| Quite abundant nutrients dissolved in terrestrial fresh groundwater were discharging into the coastal sea via submarine groundwater discharge.The freshwater and seawater mixing caused by the density difference between terrestrial freshwater and seawater has an important influence on nutrients transport and reactions in the coastal aquifer.An upper saline plume that forms due to tide-induced seawater recirculation.A lower saltwater wedge that forms due to density difference between freshwater and seawater-driven seawater recirculation.Previous studies on the nutrient transport and reactions in the upper saline plume have been studied in depth,and that in the lower saltwater plume have been discussed little.In this paper a numerical variable-density groundwater flow and reactive transport model MARUN was utilized to simulate groundwater flow and reactive transport in a vertical two-dimensional section of a homogeneous coastal aquifer perpendicular to the coastline under the action of tides,considering aerobic respiration and denitrification.The aquifer hydraulic conductivity is set to 3×10-4m/s and the inland freshwater supply is set to 8 m2/d.We consider the semi-diurnal tide of simple harmonic motion,with an amplitude of 0.75m and an average seawater level of 30m.Specific analysis of the nutrient transport and reactions in the traditional saltwater wedge of an unconfined coastal aquifer is exhibited.A sensitivity analysis with respect to hydraulic conductivity and inland freshwater supply is performed.In this paper,the sensitivity analysis of aquifer hydraulic conductivity values are 3×10-5m/s,8×10-5m/s,3×10-4m/s,5×10-4m/s and 8×10-4m/s,and the inland freshwater supply values are 3 m2/d,5 m2/d,7 m2/d,8 m2/d and 9 m2/d.Results show that seawater-freshwater mixing in saltwater wedge is much stronger than that in upper saline plume.Nitrogen removal rate reaches the highest in the zone bounded by the salinity ratio?i.e.,the ratio of the groundwater salinity to the seawater salinity?contours of 0.1 and 0.5,and declining to both sides in the transverse direction of the salinity ratio contours.Dissolved oxygen in groundwater is quickly consumed by dissolved organic carbon,creating a good anaerobic environment in seawater-freshwater mixing zone.NO3is removed in large quantities in seawater-freshwater mixing zone prior to its discharge to the sea and its removal efficiency is 77.8%.The greater the hydraulic conductivity in aquifer,the more the lower seawater-freshwater interface moves landward,and the wider the range of the lower seawater-freshwater mixing zone?i.e.,the horizontal distance between the salinity ratio contours of 0.1 and0.9?,which increases the range of the denitrification and makesNO3<sub>more likely to be removed prior to its discharge to the sea.The greater the inland freshwater supply,the more the saltwater-freshwater interface moves to the sea,and the narrower the range of the seawater-freshwater mixing zone,which decreases the range of the denitrification and makesNO3<sub>more difficult to be removed prior to its discharge to the sea. |
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