Discussion by Jan Barica
Department of the Environment, Canada
Utilization of the filtering ability of soils to purify water is a common practice for the recharge of
groundwater resources for drinking water supply in many countries in near-river catchment areas. It
should however be kept in mind that this method is successfully used for infiltration and consequent
purification of slightly polluted surface water only, with low levels of saprobity (usually not beyond
j3-mesosaprobic). In this case, the purification capacity of the filtering medium (usually highly
permeable suballuvial sandy deposits) is sufficient enough to remove inorganic and also some organic
pollution from the water and at the same time to regenerate its purifying capacity. Similarly,
pre-treated domestic sewage effluents are used for irrigation of nutrient-poor soils in various parts of
the world quite successfully, as long as the steady removal of nutrients and organic matter by plants
and microorganisms is adequate.
The above applications can lead to a temptation to take advantage of the purifying ability of soil
for a direct waste water treatment, as an inexpensive method applicable mostly in countries with low
density of population and vast uncultivated land resources available. The authors mention the possible
hazards of such oversimplification of the problem and the possible dangers in long-lasting
contamination of groundwater by organic pollutants. In order to prove this, they attempted to study
dispersion of organic matter as synthetic sewage on a model using isotropic and homogeneous soil of a
good permeability and texture. A system with fixed water table levels was used and the convective
dispersion equation for longitudinal as well as radial movement of the tracer applied. The similarity of
the results obtained by comparing the KMn04-oxygen values and the results of bacterial dispersion
and their approximate fittings to a fourth degree polynomial equation was shown.
In their consideration, the authors neglected the molecular diffusion constant. They might have
been justified in doing so in their particular study with constant water level conditions and flow rates.
However, in practice and particularly under semi-arid or arid climatic conditions, with intensive land
irrigation and resulting fluctuations of groundwater table and soil moisture content, the conditions can
be even more complex. Here, any application of water, even fresh irrigation water from unpolluted
surface sources, will inevitably lead to an increase in salt content of the soil or groundwater, or both -
depending on the climatic conditions and techniques of application. This phenomenon was described
by Marshall (1954): as water in the liquid phase moves upward through soil profile due to the capillary
action, the bulk of dissolved salts moves simultaneously with it; but when the water evaporates at the
soil surface, the salts remain in the soil. Rate of salt accumulation thus depends directly on the rate of
evaporation of water from the soil, on its flow rate to the surface and on the concentration of salts in
the rising capillary water. Degruyter (Buhring, 1960) estimated that fresh irrigation water adds 3
million tons of salt to the soils of Iraq per year (Na and Cl content of the Euphrates and Tigris varies
from 6-15 mg/1 Na and 9-20 mg/1 Cl).
This process, however, is not as straightforward as it seems to be. Even in the case where there is no
evaporation from the soil, i.e., under equilibrium conditions when capillary saturation is reached and
capillary movement stopped, the movement of salts in the soil profile can go on. This is caused by
diffusion due to the difference in the concentrations of salts in the groundwater and the soil water.
Only when the concentrations are equal will the molecular diffusion of salts eventually stop (Benetin
and Cervenkova, 1966). From this equilibrium a reverse process can start: if the water content in the
surface soil layer decreases, concentration of the salts in that layer will increase; and when the
concentration in the next lower soil layer is less, a diffusive transfer of salts downward will occur, even
against the flow of capillary water. If the flow rate is sufficiently high and concentration of salts in
water very low, the amount of salts moving downward against the water flow is negligible in
comparison to the amount of salts carried upward. However, if the water has high concentration of
dissolved solids - and sewage certainly does - the concentration of salts near the soil surface increases
rapidly and the resulting concentration gradient causes a significant amount of salts to move
downward. This phenomenon can be seen especially on bare soils of arid zones. Fig. 1 shows 1-year
changes in a non-irrigated saline soil (local name "sabakh") at Abu Ghraib, Iraq, with high
groundwater table. Salts accumulated in the soil profile come from saline groundwater only - changes
among single soil layers can be ascribed to present or potential molecular diffusion, which takes place
in both unsaturated and saturated zones.
