Karst aquifers on small islands the island of Olib, Croatia seminar/Karst_aquifers_Olib_Island.pdf · Study area—geology and hydrogeology The island of Olib is a part of the Dinaric

Karst aquifers on small islands—the island of Olib, Croatia

Tatjana Vlahović & Boris Munda

Received: 25 October 2010 /Accepted: 14 October 2011 /Published online: 3 November 2011# Springer Science+Business Media B.V. 2011

Abstract Water supply is a major problem in theAdriatic islands, especially during the summer tour-ism season, and represents a limiting factor to theislands’ further economic development. Much atten-tion has been given to water supply solutions,primarily in terms of attempting to use the existingisland water. Unfortunately, few islands have favour-able hydrological conditions to accumulate significantquantities of surface water or groundwater. In theperiod from 2001 to 2004, investigations wereconducted on many islands to define their ownfreshwater or partially brackish water resources sincedesalinisation technology could resolve a significantpart of the water supply demand on small and distantislands. Due to the specificity and complexity ofresearch in karst areas, the study was conducted inphases and included the geological and hydrogeolog-ical reconnaissance of the island, aimed at locatingpossible areas on the island where the necessaryquantities of groundwater of adequate quality couldbe captured; a detailed hydrogeological mapping of

the specified areas, geophysical investigation and testdrilling; and, over several days, test pumping of themost promising borehole. One of the islands investi-gated was the island of Olib. The conducted surveysindicated that it is possible to pump about 3.5 L/s ofgroundwater from the karst aquifer of the island ofOlib, which fully complies with the sanitary quality ofdrinking water.

Keywords Groundwater . Hydrogeological research .

Island of Olib .Water supply

Introduction

Most Adriatic islands in Croatia are built predomi-nantly of karstic carbonate rock with the surfacehydrographical network poorly developed. In suchterrains, due to increasing karstification, major waterquantities infiltrate and flow underground. The fresh-water systems on the islands are also limited due tothe wide, open influence zone of the sea, whichcauses large freshwater quantities to flow diffuselyinto the sea. At present, only three Adriatic islandsmeet their water supply needs exclusively from theirown sources (Cres, Lošinj and Vis). All the others,particularly smaller and more distant ones, have eitherpartial water supply from own sources (surface lakes,spring and groundwater captured through wells andgalleries), with the remaining necessary quantitiestransported by submarine pipelines from the

Environ Monit Assess (2012) 184:6211–6228DOI 10.1007/s10661-011-2414-y

T. Vlahović (*)Croatian Natural History Museum,Demetrova 1,Zagreb, Croatiae-mail: [email protected]

B. MundaGeoaqua d.o.o,Hanamanova 16a,Zagreb, Croatia

mainland, or their total drinking water demand mustbe transported from the mainland by water tankers. Inthe period from 2001 to 2004, investigations werecarried out on a dozen small and distant islands aspart of a study on potential solutions for water supplyby using the islands’ own freshwater resources or bydesalination of slightly brackish groundwater (Starc etal. 1997). Due to the unique, complex nature of karstand attempts to find the most feasible solutions towater supply, investigations were carried out in threephases. The first phase included the identification ofpromising areas on the islands that could enableabstraction of groundwater quantities of adequatequality, the second included detailed hydrogeologicalinvestigations of the identified areas, geophysicalinvestigations and investigation drilling, and the thirdcomprised several days’ investigation pumping themost promising boreholes.

One of the islands included in the investigationswas the island of Olib, which belongs to the Zadararchipelago (Fig. 1). The island has a surface area of

25.63 km2, measures 9.3 km in length and up to6.3 km in width, narrowing down to 1.4 km in itscentral part. It stretches in the meridional directionnorth−south, is built of carbonate deposits, mostlylimestone with some dolomite, and belongs to theDinaric karst belt (Fig. 1). All permanent inhabitantslive in the only settlement, Olib, located close to thesea on the southwest part of the island. Thepopulation is 147 according to the 2001 census,increasing up to 2,000 in the tourist season. There areover 100 private water tanks and a public water tanknear the church which is presently used only astechnical water for sanitary use (Magdalenić 1991).Groundwater is abstracted from numerous privatelyowned shallow boreholes (over 20) and three exca-vated wells (KZ-1, KZ-2, KZ-3) located in thesettlement (Fig. 2a, b). These boreholes are drilledmechanically, with a hammer (no core removal),mostly to a depth of 1–2 m below sea level. Boreholedischarge is about 0.2 L/s. The salinity of water fromthe boreholes also generally decreases inland, whilst

Fig. 1 Geographical locationof the island Olib, Croatia, inthe Dinaric karst belt

6212 Environ Monit Assess (2012) 184:6211–6228

in boreholes at the highest levels, water salinity inJuly is <200 mg/L chloride (Munda and Vlahović2006). Public excavated wells are located in thesettlement, 80 to 500 m from the sea, with chlorideconcentrations in well water decreasing proportionateto the distance from the sea (Munda and Vlahović2006). Well KZ-1 is 4.5 m deep, with a diameter of1.8 m and a 0.5-m water head above sea level. Thewater chloride content is about 400 mg/L, indicating theinfluence of seawater, which increases proportionally todrawdown. The chloride concentration of water in wellKZ-2 is about 360 mg/L chloride. Well KZ-3 is 12 mdeep, with a diameter of 1.4 m. The chloride content,with a low yield of about 2–3 m3/day, is within thelimits permitted for drinking water, i.e. <200 mg/Lchloride.

