Geochemical Behaviors Of Uranium In Estuarine And Coastal Zone

The radioactive disequilibria characteristics and half-life periods of parent-daughter nuclides within a naturally occurring U-series decay chain can provide information in research about different timescales of the processes in material circulation,residence,transportation and sedimentation of aquatic environment,which greatly benefits our understanding of global biogeochemical cycles.Due to the uranium content in the enviroment works as a criterion for evaluating the degree of parent-daughter nuclides disequilibrium,the varing pattern of temporal and spatial distributions of uranium powerful allies in the quest to study the biogeochemical processes.Fundamentally,estuaries and coasts containning complex environmental factors are zones of mixing between terrigenous freshwater and saline ocean water,thereby influencing alterations within uranium geochemical behavior and fluxes into the ocean.Hence,understanding the controlling factors of uranium behavior in estuarine and coastal zone is critical to effectively untilizing U-series nuclides in tracing processes of aquatic environment.Therefore,natural concentrations of uranium and 234U/238U ratios are determined in water column of the ChangJiang Estuary(freshwater end member,river-seawater mixing zone and intertidal zone)and Laoye Lagoon to investigate the uranium transport phenomena across land-sea margins and reveal the effects of particle-water interaction,recirculation seawater discharge and redox environment on the uranium behavior.(1)Water samples were collected from January 2010 to December 2011 at freshwater end member of the Changjiang Estuary once a month to estimate dissolved uranium flux and demonstrate the controlling factors of dissolved uranium concentrations by combining the major ions parameters.The dissolved uranium concentrations range from 1.32 to 4.06 nmol/L,which follow the seasonal variation with high values occurring during the dry seasons and low values occurring during the flood seasons.A strong negative correlation was observed between dissolved uranium concentrations and discharges(r=-0.83,p<0.05).The 234U/238U activity ratios are 1.424± 0.007 in dry season and 1.374±0.007 in wet season.The strong positive correlations between the dissolved uranium and Na+,Mg2+,HCO3-ions indicate that the weathering of carbonate and evaporite together form the main sources of the uranium.The accumulation and transportation of dissolved uranium are controlled by the carbonate ligands in the dry season and phosphate ligands in the wet season.Sequential extraction experiments suggest the soils of the Huanghe Basin contained four times more leachable uranium compared to the Changjiang Basin,which is one of the important reasons for lower uranium concentrations in the Changjiang River in comparison to the Huanghe River.Annual dissolved uranium fluxes drived from the Changjiang River are(2.3 ± 0.2)×106 mol/yr accounting for 10%of the global riverine uranium flux,which is far more than the percentage of 2.4%contributed from the Changjiang River to the global riverine runoff.This study gives a small uncertainty 8.7%for the dissolved uranium flux of the Changjiang River.(2)Water samples were collected in the river-ocean mixing zone of the Changjiang Estuary during low(March 2012,March 2013 and January 2014)and flood(July 2011 and 2012,and August 2013)discharges of riverine input.The results show that the non-conservative behavior of dissolved uranium have seasonal and interannual variability.During July 2011 and 2012(the flood season),dissolved uranium fluctuate basically along the conservative mixing line but the 234U/238U ratios are elevated systematically relative to the conservative mixing curve.However,another cruise of the flood season,during August 2013,dissolved uranium removal extended across entire salinity gradient but the 234U/238U ratios are consistent with the conservative values.During low discharge periods,the measured dissolved uranium concentration points fall on or above the conservative mixing line,and 234U/238U activity ratios of the water column fall below the conservative mixing curve at low salinity region;the excess uranium is observed in the mid-high salinity region but the 234U/238U ratios are consistent with the conservative value of the water column;The excess dissovled uranium and suspended particle matter showed a significant positive correlation(r=0.95,p<0.01).Sequential extraction experiments of surface sediments indicate around 2.0 ?mol particulate uranium with low 234U/238U ratio is released from per kilogram particle into aqueous phase through the processes of desorption and calcium carbonate dissolution,which results in the phenomenon that the measured 234U/238U ratios of the water column are less than the conservative values.At the mid-high salinity region,the releasable 234U/238U ratios of the particles are close to the measured 234U/238U ratios in water column,which may result from frequent exchange of uranium between the particulate and aqueous phases.The release flux of particulate uranium was estimated as(1.2±0.4)× 105 mol/yr in the Changjiang Estuary.