Salt Dynamics in Coastal Marshes: Formation of Hypersaline Zones

Abstract

Salt is a key solute in salt marshes and under the influence of evapotranspiration can accumulate to a high concentration level in the marsh soil and precipitate in the solid form to become a significant stressor for plants, affecting marsh plant productivity and ecological zonation. Numerical simulations of coupled pore water flow and salt transport were conducted to examine how spring-neap tides and evaporation combine to influence salt dynamics and distribution patterns in marshes. The salt pan formation was simulated with a sandy loam marsh soil subjected to a medium rate of potential evaporation. The critical condition for the salt pan formation was underpinned by hydraulic connection between the marsh surface and water table to sustain evaporation in the supratidal zone. Both low soil permeability and overly high potential evaporation were found to break the hydraulic connection. In this case, the surface soil salinity increased gradually over the intertidal zone to a maximum around the spring high tide mark followed by a rapid decrease to a lower constant level across the supratidal zone. This salinity distribution pattern has also been observed in the field. In both salt marshes with and without salt pans, excessive salt accumulated on the marsh surface due to evaporation was removed by tidally induced circulating flow and/or flow driven by density gradients associated with the accumulated salt. The salt dynamics and distribution patterns revealed here, especially the salt pan formation simulated for the first time, have important implications for studies of marsh plant growth and overall eco-functions.

Effects of large macropores on soil evaporation in salt marshes

“…Recently, intensive studies were conducted to examine surface water and groundwater interactions in creek-marsh systems subjected to tides (Adams, 1963;Cao et al, 2012;Moffett et al, 2012;Shen et al, 2018;Ursino et al, 2004;Wilson and Gardner, 2006;Xiao et al, 2017;Xin et al, 2013). They demonstrated a near-creek pore-water circulation, in which water infiltrates from the marsh platform during flood tides and seeps out of the creek bank and bottom during ebb tides (Cao et al, 2012;Wilson and Gardner, 2006;Xin et al, 2011).…”

A three-dimensional palaeohydrogeological reconstruction of the groundwater salinity distribution in the Nile Delta Aquifer

“…As an example of a study based on groundwater mechanics, Kashef (1983) used the Ghyben-Herzberg relationship to demonstrate that the seawater wedge reached far inland. This was further confirmed by the 2-D numerical model constructed by Sherif et al (1988), who concluded that the width of the dispersion zone may be considerable. Sefelnasr and Sherif (2014) showed the sensitivity of the area to sea-level rise with a 3-D numerical model, as the low topography of the Nile Delta allowed for a far-reaching land surface inundation that can be detrimental to freshwater volumes (Ketabchi et al, 2016;Kooi et al, 2000).…”

“…Hydrogeological research dealing with this area generally assumed that the connection between aquifer and sea is completely open (e.g. Kashef, 1983;Mabrouk et al, 2018;Sefelnasr and Sherif, 2014;Sherif et al, 1988), but there exists ample evidence against this assumption from both seismics (Abdel-Fattah, 2014;Abdel Aal et al, 2000;Samuel et al, 2003) as well as from the fact that offshore borelogs contain more clay than sands (Salem et al, 2016). These observations can be explained by the fact that the fraction of marine clays in deltas generally increases seawards (Nichols, 2009).…”

“…Instead, they used longitudinal dispersivities exceeding 70 m to simulate the large brackish zone in the groundwater system ( Fig. 1) (Sefelnasr and Sherif, 2014;Sherif et al, 1988). The hydrogeochemical studies, however, attribute this zone to a former, more landward extent of the seawater wedge during the Holocene transgression, since a large fraction of the brackish zone is presumably freshening (Geriesh et al, 2015;Laeven, 1991) and the extent of this brackish zone coincides with palaeo-shorelines (Kooi and Groen, 2003;Stanley and Warne, 1993) (Fig.…”

A three-dimensional palaeo-reconstruction of the groundwater salinity distribution in the Nile Delta Aquifer

“…7 and 8), after which they reach equilibrium conditions until the next time slice. This may seem very rapid, but experiments with smaller scale models which fully accounted for salt precipitation and dissolution, evapo-concentration, variable-density groundwater 5 flow and the unsaturated zone, showed that higher salinities ( > 200 g/L) are possible under evaporation rates similar to those in the Nile Delta (Shen et al, 2018;Zhang et al, 2015). Given that the Late-Pleistocene sabkhas did not migrate , we deem our T-model scenarios to be conservative.…”