Research On The Sources Of Freshwater Lens In Quaternary Aquifer Of Ningbo Basin

The Ningbo Basin is a faulted basin with the Jurassic-Cretaceous volcanic-sedimentary rocks serving as the basement basin.Since the Middle Pleistocene,a series of terrestrial and marine stratums have been accumulated and the process was complex.Due to the several times of large-scale transgressions in the Late Pleistocene and Holocene,most of the groundwater in the Ningbo Basin has been salinized.While,a part of the freshwater lens is retained in the alluvial aquifers of the Middle Pleistocene and Upper Pleistocene in the Ningbo Basin.Based on the full understanding and analysis of the geological conditions and hydrogeological conditions of the Ningbo Basin,this paper focuses on the nature of the concealed faults in the Quaternary and its relationship with the aquifer through data collection and analysis.A total of 25 deep confined water sampling points were designed.The samples were collected twice in November 2017 and May 2018.The samples were tested for isotopes(²H?¹⁸O?³H?¹⁴C?²²²Rn)and conventional water chemical ions.The source and evolution of deep confined water in the study area were studied by ion correlation analysis,ion proportional coefficient analysis,and isotope hydrogeology methods.The main research contents and results are as follows:?1?The sedimentary environment and its evolution process in the Quaternary in Ningbo Basin are summarized.The three-dimensional geological structure model of the Quaternary strata in the Ningbo Basin is established by using the rich borehole and profile data of the study area.According to the regional geological tectonic setting,the evolution of the geostress field,the characteristics of the concealed faults and the evolution of the groundwater dynamic field,the sources of the deep confined freshwater lens Ningbo Basin are inferred.Based on the previous understanding of it,it is proposed that there might be a new possible recharge source.It is believed that the confined aquifers retain part of the Pleistocene land-source freshwater,and at the same time there is modern water recharge.Modern atmospheric precipitation enters the outcrop area of the faults,and is driven by the hydraulic gradient from the mountainous area to the central area of the basin.After the deep confined water in the plain area is exploited,the fissure water in the faults moves upwards and the deep confined water is recharged.?2?The hydrochemical and isotopic test results show that the[Cl]/[Br]and Na/Cl ratio approach the standard seawater value with the increase of TDS,and the Cl^-concentration and?¹⁸O values are well correlated with TDS,indicating that the salinization of deep confined water is mainly caused by the mixing with sea water.The?²H and?¹⁸O values of the samples are significantly smaller than the modern atmospheric precipitation,deviating upward from the local atmospheric precipitation line.The age of many ¹⁴C samples is greater than 10ka.BP,indicating that the deep confined water retains much Pleistocene paleo-water components.Both?²H,?¹⁸O values and ¹⁴C ages indicate that there are recharge sources from other regions.The ³H concentration of most samples is greater than 1 TU,indicating that the deep confined water has been recharged by atmospheric precipitation after 1952.?3?Using the two compartment linear mixing model of ancient water-Holocene seawater,a reasonable explanation can be made for most water samples,but for a small number of water samples,the calculation results obtained from different indicators vary alot,indicating that there may be other sources.The river water in the mountain valley upstream of the study area was used as the end member 3 and the three compartment linear mixing model was used for recalculation.The results were in good agreement.The ²H,¹⁸O,¹⁴C,²²²Rn and TDS data showed high consistency.The ¹⁴C age of the water samples with large changes in ²²²Rn concentration tends to be young.Meanwhile,the ²H,¹⁸O and TDS values of some of these water samples also changed significantly,indicating that the groundwater at these sampling wells had a faster movement within half a year.These sampling wells are basically located near the concealed faults,indicating that the concealed faults are closely related to the fast movements of the groundwater.The above results basically confirm the speculation that the bedrock fissure water recharges the deep confined water.