As these wells do not satisfy demand, a study toidentify additional water quantities was initiated. Theminimum necessary groundwater quantity (fresh orbrackish water) in the dry period is 3 L/s, whichwould satisfy the needs of both the domestic and thetourist population in the tourism season.

Carbonates, in particular those of the karstifiedcoastal areas and islands, are characterised bycomplex hydrogeological relations. They have beeninvestigated and described, though the methodolo-gies vary. Some authors emphasize geochemistry(Capaccioni et al. 2005; Vlahović and Bačani 2005;Buljan et al. 2006; Kallioras et al. 2006; Terzić et al.2007a), some geophysics (Šumanovac et al. 2003;Šumanovac 2005; Terzić et al. 2007b), whereas otherscalculate water balance (Jones and Banner 2003;Terzić 2004; Bačani et al. 2006; Kim et al. 2006;Petalas and Lambrakis 2006; Prieto et al. 2006). Itshould be noted that contrary to the researchconducted in the alluvial (Pannonian) part of Croatia,in markedly karstic terrains, such as the Dinaric karstbelt, for which modelling is difficult, such inves-tigations have been very rare.

On the island of Olib, the investigation includedboth a geological and hydrogeological survey of theisland and a detailed geological and hydrogeologicalmapping of different perspectives of the area to thesouth of the settlement of Olib. Sites for geophysicalinvestigation were determined based on an analysis ofthe resulting maps, whilst sites for investigationboreholes were determined based on the correlationof the results of the mapping and geophysicalinvestigation. Test pumping of the boreholes was

then carried out and the physicochemical properties ofgroundwater measured.

Study area—geology and hydrogeology

The island of Olib is a part of the Dinaric karst beltwhich is characterized by deep, irregular karstificationand is strongly influenced by tectonics, compression,reverse faults and folding. The geological composi-tion of the island is dominated by Upper Cretaceouscarbonate sediments and, to a lesser degree, Paleo-gene carbonate deposits and Quaternary clasticdeposits (Mamužić et al. 1970; Mamužić and Sokač1973) (Fig. 2a). Cretaceous sediments comprisedalternating limestone and dolomite, dolomitic lime-stone and dolomite of Cenoman–Turon age (K2

1,2),and Turon–Senon rudist limestone (K2

2,3). Paleogenedeposits are almost entirely built of Eocene forami-niferal limestone (E1,2), specifically miliolid, alveoliand nummulite limestone. According to geotectoniczoning (Mamužić and Sokač 1973), Olib belongs tothe Zadar archipelago tectonic unit. In this structuralunit, vertical to slightly sloped folds stretching in thenorthwest–southeast direction are characteristic. Thefolds are interrupted by faults of the same stretchingdirection, and both are further crisscrossed bytransverse and diagonal faults. The majority of theisland is built of the Upper Cretaceous anticlinelocated in the south and southwest part of the island.The anticline ridge is built of Cenoman–Turonalternating limestone and dolomite with flanks ofTuron–Senon rudist limestone. The layers are gener-ally slightly sloped, from 5° to 35°, with the steeperones quite rare. The anticline is interrupted by a seriesof vertical and subvertical faults, of which the faultsof the northwest–southeast and north–south stretchingdirections are the most pronounced. From the south-east, through a reverse fault, the anticline structure isoverhung by the southwest, interrupted by a flank ofthe Cretaceous–Paleogene syncline. The syncline isbuilt of Cenoman–Turon limestone and dolomite, andTuron–Senon rudist limestone transgressively coveredby Eocene foramniferal limestone.

Hydrogeological relations are characterised bygeological composition and morphological features.They are a consequence of lithostratigraphic andstructural–tectonic conditions. Such terrain character-istics strongly influence hydrological conditions. In

Environ Monit Assess (2012) 184:6211–6228 6213


combination with climatic conditions, this plays a keyrole in the formation and dynamics of karst and, inparticular, island aquifers.

