CONTAMINATION OF COLLECTED SOIL-WATER SAMPLES BY THE DISSOLUTION OF THE MINERAL CONSTITUENTS OF POROUS PTFE CUPS

Sampling soil solution with porous cups is frequently used in pedological studies because this method is simple, convenient, and allows repeated sampling of waters in the same location. Ceramic cups are the most often used cups, but they affect the chemical composition of the soil solution collected. For a geochemical study, a maximum inertness of the porous material is required. For the last few years, efforts have been made to build porous P.T.F.E. (a tetrafluoroethylene polymer) cups to avoid chemical modification of the soil solution by samplers. In this paper, the chemical inertness of P.T.F.E. cups is checked after an acid pretreatment. Micromorphological observations show that the P.T.F.E. material is indeed intimately mixed with a silicated solid phase directly in contact with the solutions. The dissolution of this phase results in a release of Si, Ca, Mg, Na, and Fe. This release is increased by acid complexing agents such as found in natural waters issuing from acid soils. Therefore, these P.T.F.E. cups have to be used with great care.