Discussion
169
where qi is the volume rate of incoming groundwater. Since the salt remains in the soil after
evaporation of water from the surface, its concentration in the soil profile increases. A concentration
change in the salt content, AF, for the time period t\ - 1 2 , is
A F
= #
dF = C
# qj dt
where C is constant.
Na and Cl are two of the major inorganic ions which have higher concentrations in sewage than in
raw water because NaCl is a common article of diet and passes unchanged through the digestion and
waste water treatment system. Assuming its average content to be 130 mg/1 Na and 200 mg/1 Cl
(Winnipeg) and an average yearly evaporation (evapotranspiration in case of cultivated soil) from soil
(qi) 1000 mm (= 1000ß/sq m) the increase of salts A F on the surface of 1 sq m can be calculated by
qubstituting for ti - t 2 = 1 year as
^F = C q i
Thus, assuming that Na and Cl (and other mineral non-degradable components) from the sewage
will either remain in soil profile, or reach the groundwater and eventually again rise to the soil surface,
the yearly budget per 1 hectare for sodium will be 1300 kg/year and for chloride 2000 kg/year, not
considering other mineral components, such as sulphates, carbonates, calcium, magnesium, etc. This
increase should be considered as additional to the natural process of salinization, which takes place
continuously, as it was shown in Table 1, from groundwater only.
Returning of the sewage wastes underground in developing countries of arid zones can take place
practically in two ways:
1) The soil is constantly flooded and the saturated movement through the soil is prevalently
downward to groundwater. Although the molecular diffusion is negligible, a direct enrichment of
groundwater by mineral solids as well as non-degraded organic matter and bacteria will occur. This will
be similar in a case of a continuous discharge of sewage effluent directly into the groundwater via
distributing wells.
2) The soil is flooded only sporadically, an unsaturated flow of water takes place and salts and
other residual matter from the sewage will accumulate in soil with molecular diffusion playing an
important role. Na will replace Ca and Mg on the soil exchange complex and degrade the soil
properties. Eventually, the accumulated matter will reach groundwater by the following irrigation or
leaching action of heavy rain, and start the vicious circle again.
Thus, disposal of sewage into ground in arid zones can be even more detrimental than in temperate
regions, causing in addition to the hygienic hazards also the gradual salinization of both soil and
groundwater.
REFERENCES
BENETIN, L, and CERVENKOVA, T. (1966) Relation between evaporation and soil hydroconstants.
Proceed. Symposium on water in unsaturated zone, Wageningen.
BUHRING, P. (1960) Soils and soil conditions in Iraq. Baghdad.
DOERING, E.I., ROEVE, E.C. and STOCKINGER, K.R. (1964) Salt accumulation and salt
distribution as an indicator of corporation from fallen soils. Soil Sei. 97: 312-319.
MARSHALL, T.J., and GURR, CG. (1964) Movement of water and chlorides in relatively dry soil.
Soil Sei. 77: 147-152.
R.D. Pomeroy, USA.
The authors' conclusion that two waves of enterococci reach the test well is subject to doubt on
statistical grounds. The data plotted are "most probably numbers", and as such are imprecise. The
apparent fluctuations of the counts, as shown in Fig. 3, can be attributed to the inherent variability of
the results.
Reply
The data plotted in Fig. 3 are "most probable numbers" subject to variation. The fluctuations in
the count are of a higher order and therefore could not have been only because of statistical variations.
169
where qi is the volume rate of incoming groundwater. Since the salt remains in the soil after
evaporation of water from the surface, its concentration in the soil profile increases. A concentration
change in the salt content, AF, for the time period t\ - 1 2 , is
A F
= #
dF = C
# qj dt
where C is constant.
Na and Cl are two of the major inorganic ions which have higher concentrations in sewage than in
raw water because NaCl is a common article of diet and passes unchanged through the digestion and
waste water treatment system. Assuming its average content to be 130 mg/1 Na and 200 mg/1 Cl
(Winnipeg) and an average yearly evaporation (evapotranspiration in case of cultivated soil) from soil
(qi) 1000 mm (= 1000ß/sq m) the increase of salts A F on the surface of 1 sq m can be calculated by
qubstituting for ti - t 2 = 1 year as
^F = C q i
Thus, assuming that Na and Cl (and other mineral non-degradable components) from the sewage
will either remain in soil profile, or reach the groundwater and eventually again rise to the soil surface,
the yearly budget per 1 hectare for sodium will be 1300 kg/year and for chloride 2000 kg/year, not
considering other mineral components, such as sulphates, carbonates, calcium, magnesium, etc. This
increase should be considered as additional to the natural process of salinization, which takes place
continuously, as it was shown in Table 1, from groundwater only.
Returning of the sewage wastes underground in developing countries of arid zones can take place
practically in two ways:
1) The soil is constantly flooded and the saturated movement through the soil is prevalently
downward to groundwater. Although the molecular diffusion is negligible, a direct enrichment of
groundwater by mineral solids as well as non-degraded organic matter and bacteria will occur. This will
be similar in a case of a continuous discharge of sewage effluent directly into the groundwater via
distributing wells.
2) The soil is flooded only sporadically, an unsaturated flow of water takes place and salts and
other residual matter from the sewage will accumulate in soil with molecular diffusion playing an
important role. Na will replace Ca and Mg on the soil exchange complex and degrade the soil
properties. Eventually, the accumulated matter will reach groundwater by the following irrigation or
leaching action of heavy rain, and start the vicious circle again.
Thus, disposal of sewage into ground in arid zones can be even more detrimental than in temperate
regions, causing in addition to the hygienic hazards also the gradual salinization of both soil and
groundwater.
REFERENCES
BENETIN, L, and CERVENKOVA, T. (1966) Relation between evaporation and soil hydroconstants.
Proceed. Symposium on water in unsaturated zone, Wageningen.
BUHRING, P. (1960) Soils and soil conditions in Iraq. Baghdad.
DOERING, E.I., ROEVE, E.C. and STOCKINGER, K.R. (1964) Salt accumulation and salt
distribution as an indicator of corporation from fallen soils. Soil Sei. 97: 312-319.
MARSHALL, T.J., and GURR, CG. (1964) Movement of water and chlorides in relatively dry soil.
Soil Sei. 77: 147-152.
R.D. Pomeroy, USA.
The authors' conclusion that two waves of enterococci reach the test well is subject to doubt on
statistical grounds. The data plotted are "most probably numbers", and as such are imprecise. The
apparent fluctuations of the counts, as shown in Fig. 3, can be attributed to the inherent variability of
the results.
Reply
The data plotted in Fig. 3 are "most probable numbers" subject to variation. The fluctuations in
the count are of a higher order and therefore could not have been only because of statistical variations.