Carbonate rocks of fissure–cavernous porosity andvarying permeability were identified based on thelithological composition, genesis and structure; theintensity and depth of karstification; and the level offissure backfilling, i.e. their interconnections on theisland. In terms of their porosity, carbonate depositsare divided into highly permeable and medium tohighly permeable deposits (Fig. 2b). Highly perme-able deposits include well-karstified Turon–Senonrudist limestone (K2

2,3) and Eocene foraminiferouslimestone (E1,2). Structural elements (strata slopingtowards the sea), fissure systems and openness ofrudist limestone towards the sea indicate an unfav-ourable structural position of these deposits withregard to groundwater abstraction. As medium tohighly permeable deposits, alternating limestone anddolomite deposits (K2

1,2) were identified. Dolomite,or dolomitised limestone, is less present than lime-stone. Limestones are karstified, and their karstifica-tion level (Tišljar 2001) does not significantlydecrease even in the case of limestone undergoingthe dolomitisation process. The thin interlayer of puredolomite is very rare and has no significant role in thedetermination of the hydrogeological characteristicsof the lithologic member. However, the dolomitisationprocess of this lithologic member has significantlyinfluenced its determination as medium to highlypermeable deposits.

Due to the described geological composition, theisland does not have a surface hydrographic networkwith permanent water flows. Precipitation drainsalmost exclusively through the underground, andsurface water flows appear only after extremely heavyrains. After losing a portion of rainwater to evapo-transpiration, the remainder is mostly infiltrated intothe underground and lost via dispersal on the coast.The direction of groundwater flow is determined bythe structure spreading in the northwest–southeastdirection and a strong fault system of the samedirection, along which a depression descends in

terraces towards the sea. This depression representsthe most fertile part of the terrain, spreading throughthe central part of the settlement Olib. From the Kalacpeak in the island's central part, infiltrated rainwaterflows underground towards the sea. A portion of thecove which serves as the Olib harbour containscoastal springs and submarine springs and is also thelocation of the occurrence of wider diffuse ground-water discharges. Submarine springs also occur in theSlatina cove and the Olib harbour, and their locationscan be seen during calm seas. In the north of theisland, in Šibenska cove, 7 m off the coast and 20 moff its cape, the only brackish spring appears, with ayield of 0.1 L/s (Ivičić and Biondić 1998). Itsdischarge decreases in the recession period, becomingonly a small well in the summer months. Surfaceretention of rainwater is very rare and related to thelocally pronounced limestone dolomitisation processes.In such places, surface fens form and are used aslivestock drinking holes (Fig. 2a).

Materials and methods

Research methods

In 2001/2002, the first phase of the investigation wascarried out on the island. This included a geologicaland hydrogeological survey and focused on theidentification of promising areas for further inves-tigations. An area southeast of the settlement Olib wasidentified in this phase. In 2003, surveys resumed inthe identified area, i.e. detailed geological and hydro-geological mapping, geophysical investigation andinvestigation drilling. The analysis of results obtainedby detailed mapping (structural–tectonic character-istics, strata sloping towards the sea, openness offissure systems towards the north coast of the island)helped determine the locations of geophysical inves-tigations (Fig. 2b) comprising electrical sounding (AB/2=300 m, ten probes), electrical resistivity tomography(in the length of 3 km, six profiles), seismic refraction(in the length of 2 km, three profiles) and geoelectricalmagnetometry (in the length of 1.5 km, two profiles).These methods significantly contribute to the determi-nation of structural and lithological relationships and thegeometry of the aquifer (Šumanovac 2006), especiallyin the karstic terrains where the basic goal is tomap a narrow fracture and fault zones and potential

�Fig. 2 a Geological map of the island Olib with coastal,submarine and brackish springs, surface retention and diffusegroundwater discharge. b Hydrogeological map of the investi-gation area with the position of pumping sites and geophysicalprofiles


underground objects (Šumanovac and Weisser 2001;Engelsfeld et al. 2008). The data of 2D electricaltomography were processed and interpreted accordingto the method of Loke and Barker (1995, 1996),whilst refraction data were processed using thegeneralized reciprocal method (Palmer 1981). Thegeophysical survey locations of boreholes weredetermined according to several features:

(a) Results of the detailed geological and hydrogeo-logical surveys

(b) Analysis and correlation of results of the geophysicalinvestigation

(c) Purpose of the survey (for public water supply)(d) Need to remove the location from potential

polluters (settlement Olib has no constructedsewerage network)

(e) Seawater influence on the quality of abstractedgroundwater reduced to the minimum

Investigation drilling was carried out from 15thJuly to 15th August 2003; four boreholes were drilled(IBO-1, IBO-2, IBO-3 and IBO-4) to a total depth of203.7 m (Fig. 2b). The boreholes were drilled withcontinual core removal. Determination of the core anddetermination of the rock quality designation wasobtained by the correlation of available carotagediagrams and the drilled core. Following the installa-tion of a piezometer, short-term test pumping wascarried out in a very dry period.

Physicochemical parameters (conductivity, tempera-ture, salinity and total dissolved substances) weremeasured by depth in natural conditions and afterseveral hours of pumping. These parameters weremeasured using a LF 197 conductometer and 325TetraCon probe from WTW (Germany).

