ABSTRACT

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.

Introduction

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., 1994).

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 (1990).

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.

Methodology

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.

To assess 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 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.

Observation measurements

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.

Results

Salination

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 increase of

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).

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.

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.

Desalination

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. 7.

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 of them.

Discussion

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 agricultural uses.

Conclusions

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.

Acknowledgment