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Hydraulic connection between the river and the phreatic aquifer and analysis of the piezometric surface in the plain west of Mavrovouni, Laconia, Greece
N. Karalemas
Department of Dynamic, Tectonic and Applied Geology, Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, Panepistimioupolis Zografou, 157 84 Athens, Greece
Abstract This paper concerns the hydraulic relation between the water table hosted in the quaternary formations and the river that crosses the plain west of Mavrovouni. In terms of this study, the groundwater level in the wells was meas-ured in four periods. Also, the river flow was measured at the entry point of the plain and in the area near the river mouth. The water level data were used to create piezometric maps. These maps show that there is an underground drainage line, which could be interpreted as a movement of water from the aquifers to the river, especially at the northwestern part of the plain. However, in the entire area, the river lies at a higher level than the groundwater level; therefore, the only move-ment that takes places is that from the river to the aquifers. At the region south-eastern of Petrovouni, the piezometric lines for the three out of the four periods of study show a movement of water from the river toward the aquifers. During the fourth cycle, October 2007, the river had run dry, making the feeding of the aqui-fers impossible. These findings are confirmed by flow measurements of the river, which showed that the river feeds the aquifers as long as it has a flow. Finally, as expected, a variation of the groundwater level measurements between wet and dry seasons was observed and was measured at 1.4 m on average. It should be noted that the measurements took place during a dry hydrological year.
1 Introduction
Northwestern of Gythio lays the largest hydrological basin of the eastern Tayge-tos. The basin has a general slope toward the Southwest and ends at Laconikos Gulf, western of Mavrovouni (Fig. 1b). The river that crosses the basin is known as “Platy Potami”. The main spring that feeds this river is “Agia Marina Spring”, which is the largest spring of eastern Taygetos (Karalemas 2006). Downstream of
N. Lambrakis et al. (Eds.), Advances in the Research of Aquatic Environment, Vol. 1 DOI 10.1007/978-3-642-19902-8, © Springer-Verlag Berlin Heidelberg 2011
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the spring, the river flows for the largest period of the year. Upstream of the area of the village Archontiko, the river crosses alpine formations that are mainly schist. Downstream of that region, the river moves into Aegies plain and enters a narrow valley, at the area of Monachi Sykia. Then, it moves into the plain lying western of Mavrovouni toward South-southeast and flows into Stomio Bay. The area of study of this paper is the last part of the basin.
In terms of this project, piezometric maps were constructed for the study area. The study of piezometric maps generates useful conclusions concerning the groundwater movement, the hydraulic connection between neighbouring water systems, and the hydraulic conductivity at each position of the aquifer (Kallergis 2001, Mimikou and Baltas 2003, Lekkas and Alexopoulos 2005). Furthermore, the river flow at the area where the river enters the plain and at the river mouth was measured. The aim of the aforementioned was to monitor the hydraulic relation-ship between the river and the water tables hosted in the geological formations that cover the plain. For this reason, the groundwater level in 44 wells, which were initially selected, was measured at four periods. The altitude of the wells was cal-culated by means of topographic maps of 1:5000 scale of the Hellenic Military Geographical Service.
This work was carried out in terms of the PhD thesis of the author, a thesis that is supported by the Research Project titled “Decision Support System for the Pro-tection and Management of Water Resources of Laconia Prefecture by means of Geographic Information Systems” (PENED 2003). This project is co-financed by 90% by National and Community Funds (25% from the Greek Ministry of Devel-opment-General Secretariat of Research and Technology and 75% from E.U.-European Social Fund) and 10% by Development Corporation of Prefecture of Laconia, Regional Development Company of Parnonas and Development Corpo-ration “Parnona - Taygetou”.
2 Geological and hydrogeological setting
The geological map (Fig. 1b) shows that there are four geological formations in the study area, the lowest of which are alpine formations that consist of schist of Phyllite-Quartzite Unit, overlain by minor tectonic napes, composed of marble be-longing to the Mani Unit, according to on-site research that was conducted (Lat-soudas 1984 considers that these rocks belong to Tripoli Unit). Above these for-mations post-alpine sediments are found, which are divided to neogene sediments that mainly consist of marls and to quaternary sediments the composition and grain size distribution of which vary.