Doering et al worked out a mathematical model of molecular diffusion. It is based on the diagram
shown in Fig. 2 showing a counter-current movement of salts and groundwater. Flow of groundwater
is in an upward direction. Analysis is based on a soil column of a unit area reaching from the
groundwater table to the surface, and concentration of salts in groundwater. The rate at which the
salts enter the column from below, dF/dt is described as
167
Department of the Environment, Canada
Utilization of the filtering ability of soils to purify water is a common practice for the recharge of
groundwater resources for drinking water supply in many countries in near-river catchment areas. It
should however be kept in mind that this method is successfully used for infiltration and consequent
purification of slightly polluted surface water only, with low levels of saprobity (usually not beyond
j3-mesosaprobic). In this case, the purification capacity of the filtering medium (usually highly
permeable suballuvial sandy deposits) is sufficient enough to remove inorganic and also some organic
pollution from the water and at the same time to regenerate its purifying capacity. Similarly,
pre-treated domestic sewage effluents are used for irrigation of nutrient-poor soils in various parts of
the world quite successfully, as long as the steady removal of nutrients and organic matter by plants
and microorganisms is adequate.
The above applications can lead to a temptation to take advantage of the purifying ability of soil
for a direct waste water treatment, as an inexpensive method applicable mostly in countries with low
density of population and vast uncultivated land resources available. The authors mention the possible
hazards of such oversimplification of the problem and the possible dangers in long-lasting
contamination of groundwater by organic pollutants. In order to prove this, they attempted to study
dispersion of organic matter as synthetic sewage on a model using isotropic and homogeneous soil of a
good permeability and texture. A system with fixed water table levels was used and the convective
dispersion equation for longitudinal as well as radial movement of the tracer applied. The similarity of
the results obtained by comparing the KMn04-oxygen values and the results of bacterial dispersion
and their approximate fittings to a fourth degree polynomial equation was shown.
In their consideration, the authors neglected the molecular diffusion constant. They might have
been justified in doing so in their particular study with constant water level conditions and flow rates.
However, in practice and particularly under semi-arid or arid climatic conditions, with intensive land
irrigation and resulting fluctuations of groundwater table and soil moisture content, the conditions can
be even more complex. Here, any application of water, even fresh irrigation water from unpolluted
surface sources, will inevitably lead to an increase in salt content of the soil or groundwater, or both -
depending on the climatic conditions and techniques of application. This phenomenon was described
by Marshall (1954): as water in the liquid phase moves upward through soil profile due to the capillary
action, the bulk of dissolved salts moves simultaneously with it; but when the water evaporates at the
soil surface, the salts remain in the soil. Rate of salt accumulation thus depends directly on the rate of
evaporation of water from the soil, on its flow rate to the surface and on the concentration of salts in
the rising capillary water. Degruyter (Buhring, 1960) estimated that fresh irrigation water adds 3
million tons of salt to the soils of Iraq per year (Na and Cl content of the Euphrates and Tigris varies
from 6-15 mg/1 Na and 9-20 mg/1 Cl).
This process, however, is not as straightforward as it seems to be. Even in the case where there is no
evaporation from the soil, i.e., under equilibrium conditions when capillary saturation is reached and
capillary movement stopped, the movement of salts in the soil profile can go on. This is caused by
diffusion due to the difference in the concentrations of salts in the groundwater and the soil water.
Only when the concentrations are equal will the molecular diffusion of salts eventually stop (Benetin
and Cervenkova, 1966). From this equilibrium a reverse process can start: if the water content in the
surface soil layer decreases, concentration of the salts in that layer will increase; and when the
concentration in the next lower soil layer is less, a diffusive transfer of salts downward will occur, even
against the flow of capillary water. If the flow rate is sufficiently high and concentration of salts in
water very low, the amount of salts moving downward against the water flow is negligible in
comparison to the amount of salts carried upward. However, if the water has high concentration of
dissolved solids - and sewage certainly does - the concentration of salts near the soil surface increases
rapidly and the resulting concentration gradient causes a significant amount of salts to move
downward. This phenomenon can be seen especially on bare soils of arid zones. Fig. 1 shows 1-year
changes in a non-irrigated saline soil (local name "sabakh") at Abu Ghraib, Iraq, with high
groundwater table. Salts accumulated in the soil profile come from saline groundwater only - changes
among single soil layers can be ascribed to present or potential molecular diffusion, which takes place
in both unsaturated and saturated zones.
Doering et al worked out a mathematical model of molecular diffusion. It is based on the diagram
shown in Fig. 2 showing a counter-current movement of salts and groundwater. Flow of groundwater
is in an upward direction. Analysis is based on a soil column of a unit area reaching from the
groundwater table to the surface, and concentration of salts in groundwater. The rate at which the
salts enter the column from below, dF/dt is described as
167