as for the identification of some properties of the groundwater flow as for the ... The aim is to underline the importance of the comparative analysis of the ...
CHARACTERISATION OF GROUNDWATER FLOW BY THE APPLICATION OF THERMOMETRIC AND ELECTRICAL CONDUCTIVITY MEASUREMENTS L. Tulipano1, G.Sappa1 and B.Bianchini2 ABSTRACT: In this paper are shown the results of a study made in an alluvial deposit, outcropping in the S. Eufemia plan, in the South of Italy with the purpose of describing the potential of the thermal and electric conductivity logs both from an hydrogeological point of view and for the definition of the stratigraphy of the well in wich the measurements has been made. As a matter of fact there have been have been carried out measurements of groundwater level on more than 60 wells, and on 38 of them there have been made thermometric ane electric conductivity logs. The results of these measurements gave interesting information as for the identification of some properties of the groundwater flow as for the characterisation of the different lithological formations, which outcrops in the area under study. Dans ce papier les résultats d'une étude faits dans un dépôt alluvial, affleurer dans le S. RESUMÉ: Eufemia plan, dans le Sud d'Italie avec le but de décrire la possibilité de la conductivité thermique et électrique sont montrés tronçonne les deux d'un point de vue de l'hydrogeological et de la définition de la stratigraphie du bien dans wich les dimensions ont été faites. En réalité là été eu été emporté des dimensions de niveau du groundwater sur plus de 60 puits, et sur 38 d'eux là a été fait ane du thermometric grosses bûches de la conductivité électriques. Les résultats de ces dimensions ont donné de l'information intéressante comme pour l'identification de quelques propriétés du groundwater coulez comme pour le characterisation des formations du lithological différentes qui affleurements dans la région sous étude. INTRODUCTION The hydrogeological characterisation of small or wide areas is generally based on the interpretation of indirect research methods. This means that the combined use of different techniques can often be very useful in order to define some aspects of the groundwater flow, otherwise not easy to spot. This paper shows the results of some hydrogeological field researches made in a alluvional deposit in the South of Italy. The aim is to underline the importance of the comparative analysis of the information obtained by the application of different investigation techniques as to characterise the underground circulation as to significantly hypothese some aspects of the stratigraphy in the area. The interpretation methodology, here described, has been used to study some thermal and electric conductivity logs executed by the authors who want to show the potential of this rather economic technique, both from a strictly hydrogeological point of view, and for the definition of the stratigraphy of the measurement well. In fact, the stratigraphic information is often available, lacking of the relevant hydrogeological aspects, or viceversa (where you have either operative wells or abandoned ones where it is possible to have piezometric or temperature and electric conductivity measurements). The goal of the present paper is to describe the potentiality of the combined use of these investigation techniques.
1. Dept. Of Hydraulics, Transportations and Roads, Faculty of Engineering - University “La Sapienza” 2. Enviromental Engineering Engineering Geology for Developing Countries - Proceedings of 9th Congress of the International Association for Engineering Geology and the Environment. Durban, South Africa, 16 - 20 September 2002 - J. L. van Rooy and C. A. Jermy, editors Géologie de l'Ingénieur dans les Pays en voie de Développement - Comptes-rendus du 9ème Congrès de L'Association Internationale de Géologie de l'Ingénieur et de l'Environnement. Durban, Afrique du Sud, 16 - 20 septembre 2002 - J. L. van Rooy and C. A. Jermy, éditeurs ISBN No. 0-620-28559-1
