Evaluation of the spatial distribution of submarine groundwater discharge in a small island scale using the ²²² Rn tracer method and comparative modeling

Submarine groundwater discharge (SGD) is an important source of freshwater, nutrients, and other chemicals to the coastal water, and has significant impacts and implications for the coastal environment and ecology. Here, we combined geochemical tracers and hydrologic modeling to investigate the spatial patterns and quantities of SGD and submarine fresh groundwater discharge (SFGD) at a small island of western Japan. The results reveal large spatial variability in SGD and SFGD, significant discharge in areas with steep topography, and much lower discharge from low-lying areas. Topographic influences are likely to be a major driver of spatial variability in SFGD. The ²²² Rn mass balance model and two-end-member mixing model were used to estimate the SGD and SFGD rates. The values were ranged from 5.43 to 25.4 cm·d ⁻¹ with an average value of 16.2 cm·d ⁻¹ , and 0.39 to 1.98 cm·d ⁻¹ with a mean value of 1.17 cm·d ⁻¹ , for SGD and SFGD, respectively. The total flux of SFGD was calculated to be 8.49 × 10 ⁶ m ³ ·yr ⁻¹ , with 4.37 × 10 ⁶ and 4.12 × 10 ⁶ m ³ ·yr ⁻¹ in the northern and southern regions of the island, respectively. The results were consistent with the values estimated by the topographic-based model. The ratio of the average annual SFGD to average annual precipitation (3.69 × 10 ⁷ m ³ ) reached up to 23%, and the magnitude of annual SFGD was found to be similar to the total discharge from nine large rivers. Since the island faces a risk of water shortage, the results of the study can provide a useful insight into developing appropriate groundwater management strategies in this island's habitats.