|Resistivity Meters & Imaging Systems|
|Magnetic Susceptibility Meters|
|Magnetic Resonance Systems|
A Model Study of The Effect of Salination on Groundwater
Kamal Khair and Catherine Skokan
The excessive exploitation of groundwater aquifers leads to water table
drawdown, and subsequently to the contamination of these aquifers by the intrusion of sea
water or other hazardous sources. This worldwide environmental problem is becoming
increasingly critical in coastal agricultural areas, where the fine grained materials
develop thick fringe zone. By evapotranspiration the moisture of this zone pumps up the
salt in the dry season, which cannot be efficiently washed away in the wet season. Over
the years the salt will accumulate at different rates for different terrains and climates,
which will ultimately deteriorate the land.
The electrical resistivity methods proved to be one of the most
efficient geophysical tools in detecting and delineating salt water intrusion. The current
study investigates the possibility of an early detection, through systematic observation
of electrical resistivity in selected positions with fixed electrode arrays. The study
observations used direct current electrical profiling system of Wenner configuration. They
were carried out in a physical model of wood and plastic filled by partially saturated
sand, with constant water flow of 1.6 I/mn. The model size is 148 x 85 cm for lateral
dimensions and 25 cm of sand thickness, with a total porosity of 36%. The study
concentrated on the indirect relationship between salinity and electrical resistivity upon
salination, to about 32 g/l, and desalination back to 0.25 g/l. The results show that: the
relationship is characterized by linear logarithmic function; the slow successive lateral
change in resistivity does not reflect the velocity of the flow in the tank (0.1 ml/hr);
the resistivity values for low salinity upon desalination are much different (smaller)
than those upon salination of equivalent salt concentrations; the relative change of
resistivity upon salination and desalination involves almost equally all features of the
tank, which have distinctive resistivity values.
The problem of the salination of groundwater aquifers arises in coastal
areas, where the excessive pumping of unconfined coastal aquifers by water wells leads to
the intrusion of sea water. This negative effect of human activity has been recorded in
many areas of the world. Hence, this problem is likely to arise in areas of poor water
resources (low precipitation and high evapotranspiration); from mismanagement of water
resources; or in densely populated areas with a high consumption rate (e.g. Khair et al.,
Because of the seasonal character of sea water intrusion, there should
be constant observation to assess and delineate sea water intrusion, especially with
aggravated demand during the dry season. This observation cannot be done only through
systematic chemical analyses, as these do not provide integrated three dimensional cover
of the terrains, whereas electrical resistivity methods have proved to be the best and
most cost-effective tools to assess groundwater salinity. These methods have been applied
in the investigation of coastal aquifers in different parts of the world, such as in the
Netherlands by Van Dam and Meulenkamp (1967); in Belgium by De Breuk and De Moor (1969);
in Israel by Ginsburg and Levanton (1976); and in New England by Urish and Frohlich
In arid climates, capillary action and evaporation lead to rapid
transport of water to the surface, with the consequence that dissolved salts are left
behind to hinder plant growth (McNeill, 1986). It is extremely vital to observe the extent
of salination in agricultural areas; this is because even if the saline or brackish water
is completely swept back to the sea during the wet season, there will be an appreciable
amount of salt in the fringe zone drawn up by evapotranspiration. Neither the percolating
(laterally) groundwater in winter reaches the top of the fringe zone to wash away the
salt, nor the infiltrating (vertically) meteoric water efficiently dilutes salinity
through the capillary zone. This salt is accumulated over the years, causing a major
deterioration of agricultural lands.
The effect of the quality (mineralization) of saturating water on the
apparent resistivity has been studied by many investigators. Resistivity measurements
conducted by Sharapanov et al. (1974), showed indirect, two-segment, linear logarithmic
relationship between apparent resistivity and mineralization. For sands, the low gradient
segment corresponds to mineralizations of up to about 2500 mg/l, whereas higher
mineralizations correspond to the higher-gradient segment. Other studies (e.g., Mares,
1984; Palacky, 1988; Kui, 1990; McNeill, 1990) although implying the direct relationship
between salinity and conductivity (or indirect for resistivity), however, the nature of
this relationship has not been discussed thoroughly. Moreover, Barker (I 990) showed that
the relationship between chalk water conductivity and salinity (experimentally determined)
constructed on a bilogarithmic scale is not characterized by a straight line, but rather
by a parabola.
