Recent Hydrogeologic Study of the Vis Island

GEOLOGIJA 45/2, 419–426, Ljubljana 2002 doi:10.5474/geologija.2002.042

Recent hydrogeologic study of the Vis island

Janislav KAPELJ, Josip TERZI], Sanja KAPELJ & Mario DOLI] Institute of geology, Sachsova 2, Zagreb, Croatia

Key words: Vis Island, hydrogeology, structural geology, hydrogeochemistry, karst, seawater influence.

Abstract

The Vis Island belongs to the group of the Middle Dalmatian islands. It comprises an area of about 90.2 km2. Morphologically, three belts of highlands and two depressions with karst poljes are significant. The highest point on the island is Hum with 587 m a.s.l. the island’s water supply is organized from the water-supply station “Korita”, situated in the central part of island, in tectonically formed depression. There are two additional captured objects: the well K-1 above the Komi`a town and the spring “Pizdica”. The most important hydrogeological role on the island have two hydrogeological barriers, one in the Komi`a Bay, completely made of impermeable igneous and clastic rocks, and another one, the recently recognized relative barrier in the area of Dra~evo, Plisko and Velo polje. Since the island karst aquifer is in permanent dynamic relation with seawater, classical geologic, structural and hydrogeologic investigations have been performed with application of hydrogeochemical methods taking into account the natural chemical tracer content of groundwater and its variations in different hydrologic and vegetation conditions. Precipi- tation regime is very unfavorable with regard to the recharging of island’s aquifer, because dry periods are usually very long. During the summer tourist season, when the number of inhabitants and fresh water consumption considerably increase, amounts of island’s groundwater suitable for water supply and irrigation rapidly decrease. Sometimes, insufficient quantity of fresh water on the Vis Island causes restrictions. Concerning the development of tourist potential and the present agricultural activities, summer lack of water is a serious restrictive factor. Some results of the performed hydrogeological study, important as a basis for island’s fresh water potential assessment, will be presented.

