Abstract
The focus of this investigation was to constrain the porewater chemistry from tracer and ion profiles in a 128 m thick clay-rich sequence of Quaternary sediments comprising the clay, silt and fine sand formation in the North China Plain. Porewater are much more saline (1.1-71.0 g/L) than the lower confined groundwaters (0.3-1.3g/L). Chloride profiles from aqueous extraction (AE) (ranging from 117 to 3260 mg/L) generally keep good consistent with that obtained from squeezing test (ST). However, the sulphate and fluoride concentrations by AE (ranging from 340 to 48,700 mg/L and from 8.8 to 144 mg/L, respectively) are far greater than that by ST. Dissolution of evaporites may be responsible for the high Cl/Br ratios greater than 1000, probably indicating recharge occurred during much wetter periods. The original stable isotopic and chemical compositions in the aquitard porewater have been modified by transpiration and evaporation processes concentrating Cl and causing isotopic enrichment during their formation periods. Dissolution of fluorite, hieratite, and cryolite caused by weathering behavior and ion-exchange reactions should be responsible for the high fluoride concentrations in the aquitard porewater. High sulphate levels found in aqueous samples may arise from pyrite oxidation during sampling, storage and/or porewater extraction procedure.
Highlights
Aquitards can play a profound role on groundwater flow and solute transport in adjacent aquifers
Dissolution of evaporites can be responsible for the high ratios greater than 1000, probably indicating recharge occurred during much wetter periods
Downward hydrodynamic gradient of groundwater caused by the present pumping conditions does not change the original distribution of hydrochemistry of aquitard porewater
Summary
Aquitards can play a profound role on groundwater flow and solute transport in adjacent aquifers. The main research aspects for understanding the complex physical and chemical behavior of clay-rich aquitard systems, include multi-environmental tracers and hydraulic data; residence times, transport mechanisms, (bio)geochemical processes, and origin of porewater and solutes in an aquitard system; chemical heterogeneity in diffusion‐dominated aquitards; timing of major climatic and geologic events; estimating geotechnical/hydraulic properties. Shallow saline groundwater is distributed in most of the central and coastal plain.
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