Innovation in Environmental Solutions

Human Health Risk Assessment 8 Trenches were installed over a light non- aqueous phase liquid source with no impediments to air movement (e.g. above grade structures in proximity to the trenches). Meteorological data, with an emphasis on wind direction and speed, were continuously monitored throughout the study to understand the relationship of air exchange with meteorological factors. Two field events were conducted to evaluate trench air exchange rates, which allowed for estimation under various ambient conditions within trenches of different dimensions and orientations. During the first field event, four trenches of dimensions 3-feet wide by 8-feet long by 8-feet deep were installed and oriented in four directions. During the second field event, a longer trench (20 feet in length) was installed to determine if trench length would affect the air exchange rate. Carbon dioxide was used as the tracer gas for estimating the trench air exchange rate. Each trench was covered and filled with carbon dioxide until a concentration of approximately 50% was measured. The sheeting was then removed, and the fixed gas concentrations were recorded until carbon dioxide was measured below 1%. The air exchange rate was estimated by assuming that the decrease in the carbon dioxide concentration, as a result of mixing with ambient air, followed first order exponential decay, typical of air exchange rate studies. Study identifies risk to workers while reducing costs and redevelopment timeline The measured trench air exchange rates were more than an order of magnitude greater that the regulatory-recommended rate estimated using historical air flow between tall buildings. The trench length and orientation did not affect air exchange. Use of the measured trench air exchange rate significantly reduced the inhalation risk calculated for construction workers, and in most cases, the risk was below the acceptable threshold. For the limited areas where inhalation risks were above the acceptable threshold, naturally ventilating the trench for a period of 4-hours was sufficient to reduce risks below acceptable thresholds. If entrance into the trench was necessary prior to 4-hours of natural ventilation, then the trench could be mechanically vented. Ultimately, our trench study reduced the cost and expedited horizontal redevelopment of the former marketing terminal.