水平浄化壁による土壌浸透水の硝酸性窒素浄化技術

Abstract

The nitrate contamination level in groundwater depends on hydrogeological properties underground as well as the agricultural system aboveground. Excessive use of nitrogen fertilizers leads to nitrate contamination of groundwater under the crop area, and the nitrate concentration level will be severe if the ground is composed of coarse material and has high permeability. The impact of nitrate reaching groundwater is also influenced by the microbial activity of anaerobes in soil that contributes denitrification.In the present state of this issue in Japan, the main governmental action for this problem is to reduce the excessive use of fertilizer and manure. However, groundwater monitoring results by the Japan Environmental Protection Agency indicates that nitrate contamination is still spreading nationwide and the nitrate concentration in groundwater has exceeded the regulation level (10 mg/L) in 5% of the monitoring wells since 1995. Therefore, development of an effective cleanup measure has been required.In this paper, pilot-scale field application of a horizontal Permeable Reactive Barrier (PRB), which is a nitrate cleanup technology for the vadose zone, is introduced. The horizontal PRB installed in this study consists of an accelerative denitrifying layer with low permeability and an organic matter supplying layer embedded on it. The low permeable layer using field soil retains a lot of water in the vadose zone because of capillary force, and a reductive condition adopted for denitrification seems to be gradually formed in the barrier. In fact, the concentration of nitrate in soil water (unsaturated condition) declined from 6-8 mg/L to less than 0.1 mg/L in the accelerative denitrifying layer without an organic matter supplying layer, and an increase of inorganic carbon (IC) content from 35 mg/L to 50 mg/L was observed. An increase of IC indicates that the field soil originally contained a significant amount of water-soluble organic matter and biological mineralization of dissolved organic matter occurred in pore water. Therefore, a low permeable soil layer with a suitable organic content acts as a denitrification layer, and installation of such a layer under the ground could be an effective measure to preserve groundwater from nitrate impact.