Desalination of saline groundwater by a weakly permeable clay stratum: a case study in the North China Plain

Groundwater quality in overexploited and deep confined aquifers in the North China Plain may deteriorate due to saline groundwater leakage from the overlying saline aquifer. This study focuses on desalination mechanisms of saline groundwater through weakly permeable clay during infiltration. A hyperfiltration experiment was conducted for 212 days using an intact clay sample collected from the weakly permeable clay stratum to evaluate the desalination ability of the clay stratum. The results show that the weakly permeable clay similar to a semipermeable membrane can filter saline groundwater salt but the efficiency of this filtration was gradually weakened with the treatment of saline groundwater. Chemical osmosis during the experiment retarded the migration of the salt solution, but did not contribute to the desalination ability of the clay stratum. Ion exchange was an important mechanism of salt removal during infiltration. Especially in the later period of the experiment, the removal of Na⁺ was mainly the result of ion exchange with Ca²⁺ and Mg²⁺. Approximately, 30% of the Na⁺ removal in the experiment was caused by ion exchange.