The influence of evaporation and rainfall on supratidal groundwater dynamics and salinity structure in a sandy beach.

Evaporation has been recognized as a major driving force affecting coastal aquifer systems. However, its effects on subsurface flow and salinity structure have not been investigated in sufficient detail. This paper presents field measurements and numerical simulations of pore water flow and subsurface salinity structure in the supratidal zone of a sandy beach subjected to evaporation as well as rainfall. It was found that evaporation significantly increased pore water salinity, up to 85 g/L, within a shallow layer, approximately 10 cm below the beach surface. The induced density gradient generated salt fingers near the beach surface, which caused local groundwater circulation (i.e., fingering flow). However, unlike inland aquifers, the salt fingering was significantly diminished by tidal action that prompted the horizontal mixing of salt in the beach. The subsequent precipitation (e.g., rainfall) diluted the evaporation-induced high saline plume near the beach surface and drove the plume to migrate downward; the plume gradually dispersed and was diluted along the groundwater pathways. The simulation results indicated that evaporation as well as precipitation at the beach surface induced complex driving mechanisms for supratidal groundwater flow. Depending on the intensity at the beach surface, evaporation and rainfall significantly altered the pore water flow and associate solute transport processes in the supratidal zone of the beach. [ABSTRACT FROM AUTHOR]