The Consequences of Volcanic Activity of the Minoan Period on the Hydrogeological Conditions of the Island of Thera

As has been proved by wells, this formation consists of black grits passing downwards to fine-grained sands.

An unconfined aquifer developed at a small depth below the surface into the palaeoedaphic formation and was used for the water supply by the population of the island.

The paroxysmal volcanic activity which followed resulted in the complete covering of the above palaeoedaphic formation by pumice layers (Upper Series) 5-73 m thick.

This series, consistingof three units with different permeability (coarse-grained pumice at the base, whitish intermediate concentrations and chaotic ash current upwards) acts as a low-permeable formation (T = 10^-4 - 10^-6 m³/sec/m).

The unconfined aquifer of the palaeoedaphic formation thus obtained the hydraulic characteristics of a semi-confined aquifer, as proved by hydrogeological investigation.

Today this aquifer is the most important in the island of Thera as far as water reserves and quality are concerned. The optimal exploitation of this aquifer is urgent, since its recharging is very difficult, due to the low rainfalls in the area and the relatively low and uncontinuous permeability of the overlying pumice.

INTRODUCTION

Tourism has developed rapidly during the last fifteen years on Santorini. The impressive caldera, the volcano, and its unique Cycladic architecture have given this Cycladic island its highly impressive, unique form. The ruins of a village of the Minoan civilization excavated near Akrotiri are also of great archaeological interest.

A consequence of the above-mentioned development of tourism is the increase in the demand for water, mainly for domestic consumption. To deal with this problem, a hydrogeological investigation was carried out by the Institute of Geology And Mineral Exploration, and the approaches hitherto adopted towards the hydrogeological conditions of Thera were revised in the light of data obtained.

PALAEOHYDROGEOLOGICAL CONDITIONS

Before the beginning of the paroxysmal volcanic activity during the Minoan period, the upper surface beds of Thera consisted for the most part of pyroclastic formations, which gave the island a dark, grey-brown colour. As a result of the weathering and transportation of these materials, an almost unified alluvial bed was formed in the eastern part of the island, consisting of unconsolidated black grits in the upper areas, with a gradual transition to fine-grained sands in the lower. This natural granular graduation of the materials of the formation is developed at the surface over a small width - about 500 m to 1000 m - indicating low surface run-off conditions, similar to the present ones, during that period.

From a group of drilled wells the lenticular development of the alluvial formation was proved, dipping 5^o-7^o towards the sea (Fig. 1). Its thickness usually ranges form a few centimeters to 3 meters, in extreme cases reaching 8 m in the Exo Gonia area and 15 m in the Emporio area. The greatest thickness often occur in the small palaeogeographic basins which coincide with the present ones, while they are smaller on the ridges.

An almost uniform unconfined acquifer of important capacity was extended into this palaeoedaphic formation due to the relatively coarse-grained composition of its materials. The quality of the water in the above mentioned acquifer has deteriorated as a result of sea intrusion. One piece of evidence supporting the above is the presence of algae remains in the area of the villages of Kamari and Exo Gonia. From this acquifer, the most important on the island, and from other, smaller ones developed in the coastal areas (Emporio - Akrotiri), it is deduced that the water-supply and irrigation needs were met by wells excavated at the lower topographic points.

POST-EURPTION HYDROGEOLOGICAL CONDITIONS

The volcanic activity of the Minoan period resulted in the complete covering of the island and consequently of the palaeoedaphic formation by the deposition of the upper pumice series. In general, the morphology of the island has not basically changed, as the old morphological lines were preserved; the dimensions of the island, however, were increased in height and width. Exploratory drilling proved that the thickness of the upper pumice layer ranges from 5 to 73 m. The greatest thicknesses were observed at a distance of about 6.5 km from the volcano.

