5. GROUNDWATER QUALITY AND SALINIZATION
5.3 SPATIAL DISTRIBUTION OF SALINE GROUNDWATER FROM RESISTIVITY
5.3.4 Geoprobe direct-push geoelectric measurements
As stated before, investigating the unconsolidated aquifers in the Jordan Valley is a challenging and ambitious task. Hence direct measurements of geological composition and geoelectrical resistivities are desirable. The employment of the Geoprobe direct push technique for larger depths has proved to be a suitable tool for reasons stated before. The Geoprobe 6610DT equipment was made available by the Centre for Applied Geoscience at the University of Tübingen and shipped to the study area.
5.3.4.2 Method
The method applied works according to the direct-push principle. Direct pushing (or hammering) of a measurement or coring probe into the ground forms the principle of the applied method (hence “direct-push” technique). Since the different parameters are directly measured at the tip of the probe and
5. Groundwater quality and salinization
different parameters can be determined at the same time, the problem of equivalence and/or ambiguity can be minimized or even eliminated.A geoelectric four point “Pol-Pol” or "Wenner" arrangement located on top of a series of drill rods is pushed into the ground. Consequently the apparent resistivities of the material in the vicinity of the top of the rod is determinable and logging of apparent resitivities with depth is possible. The DC geoelectric method is described in chapter 5.3.3.2. The interpretation is straightforward. Since the electrode configuration (Wenner arrangement) is fixed the electric resistivity is calculated automatically by the recording device along with the depth of the probe and a continuous depth resistivity profile is stored in the measuring device (Fig. 5.3-13 left).
Using drilling rods of two different diameters (i.e. 2 1/8" and 1" inch rods), soil samples from any desired depth can be taken. Samples are not taken with the help of coring rods, but are taken with the help of disposable plastic tubes. Soil samples were taken from different depths and analysed for major ions.
The groundwater table was often encountered during the measurements. Since the degree of salinization of the subsurface soil and groundwater is of major interest, the depth of the water table and field parameters (pH, temperature, and electric conductivity) were determined, and water samples from various boreholes were taken.
5.3.4.3 Results
In the study area 21 direct-push geoelectric depth soundings were undertaken (Fig. 5.3-14). The lowest resistivities were found in the soundings E08, E01, E05, E10, and E20. The extremely low resistivity in the upper part of E08 can be attributed to two factors: the presence of the Lisan Formation and the high water content in the upper soil. In E05 the influence of a very shallow water table on resistivity variations can be seen very clearly. Here the groundwater table is as shallow as 18 cm below ground level. Capillary forces move the groundwater even up to the soil surface. Steady-state evaporation of groundwater in the area results in the formation of salt crusts on top of these soils as well as salt accumulation in the uppermost part of the soil. The soil in this areas consists of very fine sand to silty sand and is homogeneous in the studied section. In the profile of E05 high resistivity values are found in the top soil. These can be attributed to the low water content in this area. From the uppermost area down to the water table the resistivity decreases significantly down to 0.2 Ωm before it increases again to a value of about 1 Ωm.
However, mostly no correlation between the encountered water table and strong resistivity variations exist. Higher resistivity variations occur in the upper two to four meters. Here, the resistivity can vary up to three orders of magnitude (E05, E15), but usually is not more than two order of magnitudes.
These changes could most probably be attributed to changes in the water content of the upper soils and will be described in chapter 5.3.5 on the basis of the sounding E17.
Concerning the deeper part of the depth profiles, the highest resistivities can be found in the profiles taken west of Rama (E15 to E18), lowest west- northwest of Kafrein (E20 to E22). Resistivity variation in the deeper part of the profiles is usually smaller than one order of magnitude (between 1 to maximum 10).
5.3.4.4 Comments/ Lessons learned
The biggest advantage of direct-push geoelectrics is the direct measurement of resistivity with depth on the tip of the probe. No integration of different resistivity values of larger portions of the subsurface, as in the case of surface geoelectrics, is necessary. High resolution of the resistivity depth profiles are the result. Furthermore, more than one measurement can be performed in the borehole. It could be seen, that the water table has mostly no strong influence on the subsurface resistivity. As a result no sharp interface is developed around the water table, but rather a zone of lower resistivity.
Usually a zone of lower resistivity can be seen above the water table. Soil sampling at the sampling points shows on the one hand a correlation between the soils and subsurface resistivity (E08) and on the other hand it shows a correlation between the water saturation of the sediments and the electrical resistivity (E05). This information can be used for calibrating surface VES. In order to properly calibrate VES (to see the true depth variation of resistivity) more than one depth sounding should be
undertaken per one VES profile. One example of calibrating a VES with the help of direct-push can be seen in Fig. 5.3-13. Fig. 5.3-13 also illustrates the sensitivity to resistivity variations in the subsurface for both methods. The limitation of the direct-push geoelectric method is the small penetration depth of 20 to 30 meters compared to the penetration depth of surface VES. However, high electrode spacing and a powerful equipment for surface VES are necessary to penetrate bigger depths in a brackish environment. In boulder dominated area, like in the upper- and mid-fan areas no large penetration depths can be reached. Therefore the method is only applicable in the distal-fan area.
Fig. 5.3-13: A direct-push geoelectric depth profile versus a surface Schlumberger depth sounding. The soundings are made in the same location, about six km north of the Dead Sea.
Fig. 5.3-14: Geoprobe geoelectric depth sounding points.
5. Groundwater quality and salinization
Fig. 5.3-15: Results of the Geoprobe geoelectric soundings. In places where the groundwater table was encountered the encountered depth below ground level is plotted as a straight line.
5.3.5 Chemical analysis of soil samples