GEOL. CROAT. 50/2 279 - 288 10 Figs. ZAGREB 1997 Hydrogeological Exploration of the Rjecina River Spring in the Dinaric Karst Bozidar BIONDIC, Franjo DUKARIC, Mladen KUHTA and Ranko BIONDIC Key words: Hydrogeological exploration, Dinaric karst, Karst spring, Karst aquifer, Overthrustecl stru­ ctures, Deep retention spaces, New intake structure, Access gallery, Monitoring of the karst aquifer. Abstract The Rjccina sp ring is one of the major springs in the Dinaric K arsL It appears al the contact between permeable carbonate and impermeable clastic rocks, with a discharge of up to 120 m3/s but it dries up during the dry summer seasons. The spring occurs close to the town of Rijeka, 325 m above sea level and offers an outstandi ng op portunity to cover grav ilalionally the public water demand of a town of about 200,000 inhabitants, and the touristic needs of the who­ le region. This hydrogeological research project is a pan of efforts to solve lhc problems of water deficiency during the dry summer sea­ sons up to a maximum of three month s. It was necessary to enter the parts of a karst aquifer that are active eve n in time of any outflow from lile Rj ecina spring by complex geological, hydrogeological and geophysical exploration accompanied with deep exp loratory bore­ holes. During earlier explorat ion, it was detemlined lhat there are no active innows in the immediate hinterland of the spring and that it is necessary to discover the inflows from olher karst structures, that behave as retentions of karst spring s in the zolles of permanent dis­ charge. The presence of Tllultiple overthrusted structures in the zone around lile spring site suggest the ex.istence of deep zones of water retention, which may be reached by an access gallery from the Rjecina canyon. This work represents a substantial change in the exploration methodology for Dinaric Karst aquifers, because it directs the researchers toward deep, unknown retent ion spaces which contain large reserves of high-quality groundwater outside urban areas. 1. INTRODUCTION The RjeCina spring is the largest karst spring in the northern Adriatic area, but it is inactive during the sum­ mer dry seasons for lip to three months a year. This is a great problem for the water supply to the town of Rije­ ka (Fig. I), because all groundwater extraction sites in the coastal area have to be activated during these peri­ ods. Thi s is a risk to high water quality and a conside­ rable cost as the water has to reach 540 m above sea level. The Rjecina spring, occurring at 325 m above sea Institute of Geology, Sachsova 2, P.O. Box 268, HR- IOOOO Zagreb, Croatia. level would be an economically and technically vcry favourable gravitational water supply for the largest part of the town. Moreover, the Rjecina spring water is of extremely high quality with the potential of remaining so for future generations, as the spring catchment area in the mountains of Gorski Kotar is protected 10 a water reserve leveL The Rjecina spring occurs a substanti al distance North of the closest urban center of the Rijeka area (Fig. 2). It has only recently been of interest as part of the urban water supply as previous demand was covered by supply from coastal springs. This was also influenced by the temporary dryncss of the spring during summer dry seasons, when the dcmand for potable water reaches a maximum. The hydro-power potential of the spring initiated the first studies after World War II. Poljak and Crnolatac directed their exploration toward estimation of the possibilities of water storage in the predominant flysch deposits of the Rjecina river valley without much consideration of the karst catchment areas. SIKIC & PLENICAR (I975), the authors of the Basic Geological Map 1: 100,000 Sheet Ilirska Bistriea, provided a base for the exploration of the karst areas, as this map com­ prises the carbonate massif of a part of Gorski Kotar and Slovenski Sneznik Mt. which drain towards the Rjecina spring and other karst springs within the Rijcka region. The first hydrogeological bases of the carbonate hin­ terland of the Rjecina spring were made by Biondic and Vulic in 1969. For the first time the RjeCina spring ca­ tchment area was defined and comb ined with a very detailed hydrogeological description. This was, also, the first hydrogeological definition of a karst catchment area in the Dinarides. Within this exploration, Bozicevic performed a speleological exploration, including deta­ iled research of the Rupa ponor in the