PH DETERMINATION OF CLAY INTERSTITIAL WATER USING A FIBEROPTIC SENSOR

The disposal of high-level radioactive waste in an underground reposit ory involves prior scientific characterizations in order to determine the most appropriate geological formation as the host matrix. Basic in vestigations on the composition of the soil and its interstitial water are essential for the understanding of more complex phenomena such as transport and migration processes of radionuclides. Among the numerou s physical and chemical parameters to be considered, pH is of particul ar interest for the chemistry regulation of interstitial waters since it appears in most element speciation equilibria. Sampling techniques with postponed analysis raise the problems of pollution risks and poss ible modifications of the fluid characteristics by disturbances induce d when taking it off its natural medium. In the case of clay formation s, low water content and high interstitial pressure are additional con straints. These concerns led us to develop an appropriate analytical t ool for in situ measurements of pH. A fiber-optic pH sensor was tested in an experimental underground laboratory located in the Boom clay fo rmation at Mel, Belgium. The sensing tip (optode) was introduced via a stainless steel guideline into a chamber at the end of a 15 m long pi ezometer equipped with a stainless steel filter to let the interstitia l water in. The guideline was tightly closed in order to allow the pre ssure in the piezometer to get back to its original value, i.e. 17 bar s. The optode was composed of a semi-bead of resin impregnated with ph enol red and immobilized at the distal end of a 1 mm X 20 m optical fi ber. It was connected to an emitter-receiver apparatus (Optolec-H). Un der these conditions, the pH range of use is 7-10 with a maximum confi dence interval of 0.1 pH unit. Laboratory performances of the device a re given with a special focus upon the influence of pressure on the me asured signal. In situ results, applicability, limitations and further developments are described.