Summary and conclusions

Erfaringsopsamling på passiv ventilation til fjernelse af klorerede opløsningsmidler fra umættet zone

Since 1999, a number of facilities for passive ventilation have been established in Denmark. To date, passive ventilation has been established at 14 sites in Denmark - 11 in the Region of Greater Copenhagen Region, 2 in the Region of Zealand , and 1 in the Region of Southern Denmark. A previous project /ref. 1/ has shown that passive ventilation is an effective way to remove chlorinated solvents from the unsaturated zone. The purpose of this project is to review the experience of the 14 localities and the efficiency of the method in relation to the reduction of the concentration in the unsaturated zone, and in relation to preventing chlorinated solvents such as PCE from spreading from the unsaturated zone to the groundwater.

Passive ventilation is based on exploiting the pressure difference between the unsaturated zone and the atmosphere for ventilation of the unsaturated zone. The method can be used at locations with a relatively thick low-permeable layer with an underlying high-permeable unsaturated zone. Changes in the atmospheric pressure create a pressure gradient between the atmosphere and the unsaturated zone, which is utilized to force the polluted soil gas to the surface through wells screened across the unsaturated zone.

The source in the low-permeable layer delivers an amount of pollution to the underlying unsaturated zone per time unit – mass flux. The chlorinated solvents are transported to the unsaturated zone and in this area, passive ventilation will reduce further mass transport to the groundwater. When the concentration in the unsaturated zone is reduced by passive ventilation, the mass flux from the unsaturated zone to the groundwater will also be reduced. In the longer term, this will result in a reduction in the concentration of chlorinated solvents in the groundwater.

For locations suitable for passive ventilation, differential pressures between the atmosphere and unsaturated zone in the range between -25 to +20 mBar have been measured. The average flows registered at the various sites are in the range from 0.1 to 0.7 m³ per hour per meter screened well with an average of 0.3 m³ per hour per meter screened well. The thickness of the unsaturated zones ventilated by passive ventilation varies between 0.3-24 m.

The collection of experience from sites with passive ventilation has, for most of the sites, shown that the ventilation has had the intended effect and that soil gas concentrations at several sites are reduced by more than 70% over a period of 7-8 years. The soil gas concentration generally decreases over the first 2-3 years, and then stabilizes. The removal rate by passive ventilation in these stable conditions is estimated to be approximately equivalent to the flux from the low-permeable layer to the unsaturated zone. It is also shown that passive ventilation in the cases where the concentrations in the unsaturated zone are reduced has also prevented the pollution from spreading to the groundwater. The reduction in the groundwater concentrations has the same order of magnitude as the reduction in the concentration in the unsaturated zone.

When planning passive ventilation in the future, a flow of approximately 0.2-0.3 m³ per hour per m screened well can be expected. Furthermore, a density of approximately one well per 160 m² should be used. Concentrations in the unsaturated zone can be reduced to a minimum of 20% of the start-up concentration. If a large mass of chlorinated solvents are accumulated in the unsaturated zone or the mass flux from the overlying clay layer is large, it can be necessary to use active ventilation for a period before converting to passive ventilation.

Experience from the operation and monitoring of facilities for passive ventilation has shown that the wells should be monitored at start-up, after 6 months and after one year. Hereafter, annual monitoring should be performed until the concentrations in the unsaturated zone have stabilized. The frequency of the monitoring can be reduced to every 2 to 5 years when the concentrations are stabile. The coal filters have an adsorption capacity of 1-1.5 kg per filter, and the change of coals should also be taken into consideration when the frequency of monitoring is estimated.