In the Karterados-Vourvoulos area the pumice thickness reaches 46-47 m; in the Exo Gonia-Kamari area it ranges from 45 to 52 m, while the maximum thickness was observed in the Messaria-Vothonas area, where it reached 73 m. From the above distribution of the thicknesses of the upper pumice layer it is concluded that during the paroxysmal volcanic activity, west to north-west breezes were blowing.

As already described by other scientists, the lower part of the pumice formation consists of coarse-grained compact pumice, the intermediate of whitish pumice concentrations, and the upper part of pumice with chaotic ash currents. However, drilling has proved that the lower coarse-grained pumice is not continuous, which is of special importance to the formation of the new hydrogeological conditions.

The covering of the entire palaeoedaphic formation by a pumice layer involved considerable changes in the hydrogeological conditions of the island, which developed as they are today. The unconfined aquifer developed in the palaeoedaphic formation changed to a semi-confined aquifer, due to the low permeability of the upper pumice series.

Pumping tests have shown that the lower compact coarse-grained pumice is almost impervious, with T < 10^-5 m³/sec/m, while the intermediate and upper pumice have medium to low transmissibility, ranging from 10^-4 to 10^-5 m³/sec/m, depending on the included ash proportion. In the semi-permeable formation of the upper pumice, a low capacity aquifer is formed about sea-level, which is in contact with the aquifer of the palaeoedaphic formation. The latter has a coefficient of transmissibility (T) ranging from 10^-2 to 10^-3m³/sec/m, depending on its granular composition.

Finally, two main aquifers developed in the eastern part of the island; the first, in the palaeoedaphic formation, is semi-confined and has the most significant water capability, and the second is unconfined and has a very low capacity. These two aquifers behave as a unified aquifer with a hydraulic gradient of 0.5-1‰, since they communicate, as a result of the discontinuous distribution of the lower coarse-grained pumice.

WATER QUALITY

The quality of the waters was examined by sampling, which was carried out over the entire extent of the palaeoedaphic aquifer. A satisfactory - for the island - water quality was observed, due to the partial protection of the aquifer from sea intrusion by the presence of pumice.

Excessive pumping from the aquifer during recent years, in combination with the restricted water reserves, has resulted in the continuous brackishness of some parts of the aquifer, e.g. in the Monolithos and Karterados areas, where the Cl^- content is now greater than 700 and 600 ppm respectively; these values exceed the limits for drinking water.

WATER RESERVES

The water reserves of the palaeoedaphic aquifer in the section between Kamari and Karterados, were estimated at 4 x 10⁶ m³. However, because of uncontrolled over-pumping, a water quality abatement occurred, resulting in the decrease of the water reserves which are today estimated to be 1 x 10⁶ m³, The unfavourable hydrological factors prevailing in the area, such as the low rainfalls and the relatively high evaporation rate, reduce the potential for natural recharge of the aquifer. From the above quantities the actual amount available for the aquifer recharge could not exceed 100 x 10³ m³/year. This is due to the coarse-grained pumice layer overlying the aquifer which acts as a practically impermeable formation.

CONCLUSIONS

The volcanic activity which resulted in the deposition of the upper pumice series during the Minoan period had significant consequences for hydrogeological conditions on Thera. It was the reason for the development of a semi-confined aquifer in the palaeoedaphic formation which is now the most important aquifer on the island as far as the quantity and quality of water are concerned. The very low annual recharge of this aquifer - due to the climatological and geological conditions - combined with the contamination of the aquifer from sea intrusion, are the main factors reducing its potential for exploitation.

The annual maximum water-yield from the aquifer - without noteworthy qualitative effects - should not exceed 100 x 10³ m³, an amount sufficient to meet the needs of the surrounding municipalities.

The artificial recharge of the aquifer by means of surface run-off should be taken into account in order to obtain the maximum recharge of the aquifer, following the solution of numerous technical problems.

The long-term preservation of satisfactory water-quality in the aquifer will be achieved by the method proposed above, under optimal exploitation conditions.

---------------------------------------------

----------------------------------------------------