Study On Groundwater Discharge And Associated Pollutant Inputs In Tidal Riverine Wetlands

The biogenic element concentrations in coastal estuaries are strongly influenced not only by riverine inputs but also by submarine groundwater discharge?SGD?which has been identified as a vital process of land/ocean interactions in coastal zones.Quantitatively assessing SGD into estuaries can be helpful to understand biogeochemical processes and nutrient dynamics.Two transects monitoring groundwater dynamics perpendicular to the river banks were deployed in the upstream and downstream intertidal wetlands at Dan'ao River Estuary,Daya Bay.Samples of radium isotopes were collected along the tidal river.Groundwater-surface water exchanges and hydrochemical features are analyzed and compared.Based on the measurements of groundwater heads and salinities from two different depths in the shallow sediments and generalized Darcy's Law,the groundwater-surface water exchange rates and their characteristics under tidal pumping were determined.Simultaneously,the SGD in the tidal river estuary was quantified by the radium mass balance model.Utilizing the three-endmember mixing method,the mixing ratios of seawater,river water and groundwater in estuarine water are calculated.With the pollutant concentrations,nutrient and trace metal indexes,the pollutant fluxes through the sediment-surface water interface in the tidal river are determined.Its ecological effects on tidal riverine wetlands under eutrophication are also analyzed.A large amount(31.4 cm d^-1)of fresh groundwater discharged along the upstream transect in mangrove swamps,inhibiting salinity increasing in sediments.Relevant chemical reactions?dissimilatory nitrate reduction to ammonium,anaerobic ammonium oxidation,denitrificaiton,ammo?in its weak acid-neutral anoxia eutrophic environment contributed to the accumulation of DIN,DIP and DSi and mangrove growth.Along the downstream transect in bare tidal flat,groundwater and surface water exchanged little(SGD:1.6 cm d^-1)and respond insensitively to tidal level changes.Its alkalic oxidative environment is beneficial to the aquatic fauna.The dissolved inorganic nitrogen?DIN?,phosphorus?DIP?,and silicate?DSi?fluxes by estuarine SGD were assessed to be 8.6×10⁷,8.0×10⁶ and 2.0×10⁷ mmol d^-1.In general,the radium concentrations in brackish groundwater were higher than those in estuarine surface water by an order of magnitude.The radium concentrations in both estuarine surface water and groundwater decrease from the river mouth to the upper reach.We determined the mean water residence time?⁰.5 d?and SGD in the estuary using the simultaneous equations for ²²⁴Ra,²²³Ra and salinity mass balances and three-endmember mixing model.The SGD into the estuary was estimated to be approximately?1.99⁶.67?×10⁵ m³ d^-1 using mass balance method and?5.44⁶.33?×10⁴ m³ d^-1 using endmember mixing method.The average SGD is 2.46×10⁵ m³ d^-1,comparable to the mean river discharge rate 4.23×10⁵ m³ d^-1.The mean fluxes of SGD-derived nutrients were approximately 1.8×10⁸,2.7×10⁶ and 6.9×10⁷ mmol d^-1 for DIN,DIP and DSi,respectively.These nutrient fluxes through SGD were much higher than those from marine inputs and rivaled riverine ones in this estuary.The SGD and associated nutrient fluxes are quantitatively determined by two different methods,whose results are comparable to each other.It is found that besides the surface water carrying pollution to the tidal riverine waters,the coastal groundwater polluted by the urbanization and industrialization is also discharging a large amount of terrestrial pollutants.