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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 implications on biogeochemical processes within the intertidal zone. Key Points: A new evaporation boundary condition was proposed on seepage face with salt retained and accumulatedMultiple seepage‐faces disappear and multiple groundwater circulation cells develop due to evaporation in the intertidal zoneSeepage‐face evaporation significantly promotes groundwater and salt exchange within the intertidal zone [ABSTRACT FROM AUTHOR]

Large‐scale seepage‐faces occur on small‐slope tidal flats. All previous studies assume that the seepage‐face has only one single exit point. Here we show via numerical simulations of tidally‐influenced groundwater flow that, in a two‐dimensional vertical, homogeneous transect of a tidal flat with gentle beach slope of 1‰, multiple seepage‐faces may occur with at most four unsaturated beach surface segments which separate four seepage‐faces. Salinity‐variation induced density‐dependent flow leads to this complex phenomenon. While the seepage‐faces are the groundwater discharging zones on the beach surface, the unsaturated zones are the recharging zones. The whole aquifer beneath the tidal flat is almost occupied by seawater and forms a wall blocking the horizontal seaward discharge of inland fresh groundwater so that inland freshwater discharges mainly occur near the high tide mark. This is in great contrast with the traditional results that inland freshwater discharge occurs mainly near low tide mark. Plain Language Summary: Tidal flat with very gentle‐slopes distributes extensively around the world, for example, the coastline of Bohai, the Yellow Sea of China, and Willapa bay of USA. Due to small slopes, the intertidal zone can be as wide as several kilometers, leading to large‐scale seepage‐faces (saturated beach surface with groundwater efflux) during ebbing and low tides. All previous studies assume that the seepage‐face has only at most one segment, the upper end of which is the exit point of water table and the other end is the intersection point of seawater and beach surface. Here, we found that in a tidal flat with gentle beach slope of 1‰, multiple seepage‐faces may occur with at most 4 unsaturated beach surface segments which separate 4 seepage‐faces during low and rising tides. The locations of these seepage‐faces and adjacent unsaturated zones are quite dynamic due to the complex, dynamic density‐dependent groundwater flow in the aquifer beneath the tidal flat. Seepage‐faces are kind of complex and important boundary for coastal groundwater flow in order to accurately model the groundwater flow and solute transport in intertidal aquifers, the complex nonlinear boundary conditions on the seepage‐faces should be strictly and accurately implemented. This paper made such an attempt. Key Points: Tidal groundwater flow with strict and explicit seepage‐face boundary conditions were numerically simulatedMultiple seepage‐faces were first found to develop on gentle tidal flat with slope around 1‰Multiple seepage‐faces imply multiple local density‐dependent groundwater‐seawater re‐circulations [ABSTRACT FROM AUTHOR]