To determine available groundwater quantities, testpumping of the borehole was performed over severaldays in the following dry period by application of thestep method (three pumping rates and groundwaterlevel recovery after the end of pumping; Jacob 1946).The maximal pumping rate was Qmax=3.5 L/s.Drawdown in the nearby boreholes IBO-1 and IBO-4was continuously monitored. A plastic pipe wasinstalled in the Olib harbour in an area protected fromwaves to monitor sea level using contact electrical echosounder SEBA Hydrometrie (Germany). Measurementswere initially taken every 2 h, and later during the testpumping every 4 h.

Additionally, three physicochemical parameters ofgroundwater (electrical conductivity, total dissolvedsubstances and temperature) were measured by depthin IBO-1, IBO-3 and IBO-4 prior to, during and at theend of the test pumping. Measurements by depth priorto pumping were performed to gain insight into thestatic state, undisturbed by pumping, i.e. to determinethe boundary of fresh/brackish water and seawatermixing, whilst measurements at the end of pumpingwere done to determine changes in groundwatersalinity following test pumping. During pumping,measurements were intended to show the relationbetween the quantity and quality of pumped waterand the required limit quantity of NaCl that wouldenable the inclusion of a borehole into public watersupply following processing of raw groundwater ina desalinisation plant. Additionally, detailed physi-cochemical and microbiological groundwater analy-ses were carried out at the beginning and end ofpumping.

Results

The locations for boreholes IBO-1 and IBO-4 weredetermined based on an analysis of the results ofgeoelectric tomography and seismic refraction resultsand of borehole IBO-2 on the results of geoelectrictomography and geoelectric sounding as the obtainedresults displayed solid fractures of carbonate rockmass, low quantities of clayish component and lowsea impact on groundwater. The location for IBO-3was predominantly based on the results of geologicaland hydrogeological mapping. An interpretation ofthe geophysical investigation is provided in Table 1;the determined anomalies in the electomagneticinvestigation partially complete the knowledge onzones of intensified (amplified) conductivity that canbe attributed to the layers of higher fragmentation orclayishness. The geoelectrical tomography profilewith inserted locations of boreholes IBO-3 and IBO-4is shown in Fig. 3.

The determination of the drilled cores establishedthat all observation boreholes were made in limestoneof Cenoman–Turon age (K2

1,2), which are slightlydolomitised in places. The limestone is highlykarstified, in part very fragmented, and the fissuresare open or partially closed by the crystallisation ofcalcite and clay. At the location of IBO-1, the caving


in during drilling indicates the cavernous porosity ofthe limestone. The lithological profile of boreholeIBO-3 is shown in Fig. 4. A detailed overview of theinvestigation drilling is provided in Table 2.

In the test pumping in summer 2003, IBO-1 didnot give satisfactory results as the pumping quantitiesdropped from 0.66 to 0.16 L/s in 10 min, so pumpingwas discontinued. The measured physicochemicalparameters indicated almost fresh water, with theelectrical conductivity at the base of the wellmeasuring about 2,500 μS/cm (Fig. 5). The resultsalso showed the groundwater inflows to IBO-2 andIBO-4 to be minimal; thus, these boreholes wereassessed negatively and test pumping was discontin-ued. The results of the measurement of electricalconductivity and other physicochemical parametersby depth in IBO-2 indicated a border of fresh(brackish) and salt water between 63 and 64 m(Fig. 5). The highest levels of electrical conductivitywere recorded in IBO-4 (Fig. 5); therefore, furthertesting in this well was discontinued.

Test drilling in IBO-3 showed very good results.Pumping in two quantities (0.74 and 1.33 L/s) withslight drawdowns (0.24 and 0.43 m, respectively)indicated the possibility of obtaining larger ground-water quantities for water supply. After 7 h of testpumping and measurements of salinity and electricalconductivity, it was determined that in extremely dryperiods, this borehole can provide 1.5 L/s or possiblylarger groundwater quantities, with salinity of 0.6 orabout 200 mg/L chloride and measured electricalconductivity of about 1,500 μS/cm (Fig. 6). Othermeasured physicochemical parameters—salinity, TDSand temperature—are shown in Figs. 7, 8 and 9.

Test pumping of the IBO-3was carried out by thestep method at three different rates—0.96, 2.10 and3.20 L/s—in the period from 8 to 22 April 2004. Thetest pumping data of the most promising IBO-3borehole are shown graphically in a linear scalewhich depicts the reduced levels of groundwater asa function of time, depending on the pumped quantity(Fig. 10).

