The Effects of Varying Saline Conditions and Water Levels on Groundwater Flow and Nutrient Transport

It is well understood that the flooding and ebbing of tides in salt marsh ecosystems affect groundwater flow and nutrient transport. However, the process surrounding this dynamic relationship between a salt marsh and its adjacent aquifer is not well understood. Salt marshes create an important buffer zone that moderates the transport of nutrients and pollutants between surface water and groundwater. The mystery of this exchange has to do with the circulation of water that is primarily generated by contrasting densities between fresh water and seawater. It is important to understand this exchange to assess the impacts of climate change on the ecosystems’ soil chemistry through increased precipitation and tidal fluctuations. Lenkopane et al. (2009) considered the effects of varying saline conditions and water table levels on the interaction between surface water and the underlying aquifer. They generated a model to simulate the impacts of varying tidal salinities and annual freshwater flushing events on an estuary’s groundwater circulation. In general, simulations with higher salinity values produced larger and more predominant recirculation zones due to the greater disparity between the densities of saline and fresh water.Acadia Tucker.
Lenkopane, M., Werner, A., Lockington, D., Ling, L. 2009. Influence of variable salinity conditions in a tidal creek on riparian groundwater flow and salinity dynamics. Journal of Hydrology 375, 536–54

Groundwater flow and salt transport were determined by running a solute transport model (FEFLOW) over a five-year period, to compute the changes in fluid density within the aquifer. The model parameters were tailored by the observations of the Sandy Creek Estuary and its adjacent aquifer located in Eastern Australia. Some of the parameters used in the model included: hydraulic gradient, soil porosity, seasonal flushing, saltwater intrusion, and sediment type. The model represents a two-dimensional, vertical cross section of a shallow unconfined aquifer (open to receive or discharge water from the land above) perpendicular to a rectangular salt marsh. Lenkopane et al. tested four different scenarios with their model: a non-tidal salt marsh with constant salinity, a tidal salt marsh with a constant salinity of 0.5 ppt, a tidal salt marsh with a range of varying salinities (0-1.0 ppt, 0.25- 1.0 ppt, 0.5-1.0 ppt, and 0.25-0.75 ppt), and a tidal salt marsh with periods of fresh water flushing (2 days to represent a single event, 90 days to represent river discharge, and 0 days to represent dry season conditions). The simulations ran until the model reached a steady state, usually taking at least 14 days.
The results between the non-tidal and tidal simulations revealed that the movement of the tides produced circulation cells of salt and other nutrients. The circulation cells are powered by the increase of groundwater flow velocity, due to the fluctuating tides and the changing densities of water beneath the bank of the estuary. These underground circulation cells occur when dense seawater sinks to the bottom of an aquifer and less dense freshwater floats to the surface where it picks up more salt, changing density and becoming heavy enough to sink again. This drives a circulation cycle that discharges water into the salt marsh bed and moves salt and nutrients through the soil.
The tidal salt marsh held at the constant salinity of 0.5 produced one circulation cell near the top of the aquifer, close to the salt marsh bank, and one cell deep in the aquifer up to 60 m inland. The circulation cells were more prominent and extensive for a constant salinity of 1.0 than with a fixed salinity of 0.5. In addition, the increased velocity of groundwater flow created a larger mixing zone of salt dispersion that resulted in a 8% reduction of total percent, by mass, of salt in the aquifer and a 47% increase in aquifer discharge into the estuary above compared to the non-tidal simulations.
When analyzing the differences created by varying salinities Lenkopane et al. found that simulations with higher salinities produced larger and more driving circulation cells and therefore greater discharge into the salt marsh. The percent of salt within the aquifer was the lowest for the salinity range of 0 to 1.0 and was 33% lower than the percent of salt at a constant salinity of 0.5. This suggests that the changes in salinity affects the retention time of salt ions in the soil. The soil salinity within the salt marsh was the lowest during incoming tides. The highest soil salinity occurred during discharge episodes from the aquifer because it was redistributing concentrated saltwater from the previous high tide.

The yearly cycling of a flushing event proved to significantly disrupt the movement of groundwater, altering flow patterns and circulation cells. Both of the circulation cells vanished under a 2-day freshwater flush. The rapid increase of freshwater recharge into the aquifer created a freshwater barrier or lens beneath the surface of the salt marsh. This interrupted the communication between the salt marsh and the aquifer for a short time. The lower circulation cell reemerged under the river discharge simulation but was seriously weakened. However, introducing a constant supply of freshwater recharge into the aquifer increased the amount of discharge from the aquifer into the salt marsh. The alteration of the circulation cells within the aquifer due to fresh water flushing could possibly disband contaminants trapped in the closed loop circulation cells and redistribute them into the surrounding soil. Therefore, with the expected increase of precipitation due to climate change, groundwater circulation loops could be ever more altered, increasing the chance of soil contamination in salt marsh communities despite environmental efforts to reduce pollution. 

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