| Surface water-groundwater?SW-GW?interaction is the exchange of water and solute between surface water and groundwater in various spatial and temporal scales.It is an important and non-negligible component in the whole hydrologic cycle.Researching SW-GW interactions can provide valuable guidance for water resource,water quality,environmental health,etc.Jianghan Plain?JHP?is located in the middle stream of Yangtze River,and is an important base for marketable grain,cotton and oil production in China.In the plain,there are abundant surface water systems with dense river net,which have close hydraulic links with ground waters.Under the coupled influence from natural conditions and anthropogenic activities,JHP is facing a set of environmental problems,such as wetland degradation,water pollution,geogenic inferior groundwater,cold waterlogged paddy field,etc,which are all related to the SW-GW interactions.SW-GW interactions may be integral to understanding water quality and ecological processes in the JHP.However,the interaction characteristics and magnitude in JHP have not been well known.There is an extensive aquitard between surface water and groundwater in JHP,and similar hydrogeologic condition can be also found in other low-plain areas in the world.Existing studies on SW-GW interactions mostly concentrated on the exchange between streams and“directly-connected”aquifers composed of high-permeability sediments,and some other studies also focused on the interactions between streams and aquifers through a low-permeability streambed layer.However,there is little concern on the exchange between streams and aquifers through an extensive aquitard thus far.As a typical contaminant,high levels of ammonium was widely detected in surface waters and groundwaters of JHP.However,there are no detailed investigations of the sources and geochemical behavior of ammonium in surface water and shallow groundwater thus far in JHP.In the field of nitrogen pollution,Nitrate is often the focus in investigation of N pollution in natural waters.Recent studies however indicated the form of ammonium was more important in some regions.High ammonium contents in surface water and shallow groundwater is typically attributed to anthropogenic sources such as sewage from industrial emission or leakage of manure and fertilizer from agricultural activities.However,elevated ammonium levels of natural sources also have been identified in recent survey,which are still poorly documented.Thus it is becoming increasingly important and complex to identify ammonium sources and the processes controlling ammonium concentrations in hydrologic systems.Given the background above,this study takes JHP as the study area,SW-GW system and ammonium occurring in it as the study objects,and tries to solve two problems:?1?how surface water and aquifer interact through an extensive aquitard;?2?how the interaction pattern affects the fate of ammonium.This study aims to:?1?characterize and quantify the SW-GW interactions through multiple approaches,?2?find out the distribution,sources and geochemical behaviors of ammonium in the SW-GW system,and?3?assess the mass balance of ammonium in the SW-GW system.The main knowledge can be summarized as follows:1.Hydrochemistry,water isotope and tritium were jointly used to identify and initially characterize the SW-GW interactions in JHP.In surface water and different layers of groundwater,Cl-and SO42-were believed to be anthropogenic sources,whereas HCO3-,Ca2+and Mg2+were believed to be mainly from the dissolution of carbonate minerals.There was a gradual gradient for chemical compositions and Cl/Br mass ratios from surface waters,upper aquitard?0-18 m?,lower porous aquifer?19-100m?to the porous-fissure aquifer?>100 m?.The samples in upper aquitard were overlapped with surface water samples and some other were overlapped with those in lower porous aquifer,implying that the aquitard functioned as an intermediate pathway between surface waters and lower porous aquifer.Hydrogen and oxygen stable isotopic signatures suggested two main processes,near-surface evaporation and complex hydrologic exchange among surface waters and different layers of groundwater.The spatial distribution of tritium suggested that more active hydrologic exchange had occurred near the river in the interior plain.2.Hydrogeologic simulation and calculation coupled with tritium modeling were jointly applied to quantify the interactions between the surface waters and the aquifer through an extensive aquitard in the interior of JHP.The hydrogeologic simulation suggested the influencing lateral distance by the river was within 1000 m;vertical flow in the aquitard followed by lateral flow in the aquifer contributed significantly more?90%?than lateral bank storage in the aquitard and following infiltration to the aquifer head change near the river.The hydrogeologic calculation produced vertical fluxes with an order of 0.01 m/day both near and farther from the river,suggesting similar shorter-lived?semi-monthly?vertical fluxes occurred between the river and aquitard near the river,and between the surface end members and aquitard farther from the river.Tritium simulation based on the OTIS model produced an average ground-water residence time about 15 years near the river and a resulting vertical flux in the order of0.001 m/day.Another tritium model based on a dispersion model produced a vertical flux in the order of 0.0001 m/day away from the river coupled with