| Jianghan Plain(JHP)is not only an important base for agricultural and fisheries production in China,but also one of the densely populated areas.Groundwater is regarded as the significant resource for industrial and agricultural development,residential water and strategic reserve.However,with the development of urbanization and the intensification of human activities,coupled with the wide distribution of native inferior groundwater,the groundwater environment in JHP is deteriorating,which seriously restricts the coordinated development of ecological environment and social economy.Therefore,scientific understanding of the occurrence environment,circulation characteristics and evolution pattern of groundwater flow systems(GFSs)is of great significance for the rational utilization of groundwater resources and the protection of ecological environment.Based on the in-depth analysis for the sedimentary environment evolution of the Quaternary aquifer system in JHP,this study identified the distribution pattern of multi-level GFSs at the basin scale,expounded the circulation characteristics of local GFSs,and revealed the influence mechanism of the evolution pattern of GFSs since the LGM on the distribution and occurrence of arsenic-rich groundwater.The main understandings and conclusions can be summarized as follows:(1)The Quaternary sedimentary facies and paleoclimate evolution framework of the JHP were constructed by comprehensively analyzing the chronology,lithologic composition and particle size distribution characteristics of borehole sediments,combined with the pore water isotopes of aquitards and sediment geochemical indicators,so as to reveal the evolution characteristics for sedimentary environment of aquifer system in watershed scale.The middle and deep aquifer systems are dominated by the coarse-grained aquifer medium as a whole,and there is no continuous distribution of aquiclude.While the shallow aquifer system is dominated by stable aquitard medium.The closer to the hinterland of the plain,the sedimentary environment of the Quaternary aquifer system becomes more stable.The sedimentary environment of aquifer system in JHP is mainly controlled by hydrodynamic conditions driven by water potential difference(i.e.river transportation capacity).Since the last glacial maximum(LGM),the paleoclimate experienced several phased historical changes,which have affected and transformed the evolution pattern for sedimentary environment,presenting that the deep-incised landform controlled by ancient channel of the Yangtze River in the LGM period changed into the coarse-grained fluvial facies rapidly filled in the last deglaciation period(LDP),and then into the fine-grained lacustrine facies stably deposited in the Holocene warm period(HWP).(2)Based on the comprehensive analysis of hydrogeological conditions,hydrodynamic field,hydrogeochemistry,isotope information and age indicators,the distribution pattern of multi-level GFSs at watershed scale in JHP was identified.With the evolution of historical period,the sedimentary environment of the Quaternary aquifer system in JHP was gradually stable and presented a multi-level GFSs pattern.The boundary of circulation depth between local GFS and regional GFS decreases from approximately 70 m in the hilly area to 30?50 m in the hinterland of plain.Affected by the difference of recharge sources and terrain,the regional GFSs have obvious spatial zoning.Groundwater generally flowed from the west and northwest to the east,and finally flowed into the Han River and Yangtze River through the river networks.The groundwater ages became older with the increase of circulation depth and flow path migration.The groundwater in hilly areas is basically modern water,deep into the hinterland of the plain,and the ages of regional groundwater ranges from hundreds to6000 years.The multi-level GFSs in JHP seem to be spatially continuous,however,the ages in different levels of GFSs show unconformity distribution.There are obvious age characteristics of “cutting time” or “breaking time” in time scale,and some areas are also characterized by “piercing time”.(3)Based on the in-deep analysis of groundwater environmental characteristics,seasonal water level dynamic changes and groundwater CFCs age in typical sections,the distribution pattern and circulation characteristics of local GFSs were clarified.There are significant differences in the pattern of local GFSs in different regions of the JHP.The circulation depth of local GFSs in Hilly areas can reach the bottom of the basin(approximately 70 m),along with the direction of groundwater flow,the circulation depth gradually decreases(approximately 40 m),and the groundwater age increases and the renewal rate slows down.The dynamic variation characteristics of water level for surface water and local GFSs in the interfluve between Tongshun and Han River are mainly "hydrological type" and "hydraulic type",which are significantly affected by seasons and small-scale water conservancy projects,respectively.Under the significant control of rainfall and sluice,the local GFSs at different sections present different supplementary and discharge pattern in wet and dry seasons.The transmission speed of groundwater level is much faster than that of water quality.The local groundwater flow near the surface water has large circulation depth and rapid circulation rate,with the distance from the river,the local flow circulation depth gradually decreases.With the increase of depth,the groundwater age in the watershed area of the interfluve rapidly ages,indicating that there may be potential "stagnation points" of different level of GFSs in this area.(4)Combined with the distribution characteristics and occurrence environment of arsenic-rich groundwater,the evolution pattern of GFSs since LGM and its influence mechanism on the temporal and spatial distribution of arsenic-rich groundwater were revealed by numerical simulation.During the LDP,the river carried a large number of arsenic-rich sediments in the upper reaches and accumulated in the ancient channel,which laid the material source foundation for arsenic-rich groundwater.At this time,the middle GFSs began to develop,and the regional GFSs gradually declined in potential sink weakening period(stage II,13?9 ka B.P.),which drove the As in groundwater to converage to the potential discharge zone with slow flow rate.The stable clayey layer formed during the HWP created a good reduction environment for the underlying arsenic-rich minerals.At this time,the late local GFSs significantly developed in the shallow aquifer system in weak sink period(stage III,2 ka B.P.to the present).The abundant organic matter and microorganisms in lacustrine sediments were easy to enter the underlying aquifer through local circulation of GFSs,which promoted the reductive dissolution of arsenic-rich minerals and released a large amount of As.The evolution of groundwater flow pattern led to the formation of good storage conditions for arsenic-rich groundwater in the regional discharge area,which made it difficult to migrate.Controlled by the GFSs since the Holocene,the central concentration of arsenic-rich groundwater has been decreasing from 1000 ?g/L(10 ka B.P.)to 571 ?g/L(2 ka B.P.),and continually flowing to the Tongshun River area,which is consistent with the present distribution characteristics of arsenic-rich groundwater,indicating that the evolution process of groundwater flow has a significant impact on the distribution and occurrence of arsenic-rich groundwater in JHP. |
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