(3)Dissolved uranium was studied in overlying water(during atmospheric exposure),pore water vertical profile,and tidal creek water of the Dongtan intertidal flat in the Changjiang Estuary during October 2012 to October 2013 by combining with SO42-Cl-molar ratios and dissolved oxygen parameters.This study is an example about how sediment of the tidal flat during atmospheric exposure affects the dissolved uranium of the estuaries.Uranium excess was observed in the overlying water within a ?U range from 0.25 to 17 nmol/L.The increasing uranium excess with increasing SO42-/Cl-molar ratios is pronounced from low to high marshs indicating the uranium is released from surface sediment into the overlying water during sulfide oxidation process;except for the middle marsh in summer and autumn,the excess uranium level decreases and doesn't correlate with the 5O42-/Cl-molar ratios when the overlying water is more reducing.Further more,the pore water vertical profiles also show a uranium diffusive flux into the overlying water as evidenced by subsurface uranium maxia.During the ebb tide,the water column of the tidal creek shows uranium excess within a ?U range from 0.17 to 6.4 nmol/L,indicating that the ebb tidal water carrying the released uranium from sediment during the exposure period mixes into the water column of the Changjiang Estuary(4)The dissolved uranium concentrations and 234U/238U activity ratios was investigated in surface water,pore water,surface runoff and surrounding well water of Lao ye Lagoon during April 2014 to explore the controlling factors of the geochemical behavior of uranium in the coastal lagoons.Both of the dissolved uranium concentrations and 234U/238U ratios of terrigenous fresh waters(including riverine water and well water)are in the low level of global rivers and fresh groundwater,whose average values are 0.107 ±0.003 nmol/L and 1.050± 0.035,respectively.Without regard to low or high salinity of the groundwater,the 234U/238U ratios fall close to the conservative mixing curve of Lagoon water in union,which results in dissolved uranium shows excess but the 234U/238U activity ratios are still conservative in the water column of the Laoye Lagoon.A time series observation of a pore water profile on the bank suggests that the progress in which recirculation seawater with high uranium concentration discharges into the Lagoon through permeable layer of the sediments during ebb and low tide periods.A non-conservative uranium budget assessment suggests that recirculation seawater is the dominant source of the excess uranium(158 ±22 mol/d),and there are two major factors which controls the removal of uranium:(1)reducing environment(92±3 mol/d),(2)the tidal cycle(66±9 mol/d).In summary,the observations of the diagrams of dissolved uranium vs.salinity in both of the Changjiang Estuary and Laoye Lagoon exhibit uranium excess during terrigenous freshwater-saline ocean water mixing process.However,the measured 234U/238U ratios of the water column fall below the conservative mixing curve in the Changjiang Estuary,which reveal information on the excess uranium source of the releasable particulate uranium with low 234U/238U ratios;in contrast,the measured 234U/238U ratios of the water column of the Laoye Lagoon show conservative distribution as a function of salinity,which provide important evidence for the discharge of the recirculation seawater with enriched uranium.Hence,comparing with traditional method only using the dissolved uranium vs.salinity diagram,simultaneous observation of the dissolved uranium concentrations and 234U/238U ratios along salinity gradient better reveals the geochemical behavior of uranium in the estuarine and coastal zone.This study creatively utilize the sequential extraction experiment to investigate the sptial distribution of uranium contents and 234U/238U ratios in different mineral components of surface sediments,and provide evidences for the sources and environmental controlling factors of the excess uranium of water column.This study suggests the effect of the terrigenous sediment appears to be significant for the variation of the uranium behavior during estuarine mixing,while many other factors(e.g.recirculation seawater discharge)cause the variation of the uranium behavior in the coasts.This study improves our understanding about the controlling facters of geochemical behavior of uranium in the estuarine and coastal zone,in order to provide the essential database and references for application of radioactive disequilibria as tracers of material course and chronometers of environment.