Understanding nitrate contamination based on the relationship between changes in groundwater levels and changes in water quality with precipitation fluctuations.

There are growing concerns about nitrate contamination in Kumamoto City, where >700,000 people completely depend on groundwater as a source of drinking water. We found that some groundwater samples showed considerably different nitrate concentrations although their sampling locations were close to one another, and we speculated that this phenomenon was due to the differences in subsurface geological properties. In order to verify this hypothesis, we carried out temporally intensive long-term monitoring of the groundwater levels and water qualities at three of the closely related sampling wells, and the results revealed that the changes in water level and water quality were different at each well. The water level at well T1, where nitrate concentrations ranged from 12 to 26 mg N/L, showed a significantly sensitive and unique response to heavy rain, which indicated that the subsurface at this site might be highly permeable; this would have allowed for the influent water to easily reach the groundwater aquifer over a short period. However, wells T2 and T3, which were located within 0.6 and 1.9 km from well T1, respectively, had nitrate concentrations that were lower than that in well T1 (4.5-8.0 mg N/L) and showed only gradual responses to heavy rain. These observations suggest that the highly permeable subsurface properties in the vicinity of well T1 contributed to the more serious nitrate contamination in well T1 than those at wells T2 and T3. This study demonstrates the importance of temporally intensive, long-term monitoring for capturing changes in groundwater level and water quality with precipitation fluctuations, and we showed how this approach can lead to a better understanding of the nitrate contamination situation.