Passive dry barrier air circulation and mass transfer

Dry barriers are an alternative to prescriptive final cover designs for waste containment facilities in arid and semi-arid regions. This study investigated the use of passive airflow in dry barriers. Bench-scale testing, mass balance modeling, and a field-scale test section were used to develop and verify a method for predicting the magnitude of passive ventilation. In addition, testing to determine the effective diffusion coefficient of water vapor diffusing into a layer of coarse rock, and the rate of water removal from passive dry barriers was completed.; The rate of passive ventilation in dry barriers is a function of the wind velocity, the type of vent, and the properties of the coarse layer being vented. Ventilation increases with increasing wind velocity, and increasing air conductivity of the coarse layer. Ventilation may increase or decrease depending on the type of vent use to draw air through the coarse layer. The best ventilation performance was obtained using a commercial vent cap. However, some commercial vent caps produced less ventilation than a simple open pipe end. An equation was developed for predicting the ventilation rate based on the type of vent pipe, wind velocity, and coarse layer properties. The equation predicts similar ventilation rates to those measured under field conditions. Rates of water removal from a bench-scale dry barrier ranged between 0.05 mm/month and 0.5 mm/month. The rate of water removal is a function of temperature, the coarse layer airflow rate, and the inflow humidity. The calculated effective diffusion coefficient for water vapor diffusing into 25-mm-diameter stone is between 0.7 m2/day and 0.9 m2/day (30% to 40% of the diffusion coefficient of water vapor in free air).; Water removal rates from a field-scale passive dry barrier ranged from −0.2 mm/month to 3.4 mm/month. Removal rate at field scale is a function of ground temperature, wind velocity, and atmospheric water vapor concentration. Field-scale water removal rates can be predicted using a mass balance model. The accuracy of the prediction depends on the accuracy of estimates for the coarse layer airflow rate and coarse layer temperature.