11:30 AM - 11:45 AM
[HCG23-04] Impact assessment of nitrogen pollution sources for groundwater quality management on an uplifting limestone island: case study at Miyakojima Island, Okinawa, Japan
Water quantity and quality management is an important issue in islands. In Miyakojima City, which consists of six islands, the nitrate nitrogen (NO3-N) concentration in groundwater approached the drinking water quality standard in the 1980s and is still at a high level compared to other prefectures. For groundwater quality management, it is necessary to identify the dynamics of pollutants. In Miyakojima City, the NO3-N loading to the groundwater was estimated using the unit load method. However, the nitrogen (N) dynamics considering water flow and biological reactions were not evaluated. Therefore, in order to understand the N dynamics in Miyakojima City, this study aimed to assess the effects of N sources on groundwater quality through water quality observation and N balance estimation.
The study area was Miyakojima Island (165 km2), the largest island in Miyakojima City. Most of the island is covered by Ryukyu limestone with high porosity, and the bedrock is of impermeable mudstone. The island has been divided into 27 watersheds by geological conditions and five underground dams. Domestic and agricultural water for the city is pumped from the five watersheds on the island. The annual rainfall is approximately 2,000 mm. As the annual groundwater level is stable, the total annual inflow and outflow was assumed to be equal. In the water balance, the inflows were infiltration of rainfall, irrigation water and domestic wastewater, while the outflows were anthropogenic pumping and natural runoff (spring water). About half of the island is cultivated, and livestock farming is dominated by beef cattle. The rate of connection to the sewerage system was 16% in 2020. Groundwater quality, including NO3-N concentration and nitrate isotopes (δ15N andδ18O), was measured at springs and cave springs in 11 watersheds during 2022–2024. In the N budget, inputs were from soil, chemical fertilisers, livestock waste and domestic wastewater, and the loads were applied from values calculated by Miyakojima City (2021). N outputs were discharge via pumping and spring water, and denitrification.
The estimated water balance for the watersheds of domestic and agricultural water sources showed that the pumping was 2–73% of the total outflow. All underground dams on Miyako Island are for agricultural use, and N in groundwater could be returned to cultivated land as irrigation water. Water quality was observed to be 2.1–4.1 ppm for NO3-N concentration, 4.7–11.5‰ forδ15N and 1.2–2.9‰ for δ18O. Correlation analyses of these water quality results with information on the N sources, e.g., cultivated area, number of cattle and population in each watershed, were carried out. For impact of fertiliser, a correlation between the spring water quality and the cultivated area was found. In this study, spring water was sampled in thin Ryukyu limestone watersheds, where groundwater retention time is relatively short. Therefore, N from fertiliser might be reflected in the water quality of the spring water. For impact of livestock waste, no correlation was found between the water quality and the number of cattle. This result implies that either the livestock waste disposal is adequate or the load is significantly low. For impact of domestic wastewater, no correlation was found between the water quality and the population in a watershed with the sewage system. However, it was found the domestic wastewater could affect groundwater quality in other watersheds. Denitrification was excluded from the N balance equation due to the results of the water quality. As NO3-N concentration in groundwater have been decreasing or constant in recent years, it was expected that the total N input would be less than or equal to the total N output. However, the positive balances were showed in eight watersheds where livestock farming or sewerage system have developed. The results of this study suggest that the N loading, especially from livestock and domestic wastewater, have been overestimated.
The study area was Miyakojima Island (165 km2), the largest island in Miyakojima City. Most of the island is covered by Ryukyu limestone with high porosity, and the bedrock is of impermeable mudstone. The island has been divided into 27 watersheds by geological conditions and five underground dams. Domestic and agricultural water for the city is pumped from the five watersheds on the island. The annual rainfall is approximately 2,000 mm. As the annual groundwater level is stable, the total annual inflow and outflow was assumed to be equal. In the water balance, the inflows were infiltration of rainfall, irrigation water and domestic wastewater, while the outflows were anthropogenic pumping and natural runoff (spring water). About half of the island is cultivated, and livestock farming is dominated by beef cattle. The rate of connection to the sewerage system was 16% in 2020. Groundwater quality, including NO3-N concentration and nitrate isotopes (δ15N andδ18O), was measured at springs and cave springs in 11 watersheds during 2022–2024. In the N budget, inputs were from soil, chemical fertilisers, livestock waste and domestic wastewater, and the loads were applied from values calculated by Miyakojima City (2021). N outputs were discharge via pumping and spring water, and denitrification.
The estimated water balance for the watersheds of domestic and agricultural water sources showed that the pumping was 2–73% of the total outflow. All underground dams on Miyako Island are for agricultural use, and N in groundwater could be returned to cultivated land as irrigation water. Water quality was observed to be 2.1–4.1 ppm for NO3-N concentration, 4.7–11.5‰ forδ15N and 1.2–2.9‰ for δ18O. Correlation analyses of these water quality results with information on the N sources, e.g., cultivated area, number of cattle and population in each watershed, were carried out. For impact of fertiliser, a correlation between the spring water quality and the cultivated area was found. In this study, spring water was sampled in thin Ryukyu limestone watersheds, where groundwater retention time is relatively short. Therefore, N from fertiliser might be reflected in the water quality of the spring water. For impact of livestock waste, no correlation was found between the water quality and the number of cattle. This result implies that either the livestock waste disposal is adequate or the load is significantly low. For impact of domestic wastewater, no correlation was found between the water quality and the population in a watershed with the sewage system. However, it was found the domestic wastewater could affect groundwater quality in other watersheds. Denitrification was excluded from the N balance equation due to the results of the water quality. As NO3-N concentration in groundwater have been decreasing or constant in recent years, it was expected that the total N input would be less than or equal to the total N output. However, the positive balances were showed in eight watersheds where livestock farming or sewerage system have developed. The results of this study suggest that the N loading, especially from livestock and domestic wastewater, have been overestimated.