The current study aims to observe the quantitative influence of
salinity on the apparent resistivity conducted on a physical model of sand. The depth to
the water table in the model varies from one end to the other, with a vertically fixed
plastic plate in the middle signifying in reality an impermeable barrier in its upper part
(see fig. 1). There is a constant water flow through the sand of approximately 2.3 m3 /day (or 1.6 liters per minute). The respective drop and
increase of the apparent resistivity upon salination and desalination observed for the
model, could be applied in reality, where systematic measurements of the apparent
resistivity in selected positions with fixed current and potential electrodes, might
detect salt water intrusion at its lower extent, and sometimes before it reaches the
Figure 1. A schematic diagram of the tank: a-first water
reservior; b- second water reservoir; c- middle sand reservoir; l- sand leel in the tank,
m- middle plastic plate; p- pump with hose; s- slotted plastic plate; arrow indicates the
direction of coordinates from left to right; solid dots- the locatio of observation points
(center between potential electrodes) along the central profile; the numbers 51 and 95
indicate the coordinates (oigin at the left edge of the tank) of observation point; other
numbers are the dimensions of the tank in cm.
The physical model
The model utilized is a wooden tank, 148 x 85 cm, covered inside with waterproof
plastic plates and sheets. By fixing two vertically slotted plastic plates at 16 and 19 cm
from the short sides, the tank is subdivided into three parts; two small parts, used as
water reservoirs, separated by a large one in the middle filled with sand (see fig. 1).
The slotted plates hold the sand back preventing it from seeping into the water reservoirs
and secure continuous free flow of water. The large middle reservoir was subdivided in the
middle by a vertically fixed plastic plate but not extending down to the lowest 10 cm.
This plate, in reality, signifies an impermeable barrier in the upper part.
A moderate water pump is submerged in the first water reservoir (16 cm wide)
continuously pumping water to the second water reservoir (19 cm). This creates a
difference in water levels (hydrostatic head) in the reservoirs, which leads water to flow
back to the first reservoir, percolating through the sand. The amount of percolating water
is proportional to the hydrostatic head, that is, in turn, controlled by the position of
the pump hose end. The higher the end the weaker the flow and the lower the end the
stronger the flow. It is worth noting that during all observation measurements on
salination and desalination the height of water was, respectively, 10.5 cm and 21 cm in
the first and second reservoirs, whereas the water flow stabilized at about 2.3 ml/day or
1.6 I/mn. The lost amount due to evaporation was compensated on a daily basis by adding
from one to three liters of water to the tank, to keep the water levels and the flow
Figure 2. Three-dimensional illustration of
apparent resistivity along profiles, I, II, III, IV and V (Wenner configuration a =4cm),
for: normal water levels 21 and 10.5cm (a); and shallow water levels 24.5 and 13.5cm (b).
Numbers ar the lower left side indicatie the distance (coordinates) of observation points
from the left edge of the tank. (a) top (b) bottom.
|The preliminary measurements
the boundary conditions, the Wenner configurations for resistivity profiling and vertical
electrical sounding were applied. This was because the electrodes were inserted and fixed
through equally spaced openings in a thin plastic bar. Each electrode was separately
connected to an electrode board where the positions are numbered successively to be easily
recognized and connected to the instrument.
The vertical electrical sounding was carried out in 6 positions along a profile midway
in the sand tank, with the electrode configurations AB and MN spread perpendicular to it.
The vertical sounding curves showed, in all stations and for two different water levels,
moderate resistivity values at small distances ( AB = 6, 18 and 30 cm) and a significant
increase at large distances (AB more than 30 cm). This fact implies that the large spread
sounding values are affected by the tank bottom, and probably its side boundaries.
Therefore, large spreads cannot be applied in the detailed study.
Figure 3. Apparent resistivity along the central
profile for different water levels (for further details see the caption of fig. 2)
|As the sand has a high degree of homogeneity with limited
thickness, the study concentrated on the lateral rather than the vertical variations of
resistivity, which requires the application of resistivity profiling. The applied size of
the electrode configuration, was AB = 12 cm or a = 4 cm as this proved to be more detailed
than that of a = 8 cm. The profiling measurements were taken along five profiles set
midway along the sand reservoir for different water levels. Figure 2 shows that the
apparent resistivity jumps sporadically in the deep water level (downflow) side along
profiles IV and V for shallow water level, and anomalous values along profiles I and V
compared to profiles II, III and IV for normal water level. Figure 2 also shows that the
resistivity values along the central profile III are the most consistent for different
water levels. Furthermore, the abrupt jump in the middle (coordinate 71 cm) is common
along all profiles for different water levels, this is due to the artificial barrier
(plastic plate) in the middle of the sand tank. Hence, having the least boundary condition
constraints the detailed measurements are taken only along the central profile III, as it
is the best for the observation of resistivity variations.
