Groundwater comprises 30% of global freshwater reserves and in arid and semi-arid regions can account for more than 80% of the total available. It is described as often being of high quality, easily accessible, and generally less affected by short-term climatic and hydrological changes.
An underground dam is a facility that enhances groundwater capacity through the construction of a watertight cut-off wall within an aquifer, effectively raising the groundwater level and can also prevent seawater intrusion in coastal areas.
The history of underground dams can be traced back over 2000 years, but it wasn’t until the early 1980s that the first ones were built using modern civil engineering techniques. Since then they can be found in countries such as Japan, Brazil, India, South Korea, Mexico, Bolivia, the US, and across Europe.
These dams have several advantages, including:
- High-quality water preservation.
- Prevention of parasite growth and malaria transmission.
- Greater dynamic stability compared to surface dams.
- Ability to recharge shallow groundwater through rainfall.
- Better performance than surface dams due to lower evaporation rates during dry seasons and their contribution to environmental preservation.
- Being an economic and efficient way of sustainably managing groundwater in semi-arid regions.
- Enabling the development of water resources in areas where the construction of surface dams is challenging due to geological conditions.
Unlike surface dams, land use around an underground dam remains unchanged after construction as it’s not submerged by stored water, plus underground dams are not at risk of failure due to natural or human-induced factors. Their effectiveness depends on rigorous location selection criteria that consider geology, soil depth and texture, slope, and drainage network, so as to effectively minimise location errors.
With surface water resources becoming under increasing pressure due to population growth and climate change, certain schools of thought believe groundwater offers a possible way forward for the future.
Underground dams in Brazil
Brazilian semi-arid regions face water challenges that can directly impact important agricultural activities. The municipality of Capoeiras, in Pernambuco, has great agricultural potential, but depends on sustainable water collection and storage strategies to overcome scarcity, ensuring local agriculture and livestock farming continues into the future.
With the above in mind, recent research was carried out to help identify potential areas for the construction of underground dams in Capoeiras. Individual maps were drawn up considering terrain, geology, soils, and hydrology, resulting in a general map of the region’s suitability, using geoprocessing, remote sensing, and open-source software.
Different levels of suitability were discovered for the study area: 10.29% of the territory has high potential for the implementation of underground dams, while 31.39% has moderate potential, requiring more detailed planning. The remaining 58.32% of the territory was classified as having low potential and was considered a restricted area.
Concerns of underground dams
Based on the success of an irrigation project that utilised two subsurface dams as water sources on Japan’s Miyako Island in 1993, ten additional subsurface dams have now been completed. Each of these was built under distinct geological and construction conditions, with engineers having to overcome various challenges during the construction process.
Core technologies have made such subterranean dams possible – including an integrated storage model for creating water utilisation plans, and the soil mixed wall method for constructing cut-off walls.
The integrated water storage model calculates the groundwater flow and water balance by visualising daily fluctuations in the groundwater level over a 30-year period. While the soil mixed wall method 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:
- 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.
- 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.
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