Prior to the start of pumping, the values ofelectrical conductivity by depth were nearly identical(1,268 μS/cm at 32.0 m and 1,299 μS/cm at 42.0 m),whilst the groundwater interval along the entire lengthwas in the form of freshwater or a very slightlybrackish column (Fig. 11). In the course of pumpingat rate Q=0.96 L/s, the maximum drawdown of Δs=0.53 m was measured. The impact of sea leveloscillations on drawdown was minimal during pump-ing. During pumping, the electric conductivity initiallyshowed a mild increase (from 1,280 to 1,315 μS/cm),followed by stabilisation. During pumping at thequantity of Q=2.10 L/s, a maximum drawdown ofΔs=1.51 m was registered, and no significant impactof sea level oscillations on drawdown was observed.During pumping, the electrical conductivity showed acontinuous but very mild increase (from 1,319 to

Fig. 3 Geoelectrical resistivity tomography profile—Olib 3 ERT transect (one of six conducted transects)

Table 1 Interpretation of geophysical investigation

Resistivity(Ωm)

Predicted lithological determination

<100 Carbonate rock mass with fissures filled with saltwater or highly salinated water

100–450 Carbonate rock mass with brackish or freshwater,almost in the form of hovering lenses

450–1,600 Carbonate rock mass with small dimensionfissures, with or without backfill, possiblypartly filled with water

<1,600 Carbonate rock mass with increased fissuresand smaller cavities, filled with air and partlyweathering products. In the surface area,fissures and cavities are backfilled with clayand debris that significantly reduces resistivity(resistance lower than 1600 Ωm)


1,339 μS/cm). During pumping at quantity Q=3.20 L/s, a maximum drawdown of Δs=3.29 m wasregistered, and no impact of sea level oscillations ondrawdown was observed. However, it is possible that

the measured oscillations are partially consequentialto the opening of fault systems within the aquifer. Thevalues of electric conductivity during pumping,measured at 6-h intervals, showed a continuous but

Fig. 4 Lithological profile of borehole IBO-3


a very slight increase of about 10 μS/cm. Even at themaximum pumping rate, the borehole mostlyrecharges from shallower intervals saturated byfreshwater. By the end of pumping, no stabilisationof electrical conductivity was achieved. At the end ofthe test pumping, the electrical conductivity mea-sured per borehole depth increased (2,010 μS/cm,Fig. 11).

Based on the pumping test conducted at boreholeIBO-3, the Q − s/Q diagram was done where,according to Jacob (1950), the drawdown equationwas defined:

s ¼ 320Qþ 213333Q2:

In the equation, s is the measured drawdown (inmetres) and Q the pumping rate (cubic metres persecond). If we assume that the optimum drawdown atthe borehole is 7 m, according to the graphicalsolution from the Q−s diagram (Fig. 12), at boreholeIBO-3, higher quantities of the groundwater can beabstracted than was proven by the test pumping, sothe pumping rate could be up to 5 L/s. The relevantresults obtained during test pumping are shown inTable 3.

In the IBO-3 borehole, the distribution of ground-water temperature per depth shows temperaturestability from the surface to the bottom of theborehole. Groundwater temperature was 14.9°C, withthe exception of the temperature measured in the firstmetre of groundwater of 15.1°C due to warming fromthe increased summer air temperatures. The measure-ments of electrical conductivity by depth prior to thestart of pumping and at the end of pumping were alsoconducted in IBO-1 and IBO-4 (Fig. 13). Theelectrical conductivity by depth in IBO-1 indicatedequalisation, i.e. a very mild increase between 37.0and 52.0 m, ranging from 982 to 1,007 μS/cm. From52.0 to 55.8 m, the electrical conductivity markedlyincreased to 3050 μS/cm. At the end of test pumping,the measurements registered the same values measuredby depth up to 46.0 m. From 46.0 m to the bottom, theelectrical conductivity increased in relation to the pre-pumping measurements.

In the IBO-1 borehole, the temperature distributionof groundwater by depth is uniform, with a mildincrease towards the borehole bottom. At the surface,groundwater temperature was slightly higher at 15.3°C,whereas the temperature stabilises below, ranging fromT

able

2Overview

ofperformed

investigationdrilling

IBO-1

IBO-2

IBO-3

IBO-4

Geodetic

measurement

y=4,914,252.365

y=4,913,991.134

y=4,914,198.895

y=4,914,116.171

x=5,483,124.456

x=5,483,239.585

x=5,482,849.635

x=5,482,777.757

z=36.495

terr.level

z=47.722

terr.level

z=31.345

terr.level

z=31.258

terr.level

Startof

drilling

17July

2003

23July

2003

02August2003

11August2003

End

ofconstructio

n22

July

2003

01August2003

06August2003

13August2003

Initial

drillingprofile,∅

=146mm

0.00

m0.00

m0.00

m0.00

m

End

drillingprofile,∅

=116mm

60.20m

65.00m

43.00m

35.50m

Installatio

nof

PVCpipe,∅

=114mm

+0.20

to−5

6.00

m(56.20

m)

+0.30

to−6

5.00

m(65.30

m)

+0.30

to−4

3.00

m(43.30

m)

+0.30

to−3

5.00

m(35.30

m)

Desilter

ofPVCpipe,∅

=114mm

−56.00

to−5

1.00

m(5.00m)

−65.00

to−6

3.50

m(1.50m)

−43.00

to−4

2.00

m(1.00m)

−35.00

to−3

4.00

m(1.00m)

SlottedfilterPVCpipe,∅

=114mm

−51.00

to−3

6.00

m(15.00

m)

−63.50

to−4

8.50

m(15.00

m)