The preliminary measurements
were carried out at different water levels in the small reservoirs. Figure 3 shows the
resistivity values along the central profile with two different water levels having a
thickness of 13.5 and 10.5 cm in the first reservoir, and 24.5 and 21 cm in the second.
The resistivity values for the shallow level have little contrast compared to the normal
level. So, the last level is more optimal for detailed study of the influence of salinity
on the resistivity. Hence, during the whole period of measurements, the electrode array
(plastic bar) was set and fixed along the central profile with almost constant water level
and steady flow without disturbing or compressing the sand, which would significantly
affect the resistivity values.
There were several reasons (mostly technical) that excluded the use of conductive
material for the boundaries, which would have helped to reduce the influence of the
boundary effects and to simulate Dirichlet boundary condition (Mufti, 1976). However,
conducting the measurements of the experiment along the central profile of the model with
short electrode spacing made the boundary effects almost negligible.
The resistivity profiling measurements were taken for the middle 12 instead of the
total 18 positions along the central profile, to cut down the time of measurement to an
acceptable minimum (about 360 sec). The first observation was done immediately before the
disturbance in the salinity, the following second, third, fourth and fifth observations
were done in the next 0.5 hr, the sixth, seventh and eighth in the next hour, the ninth
and tenth in the next two hours, the eleventh after five hours and the last observation
(1211) after 10 hours, which will be the first observation immediately before the
following disturbance in the salinity. This procedure was applied upon all desalination
and most of the salination operations.
Salination was performed in seven stages, one day for each, to observe the influence of
salinity on the resistivity. In the first stage only 25 g of food salt were dissolved and
added to the second water reservoir. This implies a salt concentration of 0.25 g/l, as the
amount of water in the reservoirs and sand tank, for normal water level, is about 0. 1 ml
(or 100 liters). In the following days: 50, 100, 200, 400, 800, 1600 g were successively
added, to attain a total salt concentration of 31.75 g/l. This concentration was rounded,
two days later, to 32 g/l. In the following week the resistivity was observed once a day
until the beginning of desalination.
As salination was performed by doubling the amount of salt, desalination was done by
reducing salt concentration by one half, through dilution carried out also in seven
stages. This was performed by pumping out from the first reservoir 0.05 ml (half the total
amount) and simultaneously replacing these by 0.05 ml of fresh water in the second water
reservoir. So the theoretical salinity was decreased from 32 to, 16, 8, 4, 2, 1, 0.5 and
finally to 0.25 g/l.
It is not surprising that an increase in the salinity of saturating water reduces the
apparent resistivity of the sand or any other porous material. However, the salination in
the first stage (0.25 g/l) had reduced the resistivity for specific electrode positions
from 164, 87, 134 and 73 Ohm-m to 121, 61, 94 and 46 Ohm-m, respectively; whereas the
salinity from 0.75 g/l to 1.75 g/l reduced the apparent resistivity in the same
electrode positions from 79, 31, 49 and 31 to 34, 16, 35 and 20, respectively. This
implies that in the first stages of salination, the apparent resistivity was significantly
reduced in spite of the low salinity.
Figure 4 shows a decrease in the apparent resistivity along the central profile during
the third stage. This smooth decrease is clearly noticed for all other stages. The figure
also shows that the high resistivity values of the observation point (51) at the extreme
left and in the middle of the profile (71) are due to deeper water level for the first,
and the presence of the plastic plate (barrier) for the second. These values, except for
the last salination stage, remain consistently high relative to low values to the right
side (75 and 95). This implies that upon salination the apparent resistivity of the whole
model (3-D) will be affected, with almost the same ratio.
Figure 7 shows that the most significant change in apparent resistivity
is in the first 5 hours after salination and desalination. Later observations show very
slight change, except for one electrode position (51 cm) in the resistivity longer than 8
hours following the last stage of salination. Hence the 24 hr period for each stage was
satisfactory for the observations. All other stages have similar patterns of decrease (see
fig. 4 in Khair and Skokan, 1995).
Tables I and 2 show the values of apparent resistivity for all
observation points in the first three hours upon the second and fourth stages of
salination. A significant relative decrease exceeding 10% (shown in bold) of the apparent
resistivity with respect to the first observation, would signify that the high salinity
water parcels have reached the position concerned. The abrupt reductions in the apparent
resistivity value "events" for successive observations in the first 3-4 hours
indicate the following: 1- The mean
Figure 4. Three-dimensional illustrations of the
apparent resistivity along the central profile for the third stage of salination from 0.75
to 1.75 g/l. Numbers at the lower right side of each diagram indicate the time, in hours,
after disturbance (salination).