Grobnicko Poije. The Rjecina spring was also of interest to researchers of karst morphology and it was first exp lo red by Italian diver-speleologists in 1928. They reached a depth of IS m, followed by Croatian diver-speleologists in 1971 (to 34 m), 36 m in 1984 and 50 min 1996. During the most This paper was presented at the scientific meeting dedicated to the 80th anniversary of the life of Professor Milan Herak, held on March 5th, 1997 in Zagreb Pannonian basin 50 TOO km "\ ~ijek<_" vu/(O'o'.,L,,_, -'-"'\ ( <- ) Fig. I The Dinaric karst region in Croat ia showing the posi tion uf tile' study area. recent study in 1996 a lower channel was discovered at depth (KUHTA, 1998). Groundwater was twice pum­ ped ou t from the spring cave system during the dry summer seasons of 1969 and 1973 . Vast cave spaces were opened and speleologically cxp lored by BOZI­ CEV IC (1973, 1974). Under natural condi tions, these cave spaces had been fi lled with remnant water even in the summer dry periods. In the upper part of the cave system, a 20-m-long gallery was constructed to facili­ tate entry to the cave even during high water stages. Si nce 1971 , the Rjccina spri ng has formed an incre­ asing part of the hydrogeological research projects of the entire catchment area of springs discharging in the town of Rijcka. During these explorations, numerous detailed mappings and groundwater tracings were made in the carbonate hinterland of these springs, which hel­ pcd to define the catchment area. To maximise the use of the Rjecina spring, a cons iderable possibility would be to build a surface dam submerging the spring. Rece­ nt study showed tha t the immediate area around the spri ng does not conta in significant amounts of water, but the spring can be submerged without problems, i.e. wi thout water losses toward the downstream parts of the catchment area. The basic conclusion was that the close spring hinterland occurs intermittently out of the influence of deep underground retentions which control the spring discharge during the summer dry seasons . Therefore, new explorations should be directed toward those geologic structures which are potentially ac ti ve aquifers during the summer dry seasons. 2. GEOLOGICAL REVIEW Generally, the base for the hydrogeological explo­ ration of karst areas is a good knowledge of the geolo­ gical structure, but it is of a special importance for the Rjecina river spring wherc all the complexity of the o 10 km LEGEND: ,. 3 ." 5 • 4 6 Fig. 2 A tectonic sketc h of the sllldy area. Legend: I) bOllndary between the Adriatic and Dinaric units; 2) main reverse fault: 3) faults; 4) geological boundary; 5) watershed; 6) rivers; 7) Rjecina spring. structural composi tion of the Dinarides is fully displa­ yed (HERAK, 1986, 1991). The lithostratig raphy and tectonics define the geometry of karst aquifers into which the hydro-dynamics elements can be included. Within the Rjecina spring area, the aquifer is com­ posed of Cretaceous and Palaeogene carbonate rocks, while the Palaeogene flysch deposits form a barrier to groundwater flow. In this study, the most important influence were certain lithostratigraphic changes within thc carbonate mass, and the structural position of vari­ ous lithostratigraphic units (BIONDlC, 1988). The Lower Cretaceo us limestones and calcareous breccias (K ,), the oldest rocks in this area, form an anti­ cl ine which can be traced from the Rjecina river hinter­ land, to the north-western edgc of the Grobnicko Polje. The calcareous breccias are overl ain by dolomitized breccias (K 1.2), up to 150 m thick, formed by late-di a­ genetic dolomitization of tectonised transitional depo­ sits between the Lowe r and Upper Cretaceous. The dolomitizcd breccias are overlain by an Upper Creta­ ceous (K~·2) unit, 500 m thick, in which dolomites and limestones alternate. Dolomites prevail in the hinterland of the Rjecina spring, and gradually pass into a pure Upper Cretaceous calcareous complex of rud ist lime­ stones (K~·3), up to 350 m thick. At the transition to Palaeogene deposits , the sedimentary environment sub­ stantiall y changed . Continental sedim entation began over a marked karst relief, wi th occasional lagoonal and brackish lake sed iments as for example in the hinter­ land of the spring, where the Palaeogene began with Liburnian deposits. Following a marine transgression over the area typical marine foram in ifera l limestones Biondic, Dubric, Kulua & Biondic: I-Iydrogeological Exploration of the Rjetina River Spring ... 