Abstract:

An aquifer that traps discontinuous and non-flowing air is called a quasi-saturated aquifer. The trapped air can reduce the permeability of the aquifer; however, the relationship between quasi-saturated hydraulic conductivity (K_q), trapped gas saturation (S_a-t), and displacement rate is not fully understood. In this study, a displacement experiment was conducted to illustrate the effect of displacement rate on S_a-t and K_q. S_a-t was found to generally decrease nonlinearly with increasing displacement rate and remained almost unchanged at the high or low displacement rate. This S-shaped curve can be mathematically expressed by a model similar to “van Genuchten equation”. Further, a linear negative correlation between K_q and S_a-t was found for sandy media. The two empirical models above imply that the displacement rate can be used to estimate K_q, which provides another approach for determining the K_q values. These hydraulic properties provide a basis for understanding quasi-saturated groundwater flow. In addition, the relationship between displacement rate and S_a-t showed that in experiments involving fully saturated porous media, the initial saturation rate should be greater than the initial capillary rise rate when measuring S_a-t to avoid air entrapment in the media.

Key words: trapped gas saturation, quasi-saturated hydraulic conductivity, displacement rate, quasi-saturated aquifer, sandy media