Underground Travel of Pollutants
161
Water was charged at the influent end of the tank over the gravel at a constant rate.
The constant head tank provided at the effluent end was calibrated to give fixed water
table levels. Sewage was fed into the central well.
Analysis of experimental data
The following assumptions were made:(i) Soil assumed to be isotropic and homogeneous.
(ii) Since the permeability of the media was the same throughout and the hydraulic
gradient constant, the seepage velocity 'w' will be nearly constant,
(iii) the density of the sewage was assumed to be the same as that of ground water,
(iv) the dispersion of contaminant was unrestricted,
(v) the adsorption capacities had been approximately satisfied initially.
The experimental values for oxygen consumed and MPN enterococcus/100 ml after
fixed intervals of time are given in Figs. 2 and 3. Average values are given for oxygen
consumed and MPN for computing by the method of least squares so as to fit the fourth
degree polynomial equations.
Oxygen consumed
Figs. 2(a) and 2(b) are more or less identical. The maximum values have been recorded
in the inner and outer wells after 12 and 24 hours respectively.
The rise in concentration with time reveals that the pollutant requires that time to
reach the wells after satisfying the adsorptive capacities of the media. It is suggested that
the peak value at inner wells is greater than at outer wells either because of the formation
of an organic mat or because some of the organic matter has been mineralised by the time
the pollutant travelled from the inner to outer well.
Fourth degree polynomial equation could be fitted to the data which nearly define the
curve and the trend of variation.
Enterococcus
The confirmed MPN values for various wells at different time intervals are given in
Figs. 3 (a and b).
The plots contain two peaks. The first peak is sharp and is higher than the second. This
may be because initially, before an organic mat is formed, there is little obstruction to the
travel of the bacteria and that they therefore travel fast. Where an organic mat develops
the bacterial movement is resisted and the numbers drop. But there is a limit to the
formation and existence of the organic mat. At a critical point it breaks and bacteria pass
through. At this stage the bacterial population rises once again and the second peak is
formed. The smaller value of this peak may be due to the reduction in bacterial numbers
because of lateral dispersion. This is borne out by the fact that the second peaks are
higher in laterally displaced wells.
The results of bacterial dispersion are comparable with that of oxygen consumed. The
first peak is almost identical in both cases, which clearly indicates the formation of an
organic mat and the bacterial break through.
A plot for N (bacterial numbers) and t (time) on ordinary graph paper show that a
fourth degree polynomial equation could be approximately and statistically its best fit. A
similar curve could be fitted and the coefficients be determined by least square data.
161
Water was charged at the influent end of the tank over the gravel at a constant rate.
The constant head tank provided at the effluent end was calibrated to give fixed water
table levels. Sewage was fed into the central well.
Analysis of experimental data
The following assumptions were made:(i) Soil assumed to be isotropic and homogeneous.
(ii) Since the permeability of the media was the same throughout and the hydraulic
gradient constant, the seepage velocity 'w' will be nearly constant,
(iii) the density of the sewage was assumed to be the same as that of ground water,
(iv) the dispersion of contaminant was unrestricted,
(v) the adsorption capacities had been approximately satisfied initially.
The experimental values for oxygen consumed and MPN enterococcus/100 ml after
fixed intervals of time are given in Figs. 2 and 3. Average values are given for oxygen
consumed and MPN for computing by the method of least squares so as to fit the fourth
degree polynomial equations.
Oxygen consumed
Figs. 2(a) and 2(b) are more or less identical. The maximum values have been recorded
in the inner and outer wells after 12 and 24 hours respectively.
The rise in concentration with time reveals that the pollutant requires that time to
reach the wells after satisfying the adsorptive capacities of the media. It is suggested that
the peak value at inner wells is greater than at outer wells either because of the formation
of an organic mat or because some of the organic matter has been mineralised by the time
the pollutant travelled from the inner to outer well.
Fourth degree polynomial equation could be fitted to the data which nearly define the
curve and the trend of variation.
Enterococcus
The confirmed MPN values for various wells at different time intervals are given in
Figs. 3 (a and b).
The plots contain two peaks. The first peak is sharp and is higher than the second. This
may be because initially, before an organic mat is formed, there is little obstruction to the
travel of the bacteria and that they therefore travel fast. Where an organic mat develops
the bacterial movement is resisted and the numbers drop. But there is a limit to the
formation and existence of the organic mat. At a critical point it breaks and bacteria pass
through. At this stage the bacterial population rises once again and the second peak is
formed. The smaller value of this peak may be due to the reduction in bacterial numbers
because of lateral dispersion. This is borne out by the fact that the second peaks are
higher in laterally displaced wells.
The results of bacterial dispersion are comparable with that of oxygen consumed. The
first peak is almost identical in both cases, which clearly indicates the formation of an
organic mat and the bacterial break through.
A plot for N (bacterial numbers) and t (time) on ordinary graph paper show that a
fourth degree polynomial equation could be approximately and statistically its best fit. A
similar curve could be fitted and the coefficients be determined by least square data.