Abstract

In developing in situ remediation most of the focus used to be on techniques using infiltration water to supply oxygen to the location. Later, techniques were developed in which soil was flushed with air to enhance the oxygen availability to microorganisms. The aim of the study reported here was to examine the results of remediation where soil is flushed with air, as reported in the literature, and to examine the costs of such systems. In the study these techniques were compared with water-supply systems, particularly those using hydrogen peroxide, e.g. bioremediation in Asten (Noord-Brabant). The remediation techniques which use air to supply oxygen to the soil are soil ventilation, soil vapour extraction, bioventing and air sparging. Soil ventilation and soil vapor extraction are physical remediation techniques which can be employed only in the vadoze zone for removal of low molecular organic pollutants. Bioventing removes pollutants mainly by microbiological conversions in the vadoze zone. Air sparging can be employed only in the saturated zone, where the pollutants can be stripped by air, as well as converted by microorganisms. In general, these techniques are used on sandy soils, which are very permeable. The whole range of hydrocarbons can be remediated. According to the literature, remediations were generally kept in operation for two years. At some sites high removal rates, up to 20 mg.kg-1 soil.day-1, were reached. The literature did not mention anything about the remaining levels of pollution dropping to target-value level. Although sometimes only 5% of the original amount of pollution remains in the soil, the lowest levels obtained are found just below the former B value. Therefore, the remediation levels reached should always be viewed in connection with the remediation time and the amount of pollution originally present. At the remediation site in Asten, where a water-supply system with hydrogen peroxide dosing is used, lower remaining levels were obtained after a longer remediation time. This location could problably also be remediated using a bioventing technique if the groundwater table were lowered. The remediation time would probably be shorter because oxygen would be available to a greater extent. However, the final pollution levels in the soil are expected to be higher. Due to the lower soil-water content in the pores, the mineralization process would terminate earlier. It is possible that these results would not be obtained in the vicinity of the groundwater table. Another possibility might be natural attenuation after lowering the contamination levels with an active remediation technique. The Dutch target values can be expected to be attained, but only after a very long remediation time. Bioventing appears to be the most cost-effective remediation technique available.