Quantifying Seepage‐Face Evaporation and Its Effects on Groundwater Flow and Solute Transport in Small‐Slope Tidal Flat

• Published in: Geophysical research letters 2024-06, Vol.51 (12), p.n/a
• Main Authors: Luo, Manhua, Wang, Tianwei, Geng, Xiaolong, Yu, Shengchao, Li, Hailong
• Format: Article
• Language: English
• Subjects: Accumulation; Aquifers; Boundary conditions; Circulation; Coastal aquifers; Density; Evaporation; Groundwater; Groundwater flow; Groundwater salinity; Intertidal environment; Intertidal zone; Low tide; Mathematical models; numerical modeling; Numerical models; Retention; Salinity; Salinity effects; Salts; Seawater; Seepage; seepage face; Slope; Slopes; Solute transport; Solutes; tidal flat; Tidal flats
• ISSN: 0094-8276; 1944-8007; 1944-8007
• DOI: 10.1029/2024GL109173

Large‐scale seepage faces occur on tidal flats with gentle slope, which are widely distributed worldwide. Evaporation on these seepage faces, leading to salt retention and accumulation, may significantly impact the density‐dependent groundwater flow beneath the tidal flats. However, due to nonlinear complexities of the groundwater flow and solute transport on seepage faces, explicit boundary conditions and numerical models to quantify these processes are lacking. In this study, we present both mathematical and numerical models to quantify these processes. Compared to the results of our previous study, this paper shows that seepage‐face evaporation can (a) significantly increase the groundwater salinity in the upper intertidal zone, and form multiple groundwater circulation cells in the intertidal zone, (b) cause the disappearance of multiple seepage‐faces and reduce the spatial extent of seepage faces notably, (c) and intensify the groundwater and salt exchange as well as the seawater‐groundwater circulation through the intertidal zone. Plain Language Summary Tidal flats with very gentle slopes are ubiquitously distributed around the world. Due to these minimal slopes, the intertidal zone can extend for several kilometers, leading to large‐scale seepage faces during ebb and low tides. Previous studies have shown that evaporation without seepage face causes salt retention and accumulation, which not only increases groundwater salinity significantly, but also alters the groundwater density and its subsurface pathways. However, explicit mathematical expression and associated numerical models to quantify the process of evaporation on seepage faces are lacking, due to the mathematically nonlinear complexities to deal with the density‐dependent groundwater flow and solute transport. Here, a new boundary condition considering evaporation on seepage faces as well as salt retention and accumulation was proposed. Compared with the seepage‐face boundary conditions of our previous study, we found that multiple seepage‐faces disappear and multiple groundwater circulation cells develop due to evaporation. Seepage‐face evaporation significantly increases the salinity in the upper intertidal zone, reduces the scale and existence of seepage faces, and promotes groundwater/salt exchange. This study systematically reveals the effects of seepage‐face with evaporation on the density‐dependent groundwater flows and salinity distributions in coastal aquifers, which have strong i