−42.00

to−3

3.00

m(9.00

m)

−34.00

to−2

6.00

m(8.00

m)

FullPVCpipe,∅

=114mm

−36.00

to+0.20

m(36.20

m)

−48.50

to+0.30

m(48.80

m)

−33.00

to+0.30

m(33.30

m)

−26.00

to+0.30

m(26.30

m)

Boreholedevelopm

entby

airlift(h)

328

00

Testpumping

1h(0.66and1.10

L/s)

10min—no

water

12h(0.74and1.33

L/s)

10min—no

water

Measurementof

salin

ityandelectrical

conductiv

ityperdepth

From

37.0

to55.0

mFrom

48.0

to65.0

mFrom

32.0

to42.5

mFrom

32.0

to35.5

m

Groundw

ater

level

0.325m

a.s.l.23

August2003

at19:45

0.852m

a.s.l.29

August2003

at17:00

0.075m

a.s.l.23

August2003

at19:30

0.798m

a.s.l.23

August2003

at19:15


15.0°C to 15.1°C. In the final metres above the bottom ofthe borehole, a very slight temperature increase (15.2°C)was observed.

The measured values of electrical conductivity perdepth in IBO-4 show a continuous though a veryslight increase between 32.0 m (2,320 μS/cm) and

Fig. 5 Electrical conductivity of water along the depth of the boreholes (in 2003)

Fig. 6 Electrical conductivity and salinity of the water during the pumping test in summer 2003


34.0 m (2,430 μS/cm). Below 34.0 m, the values ofelectrical conductivity register a somewhat sharperincrease. At 35.0 m, the measured value was3,090 μS/cm. After pumping, the measured electrical

values were nearly identical, with the exception of thevalues measured at the borehole bottom where themeasured value of electrical conductivity was lower(2,790 μS/cm) than prior to the test pumping. The

Fig. 7 Salinity of water along the depth of the boreholes (in summer 2003)

Fig. 8 TDS of water along the depth of the boreholes (in summer 2003)—the values in borehole IBO-4 are missing because theyexceeded the limits of the measuring instrument (over 2,000 mg/L)


Fig. 9 Temperature of water along the depth of the boreholes (in summer 2003)

Fig. 10 Step-drawdown test in the borehole IBO-3. The left side of the curve corresponds to the pumping period, whilst the right sidecorresponds to the period of recharge of the groundwater levels after the end of pumping


measurement results proved the presence of a hydrau-lic connection of groundwater in IBO-4 and IBO-3.

In the IBO-4 borehole, the temperature distributionof groundwater by depth is uniform. The temperatureis constant, at 14.8°C, with the exception of ground-water exposed to air temperature which is slightly

higher and equals 15.1°C. During test pumping ofIBO-3, groundwater oscillation levels in the observa-tion boreholes IBO-1 and IBO-4 and the sea leveloscillations, i.e. tide levels, were monitored (Fig. 14).The oscillations of groundwater levels in the obser-vation boreholes range from 6 cm in IBO-1 to about

Fig. 11 Electrical conductivity of water along the depth of the borehole IBO-3

Fig. 12 Dependence of the groundwater level drawdown in the borehole IBO-3 to the pumping rate, the Q − s diagram, s = f (Q)


40 cm in IBO-4. The seawater influence on thegroundwater levels in the observation boreholes wasnot significant, particularly in IBO-1. With regard tothe locations of the observation boreholes, a certaininfluence of seawater on the pumped borehole cannotbe ruled out; however, the groundwater level oscil-lations within are masked by the dominant impact oftest pumping.

Detailed physicochemical and microbiologicalanalyses at the beginning and at the end confirmedthat the quality of abstracted groundwater from IBO-3fully complied with the requirements stipulated by theordinance on the sanitary quality of drinking water.

All physicochemical indicators in the water werebelow the maximum permitted concentrations fordrinking water both before and after pumping, whilstmicrobiological contamination was not registered(Table 4). Additionally, chloride content in the waterwas 161 mg/L before pumping and only 23.8 mg/Lafter the 14-day pumping.

Discussion

In most karstic Adriatic islands which are predomi-nantly built of carbonates, water supply represents a

Table 3 Relevant results obtained during investigation pumping

Pumpingrates,Q (L/s)

Pumpingtime, t (h)

StaticGWL(m a.s.l.)

DynamicGWL(m a.s.l.)

Max.drawdown(m)

30-hrecovery(m a.s.l.)