2" (EI.2)' Up to 200 m thick, were deposi ted. Toward the river Za la va lley, a gradual transition toward clastic sedim entation above the foraminiferal limestones can be observed. This began with 30-50 m thick trans it ional clast ic deposits ('Ez.3) and continued with n ysch depo­ sit s eE, ~ ), the thickness of which might reach about 600 m (based on their thickness in the {sl rian Dysch bas in). The Eocene-Oligocene calcareoll s breccias (lclar depos its) have not been observed within the exp­ lored arca. T hc Quaternary sediments are very impor­ tant for thc understanding of events when th e actual surface and underground hydrosystem was fonned. The occurrence of Pliocene-Pleistocene lacustrine sediments a t the norlh-weSlern edge of Ihe Grobnicko Polje indi ­ cate the ex istence of a palaeodepression before the Quate rnary. These lacustrine sedi ments (grave l and cong lomerates), and nuvioglacial fan s toward the sur­ rounding mo untainou s area are the results o f heavy glacia l act ivi ty in the catchment area. The remnants o f ri ver te rraces (sandy-clayey sediment s with cobbles) a lso have a significant role as they show the arrange­ ment of surface stream s in their gene tic sequence dur­ in g the Quaternary. Deluvia l and outwash depos its on steep slopes of the Rjecina river canyon reflec i recent erosional processes and do not affect the hydrogeologi­ cal interpretations. Intc rpreting the geological structu re, it is necessary to cons ider not only the local condi tions but a lso the reg io nal ones, as the hydrodynamics is related most c losely to the regional tectonics. For the study area, as well as for the whole of the Dinarides, it is importan t to de fin e the re lationship of thc macrostructural units. According to HERAK (1986, 199 1), in the Rijeka regi­ on this means the relationship between the Dinaric and Adriati c units. Thei r mutual relat ionship is significant for the formation of groundwater and its now toward the Adriatic coasta l area. According to our exploration, the boundary between these two macrostructural uni ts stre tches a long the thrust contact of the Obruc structural unit. Thi s is a continuation of a fault contact extending from Il irska Bistrica, in S lovenia, over the Ravno area, to the north-western edge of the Grobnicko Polje. The Grobni cko polje was formed at the intersection of num erous large perpendicular faults. They caused the south-westward transposi ti on of Dinaric structure and the covering o f Adriatic structurcs to the Vinodol nysch va lley (Fig. 2). According to this interpretation, the car­ bonate mass if of the Rjeci na spring hinterland belongs to the Adriatic megastruc tural unit , which is charac­ terised by fo lded struc tures and numerous imbricatc­ thrust struc tures and , at the carbonate massif border, large occurrences o f Palaeogene n ysch deposits. This is also con firm ed by normal lithostratigraphic transitions from areas of carbonate sedimentation to clast ic ones in the arca of Za le and Ravno to the north of the study arca. The loca l tec tonic structures of the Rjec ina spring area bes t re fl ec t the regional relationships. On the cliff above the spring, many thrusts occur within the carbo- nate complex, and also throughout the carbonate massif over the fl ysch deposits, as well as inte nse ro tati ve faultin g whi ch caused mutual movemelll s of part s of the overthrusts and the formation of struc tural blocks. The highes t thrusted structure is p laced cast of the spring, on a plateau above the canyon. Th is structure has a very important function in further hydrogeologi­ cal deliberations within this project. In this " top" thrust­ ed struc ture, the carbonate rocks are fold ed and of a very variable composition: f rom very permeable lime­ stones to poorly permeable dolomites. From a hydroge­ ological aspect, a crushcd anti c line of Dinaric strike having very penneable Lower Cretaceous limestones in its core is of special in terest. T hi s antic line lies parallel to the Rjecina river valley (canyon). In the Grobnicko Polje area, th is anticli ne is entire ly opened on one side forming the main seasonal groundwater discharge zone during the rainy seasons . From the Grobn icko Po lje toward the Rjecina spring h inte rl and, the ant icline pl unges s light ly, and the very permeable limestones have progressively th icker cover or low pe rmeab ility dolomites and dolom itic breccias . To the east of thi s anticline, a synclinal form opcn