Designing an Optimal Environmental Policy for Omura Bay, Japan: A Simulation Study on Water Quality Improvements

This study aimed to explore the trade-offs between regional economic activity and environmental policy to explore economic approaches for reducing and managing pollutant discharge while maintaining a balance between socioeconomic activities and the marine environment. A linear programming simulation was conducted to model the interactions between socioeconomic activities, pollutant emissions, and reduction policies in the Omura Bay watershed. The model was designed to maximize Gross Regional Product (GRP), using inflow pollutant loads as a constraint. The simulation showed that a 12.7% reduction in 2015 pollutant loads is feasible under total load control. However, this level of reduction would cause a 14% decrease in watershed GRP. Further analysis revealed that reductions beyond 12.4% would significantly lower GRP and increase the cost of mitigation, making a 12.3% reduction the most realistic upper limit. The estimated cost of implementing countermeasures to manage pollutant inflow was JPY 6.7 billion, which would translate to a JPY 37.6 billion reduction in the cost to maintain current conditions and a JPY 26.7 billion reduction in the maximum reduction scenario (12.3%) with minimal economic impact. This analysis highlights the tradeoff between environmental protection and economic performance. A key innovation is the proposal of a “proper nutrient management” scenario, moving beyond uniform reductions to assess region-specific targets that consider ecological needs, such as those of the fishery industry. This approach emphasizes the importance of setting realistic and ecologically balanced reduction targets.