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GEOGRAPHIC AND GEOLOGICAL FRAME OF THE AREA The area is a part of S.Eufemia’s plain (Catanzaro – South Italy), extending for about 120 square kilometres near the town of Catanzaro, which is the natural separation between the Northern and the Southern part of the Italian region Calabria. This plain is made of tertiary period soils where some streams, the most important of which is the Amato river, 56 km long, while its main tributaries are the S. Ippolito river, the Piazza, the Pesipe and the Cottola streams. The coastal areas had been submerged by the sea in the period between the miocene and the Pleistocene, so that they result made of sea sediments in facies of soft clays, sand and partially lithified gravels. The pleistocene series starts with a thick deposit of lithified gravels, which elements are crystalline scisti, alternated with limestones and sands. The recent evolution of the territory led to the formation of abundant alluvional deposits made of gravels that have a rather high permeability. The surface layer is made of muddy sands, and the muddy component tends to disappear while heading towards the coast. Beneath this layer, stands an alternance of different litology. The range of their thickness can vary between ½ meter and some tens. Clays can be found all over the area between 6 and 20 meters from the field. They form a continuos layer which thickness tends to diminish from the inland to the coast in the area south of the river Amato. Beneath the clays, stands a gravel layer with a very high permeability, which is a very rich aquifer (Ogniben, 1973). PROPERTIES OF THERMAL AND ELECTRICAL CONDUCTIVITY MEASUREMENTS Originally, temperature and electric conductivity logs had been used for the hydrogeological researches involving coastal aquifers. As a matter of fact these measurements and the following reconstruction of the termic and the electric conductivity logs are useful instruments to identify the position of the fresh water aquifer with respect to the underneath seawater intrusion. The depth and thickness of the transitional zone (where fresh and salt water are mixed in different proportion) can be revealed by the gaps in the conductivity along the vertical line of the well. Meantime, these studies showed how temperature versus depth can be an interesting indicator of other properties of the aquifer, whether it was a coastal aquifer or not (Tulipano, 1988). In fact, the basic hypothesis when using termic logs for the ground water flow characterisation, is that a long time contact between groundwaters and rocks, eases the termic equilibrium. Hence, water tends to the temperature of the rocks it crosses, due to the reciprocal relation between the specific capacities. According to the geophysical schemes, experimental based, the natural geotermic gradient is about 1°C/33m. The final consideration is that generally the deeper the waters are the higher the temperature is. A high number of logs showed that underground waters can have rather different temperatures even at the same depth. This has been interpreted as an indicator of how waters can have different velocities when crossing the rock. In fact, the heat transfer is higher the longer is the contact time between waters and rock. If the contact time is not enough, waters will tend to keep their original atmospheric temperature they have when they first infiltrate. The velocity waters have, when crossing the rock, is directly related to the permeability of the medium. Hence, the termic variations either in different points at the same altimetric quote, or in the same point at different quotes, can indicate variations in the permeability. The indications revealed had then a confirmation with the traditional techniques. Even the electric conductivity measures have the same origin as the termic ones, since they come from hydrogeological studies made in coastal aquifers. Yet, the application to continental aquifers is more recent and is related to the information acquired while studying the use of geochemical analyses for the interpretation of the hydrogeological processes. In fact, these techniques had progressively defined the laws regulating the chemical interaction between water and rock. Eventually, they helped finding out the peculiar characteristics of the chemical reaction happening during the filtration of the underground waters. Specifically in porous media, the electric conductivity of underground waters is strongly related to the granulometric and mineralogic properties of the medium itself. In fact, the filtration velocity influeces the chemical and physical reactions between water and rock. Generally, the diminishing of the velocity leads to an enrichment in dissolved anions and cations. The exchange reactions are even influenced by the petrographical and mineralogical nature of the deposits forming the medium. Not last, the electric conductivity of underground waters, especially in their most shallow layers, depends on other factors, such as the cycles of wetting and drying, the contribution of highly mineralised waters relating to anomalous termic gradients and the use of fertilising in agriculture. All these factors influence the salinity of the underground waters.
