Low head sodium phosphate and nitrate hyperfiltration through thin kaolinite and smectite layers — Application to engineered systems

Dilute sodium phosphate (35 ppm) and sodium nitrate (40–42 ppm) solutions were forced through thin kaolinite and smectite layers in five different experiments at heads ranging between 1.42 and 1.46 m in order to assess the potential of the clay to behave as a membrane. These heads were chosen to be the representative of heads in shallow, clay-lined retention ponds and shallow perched aquifers. Concentration increases within the experimental apparatus at the end of the experiments ranged between 1.25 and 1.55 for nitrate and 1.43 and 1.49 for phosphate times greater than the initial concentration and the calculated values of the reflection coefficient ranged between 0.58 to 0.84 for nitrate and from 0.76 to 0.88 for phosphate. Although further experimentation is necessary with a wider range of clay-rich material, these experiments suggest that some clay-rich engineered systems exposed to low hydraulic heads, such as clay-lined sewage lagoons, slurry walls, clay barriers, or perhaps even the clay-rich filter cake that lines rotary drilled boreholes, might exhibit membrane effects and in some cases, dissolved contaminants, such as nitrate and/or phosphate, may be concentrated to above regulatory limits.