It becomes an increasing concern that groundwater quality in exploited deep confined aquifer may deteriorate due to brackish water leakage from its overlying saline aquifer in Hebei Plain. However, the monitoring data show that the TDS does not significantly change in the exploited aquifer. Some physics or chemistry processes must have taken place in aquitards during brackish leakage. The semi-permeable membrane function of clay aquitard during the process of hyperfiltration（reverse osmosis） should be one of the most important processes. To confirm and test this hyperfiltration mechanism, a series of experiments were performed in which Na Cl solutions were hydraulically forced through different clay sampled from aquitard. The solution 7 g/L in NaC l was forced through at 20 °C by a fluid pressure of 0.5 kN. The results show that hyperfiltration indeed happens in caly aquitard. Semi-permeability is quantified by the reflection coefficient σ. The mean rejection coefficients（σ） for clay samples #1, #2 and #3 were estimated to be 0.063, 0.164 and 0.040, respectively. This behavior of clay was well explained with the theory of the diffuse double layer. The hyperfiltration effect is to the great extent responsible for the chemical process in the aquitard.