**6. Conclusions**

The solute diffusion coefficient of coral sands is a key factor affecting fresh groundwater conservation, and knowledge of the freshwater dispersion pattern in coral sands will provide both the basic parameters and a theoretical basis for numerical simulation of the formation and evolution of freshwater aquifers under islands, as well as the conservation and utilization of these aquifers. Using coral sands collected from a reef in the South China Sea, a series of tests and theoretical analyses were conducted in this study, from which the following conclusions were drawn:


size was minimal with respect to the macroscopic scale. Alternatively, the importance of pore size for the macro scale grew with an increase in d10, and the diffusion coefficient also rose. Finally, when the pore size reached a critical value, the influence of pore size on the solute diffusion gradually reduced, and the diffusion coefficient also approached a stable state.


**Author Contributions:** Conceptualization, X.C., C.Z., M.H. and X.W.; Data curation, X.C. and H.L.; Formal analysis, X.C.; Funding acquisition, C.Z., M.H. and X.W.; Project administration, C.Z. and M.H.; Resources, X.W. and H.L.; Writing—original draft, X.C.; Writing—review and editing, C.Z.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant Nos. 41877271, 41372318, 41330642, and 41572304) and the Strategic Pilot Science and Technology Special Foundation of Chinese Academy of Sciences (Grant Nos. XDA13010300).

**Conflicts of Interest:** The authors declare no conflict of interest.
