| In this study,the Geoprobe(r) direct push system was employed to obtain 15 meters of continuous soil core samples from the deep unsaturated zone of the cropland in North China Plain,so as to analyze the nitrate concentration in the soil samples and evaluate the risk of groundwater contamination by chemical fertilizer in this area.The results showed that,in the tested soil samples of medium permeability,the nitrate could be leached into the depth of more than 15 meters below ground surface.The distribution of nitrate in the deep unsaturated zone in such a typical setting could be separate into three zones: medium concentration zone in the root absorption area,high concentration zone below the root absorption area and low concentration zone in the deep subsurface area.The nitrate concentration below the root absorption area was obviously higher than that in the root absorption area.Although the nitrate concentration in the deep subsurface area was low,its accumulation might be higher than that in the root zone,therefore,the underlying groundwater was highly threatened by contamination.Soil texture obviously impacted the distribution of nitrate concentration below the root absorption area;and the nitrate concentration distribution was positively correlated with the soil moisture content in the deep soil profile. |