Relevance of membrane properties of clay -rich geologic materials to water-related engineering issues

A number of laboratory studies have shown that clays exhibit membrane properties. However, the vast majority of these studies have been done using high compaction pressures, high hydraulic heads, and pure clays. Such high compaction pressures and high heads are not representative of shallow subsurface conditions encountered in engineering applications. Therefore, the goal of this research was to obtain experimental data more representative of situations encountered in the shallow subsurface. First a series of six hyperfiltration experiments were performed at 690 kPa compaction pressure and 90 kPa head gradient. The 690 kPa pressure is equivalent to pressures obtained by compaction equipment used to construct clay landfill liners and other engineering barriers and is also representative of the shallow subsurface. The six samples tested were mixtures of bentonite and glass beads. The glass beads represented fine-grained sand. Solute rejection ranged from zero for the 100% glass bead sample to a high of more than 40% for the 100% clay sample. A similar set of experiments were performed using a 345 kPa compaction pressure representative of light to medium duty compaction equipment and shallower subsurface conditions. These experiments exhibited solute rejection rates from 5.2% for the 12% clay sample to over 30% for the 100% clay sample. Additional experiments were performed to determine if surface water samples in Missouri experienced changes in water chemistry during filtering due to accumulation of suspended clay on the filter paper. No statistically significant alterations in chemistry were found during the testing of five Missouri rivers.