Biogeochemistry Of Fe-S-As Systems And Seasonal Variation Of Arsenic Concentration In Shallow Aquifers Of The Jianghan Plain,Central China

The safety of drinking-groundwater supply is the research frontier of hydrogeology and environmental science.The endemic arsenic?As?poisoning in South and Southeastern Asia is the most severe environmental disaster in human history,and China is one of the most affected countries.It is a research hotspot to ascertain the spatial-temporal distribution of groundwater As and the underlying formation mechanisms,which is critical not only for the assessment of risks from As-exposure and sustainability of drinking groundwater resource,but also for the remediation strategies of high-As groundwater.It is generally recognized that multiple biogeochemical processes jointly control the natural enrichment of groundwater As,including degradation of sediment organic carbon?OC?,reductive dissolution of iron oxide minerals,and microbial activities.The variations of groundwater As concentrations at diverse time-scales have been reported worldwide,with the interaction between groundwater and surface water being a primary controlling factor under the monsoonal climate.During the groundwater recharge,the input of exogenous OC and oxidants?nitrate/sulfate etc.?,oscillation of redox conditions,and variations in microbiota jointly affect the temporal variability of groundwater As concentrations,whereas the underlying mechanisms are still in debate.Recently,the single mechanism of reductive dissolution/oxidative precipitation of iron oxide minerals is found to be insufficient to explain the observed variations in groundwater As concentration,more and more studies suggest the transformation of iron mineralogy is closely affiliated with the redox cycling of sulfur,in which the mobilization and attenuation of As is regulated by the coupled Fe-S redox cycling.However,due to the complex pathway of Fe-S-As transformations,the understanding of the biogeochemical processes regulating seasonal As variations under such a condition is limited.Besides,traditional hydrogeochemical monitoring pays more attention to identify the changes in groundwater chemical compositions,lacking the recognition about the co-evolution between groundwater microbial communities and hydrogeochemical characteristics.Therefore,it is urgent to reveal the response of groundwater As to coupled Fe-S biogeochemical processes under different time scales through combined methods of long-term field monitoring and microcosm incubation experiments at laboratory scale.The present study takes the Jianghan Plain?JHP?with drastic seasonal As variations as the study area,systematically clarified how microbiota relating to coupled Fe-S redox cycling regulated the seasonal As variations under fluctuating groundwater-level conditions based on long-term monitoring and microcosm incubation methods,and revealed the mechanisms controlling As mobilization under microbially mediated coupled Fe-S redox conditions through synchrotron X-ray absorption spectroscopy.The primary research progress is listed as follows:1.Based on high-resolution biogeochemical monitoring,the present study delineated the response of seasonal variations in microbial communities to the groundwater level fluctuations in shallow aquifers from the JHP,and revealed the co-evolution between the seasonal variations in iron-oxidizing/reducing bacteria and the iron redox cycling which is responsible for the seasonal As variations.The seasonal fluctuation of the groundwater level is a primary factor driving the dynamics in the suspended microbial community,which was identified through multiple statistic methods.High-throughput sequencing of 16S r RNA gene showed a good response of seasonal variations in microbial community compositions to the fluctuations of the groundwater level.During the monsoon season,the groundwater level raised with an enhancement of reducing conditions,meanwhile the order Desulfuromonadales capable of Fe?III?-reduction was predominant in the microbial community.During the non-monsoon season,the groundwater level decreased with an enhancement of oxidizing conditions,meanwhile the order Gallionellaceae capable of Fe?II?-oxidation was predominant in the microbial community.The variations in the relative abundance of microbial compositions relating to iron redox cycling were also corresponding to the seasonal variations in Fe?II?concentrations of groundwater.Seasonal variations in redox-sensitive components,As speciation,and microbial communities indicate that the microbially mediated reductive dissolution of As-bearing iron?hydr?oxides with desorption of As resulted in the increase of As during monsoon season,while the microbially mediated oxidative precipitation of iron?hydr?oxides scavenging dissolved As resulted in the decrease of As during the non-monsoon season.The seasonal variations in the relative abundance of OTUs clarified as Geobacter showed positive correlations with variations in Fe?II?concentrations,while the seasonal variations in the relative abundance of OTUs clarified as Sideroxydans showed negative correlations with variations in Fe?II?concentrations.These results further suggest that variations in the microbiota facilitated seasonal redox cycling of iron,which is responsible for the seasonal As variations under the fluctuation of the groundwater level.2.Based on combined methods of field monitoring of sulfate-reducing bacterial?SRB?community and laboratorial microcosm incubation of aquifer sediments,the present study systematically clarified the influence of microbial sulfate reduction on the seasonal As variations and the biogeochemical processes regulating As mobilization from sediments under sulfate-reduction conditions.Co-increases of Fe?II?,S?