s, and it s margin is obscured by the Dinaric Obruc overthru st. The wholc structure of the Rjecina spri ng hinterland sinks toward the north-west, hence, progressively you nger deposits are recorded in this direction, toward Ravno and MJaka, where there is a gradual transition of carbonate rocks into impermeable flysch that forms northwestern boun­ dary of the struc ture. Geomorphological phenomena, as in all karst areas of the Dinarides, arc a direct result of the litholog ical characteris tics, geological struc tu re , hydrogeological propel1ies of rocks and the changes in hydrological and meteorological cond itions, particularly during the Qua­ te rnary epoch. The relief form in g processess must be considered dynamically, through time, because the pre­ sent state is only a momentary reflex of the interact ion of endogenous and exogenous processes. Thc material traces in the shape of morpholog ica l phe nom ena and younger sediments (river valleys, river terraces, under­ ground phenomena etc.) enable at leas t a parti al recon­ struction of the geomorphological deve lopment of a ter­ rain and, importantly provide insigh t into the genesis of the ac tual hydrogeological relat ionships and groundwa­ ter dynamics. Sed imentary outcrops indicate that the Grobni cko Polje depress ion was a lready form ed by the Pliocene, when numerous karst poljes in the Dinarides were lakes. The Grobnicko Polje is possibly the starting po int of the development of karst processes for the whole catchment of all springs in the Rijeka reg ion (BION­ DIe et aI. , 1998). Present re lief fomlation is associated with the Pleistocene and the alternation of glac ia l and intcrglacial stages . Intense g lacial ac ti vity is obvious from the discovery of glacial scd im ents in the moun­ tainous area of Platak and Snjeznik. T he inte rg lac ial stages brought increased amounts of water resulting in increased erosion and fluviog lacial sedimentation. In the area of Grobnicko Polje and higher located Goman­ ca Polj e, glacial lakes were formed. These had a con­ trolling function in the development of underground karst morphology and kars t aquifers during the Quater­ nary. A study of Quaternary sediments shows thai all sur­ face streams were directed toward the Grobnicko Polje. The bed of the intermittent stream Zala still down cuts from the Ravno area to the Grobnicko Polje. The case of the RjeCina spring and the development of the lenght of this river canyon is especially interestin g. The river spring and canyon are geologically very young features. The discovery of terrace sediments on a carbonate ridge in the spring hinterland (about 220 m above the present spring site) are indicators of surface streams in the hin­ terland prior to the existence of the large karst spring at the hi gh rock foot. The terrace sediments also confirm that all surface streams, including the Rj ecina river, rJowed toward the Grobnicko Polje. Through the development of karst processes, water from the surface progressively was descending into the carbonate underground and the deep canyon was being deepened in the area composed of fl ysch rocks. The di scoveries of fossil springs hi gh in the clifr above the present spring confirm the gradual canyon opening and the water removal into a deeper karst underground. This erosional process is ongoing, as confirmed by discovery of two cave conduit s at the Rj ec ina spring (802:1- CEVIC, 1974). One of these is overflowing and the oth­ er ascending, wh ich wi II result during future canyon development , further deepen ing and water springing at another, somewhat lower level. The Rjecina strea m, during its first deve lopment ph ase, flowed into the Gro bnicko Polje. The runoff from the Grobnicko polje 10 the present Bay of Kvar­ ncr, which was not submerged in thaI time (S EGOTA, 1968), was effected only through the karst under­ ground. The main water communication was directed toward the Zvir spring in Rijeka. The erosional bed of thi s spring is much deeper than the bed of the Rjecina river. This is now the main drainage direction of a large kars t catchment area of springs in the lown of Rijcka. The deep Rjecina ri ver canyon dominates the mor­ phology of the study area. It has been mostly fo rmed in Oysch sediments, whi le massive carbonates oecure abo­ ve and surrounding the hinterland of the spring. The plateau is characterised by elongated valleys and struc­ tures directed toward the Grobnicko Polje. An expres­ sive morphological step is formed by the Obruc over­ thrust, that is also a boundary of the Adriatic and Dina­ ric macrostructural units. Numerous speleological phenomena have been rec­ orded in the study area. The largest phenomen arc the cave system of the Rjecina river spring, extending hori­ zont ally over more than 200 111, and the Rupa ponor in the Grobnicko Poljc, which is part of the main zone of ponors toward the coastal springs. ......... v .vb'" ... 