Faecal bacteria transfer through a soil column to the groundwater in an equatorial region: influence of the applied surface water load
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.