From the hydrogeological point of view, schistolithic formations have very low permeability, thus are practically impermeable compared to the adjacent forma-tions. Also, the Neogene formations are practically impermeable, because the clay in the marls obstructs the groundwater movement into their mass. On the contrary,
Hydraulic connection between the river and the phreatic aquifer 285
the quaternary sediments are loose and consist mainly of sand, gravel and pebbles, and therefore, they are a formation with a remarkable permeability, which varies horizontally and vertically. Consequently, it is expected that the quaternary sedi-ments host water tables, since they overlie impermeable formations. Finally, the marbles are karstic aquifers, which have very high permeability, but extremely limited appearance; as a result they do not play an important hydrogeological role in this area.
Fig. 1. a. Location map. b. Geological map (Psonis and Latsoudas 1983, Latsoudas 1984; Dimadis 1989, modified) of the region area that shows the boundary of the hydrological basin of Platy Potami River.
From all the above, it is concluded that the aforementioned area is ideal for study, since the permeable clastic formations are generally surrounded by imper-meable, and therefore are considered as hydraulically isolated (Fig. 3). An excep-
a
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tion is the southern part of the plain, where a small area of the quaternary forma-tions is in contact with marble. The marble extend to the sea, but does not actually affect the hydraulic heads of the plain, due to the small area of contact.
3 First measurement period (March 2007)
The first period of measurements took place from 17 to 19 March 2007. For this time of the year higher levels of groundwater were expected. Figure 2a shows the piezometric map of the plain of Mavrovouni with a half-meter interval of subse-quent piezometric lines. An underground drainage line, which roughly follows the present riverbed and has general direction NNW - SSE, is observed. Observing the north-northwestern area, the piezometric lines appear dense in both regions sepa-rated by this axis, especially in the eastern; this suggests that the permeability of the formations near the river is higher than that of the adjacent formations. In the centre of the plain, southeastern of Petrovouni, the underground drainage line presents a small offset to the West and does not follow the current riverbed. Furthermore, east-ern of this axis, there is an underground watershed of the same direction, and even more eastern, an even less distinct drainage line is formed. This setting continues to the Southeast until the centre of the plain, where the underground watershed gradu-ally disappears, the two underground drainage lines are joined and the distance be-tween the piezometric lines becomes greater. However, the absence of wells that are apt for groundwater level measurements in the region eastern of the river does not allow a detailed depiction of the piezometric surface. Even more southern, near the main road leading from Mavrovouni to Areopolis, the piezometric lines tend to straighten (transect virtually the river) and the distance between them becomes smaller. In the southern part of the map, northern of the carbonate formations, an additional couple of an underground drainage line and an underground watershed of short length are found. They are almost parallel to the main underground drainage line. These axes are formed by the piezometric line of 4 m. It must be noted that the piezometric lines of 3.5 m and 4.5 m are not accurate, due to lack of data.
The fact that the piezometric lines roughly transect the contact between the quaternary and the impermeable formations indicates the absence of hydraulic communication between them. In contrast, in the western part of the plain, south-ern of Petrovouni, the piezometric lines are almost parallel to the border of the plain, possibly due to the limited supply of quaternary formations from a small stream passing nearby. It must be noted that there is not a sufficient number of wells in this region to enable the accurate mapping of the piezometric surface. In the southern part of the map, piezometric lines are expected to form a small angle with the contact between quaternary and carbonate formations. Specifically, ac-cording the data, the groundwater level in the quaternary formations is above sea level, while the altitude of the karstic water should be at the sea level. Therefore, the aquifers of quaternary formations are expected to feed the karstic.
Hydraulic connection between the river and the phreatic aquifer 287
Fig. 2. Piezometric map of a. March 2007, b. June 2007, c. October 2007, d. January 2008 of the study area.
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Fig. 3. Geological cross-section.
The flow of Platy Potami River was 665 m3/h the area entering the plain and 515 m3/h at the river mouth. Hence, along the plain, the river feeds the water ta-bles that are hosted in the quaternary formations.