Suctiondepth (m)

EC—at thebeginning(μS/cm)

EC—at theend (μS/cm)

IBO-3 0.96 72 0.25 −0.35 0.53 41.40 1.280 1.306

IBO-3 2.10 96 0.25 −1.26 1.51 41.40 1.306 1.339

IBO-3 3.20 168 0.25 −3.04 3.29 41.40 1.339 1.404

IBO-3 0.25 −1.2

Fig. 13 Measurements of electrical conductivity by depth prior to the start of pumping and at the end of pumping of boreholes IBO-1and IBO-4


great obstacle to socioeconomic development andprosperity (Magdalenić 1991; Terzić 2004; Bačani etal 2006; Terzić et al. 2007b; Stevanović 2010). Watershortages and significant reductions are most pro-nounced during the summer season when interrup-tions in water supply commonly occur (Stevanović2010). Therefore, using present island water resourcesas one of the possible solutions has nowadays gainedmuch attention. Unfortunately, only a few islands,especially those small and distant ones, have hydro-geological conditions to accumulate sufficient surfacefreshwater or groundwater quantities which aresatisfactory in terms of quality and quantity to beengaged in public use. The conducted research at theisland Olib displayed that even at small and distantislands, sufficient quantities of groundwater can bepumped to be satisfactory for public use.

Based on the results of the geological, hydro-geological and geophysical investigations, four bore-holes were drilled on which further investigation wasconducted. The obtained results of the geophysicalinvestigation helped determine the physical distribu-tion of the fault systems that predispose groundwaterflow, seawater impact on its salinity and approximatebackfilling of fissure systems by the clayish component.

Although the results of the geophysical investigationwere promising, research drilling results did not confirmthe assumptions derived by geophysical investigation.The location of the IBO-3 borehole displayed the bestresults for water supply, even though it was assigned theleast perspective by the results of the geophysicalinvestigation.

The results of the test pumping in the IBO-1borehole were not satisfactory, even though severalparameters such as equipment passing during drilling,cavernous porosity, core determination and the rela-tively fast recovery indicated a promising location. Asgroundwater inflow was too low and fissures werebackfilled with clay, accessing borehole by airlift wasnot allowed, and therefore the test pumping wasdiscontinued. Additionally, the obtained results suchas minimal groundwater inflows in boreholes IBO-2and IBO-4 and the highest levels of electricalconductivity in IBO-4 showed that further testing inboth boreholes needs to be discontinued.

Test drilling at the last borehole, IBO-3, and themeasured water quantities, salinity and electricalconductivity displayed an opportunity to includeIBO-3 borehole into the public water supply on theisland Olib. In this, the most perspective location, the

Fig. 14 Electrical conductivity of water per depth of boreholes IBO-1 and IBO-4 (in summer 2004). Groundwater levels in theobservation boreholes and sea level oscillations, i.e. tide levels, during the pumping of borehole IBO-3


electric conductivity during pumping showed a mildincrease followed by stabilisation, which can beattributed to a dominant recharge from the surfaceinterval of the relatively fresh water column. These

indicators display a stronger impact of pumping rateson drawdown than the impacts of tide levels. Whilstobserving the tidal effect on groundwater and seawa-ter flow in the coastal aquifer on Jeju Island, Kim etal. (2006) concluded that water-level variation is moresensitive to tidal fluctuations near the coast and moredependent on rainfall towards inland. During pump-ing at the quantity Q=2.10 L/s with a maximumdrawdown of Δs=1.51 m, the electrical conductivityshowed once again a continuous and mild increase,indicating that recharge at this pumping rate stilloccurs with a significant inflow of freshwater fromthe surface. At the end of the test pumping, theelectrical conductivity per borehole depth increasedup to 2,010 μS/cm and indicated potential of asatisfactory water quality in long-term pumping ofIBO-3 for public water supply.

The Q−s diagram at borehole IBO-3 assumed thelimit pumping rates for IBO-3, but did not define thegroundwater quantities in the case of increasedabstraction, which may be a limiting parameter inplans to include the borehole in the island’s watersupply system (Ford and Williams 1989; Motyka1998; Urumović 2000, 2003; Terzić 2004). Consider-ing that the temperature, electrical conductivity andtotal dissolved substances curves showed no anoma-lies, we can conclude that the freshwater and seawatermixing boundary in the stationary state is locatedbeneath the final depth of borehole IBO-3. Temper-ature distribution of groundwater by depth is uniform,with the surface groundwater temperature slightlyhigher due to the influence of air temperature andabove the bottom of the borehole slightly higher dueto the impact of seawater temperature.

By a verymild increase between 37.0 and 52.0 m, theelectrical conductivity by depth in the IBO-1 boreholeindicated equalisation. In depths from 52.0 to 55.8 m,the electrical conductivity markedly increased up to3.050 μS/cm, indicating a considerable influence ofseawater due to the vicinity of saltwater mixing. Theboundary of freshwater and seawater mixing was raisedby nearly 6.0 m in comparison with the start ofpumping. These measurements confirmed the hydraulicconnection, i.e. impact of pumping of IBO-3 on IBO-1.The difference in the measured values of electricalconductivity per depth with regard to boreholes IBO-1and IBO-3 is likely due to more open fissure systems inthe higher intervals of the borehole, thus resulting in thedominance of freshwater inflow over water mixed with

Table 4 Analytical data (physicochemical and microbiological)for the groundwater from borehole IBO-3

