Temperature logging to describe the movement of sewage-polluted surface water infiltrating into a fractured rock aquifer

Abstract

In 1992–1993, temperature logs were used to study the movement of sewage-polluted surface water infiltrating into the fractured limestone of an experimental site located in the southeastern part of the Lez Basin (Southern France). The wells investigated were located on either side of a sewage-polluted stream and intersected water-bearing fractures characterised by large contrasts in hydraulic conductivity. From the results of temperature-depth profiles measured in four closely spaced wells of 60 m depth (W7, W8, W 10 and W 16) during the period February 1992–June 1993 and the findings of a previous and more extensive geothermal survey, we examined the spatial distribution and the temporal variability of ground-water temperature during periods influenced or not influenced by percolating sewage-polluted water. Results of this thermal survey, which were in good agreement with those of a physico-chemical and bacteriological survey simultaneously carried out at the site, provided a substantial amount of information on the distribution of contaminant flow 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 infiltrating water. Ground water in this conductive opening also had a low physico-chemical and bacteriological ‘stability’ and the highest average contaminant concentrations. In contrast, Wells W10, W16 and, to a lesser extent, Well W7 displayed only low ground-water temperature anomalies during periods influenced by percolating sewage-polluted water. Ground water circulating through the thin and rather closed fissures intersected 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 profiles in wells could effectively be used to described the effect of the structural features of fractures limestone aquifers on the movement of infiltrating contaminants.