples of ineffective vapor seals in plumbing systems include dry P-traps (in sinks and floor drains), breached toilet wax rings, cracked plumbing drain pipes, loose pipe fittings
containing water and VOC-containing vapor infiltrate the breached sewer pipes. While VOC-containing fluids flow downgradient in the sewer pipes toward the wastewater treatment plant, the VOCs contained in the groundwater, pipe debris/solids and soil vapor have an opportunity to volatilize into the sewer air, and once in the sewer air, the con- taminants in sewer air migrate within the connected sewer pipes, but without the force of gravity which provides the downgradient flow direction of the liquid and solids waste stream. Sewer air movement will be dependent on a number of variables, but VOCs in vapor form (sewer air) may exit the sewer at any point where the sewer or plumbing is not vapor (or water) tight. Sewer air outflow will be increased with any nega- tive pressure on the pipe.
Figure 3: Variations in wastewater flow within a sewer pipeline (modified after SASM, 2010).
and gaskets, improper repairs or addi- tions and settlement. Examples of vapor leak locations (Figure 5 on page 31), a close-up of a P-trap (Figure 6 on page 32) and the migration pathway of sewer gas and VOCs into indoor air (Figure 7 on page 32) illustrate how VOCs in sewer air can migrate into indoor air.
Breached Sewer Lines Intersecting Subsurface VOC Plumes
When breached sewer collection pipes intersect contaminated soil and groundwater which contain VOCs, VOC-
Sewer air contains gases such as methane, ammonia, hydrogen sulfide, and low levels of carbon dioxide as well as any other volatile non-sewer deriving compounds. Depending on the concentrations of the VOCs in the intersected contamination plumes, con- taminant VOCs such as PCE, trichlo- roethene (TCE), benzene, and other chemicals (as well as the sewer gases mentioned above) migrate by several methods throughout the wastewater system. The contaminants migrate through the system, including the main sewer lines, the attached sewer laterals
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