Groundwater geochemistry of a nitrate-contaminated agricultural site

Groundwater samples were collected from several soil depths down to 50 m below soil surface to investigate vertical profiles of NO₃ ⁻ and hydrogeochemical characteristics of the experimental site. The experimental site is located in the Shimabara City, Nagasaki, Japan, where nitrate contamination in groundwater is severe due to intensive agricultural production. A transition zone regarding dissolved ions was found between specific depths caused by differences in the permeability of soil layers. Though NO₃ ⁻ concentration decreased significantly in the transition zone, the entire soil depth exceeded permissible levels (50 mg L⁻¹) for drinking purposes. Comparing the temporal NO₃ ⁻ fluctuation 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 NO₃ ⁻ concentration for the shallow groundwater. After some time, however, increase in NO₃ ⁻ concentration occurred due to leaching of accumulated NO₃ ⁻ in the soil matrix. This indicated that temporal NO₃ ⁻ fluctuation is mainly controlled by natural impact and occurring crop system. Results of principal component analysis suggested that application of chemical fertilizers [(NH₄)₂SO₄, NH₄NO₃, and KCl], dissolution of minerals (feldspar, calcite, and dolomite), and ion exchange are the predominant factors resulting in the observed vertical groundwater chemistry. The relative magnitude of these three principal component scores changed across the transition zone. Below the transition zone, groundwater geochemistry reflected application of NH₄NO₃ and KCl fertilizer and dissolution of albite and orthoclase.