Abstracts
Résumé
L’alimentation des eaux souterraines procède entre autres par des apports d’eau d’infiltration à travers les différents horizons du sol qui séparent la surface du sol du toit de la nappe phréatique. Une étude a été réalisée au laboratoire sur le rôle de la charge d’eau introduite dans une colonne de sol de 1 m de hauteur située au toit de la nappe, dans le transfert à l’eau souterraine, des bactéries indicatrices de pollution de l’eau de boisson. Les charges d’eaux usées de 50 mL, 100 mL et 250 mL ont été appliquées. Les analyses des eaux avant et après percolation ont concerné les coliformes thermotolérants et streptocoques fécaux, pour les paramètres bactériologiques, et le pH, NH4+ et la conductivité électrique, pour les paramètres chimiques.
Les résultats révèlent une réduction du nombre de microorganismes dans les eaux qui ont percolé à travers la colonne du sol. Cette réduction est imputable à la rétention de ces cellules par la colonne de sol. À la charge de 50 mL d’eau appliquée au-dessus de la colonne du sol, cette réduction a été de l’ordre de 7 unités logarithmiques pour les coliformes thermotolérants, et de 6 pour les streptocoques fécaux. En appliquant la charge de 250 mL, la réduction a plutôt été de l’ordre de 6 unités logarithmiques pour les coliformes thermotolérants, et de 7 pour les streptocoques fécaux. Cette réduction de la concentration microbienne circulante observée dans les eaux percolées a été de l’ordre de 7 unités logarithmiques pour les deux groupes de bactéries lorsque la charge de 100 mL a été appliquée. À charge d’eau usée élevée, la colonne de sol semble ainsi retenir plus de streptocoques fécaux que de coliformes thermotolérants. Ce comportement de la colonne de sol semble s’inverser lorsque la charge d’eau appliquée est relativement faible. Les éléments chimiques sont également retenus par le sol. Les caractéristiques des eaux qui percolent évoluent dans le temps, montrant que la rétention des polluants des eaux d’infiltration par une colonne de sol est un processus dynamique.
Mots clés:
- coliformes thermotolérants,
- streptocoques fécaux,
- charge d’eau,
- colonne du sol,
- eau d’infiltration,
- nappe phréatique
Summary
Water percolation through different soil horizons is one of the main mechanisms contributing to the improvement of the microbial quality of ground water. These soil horizons separate the soil surface from the groundwater table. Wastewater often contains chemicals and microbial pollutants, generally at high concentrations. On the other hand, ground water constitutes a major natural resource in most regions of the world. The present study was carried to examine the transfer of bacterial pollutants to the ground water, with the objective of evaluating the influence of the rate at which wastewater percolates through a soil column overlying the groundwater table.
The soil column was 25 cm in diameter and one meter high. It was composed of two horizons of different heights with pH values that varied from 4.43 to 4.56. Wastewater percolation tests were carried out with volumes of 50 mL, 100 mL and 250 mL, which were introduced every 30 minutes for each experiment. Chemical analysis was performed for pH, NH4+ and electrical conductivity. Bacteriological analysis was also carried out for thermo-tolerant coliforms and faecal streptococci quantification. These analyses were first carried out on each wastewater sample before introduction into the soil column, and then again after their percolation through the soil column.
Results showed that the lapse of time needed to observe the first percolated water drop was longer for low water loads than for the higher water loading rates. On the other hand, the time necessary to collect an adequate volume of percolated water for analysis was shorter for low water loads than for the high water loads. The average volume of percolated water per hour was thus high at low water loads, and relatively low at high water loading rates.
A comparison of the microbial characteristics of the introduced wastewater and those of the percolated water showed that the bacterial load in the percolated water was lower. This reduction was due to bacterial retention by the soil column. At an applied load of 50 mL, this reduction was of the order of 7 log units for thermo-tolerant coliforms, and 6 log units for faecal streptococci. When water load of 250 mL was applied, the reduction was of the order of 6 log units for thermo-tolerant coliforms, and 7 log units for faecal streptococci. The reduction was in order of 7 log units for both bacterial groups at an applied wastewater loading of 100 mL. It thus appears that, at high applied wastewater loadings, the soil column retained faecal streptococci better than thermo-tolerant coliforms. The soil column behaviour was reversed when low wastewater loads were applied. It was also noted that for electrical conductivity, a reduction varying from 6,240 to 6,550 µS/cm was obtained in the water leaving the soil column. The concentration of ammonia decreased from 44‑50 mg/L at the entrance to the column to around 1 mg/L at the exit of the column. Average pH values of water percolated through the two soil horizons varied from 5.70 to 7.32, whereas pH values of water introduced into the soil column varied from 7.43 to 8.02. It thus appears that the pH of these two soil horizons strongly influenced the pH of the percolating water that would enter the underlying ground water. The chemical and bacteriological characteristics of percolating water exhibited temporal variations, showing that the retention of pollutants by the soil column was a dynamic process.
Key words:
- thermo-tolerant coliforms,
- faecal streptococci,
- water percolation,
- soil column,
- water loading,
- ground water
Appendices
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