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The conclusion is that interesting indications on groundwater flow can be achieved from an interpretation of the electric conductivity logs interpreted together with other information such as the termic logs and the description of the main geolitological characteristics of the aquifer. Hence, it is possible to spot the layers with different permeability and those richer in the clay mineralogical component, being equal the other parameters. The need here is to use multitracers techniques in the aim of matching different kinds of information coming from the different measurements. The use of the thermal and electric conductuctivity measurements for the hydrogeological characterisation of the alluvional flat, marked the application in the environmental field. Thanks to this study, it has been verified through thermal and electric conductuctivity measurements that it is possible to hypothise not only the scheme of the underground circulation, but can give the information to reconstruct the litological characteristics of the layers that the measurement well crosses. The information is due to a reciprocal acknowledgement to assume the required attendibility. Yet the low costs and the rapidity of the measurements, versus the significance of the information, is enough to say that these investigation techniques are a very useful instrument to investigate a hydrogeological system, especially when coupled with traditional methods such as continuum borehole tests and pumping tests. The whole interpretation of data requires effort, experience and sensitiveness for hydrogeological matters; yet it is possible to find the presence of high or low permeability layers through a simple crossed interpretation of the termic and electric conductuctivity logs. Moreover, the need of a significant number of stratigraphies, allows hypotheses on the litological nature of the layer; through the reconstruction of vertical sections and the mapping of data, the interconnections between the different aquifers can be established. LEGEND Wells in wich are made piezometric measuremements Wells in wich are made temperature and electric conductivity logs Wells in wich the water has been taken and analysed 54
53
11
49
50 56
13
32 44
55 10 9
48
14 58
24
31
843
41
25 62
51 36 35 60
34
7 42 6
15
40
52
16 37
17 57
30
29
28 45
23
22
33
46
5 27
18 59
3 61 12 2
38
19
26
20 21
4 1
Figure 1. S.Eufemia’s plain - Distribution of the wells DESCRIPTION OF THE FIELD INVESTIGATION Before carrying on the thermal and electric conductuctivity measurements, the census of the wells in the studied area has been performed through the analysis of the information obtained from the previous studies in the area itself. The selection of the measurement points was based upon the depth of the well (generally > 30 m); in more superficial wells depended on the distance from the sea. The choice of wells closer to the coastal line is justified by the need of verifying whether sea intrusion was on. Static measurement coupled
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the thermal and electric conductuctivity ones, using a piezometric device, in order to be able to reconstruct the groundwater level. The total number of measurements was of 62 groundwater level and 38 thermal and electric conductuctivity logs, all of them being distributed among the wells in the site; the geometrical characteristics and their use were found for each well. The results obtained from the logs and the stratigraphic profile of the well (when available) had been reported on sheets like the one in Figure 2, then used for the interpretation. Figure 1 shows the planimetric distribution of the measurement wells. The instrument for this set of test is a portable one, Profile LF 197, produced by WTW, which was set by the measurement cell, a transmission wire and a recording unit that gives the data back in a digital format. The recording unit not only allows the instant reading of the data, but gives the automatic calculation through appropriate algorhythims, both of the Total Dissolved Solids expressed in mg/l and of the salinity expressed in g/l. W ell n.46 Site: Aglioca
3,0m
9,0m
13,0m 14,0m
21,2m
30,0m
Stratigraphy (n.114) +10m s.l.m.
16
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
17
18
Borehole diameter (mm): 300 Date: 30-9-98
C onductivity (µ S/cm )
Tem perature (°C ) 15
600
19
depth (m)
P.C. 0,6m 1,4m
Altitude (m s.l.m.): 10,0 Depth of the w ell: 28,0m
800
1000
1200
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
1400
Altitude Depth Temper. Conduct. (m s.l.m.) (m dal P.C.) (°C) (µ S/cm) 6 4 18 825 5 5 17,7 902 4 6 17,4 972 3 7 17,2 1054 2 8 17,1 1110 1 9 17 1132 0 10 17 1155 -1 11 17 1163 -2 12 17 1169 -3 13 17 1172 -4 14 17 1175 -5 15 16,9 1175 -6 16 16,9 1176 -7 17 16,9 1177 -8 18 16,8 1178 -9 19 16,6 1183 -10 20 16,3 1195 -11 21 16,2 1211 -12 22 16 1233 -13 23 16 1258 -14 24 15,8 1281 -15 25 15,7 1291 -16 26 15,6 1300 -17 27 15,6 1303
SO IL
MUD
CLAY
SAN D
M U D D Y SAN D S
M U D D Y C LAY
Figure 2. Example of sheet used for interpretation of the results ANALYSIS OF THE RESULTS The analysis of the thermal and electric conductuctivity measurements Some of the sheets associate a stratigraphy at the thermal and electric conductuctivity logs. The stratigraphy is chosen among one of those from a borehole test close enough to the measurement point (distance R
