| Underground construction
was applied to the first large scale subsurface dam in Japan, the Sunagawa Dam. This method has since evolved and been applied to saltwater intrusion prevention type subsurface dams, surface water- groundwater storage type subsurface dams, and deep subsurface dams of 70m or more. Recent research shared by Imaizumi Masayuki has highlighted what has been called one of the most concerning issues regarding the environmental impact of subsurface dams, and this is their effect on the NO3-N concentration in groundwater. In the early 1990s, when construction of the Sunagawa Dam began, the concentration of NO3-N in groundwater steadily increased in some areas of Miyako Island and the southern part of the main island of Okinawa, and were predicted to exceed environmental standards. As Masayuki explains, the increase in NO3-N concentration in groundwater in a subsurface dam basin consists of two components: (1)The NO3-N concentration component due to increases in load sources (chemical fertilisers, livestock waste treatment, and domestic water) that occur commonly throughout Miyako Island, including the subsurface dam basin.
(2)The NO3-N concentration component caused by changes in the natural groundwater flow due to the subsurface dam and repeated irrigation of groundwater from the reservoir area. It’s important to note, the author adds, that the increase in NO3-N concentration caused by component 1 occurs even in basins without subsurface dams, and can be reduced by reducing the load source. The increase in NO3-N concentration caused by component 2 is a phenomenon that occurs only in subsurface dam basins. As Masayuki explains, when a subsurface dam is constructed, the natural flow of groundwater is artificially altered, resulting in congestion of the groundwater flow below the full water level in the basin behind the dam. Furthermore, if groundwater pumped from the subsurface dam basin is repeatedly used to irrigate farmland in the same basin, part of the groundwater will be recycled through the same upland soil. This
recycling process repeatedly loads groundwater with nitrogen leached from farmland soil, leading to an increase in the NO3-N concentration in stagnant groundwater. To stop such an increase it is necessary to adjust the height of the cut-off wall crest and land use above the dam reservoir.
Two components
When assessing the impact of a subsurface dam on NO3-N concentration, it is important to clearly distinguish between the two components above. As some literature, Masayuki says, does not mention this prerequisite and attributes the increase in NO3-N concentrations solely to dam construction. He went on to add that the results of an investigation using the TRAM (total readily available moisture) in irrigation engineering, confirmed that irrigation water below the TRAM does not affect groundwater quality. While results of an investigation using causal inference have statistically demonstrated subsurface dams do not affect the increase in NO3-N concentration. Indeed, subsurface dams further reduced the NO3-N concentration trend because large amounts of groundwater pumped from the reservoir refreshed groundwater below the full water level.
Above: Sub-surface dam on Miyako island, Okinawa, Japan © Khun Ta /
Shutterstock.com
References
A review of studies on underground dam site selection by Ali M. Rajabi, Shayan Alizadehnia, Abdollah Sohrabi. Physics and Chemistry of the Earth, Volume 140, October 2025, 103995.
https://doi.org/10.1016/j. pce.2025.103995Get rights and content
Geotechnologies in the identification of areas suitable for the construction of underground dams in the Brazilian semiarid region by Jacqueline Santos de Sousa, Gledson Luiz Pontes de Almeida, Héliton Pandorfi, José Amilton Santos Júnior, Abelardo Antônio de Assunção Montenegro, Géssica de Paula Alves Marinho, Adélia Maria Oliveira de Sousa, Marcos Vinícius da Silva. Journal of Hydrology Volume 662, Part B, December 2025, 134017.
https://doi.org/10.1016/j. jhydrol.2025.134017Get rights and content
Masayuki, I. Review of Subsurface Dam Technology Based on Japan’s Experience in the Ryukyu Arc. Water 2024, 16, 2282. https://
doi.org/ 10.3390/w16162282
Left: Japan’s Miyako Island where groundwater development and underground dam construction have been of interest since the 1970s
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