Présentations Colloques

Targeted provisional session No 7.03**Groundwater samples were collected from several depths down to 50 m below soil surface to investigate vertical profiles of nitrate and hydrochemical characteristics of the experimental site. The experimental site is located in the Shimabara City, Nagasaki, Japan, where nitrate contamination in groundwater has occurred due to intensive agricultural activities. **The transition zone between dissolved ions was found between specific depths caused by differences in the permeability of soil layers. Though nitrate concentration decreased significantly in the transition zone groundwater, the entire soil depth exceeded permissible level (50 mg L-1) for drinking purposes. Comparing the temporal fluctuation of nitrate concentration above the transition zone with that of the below, distinct fluctuations were observed depending on sampling campaign. High rainfall amounts typically lead to initial decrease in nitrate concentration for the shallow groundwater. After some time, however, increase in nitrate concentration occurred due to leaching of accumulated nitrate in the soil matrix. This indicated that temporal fluctuation of nitrate concentration is mainly controlled by natural impact and occurring crop system. **Results of principal component analysis suggested that application of chemical fertilizers (ammonium sulfate, ammonium nitrate, and potassium chloride), dissolution of minerals (feldspar, calcite and dolomite), and ion exchange are the predominant factors resulting in the observed vertical groundwater chemistry. The relative magnitude between these three principal component scores changed across the transition zone. Below the transition zone, groundwater chemistry reflected application of ammonium nitrate and potassium chloride fertilizer and dissolution of albite and orthoclase.