Figure 5. Change of apparent resistivity (central profile) relative tot
the first observation (before disturbance), in the first five hours after salination: a)
from 3.75 to 7.75 g/l (5th stage)(top); and b) from 7.75 to 15.75 g/l (6th stage)(bottom).
velocity of water percolation through
the sand, calculated by the above mentioned events is in the order of a few decimeters per
hour, the time during which the water in the tank completes a whole cycle; 2- The
resistivity reduction velocity ranges from 8-12 cm/hr across the barrier, to about 24
cm/hr in the upstream side and to about 30 cm/hr on the downstream side; 3- The low
velocity in the middle of the profile is due to the plastic barrier, whereas the
relatively high velocity in the downstream side, is due to the higher hydrostatic head
between the two sides caused again by the barrier.
Figure 5 shows the change of apparent resistivity relative to the first
observation in the first five hours after salination for 5 th and 6th stages. This figure
shows higher relative change (decrease) of the apparent resistivity in the right
(upstream) side as it is closer to the high salinity source than the left side.
The figure also indicates that the high
relative decreases for the first 5 hours of observations are propagating from right to
left or parallel to the water flow. All other stages have similar patterns of resistivity
decrease upon salination.
Figures 6a and 9a confirm the indirect proportional relationship between the salinity
and apparent resistivity, characterized by almost linear logarithmic function. On the
other hand, the anomalous decrease of apparent resistivity in the electrode position to
the extreme left (downstream) is ambiguous and could be due to some complications related
to measurements and/or the stability of the electrode plastic bar over the sand,
particularly this phenomenon is not observed upon desalination (see fig. 6a).
The influence of salinity on the resistivity of the sand is also
evident upon desalination (dilution) stages. Figure 8 shows the direct relationship
between the dilution and resistivity, which increases smoothly throughout each stage and
between successive stages in a similar, but reverse, way to salination (see fig. 4). The
remarkable difference is that the resistivity upon desalination does not attain high
values comparable to those upon salination, especially for low salinity stages. For
example, in the last stage of desalination, with a drop in the theoretical salinity from
0.5 to 0.25 g/l, the resistivity does not attain the high values of the first or second
stages of salination, and its values are roughly comparable to those of the third stage
with a salinity increasing from 0.75 to 1.75 g/l. This difference is clearly shown in fig.
This phenomenon is explained by the fact that the salt pumped to the
fringe zone was not diluted upon desalination, as only the saturated zone is involved in
this process. Hence, the fringe zone was pumping up the moisture from the saturated zone
with its respective salt content. Thus, because of evaporation, the fringe zone moisture
is always streaming upward, pumping up the salt, vaporizing and leaving it behind. Indeed,
after the completion of all measurements, the water was totally pumped out of the tank,
and as the uppermost sand particles dried out a thin white cover of salt appeared on top
Tables 3 and 4 show the values of apparent resistivity for all
observation points in the first three hours upon the second and fourth stages of
desalination. The appearance of jumps (shown in bold) in resistivity "events" of
more than 10% upon desalination, support the remarks concluded upon salination concerning
the decrease in the velocity of water flow in the middle of the profile because of the
plastic barrier. However, in contrast to the salination process the "events"
velocity in the left (downstream) side, here, is less than that of the right side. It is
worth noting that the process of desalination required 15-20 mn to dilute, by half, the
water of the tank, whereas salination was impulsive and only required a few seconds to add
salt solution to the tank.
Figure 7. The change of apparent
resistivity over time (central profile) after disturbance for four electrode positions,
with coordinates 51, 71, 75 and 95 cm, upon the first stage of salination (a) (top); and
last stage of desalination (b) (bottom).
Figure 8. Three dimensional illustrations of the apparent resistivity
along the central profile for the third stage of desalination. Numbers at the lower right
side indicate time, in hours, after distubance (desalination).
|Figure 10 shows the change of apparent resistivity
relative to the first observation in the first five hours after desalination for 5th and
6th stages. This figure shows higher relative change (increase) of the resistivity in the
right (dilution) side than in the left side. It also shows that the high relative
increases for the first 5 hours of observations are propagating from right to left
parallel to the water flow. All other stages have similar patterns of resistivity increase
upon desalination. Finally, figs. 6b and 9b show almost a linear logarithmic function of
the indirect relationship between salinity and apparent resistivity upon desalination.
