NUMERICAL STUDY ON GROUNDWATER POLLUTION MECHANISM IN THE TWO SHIRASU PLATEAUS IN THE SAME CATCHMENT AREA

Abstract

Groundwater pollution by nitrate due to agricultural activities has become serious environmental issues in the two shirasu plateaus of Kasanohara and Kanoyabaru of Osumi peninsula, Kagoshima prefecture. We developed a groundwater recharge model and examined the water budget characteristics. Furthermore, two-dimensional numerical simulations were carried out to evaluate the effects of countermeasures for the contaminated source. The results of water balance analysis showed that runoff is 10-15%, groundwater recharge is 65%, and evapotranspiration is 20-25% of precipitation in the catchment area. The numerical results indicate that it takes 50 years for Kasanohara and 80 years for Kanoyabaru to display the effects of countermeasure of 80% reduction in concentration of the source zone. The long-term numerical simulation suggested that detailed recharge rate fluctuation rather than constant recharge rate should be applied.