Transfer Of Solutes Across The Sediment-Water Interface In Estuaries

Estuaries are transition zones connecting rivers and the ocean.Estuarine sediments are a key reservoir of chemicals?e.g.,carbon,nutrients,trace metals?.Solute exchange between sediments and the overlying water column strongly modifies the compostion and concentrations of chemicals in estuaries,and eventually influences the fluxes to the ocean.The transfer processes between sediments and the water column can be categorized as pore water exchange?PEX?with scale lengths of "milli-meters to meters"and submarine groundwater discharge?SGD?with scale lengths of "meters to kilimeters".PEX and SGD represent the autochthonous and allochthonous sources of chemicals in estuaries,respectively.As such,accurate and quantitative assessment of PEX and SGD components is essential.However,due to the limitations of tranditional methods,the input from bottom sediments was generally ascribed to the transport by SGD in previous studies.The importance of PEX has not been adequately evaluated.The ²²⁴Ra/²²⁸Th disequilibrium approach is a unique method to quantify the benthic fluxes of solutes,as it circumvents the difficulty of imposing interference on the natural sediment system.By taking advantage of this method and constructing a full mass balance of ²²⁴Ra in the overlying water column,the relative importance of PEX vs.SGD can be evaluated.This will help to better understand the geochemical cycle of dissolved chemicals in estuaries.It also has clear significance for the water quality management.The objectives of this study are:?1?quantification of the relative importance of PEX vs.SGD in the delivery of dissolved chemicals into estuaries;?2?evaluation of the influence of PEX-driven benthic input on the geochemistry of solutes in estuaries.This study is based on the data collected conducted in the Jiulong River Estuary?JRE?and the Pearl River Estuary?PRE?.The main conclusions are:In the JRE,PEX fluxes of²²⁴Ra were highly variable,both temporally and spatially,and can change by 1-2 orders of magnitude.The strong correlation between ²²⁴Ra-based irrigation rate?FRa?and 234Th-based sediment mixing rate?FRa=?3.48±0.42?DB+?0.00±0.08?,R2=0.93,P<0.001?,highlighted irrigation as the predominant PEX process for solute transfer across the sediment-water interface.This strong correlation indicated that sediment mixing?or more specifically,bio-turbation?and irrigation are two inter-correlated processes.By constructing a full mass balance of water column ²²⁴Ra,we were allowed to conduct the first quantitative assessment of the relative importance of PEX vs.SGD in the delivery of solutes into an estuary.Total PEX flux of²²⁴Ra?in 1010 dpm d-1?into the JRE was estimated to be 22.3±3.0 and 33.7±5.5 during the winter and summer surveys,respectively.In comparison,total SGD flux of ²²⁴Ra?in 1010 dpm d-1?was 11.3±8.6 and 49.5±16.3 in the respective seasons.By multiplying the PEX fluxes of ²²⁴Ra by the ratio of the concentration gradients of component/²²⁴Ra at the sediment-water interface,the total PEX fluxes of dissolved inorganic carbon?DIC?and nutrients?NH4+,NO3-,and H4SiO4?into the JRE can be quantified.In the meantime,net export of DIC and nutrients via SGD were estimated by multiplying the SGD fluxes of ²²⁴Ra by the DIC?nutrients?/²²⁴Ra ratios in the SGD end-members around this area.The results revealed that PEX-driven fluxes of solutes rival net SGD input and river input into the estuary.An additional finding is that NO3-in the water column was effectively sequestered due to SGD.Overall,PEX,which should be regarded as a separate pathway for the autochthonous dissolved components,is a major mechanism for the input of solutes in estuaries.It significantly influences the biogeochemistry of DIC and nutrients in the water column,and must not be overlooked.In the PRE,significant deficit of ²²⁴Ra relative to ²²⁸Th was commonly observed in the upper 0-15 cm sediment in summer.The ²²⁴Ra fluxes increased downstream off Humen,and was consistent with the spatial distribution of excess ²²⁴Ra in the water column.This indicated that benthic input is a key source of ²²⁴Ra in the estuary.In contrast to the JRE,the ²²⁴Ra mass balance in the water column showed that PEX processes were the predominant mechanism for solute transport between sediments and the overlying water in the PRE.The total PEX fluxes of