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Nitrate nitrogen pollution in groundwater can be understood from three viewpoints: generation sources, mobility, and elimination effects (natural purification effects) . Alluvial fans are often utilized for farmland where nitrate fertilizer (pollutant source) may be used. Nitrate pollutants readily spread in groundwater at alluvial fans where two controlling factors for pollutant mobility are high: permeability and hydraulic gradient of groundwater. Moreover, the denitrification process, which is one of the most important natural purification effects, is not expected to occur readily in alluvial fan groundwater, which is seldom under anoxic conditions and contains little organic matter. In numerous previous studies, groundwater at alluvial fans has been researched using four main methods: water quality monitoring, calculations based on pollutant load per unit activity of source, stable isotopic tracers, and computed simulation models. As a result, processes and reactions of nitrate nitrogen in groundwater can now be evaluated quantitatively and visualized. Of these, because an oxidation reaction of ammonium sulfate, which is one of the main sources of nitrate nitrogen, multiplies the effects on groundwater chemistry, it is noted that NO 3 -：SO 4 2molar ratios and δ 13 C DIC values are secondary indicators for identifying this process in terms of groundwater pollution.