General data are Orlovica, Mali Hum and Široko brdo as well. The territory of Vis island has a dis- The Island of Vis, belongs to the group of tinguished Mediterranean climate. The char- the middle Dalmatian islands. Its area is acteristics are long and hot summers with 90.2 km2, which places it on the ninth place very high medium air temperatures in Au- among the Adriatic islands. It is situated at gust (26.0 °C) and droughts during several 55 km from the land, so at the same time it months, so the years without rain for 3-4 is one of the farthest inhabited Adriatic is- months are not rare. The winters are soft lands. Morphologically, three hill chains and (medium air temperature in January is two valleys containing several smaller karst 10.1 °C). Mediterranean evergreen vegetation fields are well distinguished. The mentioned of low bush type is dominant. Other agricul- morphological entireties strike east – west. tural plants are olives, lemons, mandarins The highest peak is Hum (587 m), located and several indigenous sorts of grape vine. about 2.5 km southeast of Komià, and there Similar to other Adriatic islands, there are 420 Janislav Kapelj, Jospi Terzi}, Sanja Kapelj & Mario Doli} no surface waterflows. Periodical torrent flows The youngest Quaternary (Pleistocene) se- can emerge after heavy rainfalls, mainly in diments are found in joint systems, local the area of Komià. depressions and karst fields. There are silty The basis for water supply of the Vis is- clays (terra rossa – ts) and talus breccias – land is the pumping site “Korita” which feeds (sbr). Characteristic sediments on the island the water supply system. The pumping site are bedded sands – p that are, most likely, is located in the middle part of island and of eolian origin (Markovi}-Marjano- it consists of six deep drilled wells, where vi}, 1978). The hypothesis of eolian origin four of them are used for water supply. Be- of these sediments presumes their sedimen- sides the mentioned, there is the pumping tation from northeast direction, from the site “Pizdica” in Komià bay, which is used surfaces that are today under the sea. These periodically. The problems of groundwater sands there are finds of gastropods (Pupa shortage emerge in dry periods when the muscorum L.) characteristic for Riss and early natural inflows from rainfalls are decreased Würm. to minimum, or they are completely absent, The island of Vis belongs to tectonical whereas the consumption is increased, what unit called “Middle Dalmatian islands” is emphasized during the tourist season when (Borovi} et al., 1977) which is divided in the number of inhabitants is multiplied seve- three smaller units: Bra~ island, Hvar island ral times. During the last several years, hydro- and Vis island. These units are part of the geological investigations were performed on Adriatic carbonate micro plate that was for- the island. On that basis, several variants of med during several tectonic phases, from water supply system improvements have been Laramian at the turn of Cretaceous to Ter- proposed. tiary as the oldest phase, to Pyrenean that dates in upper Eocene and that left the most distinguished marks. The Pyrenean phase Geology and structure caused significant tectonic deformations of terrain with evolution of scaly structures On the Vis island occur rock from Juras- due to the northeast stress. Regional stress sic (Triassic) up to Quaternary age. The old- orientation change during neotectonic phase est deposits are clastic sediments with gyp- of evolution lead to dominantly horizontal 3 sum interbeds of upper Malm age (J3 ?). With movements along the existing joint systems, them, occur spilites and diabases (bb), as as well as to creation of new joint systems well as pyroclastics (w), in the Komià bay. generally parallel to basic axis of anticline The age of these rocks is not yet determined, structure with strike east – west, which sub- so in older papers in literature (Crnola- sides towards the east with average angle of tac, 1953) the mentioned clastics are said 10 degrees. The anticline core is built of to be of lower or upper Triassic age. In newer clastic rocks with gypsum and eruptives pro- papers (Borovi} et al., 1977) they were bably of upper Jurassic age, while the limbs described as clastics of upper Malm age. are built of Cretaceous carbonates: limesto- These rocks are overlain by Cretaceous nes and dolomites. Anhydrites are located in carbonate rocks whose age is well documen- deeper layers (probably below lower Creta- ted paleontologically (Borovi} et al., 1977). ceous) in much wider area than the area of Occur brown-grey limestones of Berriasian Vis Island (Šu{njar, 1967). Regional com- 1 1+2 age ( K1 ) and on them grey dolomites of pression caused diapiric gypsum and anhy- 2 1+2 Neocomian age ( K1 ) lie. The upper stages drite movements, which along with other of lower Cretaceous age (Barremian, Aptian, factors, created tectonic framework for de- Albian) are represented with grey-brown li- velopment of karstification processes. The 3-5 mestones (K1 ). Upper Cretaceous (Cenoma- described structure was cut with several lon- nian, Turonian) is represented by dolomites gitudinal faults (with strike subparallel to that contain interbeds and lenses of lime- fold axis). In the anticline apex, which is 1,2 stone (K2 ). The final unit of Cretaceous is presumed to be practically in the middle of represented with limestones of Senonian age the island and can be traced along the whole 3 (K2 ). Total thickness of Cretaceous carbon- islands length, significant extension stresses ate beds is over 2000 m, and they are the and horizontal block movements existed, tra- most represented on the surface. ces of which can be seen on the numerous Recent hydrogeologic study of the Vis island 421 outcrops, which lead to significant fractur- attention should be given to present karsti- ing of rock mass. Within the recent investi- fication processes and to the processes in gations, structural elements were measured immediate geological history – Quaternary, on totally 62 outcrops, which enabled deter- especially Holocene. Probably the most cru- mination of the field of local stress. In the cial data for such research is the data on sea zones of extension and fracturing of carbon- level variations during the same period. For ate rocks, it came to intense karstification in example, it is presumed that the sea level at rocks liable to such processes, which en- the end of the last glaciation, Würm, was, on abled creation of significant joint pore space the average, about 31 m below today’s sea in the ground. Generally, in the karst ter- level (Šegota, 1968). The tectonic factors rains, anticlines are the suitable environ- of rising and lowering the rock masses also ment for accumulation or directing of ground- influence the sea level in a particular area. water flows. The data exist that the sea level in Vis sur- rounding was lowered for almost 100 m (Markovi}-Marjanovi}, 1978) during Hydrogeology Würm, so the karstification base was about 100 m lower than today’s. Therefore, the The island of Vis is a separate hydro- karstified and permeable rocks could be ex- geological entirety, whose properties depend pected to lie significantly below today’s kar- on local lithological and structural relations, stification base. This fact is very important proper input (effective precipitations) and from the water supply aspect, i.e. the exist- output