Regulatory Mechanisms Of Riverbank Filtration(RBF) On Ammonium In Groundwater

With the population growth and the rapid development of industrial and agricultural activities,high levels of ammonium（NH₄-N）was widely detected in groundwater（GW）throughout the world.NH₄-N is usually enriched in GW of riverside aquifer,which is considered as one of the most important hotspots for nitrogen（N）cycle.The complexity of the dynamic distribution mechanism of ammonium in riverside aquifer is always a bottleneck for regional water pollution prevention and control.Riverbank filtration（RBF）is the major form involved in the interaction between river and GW,which has a strong and dynamic impact on riverside aquifer environment.Previous studies have reported that RBF is a more efficient purification process,but more scholars have proposed that exogenous substances inputted by RBF can directly or indirectly induce a rang of biodegradation and redox reactions,these processes further lead to the enrichment of harmful components in filtration water,ultimately posing a continuous threat to GW quality.In recent years,high levels of ammonium in riverside aquifer has been increasingly reported,indicating some reactions that induced the enrichment of ammonium must have occurred here.Nevertheless,the effects and mechanisms of action of RBF on the enrichment of ammonium in riverside aquifer have not been carried out so far.The Jianghan Plain（JHP）is located in the middle reaches of the Yangtze River.The plain is characterized by abundant surface water（SW）systems（rivers,lakes and ponds）,which have close hydraulic links with GW.Natural environment of JHP is also altered and destructed by anthropogenic activities.All these strongly affected the water flow pattern and hydrological cycle between river and riverside aquifer,which controlled the occurrence and fate of ammonium in GW.Furthermore,previous surveys revealed that there were high ammonium concentrations of GW in the JHP,especially in the regions of riverside aquifer.Hence,in the JHP,it is a typical zone to study the spatio-temporal variations and mobilizing mechanisms of ammonium in GW under the control of RBF.Given the background above,this study chooses the JHP as the study area,and tries to solve the key scientific problem:how RBF affects the spatio-temporal variations and mobilizing mechanism of ammonium in GW.Based on the regional surveys,hydrological and biogeochemical monitoring,batch-incubation and RBF simulation experiments,four related studies were conducted in this work:（1）identified the distribution and control factors of groundwater ammonium hotspots under the control of RBF,（2）recognized the characteristics of nitrogen buried in water-bearing medium and the potential of the in-situ release of ammonium in the typical monitoring field,（3）revealed the characteristics and the underlying control mechanisms for temporal variations of ammonium in GW and（4）explored the effects and mechanisms of action of RBF on the enrichment and mobilizing of ammonium in GW.The main understandings and conclusions can be summarized as follows:1.Ammonium,hydrochemistry,hydrogen and oxygen isotope and water level monitoring data were jointly used to characterize the typical RBF process of JHP and to identify the distribution,control factors and potential genesis of groundwater ammonium hotspots under the control of RBF.The river stages were generally higher than GW levels during the wet season（June-August）,indicating a continuing hydraulic gradient from river to GW（i.e.,GW Recharge）.There was a gradual gradient forδ¹⁸O and DO values from river to the riverside aquifer,further confirming the presence of strong natural RBF process at the point.The NH₄-N concentrations in rivers were relatively lower with the average value of 1.13 mg/L,but most GW samples in riverside aquifer had high NH₄-N concentrations（average,3.92 mg/L）,which increased with depth in vertical direction（peaking at a depth of approximately 20～30 m）.The distribution of groundwater ammonium hotspots showed a high degree of heterogeneity,which were located on both sides of the separate reaches of the Han River,DJR and TSR with the average NH₄-N value of 6.11 mg/L.The correlation analysis between NH₄-N concentrations and other variables of groundwater ammonium hotspots suggested that high NH₄-N concentrations were predominantly from geologic origin（natural organic matters-NOM mineralization）,which was controlled by organic matter-rich environment and local highly reducing condition.Based on the Random Forest regression model,the dissolved organic matters（DOC）values were recognized as the most dominating factor for controlling the distribution of groundwater ammonium hotspots（importance radio,58.50%）.Due to the presence of abundant clay minerals in the water-bearing medium,NH₄-N produced by mineralization in the shallow subsurface cannot be easily transported to the aquifer,indicating that the“NH₄-N inputs”hypothesis does not hold true.However,exogenous substances inputted by intensive RBF process could further induce the in-situ enrichment of NH₄-N in riverside aquifer through microenvironment controls.All these implied that the“NH₄-N release”hypothesis is the primary mechanism for the formation of groundwater ammonium hotspots in the JHP,with prevalent natural RBF process as a key external driving force.2.Lithologic properties,mineral compositions,exchangeable N concentrations and other geochemical indicators of water-bearing medium couple with N mineralization experiments were used to recognize the characteristics of nitrogen buried in water-bearing medium,and to quantify the mineralization capacity and the potential for in-situ NH₄-N release of different water-bearing medium.In the water-bearing medium of the monitoring sites,the exchangeable ammonium（EX-NH₄-N）concentrations were overall lower（0.89～22.05 mg/kg）,and extremely high EX-NH₄-N concentrations were only present in the fine sediments with low clay minerals.While the organic nitrogen was the dominant nitrogen species of the buried N（generally over 90%）with a range from 176.44 to 1517.48 mg/kg,implying that the sufficient organic nitrogen could be used as a potential source of N for GW.The EX-NH₄-N concentrations had good positive correlations with the total organic carbon（TOC）and the total organic nitrogen（TON）,indicating the EX-NH₄-N concentrations in water-bearing medium were mainly controlled