The Ground Water Potential of Santorini Island

The mean annual precipitation of rain is 378 mm for a period of 30 years with a very dry summer period. The mean air temperature is 17 ^oC.

The climate is thus subarid but the relative humidity is abnormal with rather high values even during summer (60 %); this situation is related to the geomorphological aspect of the island; more precisely, it is due to the caldera zone with its high cliff, and forms favorable conditions for agriculture due to the retention of dew by the hygroscopic soil of the volcanic cover of the island.

Hydrologically the runoff can not be seriously taken into consideration in the assessment of the hydrological balance. On the other hand the nature of the volcanic cover of the "pumice sequence", even though it presents some porosity and sometimes has been reworked, does not permit, in relation to the hydrometeorological conditions, any notable efficient infiltration for groundwater development. A further variation but without essential difference occurs in the peripherical more reworked island of Thera, where a heterogenous phreatic aquifer of very low potential is developed; there the waters are of medium to bad quality due to the sea intrusion. A quantitative approach to the possibilities of the volcanic cover groundwaters results in real annual evapotranspiration losses of 340 mm and to 1.10⁶ m³ per year groundwater renewed reserves; the water is governed by a divergent flow and does not permit the possibility of any serious exploitation.

The only rocks which could be of some importance are the marbles of Profitis Elias on Thera island, that are in great part isolated from the risk of sea intrusion and present, due to karstification, a notable coefficient of efficient infiltration. The main groundwater exploitation work is located in that area, supplied by karstic transfusion, fairly attacked by the sea. The quantitative approach indicated that there is an annual groundwater movement of 0.3.10⁶ m³, but localised in a distinct limited area, with the possibility of a slight augmentation of the actual eXploitation quantities.

In conclusion, the groundwater potential of Thera is not able to satisfy even the actual water necessities of the island and other solutions have to be  found (more systematic collection and storage of small quantities of rain and surface runoff, desalinization schemes).

INTRODUCTION

Santorini, situated in the southern part of the Cycladic group of islands, is universally known for its famous volcano and its archaeological treasures, dating mainly to the Minoan age, which were covered by the disastrous eruption - Krakatau type - around 1500 B.C. The volcano itself belongs to the south Aegean volcanic arc of calcalcaline formations.

Santorini constitutes a group of islands (fig. 1), the largest being Thera island with a crescent-like shape; Thera, together with the islands of Therassia and Aspronissi, forms a ring enclosing the important gulf of Santorini (83 km ²) which is one of the greatest and most imposing calderas in the world, dating to the 1500 B.C. eruption. The small volcanic islets of Kamenae, of recent to contemporaneous age (196 - 1951 A.D.), emerge from the centre of the caldera.

• The geological environment

The geological, volcanological and petrographic studies done on Santorini, and a list of references, can be found in the First International Congress of Thera in 1969 (Acta) and in this second Congress; some recent papers are also mentioned in the reference list of the present study. Here, for the sake of completeness, we will try to depict briefly the general geological conditions, omitting the details which do not concern our principal subject, the ground water potential.

The core of the group of Santorini islands, occuring in Thera, consists of the semi-metamorphic rocks of the prevolcanic island, which had been formed from the breaking-up of Aegeide (geological Island of Strongili); that is, low grade metamorphic marbles of triassic age (with Megalodon) and phyllites of eocene age (with foraminifera). The main outcrop of this basement forms the highest point of Thera, the mountain of Profitis Elias (fig. 1).

The other formations are almost all of volcanic origin of recent and contemporeneous age: various andesites to rhyodacites in domes and lava flows, pyroclastic material (tuffs-ashes) and ignimbrites. These formations are the results of successive eruptions of different distinct sectors of this volcanic center of the Cyclades. At about 1500 B.C. the terrible and destructive eruption took place in high frequency phases, during which the unique island of that time - "Strongili" - broke into pieces, at the same time as the famous caldera was formed. The three islands Thera, Therassia and Aspronissi were left like vestiges. The products of the eruptions covered the area of these islands with layers of pumice and volcanic ash at the very time that its civilization flourished. Later on, probably from 196 A.D. (or 49 B.C.) up to the present time, the eruptions of the volcano formed the dacite-made islets of Kamenae, situated in the centre of the caldeira.

At the present time the whole surface of the three main islands of Santorini is covered by the layers of pumice, by ash ejecta of an overall thickness in some places of more than 60 m. The most ancient of the volcanic rocks are visible only in some high zones and particularly in the inner steep cliff of the enormous caldeira.

The lower part of this cover from the great eruption, immediately on top of the Minoan soil, consists of a layer of pumice of large dimensions, 4 m thick. Other layers of similar constitution, but with materials of smaller size, follow, and finally the upper layer consists of small size pumice and volcanic ash; this is the "Thera soil or earth", a type of pozzulana exploited for the cement industry etc.