of water from the system. The output ence and in-depth strike of fresh water lense consists of evapotranspiration, pumping quan- on Vis, and from the aspect of determination tities on wells and outflows on springs. Sig- of the nature of fresh – salt water contact. nificant springs are the ones in Komià bay, The importance also lies in the possibility but smaller appearances in Vis bay and along for deep intrusion of salt water or even the the east and west coast have been registered possibility for existence of old (trapped) sea that are supposed to be connected almost water below the fresh water lens. However, exclusively to surface and shallow ground taking all the facts into consideration, it is waters that outflows after the heavier rains. possible to conclude that tectonics had the Karstification is the natural process that sig- biggest influence on the depth of Quater- nificantly changes hydrogeological medium nary karstification, rather than the eustatic in which it is developed. When this process changes. From the hydrogeological aspect it advances, which is the case in Dinaric and is also very important to emphasize the fact Adriatic belt respectively, the porosity of that the karst forms and cavities of larger rocks is not only of joint type, but also of dimension on the Vis island are mostly filled joint-solution type. Since the space arrange- with fine grained material. This means that ment of carbonate rocks and basic struc- after the phase of intense karstification a tural-tectonical characteristics of the area different phase followed, during which the were pointed out above, it is necessary to water circulation lead to filling the caverns emphasize also the fresh water influence on and joints with silty-clayey and sandy mate- karstification processes. Today’s level of rial. These circumstances lead to evolution of karstification reaches the same depth as the the Vis aquifer, because the groundwater out- fresh water. That surely is not today’s sea flow significantly slowed down owing to se- level because it must be presumed that the condary filling of caverns, cavities and di- fresh water lens stretches below sea level scontinuities, and also the sea intrusion slo- also. Through the geological history the kar- wed down. In this way the dynamic stification process has developed more or equilibrium of these two most significant less continuously so for the describing the water bodies was established. Hydrogeolo- karstification phases through geological hi- gically, the following units can be distin- story we use the term paleokarst. For hydro- guished on the island: the carbonate, pre- geological considerations, the paleokarst ge- dominantly calcareous water bearing rocks nerally is not interesting because its struc- of Cretaceous age, which function as the tures and forms are mostly lost during the permeable area; impermeable igneous, py- later geological processes. Therefore, the roclastic, and clastic rocks, which function 422 Janislav Kapelj, Jospi Terzi}, Sanja Kapelj & Mario Doli} as the hydrogeological barrier, and dolomitic the management optimization of the exist- limestones in the base of sequence of karst ing fresh water reserves on the island and to poljes on the south and north brim of island. find the most perspective areas for new fresh The latter function as a relative hydrogeo- water catchments. The results of such inves- logical barrier that prevents the outflow from tigations are mostly based on determined the aquifer towards the north and south coast, variations in chemical and isotope compo- and at the same time prevents the sea water sition of groundwater, on ionic ratios and on intrusions into the aquifer (fig. 1). The area calculations obtained by geochemical mod- of karst poljes that stretches along the south elling. This paper shows the results gath- side of the island (Plisko polje, Velo polje, ered during the initial phase of hydrogeo- Dra~evo polje) should be especially empha- logical investigations of water potential on sized. In the karstified carbonate rocks un- the Vis island. der the poljes, the filling in is significantly increased, which lead to the decreased hy- draulic conductivity of rock masses which Hydrogeochemical facies today it function as a relative hydrogeological barrier (Terzi} et al., 2002). The measured groundwater temperatures in December 1999 are in accordance with average annual air temperatures on the Vis Hydrogeochemistry island. In June 2000 the higher temperatures were determined on all springs and the The usage of hydrogeochemical methods largest deviations were determined on the and of natural tracers for the investigations springs Batuda, Draga Voda, Velo @alo and of coastal and island aquifers is becoming Gusarica, as the results of higher portion of an irreplaceable approach that enables de- warm water contribution, hence the drain- termination of connections of island aquifer age of shallow parts of island aquifers. The with sea, of salinity origin, of conditions for totall dissolved solids (TDS) in the samples renewal the groundwater and of hydrodyi- collected on the Vis island vary from 380 mg/l namic conditions in the island aquifers. As to over 1300 mg/l (fig. 2). the most common natural tracers the ground- In the groundwaters distant from the coast, water temperature, electrolytic conductiv- the values are lower and the deviations from ity, total dissolved solids and chloride con- two measurings are smaller. In the coastal tent are used. In the recent time the ionic springs the highest concentrations of total ratio, stable isotopes and inert gasses com- dissolved solids, accompanied by wider de- position are also used. However, the ground- viation regarding the time and hydrological water salinity in the coastal area and on the conditions during sampling, were recorded. islands of the Adriatic Sea can be of various Thus, in the samples collected in June, after origins. The most common case is mixing the dry period of several months, the ground- with today’s sea water, then mixing with old water mineralization was higher on all the or so called aged sea water and rarely, the locations except on Batuda and the well Velo regionally limited cases are a consequences @alo in Komià. of evaporites (gypsum, anhydrite) in the li- The basic chemical composition of the Vis thology of karst aquifers. The procedures, island groundwaters mostly originated from such as groundwater overexploitation in dissolution of rocks that build the aquifer. coastal and island aquifers for various needs, The vicinity of the sea caused that the hydro- agricultural activities accompanied by fer- geochemical water types on the Vis island tilizing and often irrigation procedures can belong to the so-called mixed type – from cause the increase of groundwater salinity CaNaMg-HCO3Cl to NaMgCa-ClHCO3. The (Richter, B. C. & Kreitler, C. W. 1992; presence of gypsum and anhydrite in the Fidelibus, M. D. & Tulipano, L. 1986, area of Komià causes the increase of sul- 1990; Gimenez, E. & Morell, I. 1997; phate percentage in waters that outflow in Kapelj, S. 1997; Kapelj, J. et al., 1997, the coastal springs Gusarica and Kamenica 2000). Therefore, on the Vis island, it is ex- II, so the waters adopt a partial sulphate ceptionally important to find the origin and character, and the they are of the CaNaMg- the age of salinization. This is important for SO4HCO3Cl type. Recent hydrogeologic study of the Vis island 423