Figure 9. Stabilized values of apparent resistivity (central profile) for all
stages upon salination (a) and desalination, top (b) for four electrode posistions,
Figure 10. Change of apparent resistivity (central profile) relative to the
first observation (before disturbance) in the first five hours after desalination: a) from
2 to 1 g/l (5th stage)(top); and b) from 1 to 0.5 g/l (6th stage)(bottom).
Figure 11 shows that during 8 days of quiescence (no salinity disturbance) separating
between salination and desalination, the resistivity in all electrode positions remained
almost constant. This indicates the stability of the system, keeping constant water level
and flow and stable electrode array. Furthermore, fig. 7 indicates some stability 5 hours
after the disturbance in all stages of salination and desalination. This means that 5
hours are needed to attain a high degree of water co-mingling. On the other hand, the pump
capacity at the measured level was in the order of 100 1/hr and as the total amount of
water is about 100 1, hence the water flow cycle is one hour. This means that 5
circulations of tank water are needed to attain the above mentioned co-mingling. This fact
invites the suggestion that upon salination or dilution the intruding water parcels of
different salinity do not displace the existing ones, but are mixed with them, creating a
gradual change of salinity across the intruded water front. Consequently, salt water
intrusion can be detected before an appreciable increase in salinity appears in the pumped
water if systematic measurements of resistivity are carried out in the area surrounding
the producing wells, especially towards the sea.
The early stage detection of salt water
intrusion is of great importance even in areas of moderate seasonal ground water flushing.
Because during the dry season the evapotranspiration occurring in the fringe zone draws
the high salinity water upwards and accumulates the salt in its upper part near the
surface, this will not be diluted or flushed away in the wet season, because it is higher
than the water table. This problem is severely aggravated in coastal areas of low
precipitation as here the salt would not be sufficiently washed down to the water table
for further lateral dilution and flushing. The thickness of the fringe zone, which is
indirectly proportional to the particle size, is also critical. Hence, fine grained
material such as silt and clay would develop thick fringe zones, which could extend from
the surface to the water table. Thus, the continuous drawing up of salt by
evapotranspiration in the dry season, can not be simply compensated for by discrete
downward flow from heavy rain storms in the wet season, due to the low permeability.
According to the above discussed mechanism, over a long period of time, the salinity will
ultimately increase in the soil to the extent that it loses its value for most of
The accumulation of salt in the soil profile seems to appear only in
arid and semi arid areas. However, in the future this phenomenon is likely to extend to
other coastal areas, where the problem of sea water intrusion is aggravated and not
seriously dealt with. This is because the infiltration during wet season is not able to
significantly dilute salinity downward through the capillary zone. Finally, a systematic
observation of the resistivity using fixed electrode arrays, the well casing for instance,
would detect any increase in the salinity and provide reliable tools to control the well
Table 3. Values of apparent resistivity upon desalination from
16 to 8 g/l, italics - observation times, in hours, after desalination.
Table 4. Values of apparent resistivity upon desalination from 4 to 2 g/l, italics
- observation times, in hours, after desalination.
Maintaining constant water levels and steady flow with a fixed electrode array,
resistivity observations in the sand tank indicate the following:
- Low resistivity values upon desalination compared to equivalent salt concentrations
upon salination, and the appearance of salt on top of dried sand particles, indicate that
the evaporation of fringe zone moisture draws up the salt and accumulates it in the upper
part of the sand.
- The relative change of apparent resistivity upon salination and desalination over the
deep and shallow water zones and over the barrier, indicates the potential capability of
this method to detect any change in salinity in the underlying material.
- The velocity of water flow depicted on resistivity variations in the sand tank are in
the range of a few decimeters per hour. These values disagree with the fact, that the
water in the tank completes a whole cycle in only one hour.
- The apparent resistivity measurements upon salination or dilution, show that the
intruding water parcels of different salinity, are mixed with the existing ones rather
than displacing them, and create a gradual change of salinity across the intruded water
|Finally, in view of the serious problem of
groundwater salination in most coastal areas and the need to apply the most efficient
preventive measures, the first of which would be early detection, the sand tank model
could be considered as an analog to this problem, as it indicates the capability of
resistivity methods in the delineation and timely detection of sea water encroachment into
the groundwater aquifers.
This study was made possible by a Fulbright Grant to K. Khair to visit the Department
of Geophysics at the Colorado School of Mines. Special thanks go to Dr. A.W. Ibrahim and
Dr. J. Skokan for valuable suggestions. We also thank Mr. H. Schneider for his help.
Figure 11. Three-dimensional illustration of the apparent resistivyt (central
profile) over the period between salination and desalination. Numbers at the lower right
side indicate the time, in days, after the last tage of salination.
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