Components Beginningof pumping

End ofpumping

EC (μs/cm) 1,314 1,428

TDS (mg/L) 672 719

HCO3− (mg/L) 494.1 493.4

Alkalinity (mg CaCO3/L) 405

SiO2 (mg/L) 4.995 3.13

NH4+ (mgN/L) 0.02 <0.03

NO3− (mgN/L) 0.1 0.3

Cl− (mg/L) 161 23.8

Pb2+ (μg/L) <0.5 <0.5

Cd2+ (μg/L) 0.108 <0.1

Hg2+ (μg/L) <0.01 <0.01

Zn2+ (μg/L) 17.27 <5

Total Fe (μg/L) 4.49 <1

Total Cr (μg/L) <0.4 <5

Mn2+ (μg/L) <0.4 1.8

Na+ (μg/L) 37485.0 60100

K+ (μg/L) 1286.2 1090

Phenols (mg/L) <0.001 <0.0005

SO42− (mg/L) 26.6 3.7

PO42− (mg/L) 0.003 <10

Total hardness (°dH) 26.32 515.2

Carbonate hardness (°dH) 22.68 384

Calcium hardness (°dH) 22.14 222.2

Magnesium hardness (°dH) 4.18 48.2

Total oils and fats (μg/L) Under detectionlimits

28.3

Mineral oils (μg/L) Under detectionlimits

–

Use of KMO4 (mg O2/L) 0.8 0.67

Anionic detergents (mg/L) <0.001 0.0256

No. of aerobic bacteria(1 mL/22°C)

10 12

No. of aerobic bacteria(1 mL/37°C)

7 7

Total coliforms (100 mL) 0 0

Faecal coliforms (100 mL) 0 0

Sulfido-reductive clostridia(20 mL)

Not isolated 0

Pseudomonas aeruginosa(100 mL)

Not isolated 0


seawater. According to Ng et al. (1992), open fissuresystems and joints developed under tectonic stressand karst development associated with sea levelfluctuations are the two most important causes ofporosity and permeability in the aquifers on GrandCayman.

Measurements of the groundwater level in theobservation boreholes IBO-1 and IBO-4 confirmed thehydraulic connection of groundwater with the pumpedboreholes, although this was weakly pronounced (Ferriset al. 1962; Urumović 2003). Oscillations of ground-water levels of 6 cm in IBO-1 to approx. 40 cm inIBO-4 are due to the distances between the observa-tion boreholes and the pumped boreholes (280 m inIBO-1 and 110 m in IBO-4), rock permeability andthe change in structural–tectonic conditions at thelocations of the observation boreholes. In addition,temperature distribution by depth displayed that thereare no groundwater inflows with different physico-chemical characteristics, i.e. that the aquifer at thelocation of the borehole IBO-4 is poorly permeable.

Therefore, in terms of quantity and quality, ground-water resources from IBO-3 are so far the only ones thatcompletely fulfill conditions for public use as thesecompletely meet the requirements assigned by theCroatian Ordinance on the sanitary quality of drinkingwater (Official Gazette, 182/2004). It was proven that noprocessing of raw water is required prior to its inclusioninto the Olib water supply system. In the case ofincluding borehole IBO-3 into the public water supplyin the future, it is necessary to develop a groundwaterexploitation programme and abstraction monitoring formaintaining the quantity and quality of raw groundwaternecessary for water supply as well as measures forprotection of the recharge area of the abstracted aquifer.

Conclusions

The conducted test pumping of borehole IBO-3,including measurements of the electrical conductivityof water per depth and during pumping in the summerof 2004, indicates a promising location and conse-quentially a possible inclusion of the borehole withthe capacity of 3.2 L/s into the public water supply ofthe island Olib. The groundwater quality determinedon the basis of the physicochemical and microbiolog-ical analyses and additional measurements of chloridecontents and electrical conductivity per depth at the

start and at the end of the pumping fully complieswith the provisions of the Croatian Regulation on theSanitary Quality of Drinking Water. The water doesnot need processing prior to its inclusion into thepublic water supply. At the pumping end, the chlorideconcentration in water was only 23.8 mg Cl− per litre.In the case of the exploitation of borehole IBO-3,drilled observation boreholes IBO-1, IBO-2 and IBO-4should be used as the observation boreholes formonitoring oscillations in groundwater levels andquantities. If there is a need for larger groundwaterquantities for water supply, analysis of the measured andcalculated drawdown and pumping rates, the drawdownvs. pumping rates graph, determined that in terms of theborehole yield, it is possible to abstract the quantity ofabout 5 L/s, with the expected optimal drawdown ofabout Δs=7 m. A drawdown by the abstraction of3.2 L/s (Δs=3.29 m), the height of the relatively freshwater column and measurements of electrical conduc-tivity along the depth after pumping and beforeincreasing pumping rate show that it is necessary toconduct a test pumping with the proposed quantity andto monitor groundwater quality. Additionally, a possi-bility of providing accessory groundwater quantitiesthrough further hydrogeological investigations on theisland Olib is advisable.

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