'v." ..... " ."" o. 3. HYDROGEOLOGICAL REVIEW The general hydrogeological situation of the who le catchment area depends 011 the tectonics and the signifi­ cant lithological variation of this area. The lithological characteristics directly affect the hydrogeological prop­ erties of these rocks, whi le their vertical and lateral changes inOuence the spati al arrangement of potenti al aquifers. Within the carbonate complex, in the area o f the Rjecina spring, two groups of rocks of different hydro­ geological characteristics may be distinguished. While the limestones represent a very permeable med ium and generally the main grou ndwate r col lectors, the dolo­ mites arc much less permeable and very often form bar­ riers to groundwater flow, within very permeable lime­ stones or they direct thi s nows a long the bounding structures. The flysch sediments are generally impenne­ abl e and they, depending on their structural pos iti on, also form barriers to gro undwater flow in karst areas. The Rjecina spring appears at the tectonic contact between very permeab le karstified limestones and the Eocene flysch. However, groundwater flow is recorded be low impermeable fl ysch sediments at some other sites. The Quaternary deposit s, depe ndin g on the ir lithological characteristics, are of variable penneability; however, they have no major significancc in thi s area due to their relatively small thickness. It would be too simple to assume that very penne­ able limestones are always a lso acti ve aquife rs. T his depends both on the position of the rock units and the geological structure of the terrai n, and those exp lains the complex ity involved in the hydrogeological ex pl o­ ration of karst terrains, especially in the high zones of karst catchmcnt areas. Considerable changes in the con­ ditions of groundwater dynamics of the study area have to be connected with the lates t vertica l and rotary movemen ts of the tectoni c forms that were originall y tangentional (overthrusted geologie st ructu res). All larger spring sites in this catchment area are associated with the latest tectonic movemen ts. This refers to the Rjecina spring, to the intermillent springs at the north­ western edge of the Grobnicko Polje and to the penna­ nent coastal group of springs as Zv ir and Martinsc ica. Even the fl ysch zone, stretching almost continuous ly from the Bay or Trieste to the town of Novi Vinodolski , does not function as a hydrogeological barrier along its full length. Thi s is caused by its variable depth due to the latest tectonism and eros ional processes. The rcsult is that groundwater flow s through the Upper Cretaceous and Eocene hi ghly permeable karstified lim esto nes beneath the fl ysch. A s imilar case is the underground fl ow from the ponors in the Grobnicko Polj e to the coastal springs Zvir and Martinscica. The Rjecina spring is an int ermittent karst spring reaching a maximum di scharge rate of 120 m3/s (annual average about 8 m3/s). It is a part of the intermittent dis­ charge zone from the catchment area of the Rijeka town springs (BIONDlC, 1988; BIONDIC et aI., 1995) thaI Biondic. Dukaric. Kuhta & Biondic: Hydrogeological bploration of the Rjecina River Spring .. 283 N A = 0 10 km LEGEND: D ., 8 0 15 • 22 D 2 N 9 • 16 z: 23 LJ 3 N 10 ~ 17 ID1 4 N 11 0 18 D 5 '" 12 19 D 6 N 13 0 20 D 7 0 14 :.: 21 Fig. 3 The regio nal hydrogeological map. Legend: J) Quaternary lay­ ers of variable permeability, relativel y small thickness; 2) Qua­ ternary layers of low permeability, relatively small thickness; 3) hi ghly permeable alluvial deposits; 4) high ly permeable carbon­ ate rocks; 5) medium permeable carbonate rocks; 6) low perme­ able carbonate rocks; 7) impermeable clas tic rocks; 8) ma in reverse fault; 9) fault; 10) geological boundary; 11 ) groundwater now directio n tes ted by tracing; 12) watershed; 13) river; 14) in take structure; 15) dug well; 16) drilled well ; 17) hole with water; 18) ponor; 19) cave; 20) hole; 2 1) estavelle ; 22) spring; 23) submarine spring. compri ses a wide mountainous area in Gorski Kotar and a part of the Slovenski Sneinik Mt. The penmanent springs of this catchment area are: Zvir, extraction site Zvir II , pumping site Martinscica and coastal springs in the town of Rijeka. This is the most productive catch­ ment area in the northern Adriatic region and the main potable water supply for the town of Rijeka, and a major part of the Bay of Rijeka (approximately 3 m3/s during the dry summer seasons) . The Rjecina spring occurs high in the catchment area (325.2 1 m above sea N A' ~ 11m m LEGEND: c:::::::::J 8 15 LJ 2 9 r--- 16 c:::::::::J 3 10 = 17 .. 