This succession in the cover does not present the same order everywhere, but in any case it points to about three or more violent and successive eruptions, from the time of the caldera formation.

At the peripheral zone of the islands exogenous factors have produced either a kind of alluvium formation through transposrtation of the cover material, or a deterioration through modification of the initial structure of the volcanic deposits.

• The climatic-hydrologic environment

The climate of Santorini could be characterised as subarid if temperature and precipitations were taken into account (for example index de Marton: 14); but it is not entirely so if the relative humidity is considered. This is very important for the vegetation, because of the decline in the needs for irrigation during the dry season of the year.

More precisely, the mean annual precipitation goes up only to a height of 355 mm (45 years of observation in Thera itself) or a monthly height of 85 mm; 30 years of observations give a 378 mm average. The mean number of rainy days is about 60 (12 years of observations); in this number the days with atmospheric water traces have been included. Sometimes during summer it is possible that there is absolutely no precipitation for more than 4 months. Generally, 70 % of the rain falls from November to February (fig. 2). The mean annual air temperature is 17^o C (30 years of observations) with the extreme average of 24,5^o and 10^o C.

On the contrary the mean relative humidity goes up to 68 % and during the hottest months of summer it remains in the order of 59 - 60 %. In other words it is greater than the values of humidity in the surrounding regions in Greece, where the rainfall is much higher.

This situation could probably be attributed to the fact that the vapors, during the daily evaporation of the seawater of the caldera, reach the height of the cliffs of the interior face of the island, reaching a height of up to 350 m.

So the vapors are dilated due to the lowering of the atmospheric pressure and the temperature falls adiabatically, reaching their saturation point. This fact, reinforced by the low wind intensity during a long time of summer, causes the formation of dew which soaks the soil to a notable depth (for more details see Stoyannis 1971).

In addition the water that falls on the ground cannot generaly infiltrate to deeper horizons, although the soil does not appear to be impermeable, because of the absorption caused by the hygroscopic character of the cover of the island.

This fact plays obviously an important role in the establishment of the hydrogeological conditions of the island.

THL HYDROGEOLOGICAL CONDITIONS: POTENTIAL AND POSSIBILITIES

• The phreatic waters aquifer: the porous media; general situation.

The islands possess - apart from the mountain of the prevolcanic rocks of Profitis Elias and some other individual points - a smooth morphology extending from the sides of the cliffs of caldeira towards the peripherical coasts.

This gentle slope is transversed by beds of small torrents, sometimes well notched, which however are deprived of water even during the rainy period of the year. Limited flows could only occur sometime after a heavy rain. (A different system in recent past times is evidenced by the torrential materials in the Akrotiri area on the pumice sequence).

As a result of this situation the runoff cannot be seriously taken into consideration in the assessment of the hydrological balance. On the other hand the "pumice sequence" cover of the islands has a high apparent porosity and, due to the rework, appears at the same time sufficiently loose (evidence from granulometric data, Bond & Sparks 1976). However no considerable infiltration can be expected here.

It is precisely the nature of this cover, typical of the island, that governs everything, because the hygroscopic role of its constituents - produced by alteration and evolution - is decisive, as has been mentioned in the previous paragraph. In addition, the retention of water is favoured by the "blind" porosity of the pumice.

This situation decreases to an extreme degree any efficient infiltration, favours evapotranspiration and, together with the meteorological conditions, does not permit the development of any phreatic water table of importance in Santorini.

A certain variation of this general situation, but without any essential difference, occurs in the eastern peripheral zone of the island of Thera (fig. 1), where rework and alluvial transportation is more important and the cover looser. So a phreatic-water table, heterogenous in the horizontal sense, but of very low water bearing and storage capacity, has been developed. This is the situation in the areas North of Oia and Megalo Vouno, East of Merovigli - Fira - Pyrgos and in the coastal region of Emporio. The ground water of this aquifer is of medium to bad quality due to the sea intrusion (Cl^-: 1000 ppm in the area of Emporio). Certain wells with very low yield are capable of providing drinking water, but the Cl^- ion concentration does not fall below 200 ppm.

It is therefore evident that this periferal aquifer of Thera can only satisfy local domestic needs.

• A quantitative approach to the porous media water potential

The above qualitative observations are founded on and proved by an indicative calculation of the infiltration by means of the annual hydrological balance of the island in the area of the "pumice sequence" cover:

P = E + R + I ± ΔW (with P the rain precipitation; E the evapotranspiration; R the runoff; I the efficient infiltration and ΔW the variation of water volumes, which has a zero value for an annual hydrologic cycle).

The mean annual precipitation, P, for a 30 years period is of 378 mm.

