TEMPERATURE LOGGING TO DESCRIBE THE MOVEMENT OF SEWAGE-POLLUTED SURFACE-WATER INFILTRATING INTO A FRACTURED ROCK AQUIFER

In 1992-1993, temperature logs were used to study the movement of sewa ge-polluted surface water infiltrating into the fractured limestone of an experimental site located in the southeastern part of the Let Basi n (Southern France). The wells investigated were located on either sid e of a sewage-polluted stream and intersected water-bearing fractures characterised by large contrasts in hydraulic conductivity. From the r esults of temperature-depth profiles measured in four closely spaced w ells of 60 m depth (W7, W8, W10 and W16) during the period February 19 92-June 1993 and the findings of a previous and more extensive geother mal survey, we examined the spatial distribution and the temporal vari ability of ground-water temperature during periods influenced or not i nfluenced by percolating sewage-polluted water. Results of this therma l survey, which were in good agreement with those of a physico-chemica l and bacteriological survey simultaneously carried out at the site, p rovided a substantial amount of information on the distribution of con taminant how pathways. Well W8, which showed high fluctuating ground- water temperature anomalies, intersected a solution-enlarged part of a bedding joint which seemed to carry much of the sewage-polluted infil trating water. Ground water in this conductive opening also had a low physico-chemical and bacteriological 'stability' and the highest avera ge contaminant concentrations. In contrast, Wells W10, W16 and, to a l esser extent, Well W7 displayed only low ground-water temperature anom alies during periods influenced by percolating sewage-polluted water. Ground water circulating through the thin and rather closed fissures i ntersected by these wells was less sensitive to pollution, as it had a greater thermal, physico-chemical and bacteriological 'stability' and the lowest average contaminant concentrations. Thus, we suggest that in advance of more focused monitoring programmes, temperature-depth pr ofiles in wells could effectively be used to describe the effect of th e structural features of fractured limestone aquifers on the movement of infiltrating contaminants.