solutes were one order of magnitude higher than those transported by SGD.These results were consistent with those in the autumn.The total PEX fluxes of DIC and nutrients in the summer survey were<1/3 of those in the autumn.It was probably related to the less enhancement of irrigation during summer.The PEX fluxes of DIC corresponded to-5%of the riverine load in the summer and-30%in the autumn,respectively.The addition rate of DIC to estuarine water by PEX was comparable to or slightly higher than those induced by aerobic respiration or denitrification.The PEX flux of NH4+ in the summer was lower than that in the autumn,but they rivalled or overwhelmed the river input.In contrast to DIC and NH4+,NO3-in the water column was effectively sequestered into sediments by PEX processes at a lower or comparable rate of the river input.The PEX input of NH4+ can support>1/3 of the NH4+ consumption by nitrification,and were higher than the removal of NO3-.As such,nitrification of NH4+ in the water column can compesate the removal of NO3-.Overall,PEX did not apparently influence the distribution of DIC and nutrients in the PRE.However,the transport rate of PEX was comparable with or slightly lower than some biogeochemical processes?e.g.,aerobic respiration,nitrification?in the water column.As such,if PEX processes have not been considered,our understandings of the biogeochemical processes and rates of carbon and nutrients might have been biased.Strong correlations between dissolved ²²⁴Ra and DIC,nutrients,as well as trace metals?Ba,U,Mn,and Fe?in pore waters were commonly observed in the PRE.This likely reflects a close linkage of geochemical cycling of alkaline earth elements?e.g.,Ra and Ba?and redox sensitive elements?like U?to diagenetic reactions of manganese and iron oxides in sediments.This linkage makes it possible to quantify benthic fluxes of alkaline earth elements and redox sensitive metals using the ²²⁴Ra/²²⁸Th disequilibrium proxy in sediments.Benthic Ba flux based on ²²⁴Ra/²²⁸Th disequilibrium was found to vary from virtually nil to 320 ?mol m-2 d-1 within the PRE.The highest flux was identified at S=3.0-7.8 and could lead to an elevation of 54 nmol Ba l-1 in the water column,which well reproduces the Ba addition frequently observed in the low salinity zone of the estuary;Benthic fluxes of redox sensitive U ranged from-0.42?"-"denotes flux into sediment?to 1.3 ?mol m-2 d-1.This could only cause a change of-0.1 to 0.3 nmol U l-1 in the water column,which is very small compared to the U concentration in the PRE and that in the Northern South China Sea?13-14 nmol l-1?.Therefore,U in the water column of the PRE must behave conservatively during mixing.This prediction is consistent with historical measurements of water column U concentration in the PRE,which suggested a conservative mixing behavior;Large benthic fluxes of Mn and Fe were generally identified with the ²²⁴Ra/²²⁸Th disequilibrium method,which varied from virtually nil up to 97 mmol m-2 d-1,and from zero to 27 mmol m-2 d-1,respectively.It may explain the sharp addition of dissolved Mn at S=3-5.3 in an earlier observation.The estimates of Fe flux are 1-2 orders of magnitude higher than historical measurements based on the traditional incubation method in other coastal settings.Nonetheless,they are in excellent agreement with a simple consideration of highly reactive Fe mass balance in sediments.Overall,the results highlights the ²²⁴Ra/²²⁸Th disequilibrium approach as a reliable method to quantify benthic fluxes of trace metals.In the meantime,the role of sediments in estuarine geochemistry of trace metals needs to be re-evaluated.To sum up,PEX is a key pathway for the transfer of DIC,nutrients and trace metals between bottom sediments and the overlying water column.It represents the autochthonous source of the regenerated components of benthic respiration in sediments.The PEX fluxes rivals or overwhelms the net SGD input and river input,and must not be overlooked in the overall budget and the biogeochemistry of chemicals in estuaries and other coastal settings.Accurate quantification of PEX fluxes of solutes and quantitative assessment of the importance of PEX vs.SGD provide substantial understandings on the geochemistry of solutes and the impacts of estuarine exports on shelf waters.Moreover,it has clear significance for water quality management in these systems.