Figure 1. Schematic hydrogeological map of the Vis island. 1 – fractured, karstified, well permeable limestones; 2 – fractured dolomitic limestones, less karstified; 3 – dolomites of very low permeability; 4 – clastic and igneous rocks, barrier; 5 – Quaternary deposits, karst poljes, barrier; 6 – swallow hole; 7 – spring Q<1 l/s; 8 – intermittent spring; 9 – main drainage direction; 10 – capped spring; 11 – spring; 12 – water supply well; 13 – borehole; 14 – estimated underground connection; 15 – water divide; 16 – underground water divide; 17 – estimated groundwater level; 18 – estimated transition zone. 424 Janislav Kapelj, Jospi Terzi}, Sanja Kapelj & Mario Doli}

Figure 2. The content of totally dissolved solids in the groundwater samples of Vis Island

The springs Pizdica and Velo @alo in the Conclusions and propositions coastal area of Komi`a have the biggest percentage of chlorides. The interesting, and, Synthesis of the existing understanding due to insufficient investigations and moni- of the Vis island hydrogeology makes obvi- toring, up to now unexplainable fact is a ous the fact that the areas of various ground- greater percentage of chlorides during the water qualities can be separated. The middle hydrologically higher waters in December part of the island, the most distant from the 1999. To which extent the mentioned phe- sea, represents the most appropriate area nomenon is connected with sea influence for the groundwater exploitation, because will have to be determined, because the ori- there the ground water has the lowest salin- gin of chlorides can be connected with wash- ity and the smallest deviations of dissolved ing of salt from the surface, where the salt salts in the two hydrologically different parts developed there by sedimentation of the so of the year. The springs in the Komi`a bay called salt-spray water (the sea water parts are all salted; the salinity of the springs Gu- of aerosol dimension), and/or it is the reflec- sarica and Kamenica II mostly belongs to tion of washing out the pollution from the the increased percentage of sulphates, and ground and epi-karst zone by precipitation in the springs Pizdica and Velo @alo, located (fertilizers, septic tanks etc.). However, the on the edge of southern part of the bay, it percentage of sulphates is higher during the belongs to the increased percentage of chlo- hydrologically lower waters. Then, the in- rides. According to the acquired knowledge, creased sulphate percentage in the ground- the problem of shortage of installed pump- waters is the result of drainage of deeper ing capacity in the water supply system of parts of aquifer, where the evaporite sedi- the Vis island does not exist. The problem ments are in the contact with carbonate parts lies partially in the losses in water supply of the aquifer. system and partially in the fact that the Recent hydrogeologic study of the Vis island 425