4 11 • 18 c:::::::::J 5 e 12 B B' 19 c:::::::::J 6 e 13 c:::::::::J 7 e 14 Fig. 4 A detail hydrogeological map of the study area. Legend: I) highl y penneable debris; 2) low permeable debris; 3) highly per­ meable alluvial deposits; 4) highly penneable carbonate rocks; 5) medium penneable carbonate rocks; 6) low permeable carbonate rocks; 7) imperm eable clastic rocks; 8) main reverse fault; 9) reverse fault ; 10) fault; 11) geological boundary; 12) Rjecina spring; 13) est ave li e; 14) small sp ring; 15) groundwate r flow direction tested by tracing; 16) river; 17) proposed gallery; 18) exploration borehole; 19) cross section 8-8 ' . level) and has the function of an overflow for medium and high groundwater, for which the nat ural under­ ground flow is directed toward the Grobnicko Polje and the permanent coastal springs (Fig. 3). The springs at the north-western edge of the Grobnicko Polje are also intermiltent , but they are active for a much shorter peri ­ od than the Rjecina spring, hence, watcr only flows out from those springs during the high stages. During dry seasons, the Grobnicko Polje springs are without water, but the water flows through the deep karst underground towards the permanent coastal springs. This flow is monitored in several observation boreholes and has becn confirmed by numerous water traci ng tests (BIO­ NDle et aI., 1979). The immediate hinterland of the 2<, Geologia CrO:l\ica 50/2 _69 " 700 a.s.!. S' Na Sten i .00 S K , ", K ' .' 0- --200m U s D K 2 1,2 5 10 11 12 O t 2 O K" , 6 D E", 3 B K, 7 _ E" 1 8 BlK 2 2 ,3 4 ~ 9 13 Fig. 5 Cros~Hcctjoll through the nclive karst aquifer with the proposed intake gallery. Legend: I) intergranular porosity. high permeability; 2) intcrgranular porosity, high to variable permeability; 3) impcnncable clastic rocks: 4) high permeable carbonate rocks; 5) med ium perme­ able carbonate rocks; 6) low permeable carbonate rocks; 7) high permeable carbonate rocks; 8) geological boundary; 9) erosional boundary; 10) fault; 11) presumed fault; 12) proposed gallery; [3) borehole SP-3. Rjccina spring also lacks an active groundwater circula­ tion, although its cavernous conduits are filled with delayed groundwater. The described water dynamics in the catchment area of the Rijeka town springs has been defined by numer­ ous groundwater tracing tests. For the high parts of this area, 10 which the Rjecina spring belongs, the tracing of a sma ll ponor al Trslenik village in the mountainous parI of the catchment area is particul arly interesting. It was performed at Ihe beginning of a summer dry sea­ son, when the flow from the Rjecina spring began to decline and the intermittent spri ngs on the north-west­ ern edge of the Grobnicko Polje were dry, while about 1.5 m)/s flowed out from the permanent coastal spring Zvir. The observation boreholes at the north -western edge of the Grobnicko Polje and the permanent coastal springs of Ihis catchment area werc monitored. The appearance of Ihe tracer in the boreholes and permanent springs confirmed groundwater connection between the mountainous area of high precipitation (more than 3,000 mm per year) and thc zones of intermiltent and permanent springing. Similar results were also pro­ duced by the tracing of ponors in Mlaka situated adja­ cent to the intenniuent springing zonc. For thc Rjecina spring, what happens in the zone of intermitlent spri ngs is the most important. This zone belongs structurally to the Adriat ic edge. It is a folded area of Dinaric strike, where each fold thrusts over the other, as is clearly visible on the cliff abovc the Rjecina spring. In these folds, limestones and dolomites alter­ nate. A combination of the folded masses of different hydrogeological characteristics, combined with a north­ westwards sinking of the whole structure under the impermeable flysch deposits, forms a basis for the for­ mation and flow of surfacc and groundwater. The whole carbonate