The real annual evapotranspiration, E, can be calculated by empirical well-known formulas which are convenient for porous formations.

The Turc formula gives: E (mm) =  P (mm) / (√0.9 + P²/L²)

with L = 300 + 25 T + 0.05 T³

and T = 17^o C : the mean annual temperature of air on the island for the period of 30 years.

The resulting evapotranspiration is 368 mm. The less common formula of Contagne gives: E (m) = P (m) - LP ², with L = 1 / (0.8 + 0.14 T) or E = 333 mm.

Although the Turc formula gives generally the better results, it is necessary - in the case of Thera - due to the wind system and the relative air humidity to reduce the value of 368 mm. So it appears that a 340 mm evapotranspiration is a realistic approximation.

There remain, consequently, just 38 mm for efficient infiltration and runoff.

From this quantity it is certain that the infiltration cannot exceed 20 mm per mean hydrological year.

This level of infiltration is extremely low; it corresponds, in the area of 50 km ² which constitutes the "impluvium of the pumice sequence", to an annual quantity of 1.10⁶ m ³. In addition, if it is considered that the groundwater flow does not converge on any particular zone, but it is dispersed peripherally with a final exit into the sea (fig. 1), no exploitation of even moderate significance can be expected.

The following table summarises the above analysis:

The hydrological balance of Thera.

Prec. precipitation, Ev-Tr. evapotranspiration, Eff-Inf. efficient infiltration, Imp.-Pr. impluvium principal, Tot - Ann - Q. total annual quantities.

• The karstic aquifer: general situation

The only water-bearing rocks which could be of a certain importance are the marbles of Profitis Elias mountain, even though a part of the latter is covered by volcanic ash and pumice layer (fig. 1).

This massif, fairly karstic, with a karst network below sea level also, as a result of the isostatic and eustatic movements is for the most part blocked and isolated from the sea. The lateral isolation is caused by the recent volcanic cover; the frontal one is formed of a band of phyllites of tectonic origin.

Thus an anastomose and elevation of the groundwater before the coastal area is confirmed by the presence of some overflow springs of low discharge (20 - 30 m³/day). It is there that the only water supply work for the island of Thera has been effected; it opens into the volcanic cover, but it seems to be supplied by karstic transfusions. Its yield however is not sufficient (a few tens of m³/day), and on the other hand the quality of the water is only just acceptable, (approx. 300 ppm Cl^-) and is liable to further deterioration due to the exploitation rate.

• Quantitative approach to the karstic ground water

To complete these data and to have a more precise estimate of the ground water possibilities, the following calculation was made: The surface of the main mountain of Profitis Elias, which is partially isolated from the sea and constitutes the main impluvium of the exploitable karstic aquifer, is 3 km² approximately.

The efficient infiltration can be estimated, from general Greek experience on karstic terrains, as 40 % of the mean annual precipitations. This quantity is reduced to 30 %, because of some volcanic tuffs on the slopes of the mountain and its rather steep morphology. So the ground water quantities which are renewed annualy are 0.35.10⁶ m³. 

From this quantity a portion goes inevitably to the sea (due to the partial isolation), another is exploited for limited water supply purposes, and a last supplies small springs or is transfused in the volcanic lateral cover. The conclusion is that only a small quantity, of the same order as that exploited today, can be additionally tapped, and it is not possible to satisfy current necessities.

Nevertheless, it is certain that the overall water situation and potential was much better before the Minoan eruption. In brief, the surface extension of the karstic marble was greater, lavas were not covered by the pumice sequence, and the fissures and discontinuities could have had the role of drains, and of concentrating water in several zones.

CONCLUSIONS

In conclusion it is clear that actually the ground water potential of the Thera island is very low and unable to satisfy the basic necessities. The population (1971) is 6.500 habitants, and during 1972 8.300 tourists passed the night on the island. The annual requirement of the city of Fira was 50.000 m³ (1971), with a daily consumption of 230 m³; with the touristic development of the island it is expected that by the year 2.000 the annual requirement will be 230.000 m³ with a day-maximum of 1000 m³ (data from the Ministry of Interior).

For a radical solution, and before deciding on a desalinization scheme, the possibilities of creating small dams and rain-reservoirs has to be examined. This means a detailed study of the hydrological system of streams and of the possibilities of exploitating the small (if it is true) runoff quantity in connection with the reservoir-evaporation losses. It is useful to note here that on Myconos, which has slightly better hydroclimatic conditions, and where a very low ground water potential occurs, a surface water exploitation solution is already projected (1977).

To conclude this paper we will just mention also the presence of various mineral springs, either alcaline or gaseous and thermal, of the submarine bank of the volcano of Kamenae and in the periphery of the caldeira.

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