Temporal variability in volatile organic compound concentrations in sanitary sewers at remediation sites

In recent years, there has been a growing recognition of the importance of sewers as preferential pathways for vapor intrusion. This pathway involves volatile organic compound (VOC)-impacted vapor migration through the interior of pipes or conduits towards the potential receptor. A number of research initiatives have been undertaken to improve the understanding of this pathway which, in turn, supports development of sampling methods to evaluate whether it is a concern at a given site. One aspect of the conceptual model that is important to understanding the potential risk of sewer vapor intrusion is the degree to which VOC concentrations vary over time. This paper presents results of field sampling conducted in Texas, Utah, and California to explore temporal variability of chlorinated VOC concentrations in sewer vapor over different timescales. These results, along with results from other studies, suggest that wide ranges of VOC concentrations can occur at given sampling locations. However, long-term temporal variability (months to years) is much higher than short-term variability (days). The timescale over which the variations occur has implications on sampling methods and frequency. The results suggest that multiple grab samples collected over a longer time period, such as on a quarterly basis, are more appropriate for characterizing the long-term average sewer vapor concentrations than single time-integrated samples collected over a period of hours to days. Because time-integrated samples can often be logistically more difficult to collect, an understanding of the timescale of variability may help practitioners balance field logistics considerations with data adequacy when characterizing VOC concentrations in sewers.