-II?,and As concentrations in groundwater were observed from the seasonal monitoring,indicating a concurrent occurrence of iron-reduction and sulfate-reduction during the seasonal increase of As.Positive correlations between the dsr B gene abundance and Fe?II?/As concentrations were also observed,which showed a good response to the seasonal fluctuations of the groundwater level.High-throughput sequencing of dsr B gene revealed the relative abundance of predominant SRB Desulfocapsa increased concurrently with Fe?II?concentrations in groundwater,suggesting the potential participation of SRB in the seasonal redox cycling of iron.The underlying mechanism could be attributed to the promotion of bacterially generated HS^-on the abiotic reduction of As-bearing iron?hydr?oxides and thus resulted in the seasonal increase of As in groundwater.This process was further evidenced by the sediment incubation,in which we observed significant As mobilization from the anaerobic incubation of aquifer sediments under bacterial sulfate reduction conditions.Instead of following the redox sequence of iron/sulfate/arsenic-reduction,the microbially mediated reduction of iron,sulfate,and arsenate occurred concurrently during the mobilization of As.The sequential extraction of Fe/As fractions in sediments suggest that both biotic and abiotic reductive dissolution of As-bearing iron?hydr?oxides in combination with arsenate reduction are responsible for the As mobilization.During the release of As from sediments,the increase of dsr B gene abundance and the relative abundance of SRB Desulfomicrobium in the microbial community also highlighted the influence of SRB on the transport and fate of As.In addition,thiolated-As speciation?H₃As S₄^-?was detected and showed a corresponding variation with the total As and As?III?concentrations.These results suggest microbial sulfate reduction can facilitate seasonal enrichment of As through both the abiotic reductive dissolution of As-bearing iron?hydr?oxides and the formation of thiolated-As species.3.Through the co-incubation of an isolated SRB strain JH-1?with the capability of iron-reduction?and artificial As-coprecipitated/adsorbed ferrihydrite under the stimulation of a gradient of sulfate concentrations,the present study revealed the mechanisms regulating As mobilization from the ferrihydrite under coupled Fe-S redox conditions.We further isolated a bacterial strain JH-1 with the capability of Fe?III?/sulfate reduction,and this strain was co-incubated with As-coprecipitated/adsorbed ferrihydrite under a gradient of sulfate?4/0.4/0.04/0 m M?.The results of synchrotron X-ray adsorption spectrum?XAS?of Fe/As and variations in aqueous Fe/S species indicated that the JH-1 significantly reduced ferrihydrite in low?0.4/0.04 m M?or no sulfate systems,in which the ferrihydrite transformed into magnetite under the catalyzation of aqueous Fe?II?.In the meantime,the adsorbed/coprecipitated As?V?was incorporated into newly formed magnetite,and it was not reduced into As?III?.In the 4 m M sulfate system,ferrihydrite was primarily reduced by bacterially generated HS^-instead of bacterially iron reduction.During the reductive dissolution of ferrihydrite,the transformation of ferrihydrite to magnetite was inhibited.Instead,lepidocrocite was the primary-secondary phase with a partial generation of mackinawite.In contrast to the low sulfate systems,the adsorbed/coprecipitated As?V?were partially reduced into As?III?under the BSR condition.The As?III?was preferentially retained on the surface of lepidocrocite instead of incorporation into iron sulfide minerals.These results are of great importance for understanding the transport of As in aquifers under the coupled Fe-S redox conditions:In the environments lacking of sulfate,JH-1 can serve as a dissimilatory iron reducer facilitating the transformation of ferrihydrite to magnetite,and inhibit the mobilization of As.In the environment with strong input of sulfate,JH-1 can reduce the adsorbed As?V?to ferrihydrite through generating HS^-,which decreases the absorbability of arsenic as a prerequisite for As mobilization in groundwater.In summary,the new findings of the present study are?1?the response of seasonal variations in the suspended microbial community to groundwater-level fluctuations in the JHP and co-evolution between microbial community and hydrogeochemical characteristics,and?2?the mechanisms regulating seasonal As variations under coupled Fe-S redox conditions.These results provided new insights into the influence of bacterial sulfate reduction on the mobilization and transport of As in aquifer systems,which is of great importance for understanding the biogeochemical processes controlling temporal As variations in alluvial-lacustrine aquifers under monsoonal climates.