Situated at the confluence of terrestrial and marine environments along estuarine coasts,mangrove wetland ecosystems play a crucial role in regulating climate and ecological conditions.Submarine groundwater discharge(SGD)serves as the most significant hydrological process in coastal seawater-groundwater interactions,directly affecting mangrove growth and regional ecological health by controlling biogenic element cycling within mangrove wetland systems.Existing research on seawater-groundwater interactions in coastal aquifers primarily focuses on structurally homogeneous sandy tidal flats and theoretical numerical models.In comparison to sandy tidal flats,mangrove wetlands exhibit longer tidal flats,more gradual topography,and stronger aquifer heterogeneity,rendering the intertidal mixing of saline and fresh groundwater discharge,as well as biogenic element cycling,potentially more complex.Previous studies on the mixing of saline and fresh water in mangrove wetlands have been limited to areas within 3 meters of the intertidal zone and have hypothesized freshwater discharge in deeper aquifer layers.However,due to the lack of definitive observations on the saline-freshwater interface,current understanding of deep mixing processes and biogenic element cycling in mangrove wetlands remains limited.Based on this,the study selected Hainan’s Dongzhai Harbor as the research area,conducting periodic observations and constructing multi-level monitoring cross-sections in a typical mangrove wetland.The research revealed the mixing of saline and fresh water in mangrove wetland aquifers and the associated biogenic element cycling processes.Numerical models were employed to quantitatively investigate the impact of key factors on the mixing process in mangrove wetlands,as well as the migration,transformation,and flux of biogenic elements in hotspots and hot moments.The main conclusions drawn from the study are as follows:(1)A distinct saltwater-freshwater mixing zone exists within the aquifer of the Dongzhai Harbor mangrove wetland monitoring profile,with saltwater above and freshwater below.The shallow groundwater in the dense mangrove area is influenced by plant transpiration,resulting in salinity levels 2-10 ppt higher than seawater.The extension of the saltwater-freshwater interface coincides with the high-permeability sand layer,and there is no apparent tidal pattern in the overall spatial distribution of salinity and biogenic elements(C,N,S,Fe)in the groundwater.The average concentration ranges of DOC,ammonium nitrogen,nitrite,nitrate,sulfate,sulfide,ferrous iron,and total iron across multiple sampling periods are 0.3-1.3 mmol/L,0.7-26.1 mmol/L,0.1-0.3μmol/L,4.9-33.8μmol/L,0-26 mmol/L,0.4-576.8μmol/L,0-0.25μmol/L,and 0-3μmol/L,respectively.High molecular weight components account for an average of 47-73%of DOC.DOC,high molecular weight dissolved organic matter,and sulfides are mainly concentrated in the shallow areas of the dense mangrove zone,ammonium nitrogen is primarily distributed in the middle aquifer,nitrite and nitrate are mainly found in the shallow parts of the aquifer,sulfate spatial distribution aligns with salinity,and the depth of iron enrichment in groundwater is consistent with that in sediments.(2)Based on the analysis of the equivalent water-salt transport numerical model,the proportion of groundwater discharge is highest on the ocean side of the mangrove wetland monitoring profile(45%),followed by the mangrove area(33%)and the tidal channel closer to the inland(22%).Of the freshwater discharged from the inland,1%flows into the interior of the mangrove wetland,while 90%of the saltwater entering the mangrove wetland circulates within the shallow range of the mangrove root system,with only 10%entering the deeper aquifer.Combined with different case models,the low permeability medium(silty clay)in the wetland results in spatially independent saltwater circulation within the mangrove root system,controlled mainly by tidal fluctuations,topography,and surface pore development.The distribution of deep high-permeability sand layers and the permeability of low-permeability aquifers primarily affect the deep salinity distribution and the discharge of underground freshwater.When the vertical anisotropy of the deep silty clay layer is small,the high-permeability sand layer acts as a hydraulic barrier for salt migration to deeper layers,determining the depth of the saltwater-freshwater interface.When it is larger,the mangrove wetland may not exhibit a USP.Based on the average net outflow of the tidal channel on the monitoring profile and considering the area and tidal channel density of the Dongzhai Harbor mangrove wetland,the estimated groundwater discharge within the wetland is 1.2×10~4 m~3/d,which is 2.5 times the discharge in the surrounding tidal creeks.(3)The established reaction-transport numerical model provides a reasonable characterization of the cycling and discharge processes of carbon,nitrogen,sulfur,and iron.The model results demonstrate that groundwater discharge and various biogeochemical reactions during the mixing process of saline and freshwater lead to the enrichment of NH4~+and HCO3~-in the mixing zone,while the increased iron content in the sediments may also result in iron enrichment in the groundwater.Aerobic respiration,denitrification,nitrification,anaerobic/aerobic iron oxidation,and anaerobic/aerobic sulfide oxidation mainly occur in the silty clay layer,with reaction hotspots located in areas of relatively high groundwater flow velocity.Iron reduction hotspots are primarily in the silty clay layer,while sulfate reduction hotspots are at the interface between the silty clay and muddy clay layers.The rates of anaerobic iron oxidation and sulfide oxidation increase with rising mean tidal heights,while denitrification rate changes are different from nitrification rate changes on different time scales.The relationship between groundwater flow velocity and reaction rate determines the changes in the area of reaction hotspots.(4)Low molecular weight dissolved organic matter in the mangrove wetland aquifer primarily originates from the ocean and sedimentary organic matter degradation,accounting for 52.0%and 44.8%respectively,with the remaining 3.2%contributed by terrestrial groundwater input.Of this low molecular weight dissolved organic matter,71.2%participates in various microbial reactions,with the relatively fast denitrification process and aerobic respiration consuming 50.2%and 48.0%respectively.The calculated nitrate removal efficiency within the wetland is 59.3%.Considering the area of the Dongzhai Harbor mangrove wetland,the estimated annual DOC production is 2.37×10~6 mol,with the total amount of surface water nitrate removed being 1.24×10~6 mol.The results of the reaction-transport case model indicate that when the surface water is polluted with aquaculture wastewater rich in DOC,ammonium nitrogen,and nitrate,the mangrove wetland can still maintain a high nitrate removal rate,but the discharge of DOC and ammonium nitrogen may increase.The vertical high permeability brought by mangrove roots and benthic organism burrows is beneficial for the removal of nitrate from surface water.The distinctive and innovative aspects of this study include:identifying the salinity interface structure of the mangrove wetland aquifer,constructing an equivalent variable-density,variably-saturated reaction-transport numerical model based on the PFLOTRAN code,and for the first time incorporating different dissolved organic matter components(high molecular weight and low molecular weight)into the model.This approach quantitatively characterizes the migration and transformation characteristics of biogenic elements in the mangrove wetland aquifer,and reveals the influence of intertidal geomorphology,aquifer structure,tides,and inland water levels on the mixing process of fresh and saline water in the mangrove wetland aquifer. |