by the buried organic matters.N mineralization characteristics of water-bearing medium showed obvious distinctions,and all water-bearing medium were able to release a large amount of mineral N（dominated by NH₄-N）during N mineralization experiments,with the accumulative NH₄-N amount of release ranging from 17.38 to 59.79 mg/kg.The values of easily mineralized organic nitrogen（N₀）were particularly high in water-bearing medium of Maozui site,indicating an enormous capacity for mineralization here.In the typical cell body of Maozui site,the NH₄-N mineralized from TON in water-bearing medium would lead NH₄-N concentrations to exceed China drinking water standards（0.5 mg/L）in 2220-t GW.All these implied that NH₄-N released from water-bearing medium in situ was a non-negligible source for the enrichment of NH₄-N in GW.3.The monitoring of SW and GW levels,NH₄-N concentrations and hydrochemistry coupled with a set of incubation experiments were applied to identify the water flow pattern of the monitoring sites and to characterize the characteristics and mechanisms for seasonal variation of NH₄-N in GW.The mostδ¹⁵N values of ammonium in GW ranged from 2 to 4‰,and the NH₄-N concentrations had significant positive correlations with all dissolved organic matters（DOM）components,indicating NH₄-N in GW was mainly from N mineralization and was co-regulated by multicomponent DOM.The monitoring of water levels and hydrochemistry revealed that there were different water flow patterns in the monitoring sites.Compared to GW in the“GW Discharge”（0.26～3.38 mg/L for DO and 2.10～6.56 mg/L for DOC）,the DO and DOC values of GW in the“GW Recharge”were higher,ranging between 0.26～4.98mg/L and 0.76～18.85 mg/L,respectively.It indicated the inputs of exogenous organic carbons（OC）and oxygen.Temporal NH₄-N variations in GW were correlated with changes in water flow patterns,with higher NH₄-N concentrations（up to 5.88 mg/L）corresponding to“GW Recharge”,and with lower NH₄-N concentrations（as low as0.05 mg/L）corresponding to“GW Discharge”.Batch sediment incubation experiments showed that different water-bearing medium had the potential ability to generate NH₄-N.NH₄-N concentrations released in the incubations with 10 m M glucose（0.63～2.90 mg/L）were more than 10 times higher than those in the incubations without glucose（0.01～0.24 mg/L）,indicating that exogenous OC（carbon sources）inputs could lead to abundant NH₄-N release from water-bearing medium to GW through N mineralization.In addition,oxic condition（DO inputs）could also promote N mineralization,further resulting in the rapid enrichment of NH₄-N.The RBF process determines whether the exogenous OC（carbon sources）or oxygen are inputted to the aquifer,and ultimately regulating the temporal variation of NH₄-N concentrations in GW:（1）during the period of“GW Recharge”,SW carried exogenous OC and oxygen into the subsurface,which enhanced the N mineralization to promote NH₄-N release and caused the NH₄-N concentrations to increase in GW;（2）during the period of“GW Discharge”,the pathway for supply of exogenous OC and oxygen was closed,which limited the N mineralization and caused the NH₄-N concentrations to decrease in GW.4.A soil column experiment with different types or concentrations of leachate was applied to analysis the migration and transformation processes for NH₄-N in different water-bearing medium and to explore the effects and mechanisms of action of RBF on the enrichment and mobilizing of ammonium in GW.When ultrapure water was used as the leachate,a range of concentrations of NH₄-N（up to 0.32～1.39 mg/L）was detected in all three water outlets with low NO₃-N and high Eh values.All these indicated that N mineralization was the main control process for the enrichment of NH₄-N in the soil column.This process was present in higher intensity under the oxic condition.When NH₄-N solution was used as the leachate,NH₄-N concentrations detected in three water outlets were all below the initial NH₄-N concentrations of leachate coupled with high concentrations of NO₃-N were detected and EX-NH₄-N concentrations in water-bearing medium also increased significantly,indicating that higher concentrations of NH₄-N in the leachate were consumed in the soil column and was too hard to reach the aquifer layer.The intensive ion-exchange reaction and dynamic“nitrification-denitrification”process were the main control process for the decay of NH₄-N in the soil column.When electron acceptors(NO₃^-and SO₄^2-)was used as the leachate,high concentrations of NH₄-N（up to 0.62～2.87 mg/L）was detected in all three water outlets,indicating that the inputted exogenous electron acceptors(NO₃^-and SO₄^2-)could engage in oxidation-reduction reactions in water-bearing medium of different layers,further promoting the degradation of reactive NOM（electron donors）and rapid NH₄-N release.This process was controlled by environmental factor（p H）and reactive NOM storage in water-bearing medium.When exogenous organic carbon（OC）solution（glucose,fulvic acid and humic acid）was used as the leachate,high concentrations of NH₄-N（up to0.88～2.23 mg/L）were detected rapidly in all three water outlets and there were continously and significantly increased especially in the layers of aquitard and aquifer.It indicated that the buried carbon sources in water-bearing medium were not enough to promote rapid NH₄-N release,but exogenous OC inputs could directly stimulate the rapid microbial growth that was associated with N mineralization and further enhance its reactant activity.All these significantly enhanced the intensity of N mineralization,ultimately resulting in the rapid enrichment of NH₄-N.Based on the findings from this study,we constructed a conceptual model for the enrichment and mobilizing of ammonium in GW under the control of RBF.Although the study area is characterized by agricultural activities,NH₄-N inputted by agricultural sources or produced by mineralization in the shallow subsurface（riverbed or aquitard sediments）were all consumed by the intensive ion-exchange reaction and dynamic“nitrification-denitrification”process,which was too hard to reach the aquifer.Nevertheless,under the control of RBF process,the inputted exogenous substances（DO,exogenous electron acceptors and OC）could significantly enhance the intensity of in-situ N mineralization of aquifer sediments,which was the underlying mechanism for the enrichment of NH₄-N in GW of study area.