DEVELOPMENT OF A METHOD FOR REMOVING NITRATE DIRECTLY FROM SOIL PERCOLATE BY INTRODUCING LOW-PERMEABLITY-LAYER AND SULFUR.

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The objective of this research is to develop a method for removing nitrate directly from soil percolate. We construct a low permeability layer (LPL) below the level where nitrogen is sufficiently nitrified. By "low permeability layer" we mean the soil layer whose saturated hydraulic conductivity is approximately equal to the infiltration rate of the site. As the water percolates in the soil, LPL becomes saturated and acts as a barrier to oxygen supply. In addition, we put electron donor for denitrification, such as sulfur, etc. below the LPL. Thus, below LPL, oxygen concentration will be suppressed and there exist electron donors, then denitrification will occur. Two soil columns of 0.15 m in diameter and 0.8 m in height are prepared. For the one column (LPL column) we set LPL whose saturated hydraulic conductivity is approximately 0.1 m d^<-1>(20℃) at depth from 0.16 to 0.32 m. Below the LPL, sulfur (electron donor) and calcium carbonate was added to the soil in a (mass) ratio of S : CaC0_3 : soil = 5 : 5 : 90, at depths from 0.32 to 0.48 m. For the other (control) column, LPL is not prepared but sulfur and calcium carbonate was added to the soil in the same ratio at the same depths. Potassium nitrate solution of 20 mg-N∫^1 was supplied from the top of the column continuously at several different rates.In a 3-month experiment, LPL column revealed apparent nitrate removal efficiency of 0.79, 0.84 and 0.94 for the water feed rates of 0.025, 0.05 and 0.1 m d^<-1>, respectively. Thus, we concluded this Low Permeability Layer Oxygen Barrier method really works.