The Formation And Geochemical Evolution Of Porewater In The Coastal Clay-rich Aquitards

The clay-rich aquitards are the major component of the groundwater flow systems,and play an important role in the physical and chemical evolution process of groundwater in the aquifer.Due to the low permeability,aquitards act as natural protective barriers for underlying aquifers and also provide additional palaeohydrogeology information.Although,most of the previous studies have focused on the detailed investigation of the change and evolution of regional groundwater quality,etc.By contrast,most current studies on the clay-rich sediments focused on aeration zone and the shallow saturated zone,and few researches have been conducted on porewater in the deep aquitards due to their lower water content and porewater extraction difficulties.The successful description of porewater constituents and hydrochemical processes involved in coastal aquitards will benefit the palaeoenvironmental information recognition,the sustainable exploitation,evaluation,and protection of groundwater resources in aquifers.In this study,North Jiangsu coastal plain was chosen as a typical case.Through several sets of laboratory experiments and comparative analysis of multiple methods,the forecasting method of the hydraulic conductivity?K?and effective diffusion coefficients?D_e?for low-permeability materials were determined.The hydrogeochemical and stable isotope compositions of aquitard porewater samples were investigated to determine the origin,salinization,and hydrochemical evolution of water.On this foundation,together with the preexisting knowledge of the sedimentary depositional environments,the solute transport processes of porewater in the aquitards since the Late Pleistocene were constructed,and also provided insight into the timing of major geologic events.The conclusions and understandings are as follows:?1?The formation conditions of pore waters.This study examined three cores?Sheyang:SY1-250 m and SY2-120 m,and Lianyungang:SY3-350 m?located in north Jiangsu coastal plain,China,adjacent to the south Yellow Sea.Through the analysis of the total dissolved solids?TDS?of pore water,major ions(Cl^-,Na⁺,Ca²⁺,Mg²⁺,SO₄^2-and Br^-),and?¹⁸O and?²H values,the origin,salinization,and hydrochemical evolution of water were discussed.The TDS of porewater samples highly varing from 0.03 to 26.1 g/L were observed.The higher and heavier?¹⁸O and?²H values indicate that the source of Group 1 salinized porewater was the Holocene seawater.The Group 3 salinized samples were composed of saline and brackish porewater samples,which were probably related to the remnant palaeo-seawater of the Late Pleistocene.Group 2 porewater samples had freshwater characteristics and resulted from atmospheric precipitation in a cold climate.?2?Based on the mass-balance mixing model,the maximal seawater mixing proportion of Group 1 samples was estimated to be 82%,and most of the Group 3samples contained the Late Pleistence seawater less than 5%.?3?Water-rock interaction also accounted for the differences between the initial and current porewater solutions as a result of the long residence times of the water samples.Na/Cl and Ca/Cl molar ratios in porewater samples ranged wildly.The plots of?Ca vs.?Na and(Ca²⁺+Mg²⁺–HCO₃^-–SO₄^2-)vs.?Na⁺+K⁺–Cl^-?indicated that Na⁺adsorbed on clay surfaces and albitization were likely associated with the Group 1 and 3 salinized porewater constituents,as well as cation exchange between Ca²⁺and Na⁺.By contrast,cation exchange was more noticeable for Group 2 samples with low salinity.The dissolved Ca²⁺exchanges mainly for Na⁺,and weathering of silicate minerals and Ca-bearing plagioclase may provide only a minor contribution.?4?The laboratory hydraulic conductivity tester and the radial diffusion device on undisturbed samples were designed for the low permeability of clay-rich deposits and the parametres determination.The relationship between the estimated De values and porosities of clays verified the feasibility and correctness of the radial diffusion method.Laboratory,empirical formula and chemical methods were used to predict the vertical hydraulic conductivity in saturated clay-rich deposits,and their practicability was analyzed.According to the laboratory methods,testing results show that K of deposits in the borehole SY1 ranged from 2.61 E-08 to 9 E-12 m/s,and the D_e values belonged to 1E-10 m²/s orders of magnitude.?5?The conceptual model regarding evolution processes of Cl^-migration in the aquitard–aquifer system was made of three simalution phases:the last glacial maximum?around 25–15 ka BP?–Holocene?around 10 ka BP?,Holocene–AD 1 128,AD 1 128–present.The various initial and upper boundary conditions over geological time were implemented.the fitness between simulated and measured values suggested that diffusion was the dominant transport mechanism of Cl^-in the aquitard–aquifer system,and provided estimates of the onset time of the Holocene?about 10 ka BP?and the last glacial maximum?around 25–15 ka BP?.