an average residence time around 90 years.Those results suggested an order of magnitude difference for the longer-lived?decades?vertical fluxes between surface waters and the aquifer near and away from the river.3.N isotopes coupled with ancillary chemistry was used to identify ammonium in JHP.High levels of ammonium have been detected in both surface water and shallow groundwater of the JHP.Ammonium in the Tongshun River?up to 10.25 mg/L?showed a sharp accumulation in the upstream and gradual attenuation in the downstream.The?15N values of ammonium in the TSR were high and ranged narrowly from+12.5 to+15.4‰,suggesting an anthropogenic source that was septic effluent from industrial waste discharge.Sorption and nitrification were likely to respectively serve as the principal processes contributing to ammonium attenuation in different reaches of the downstream TSR.In shallow groundwater,high levels of ammonium?up to 14.10 mg/L?occurred in an reducing environment.The narrow?15N variation with low values?+2.3to+4.5‰?in lower aquifer suggested a natural source that was organic N mineralization.The?15N values in shallow aquitard exhibited a wide range from-1.8 to+9.4‰,owing to various sources.Two types of water in shallow aquitard could be identified:?1?type-1 water with relatively longer residence time was similar to those in the aquifer where ammonium was mainly sourced from organic N mineralization;?2?type-2 water with shorter residence time was jointly affected by surface input,chemical attenuation and mineralization of organic N.The aquitard prevents prompt ammonium exchange between the surface and aquifer,and shallower part of the aquitard provides a sufficient reacting time and an active reacting rate for ammonium removal.4.The effect of seasonal hydrologic changes on ammonium behavior was investigated in a field monitoring site of JHP.In the shallow sediments,the organic nitrogen was the dominant nitrogen species?generally over 90%?.The NH3-N contents show a large vertical variation?4.0 to 187.5 mg/kg?.The?15N values of the sediments range from-0.4 to+5.3‰,with most of them within+2.0 to+5.0‰.The NH3-N contents had good positive correlations with both TN contents and?15N values,indicating the ammonium contents in shallow sediments were mainly controlled by the original buried organic nitrogen.The water level monitoring of surface waters and groundwaters showed a seasonal recharge?in wetter months?and discharge?in drier months?.The redox conditions in 10-m piezometers were variable spatially?-160 to-40mv?and showed insignificant variations temporarily,whereas the redox state in 25-m piezometers were relatively consistent spatially?-160 to-100 mv?and were more reducing in wetter months.The ammonium levels were generally higher in 10-m piezometers and lower in 25-m piezometers in the wet season comparing to the dry season.The?15N values of ammonium in 10-m and 25-m piezometers both showed wide ranges?+0.1‰-+7.9‰and-2.8‰-+11.4‰?,but with most of them between+2.0‰and+6.0‰,indicating the ammonium in 10-m and 25-m piezometers were mainly from organic nitrogen mineralization.Two seasonal variation patterns about the ammonium levels and their?15N values in 10-m piezometers were found:?1?the simultaneous increase of ammonium levels and?15N values in the wet season,which was owing to the anthropogenic input;?2?the minor increase of ammonium levels coupled with minor decrease of?15N values in the wet season,which was attributed to enhanced organic N mineralization.One seasonal variation pattern in 25-m piezometers was discovered,i.e.,the decrease of ammonium levels coupled with the increase of?15N values in the wet season,which was likely to be resulted from anammox.5.A conservative tracer experiment along with the physical,chemical and biochemical characteristics of streambed sediments was used to characterize the hyporheic flow and assess the contribution of hyporheic zone to ammonium removal in a small-size stream with high levels of ammonium and a considerably thick benthic biolayer.The OTIS-P simulation based on the bromide concentrations in the stream resulted in a reach-scale average hyporheic depth,residence time and flux of 16.4 cm,111 minutes and 1.36 m/day.The observation from the tracer experiment showed a flow path-scale hyporheic depth,residence time and flux in the stream center of 6 cm,75minutes and 0.66 m/day.On the contrast,those values in the stream margin were 3 cm,100 minutes and 0.25 m/day.The reach-scale OTIS simulation produced an average ammonium removal constant of 8.5×10-6 s-1.The flow path-scale observation showed a spatially variable reaction constant,but the constant was higher in the top streambed sediment of the margin(2.9×10-5 s-1)comparing to the center(1.1×10-5 s-1).The resulting percent removal of ammonium in the top sediment of the stream margin?5.24%?was significantly higher than that in the center?0.72%?.The ammonium removal probably occurred by the ammonium oxidation in the hyporheic zone,which can be supported by the higher abundance of detected ammonium-oxidizing bacteria?Candidatus Nitrososphaera,Nitrospira?,and AOB and AOA in the top sediment of the stream margin comparing to the center.The ammonium removal in all locations were reaction-limited,along with the analysis of reaction significance,suggesting that the whole-stream significance of ammonium removal was controlled by the local biochemical conditions. |
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