Figure 3. Piper’s diagram of major ionic composition 426 Janislav Kapelj, Jospi Terzi}, Sanja Kapelj & Mario Doli} groundwaters from the central part of is- particle spends in the ground. Also, the space land (Korita) and karst aquifer are natu- arrangement of fresh and salt water mixing rally drained generally towards the west zones should be investigated, which can be (Komià bay). There, in contact with the established with geochemical investigations hydrogeological barrier (that consists of dia- in deep boreholes that should be drilled for base, spilite and clastic deposits), they flow that purpose. out on the spring “Pizdica”, and after that into the sea. On the south (as well as on the Literature north) part of the island there are no permanent or significant periodical springs, sug- Borovi}, I., Marin~i}, S. & Majcen, @. (1977): Osnovna geolo{ka karta s tuma~em, M gests the existence of an underground rela- 1:100.000, List Vis, Beograd. tive barrier for groundwater flow in the part Crnolatac, I. (1953): Geologija otoka Visa, from Podhumlje to Plisko polje. The waters Zavod za geolo{ka istraìvanja, Zagreb. Fidelibus, M. D. & Tulipano, L. (1986): Mi- from the Quaternary deposits in this part of xing phenomena owing to sea water intrusion for terrain probably flow out in the subsurface the interpretation of chemical and isotopic data sea spring “Kut” in Komià bay. The nature of discharge waters in the Apulian coastal carbonate aquifer (southern Italy). Proceedings of of north and south natural barrie is not yet the 9th Salt Water Intrusion Meeting, Delft. sufficiently known. Between the karst springs Fidelibus, M. D. & Tulipano, L. (1990): “Pizdica” and “Gusarica” there are several Major and minor ions as natural tracers in mixing phenomena in coastal carbonate aquifers of Apu- smaller springs in the Komià bay, the so- lia. Proceedings of the 11th Salt Water Intrusion called “flysch” springs judging from the Meeting, Gdansk. hydrogeochemical properties and from the Gimenez, E. & Morell, I. (1997): Hydrogeo- chemical analysis of salinization processes in the outflow regime. Their inflow area is the local coastal aquifers of Oropesa (Castellon, Spain). En- catchment area of Quaternary slope talus vironmental Geology, 29, 118-131. deposits in the immediate hinterland. This Kapelj, J., Kapelj, S. & Biondi}, B. point at, the fact that in the Komià bay the (1997): Spatial variations of the fresh – salt water interface of a Croatian coastal aquifer in the town igneous and clastic rocks function as a com- of Biograd area, 24th Hydrology and Water Re- plete underground and surface hydrogeolo- sources Symposium, Auckland, New Zeland. Po- gical barrier, because the karst aquifer is ster presentation-Extended abstract. Kapelj, J., Kapelj, S. & Markovi}, T. drained on the boundary parts of surface (2000): Hidrogeolo{ke i hidrogeokemijske zna~aj- manifestation of the mentioned barrier (Piz- ke podzemnih voda otoka Raba, 2. hrvatski geo- dica and Gusarica). The main goal of future lo{ki kongres, Cavtat – Dubrovnik, Zbornik ra- dova, 621-627, Zagreb. activities should be the learning about the Kapelj, S. (1997): Hidrogeokemijske zna~aj- hydrogeological properties of aquifer on the ke Vranskog jezera na otoku Cresu. Doktorska island that include: the groundwater rene- disertacija. Sveu~ili{te u Zagrebu, Rudarsko–geolo{ko–naftni fakultet. Str. 96. Zagreb. wal dynamic, hydrodynamic conditions, ave- Markovi}-Marjanovi}, J. (1978): Kvartar rage retention time for aquifer groundwater na ostrvu Visu, Glasnik Prir. muzeja (A), 32, 145- and the evaluation of contribution of waters 161 (1977), Beograd. Richter, B. C. & Kreitler, C. W. (1992): of various origin with special attention to Identification of Sources of Ground-Water Salin- the salinity problem. Under the term ground- ization Using Geochemical Techniques. Project water dynamics the knowledge of the level Summary – EPA, 1-8. of new groundwater contributions influenc- Šegota, T. (1968): Morska razina u holocenu i mlaðem Würmu, Geografski glasnik, 30, 15-39, ing the total water volume in the ground Zagreb. and the seasons they are mostly related to, Šu{njar, M. (1967): Stratigrafska i struktur- has been considered. How much the new na problematika otoka Visa, Geolo{ki vjesnik 20, 175 – 189, Zagreb. contributions are homogenized with the Terzi}, J., Kapelj, J. & Kapelj, S. (2002): existing water in the ground determines the Hidrogeolo{ki pristup problemima vodoopskrbe hydrodynamic conditions in the aquifer, and jadranskih otoka – primjer otoka Visa. I. znanst- veno–stru~ni skup “Turizam, vodno gospodarstvo the average retention time in the aquifer i za{tita mora”, Opatija, poster – prezentacija i refers to the average time that the water saètak, 349-350.