Evaluating subsurface dams for the sustainable use of groundwater in remote small islands

Islands rely on groundwater for their water resources. Groundwater overdraft lowers groundwater levels and causes seawater intrusion, which results in groundwater shortage and deterioration. Many remote small islands in the Yellow Sea of Korea suffer from water shortages. To supply 100 m³/d of groundwater to 197 island residents, a subsurface dam was constructed in Anma island (5.8 km²), Korea, from Sep. 2019 to Oct. 2020. This study assessed the effect of the subsurface dam on the groundwater sustainability of the small island. The Quaternary alluvium (0.48 km²), located in the central part of Anma island, is a main aquifer and extends to a thickness of about 40 m, which rests on the Cretaceous tuff. The water table lies approximately 5 m deep below ground and fluctuates within alluvial aquifers. The subsurface dam was installed through the alluvial layer to the top of the tuff to reduce groundwater discharge to sea and was built across the coastal alluvial aquifers. Aquifer tests found hydraulic conductivity for alluvium and tuff to be 3.910^-6 and 2.8410^-6 m/sec, respectively. The average water levels measured in Sep. 2019 from the five observation wells in alluvial aquifers increased in Sep. 2021 by 0.28 m (9%) after the dam construction, but the precipitation also increased by 18% in the same period. Therefore, the increase in water level could not account for enhanced groundwater storage due to the dam. Numerical simulations were carried out with annual precipitation of 1,100 mm and a recharge rate of 7.2% to evaluate the change in groundwater storage before and after the subsurface dam. The simulation results showed that the subsurface dam contributes to an increase of only 0.06 m in water level in alluvial aquifers. This represents an increase in groundwater storage of 5,760 m³ in the alluvial aquifer (considering the specific yield of 0.2), which amounts to 15% of the annual groundwater recharge of 38,016 m³ in alluvial aquifers. The additional increase in groundwater storage due to the subsurface dam could complement the scheduled groundwater development of 36,000 m³/y from the alluvial aquifers, particularly when the recharge declines due to drought. The simulation also indicated that regardless of the subsurface dam, seawater intrusion was found to be insignificant due to the small amount of groundwater pumping at 100 m³/d.