4 Second measurement period (June 2007)
The second measurement period took place between 25 and 27 June 2007. The groundwater levels were on average 0.4 m lower than those measured in March.
The piezometric surface in June 2007 is shown at the map (Fig. 2b). The shape of the piezometric surface is similar to that of March. The only difference lies in the southern part of the map, near the carbonate rocks, where the underground drainage line and the parallel underground watershed are less distinct.
The Platy Potami River maintained a very low flow (36 m3/h) near the entrance to the plain, water that was infiltrating nearby. Also, there was stagnant water downstream of the public road leading from Mavrovouni to Areopolis, because the river mouth was blocked by a large volume of sand dunes.
5 Third measurement period (October 2007)
The third period includes measurements from 10 to 15 October 2007. This season is characterized by lower groundwater levels, because it is the end of the irrigation season and the new hydrological year has not started yet. It is significant that some wells in the northern part of the plain were dry. The piezometric surface in Octo-ber 2007 is shown in the map (Fig. 2c). In comparison to the aforementioned pie-zometric maps, the following differences are observed. Firstly, the main under-ground drainage line is more accurately defined in the southeastern part of the plain. The piezometric surface is on average 0.95 m lower than that of June 2007.
A notable change is the shift to the East of the main underground drainage line at the area southeastern of Petrovouni until it concurs with the riverbed. Finally, the underground drainage line and the parallel underground watershed in the southern part of the map are not very distinct.
Hydraulic connection between the river and the phreatic aquifer 289
The river showed no flow downstream of the village Archontiko. Shortly be-fore the river mouth, stagnant water was observed, similar to that of the previous period, but had a much lower level.
6 Fourth measurement period (January 2008)
The measurements of the fourth period were performed from 14 to 15 January 2008. Even though the hydrological year 2007-2008 was also dry, the groundwa-ter level was higher in January, as expected. The piezometric surface in January 2008 is presented on the map (Fig. 2d). The groundwater level has risen by an av-erage of 1.30 m compared to that of October; as a result, the shape of the piezo-metric surface in the southeastern part of the plain was restored, as it was in March and June 2007. Also, the shift of the main underground drainage line at the area southeastern of Petrovouni is relocated westwards of the river bed. Finally, the underground drainage line and the parallel underground watershed in the southern plain became more obvious and more similar to the situation that was “captured” last March.
The flow of the river was measured at 927 m3/h at the area entering the plain and 833 m3/h at the river mouth. As shown by the measurements, the river was feeding the quaternary aquifers.
7 Discussion - Conclusions
The piezometric maps mentioned in this paper were compared to those of two other studies at the same area (Dimakopoulos 1991, Arvaniti and Gerasimopoulos 2009). The major difference is the absence of the main underground drainage line. It is estimated that this difference is owed to the limited number of wells included in the other studies (6 and 31, respectively).
From the maps of the four successive periods, it appears that the shape of the piezometric surface changes in the central and southern parts of the plain only dur-ing the period of October 2007. This variation is possibly due to the groundwater level dropdown, which results in the piezometric surface approximating the perme-able-impermeable surface. The variation in the groundwater level of the water table between wet and dry season (March and October 2007) is an average of 1.4 m.
During the studied periods, in the northwestern part of the plain, the curves of the piezometric lines show concave towards downstream; this would normally suggest that the water tables feed the stream. However, in the entire area, the phreatic water table is located at lower elevations than the river bed. Thus, the only possible water movement is that from the river to the water tables. Therefore, the shape of the piezometric lines that forms the underground drainage line, which
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concurs with the riverbed, is due to the feeding of the formations under riverbed from the adjacent formations. For this reason, this shape is maintained throughout the year. At the area southeastern of Petrovouni, during the wet periods (March-January), the curves of the piezometric lines show concave towards upstream; this proves the feeding of the water tables from the river. These are confirmed by the flow measurements of the river, since feeding from the river to the water tables took place during all the periods that the river had flow.
In the southern part of the plain, near the carbonate rocks, an underground drainage line and a parallel underground watershed appears on the maps created for all periods. These forms are limited in length due to the absence of wells. The fact that these forms exist during the four periods is more likely owed to potential variations in the altitude of the bedrock in the region.
References
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