mass, at its north-western side, is bor­ dered with impermeable flysch (Fig, 4) and, at its south-eastern side in the Grobnicko Polje, cut by a strong perpendicular fault. Along this fault, the main drainage direction is formed toward the permanent springs of the catchment area (B10NDlC ct aI., 1995), The challenge for hydrogcological exp loration was to determine whether it would be possible to extract deep groundwater during the dry summer seasons, when the RjeCina spring is inactive. Such action has not previously occurred in the Dinaric karst terrains. Alter­ natively, onc intermittent spring must bc transformed into a permanent one in order to provide a permanent urban water supply from a high, unurbanised zone with­ in the same catchment area. Taking into account the absence of active groundwater inflows in the immediate vicinity of the spring during the dry summer seasons, the exploration had to be directed to other structural forms which, even during dry seasons, keep an active connection with the main recharge areas in lhe moun­ tainous part of the catchment area. The tangentional structural forms offered such a possible solution. Detailed hydrogeological mapping of the spring area highlighted a manifold overthrusting of carbonate rocks of different hydrogeological characteristics (lime­ stones and dolomites) at the eastern side of canyon, and the very spring zone. This suggested the possibility of the existence of active aquifers even when thc Rjecina spring is inactive. The hydrogeological map (Fig. 4) and the cross-section (Fig. 5) show three such over­ thrusts with paraclases inclined mainly north-eastwards. The first overthrust is composed of very permeable limestones lying over the flysch barrier and directly passes the spring site. The second overthrust is visible high on the slope, above the spring, and this overthrust above inspired the search for an active aquifer. It is part of a folded structure that extends south-eastwards to the Grobnicko Polje where is the huge intermittent dis­ charge zone, reaching the rate of up to 15 m3/s in rainy Biondic. Duk~lric. Kuh!;l & I3iondic: I Iydrogeological Explora!ion of !he Rjclina River Spring ... 285 seasons . T he core of the folded struc tu re is composed o f Lower Cre taceous highly permeable calcareous brec­ cias and limestones. These rocks, being covered by younger poorl y pcnncable dolomitized breccias, could be a good medium [or the formation of "capt ive" aqu ­ ifers. The th ird overthru st which covers the aforemen­ tioned thrust fold and its very permeable limestone zo­ ne is too high to be in the zone of ac ti ve g roundwater c irculation. Detail ed hydrogeolog ical mapping was followed by geophysical exploration using double grad ient mapping ancl res istivity sounding. The sounding reached depths sufficient to enabl e hyd rogeological interpretation to below the Rjecina spri ng level. By interpreting the mea­ sured geoelcclrical data , three sites o f well -marked minimums (maximal d ifferences in the rock res isti vity for two deep ho ld) were d iscovered along the core o f the fold structu re . The rcsults of the hydrogeological mapping and geophysical exploration were used to pre­ pare the forecast c ross-sectio n whi ch were a bas is for de finin g of the locations and dep ths of exploratory boreholes. The exploratory-observation boreholes were located along the co re of the fo ld structurc that fo ll ows the Rj ec in a river valley to the north- wes tern edge o r the Grobni cko Polje. One of the boreholes (SI'-3) provided da ta a ll ow in g fu ll description of the hyd rogeo logi cal cha racte ri stics of the acti vc karst aquifer (Figs. 4 & 6). Down to a depth of 145 m below su rface, thi s borehol e was d rilled through practical ly impermeabl e Lower Cre taceous dolomitic breccias, with a constant return o f drilling water. However, whe n the borehole reached hi ghl y penncab le crushed calcareous breccias a t the depth of 145 111 , an abrupt loss of drilling water oceured combi ned wi th a s imultaneous su bartis ian ri se o r groundwater level high above the contact betwecn the impermeable and pemleable rocks. The borehole ended at a depth o f 300 m in the highly permeable Lower Cre­ taceous calcareous complcx. It may be concluded that th e hi gh ly permeabl e Lowe r Cretaceo us ca lcareous brecc ias and limeslOnes form an active karst scm icon­ fin ed aqui fer, even during the summer dry seasons. Signifi cant results were obtaincd from the monitor­ ing of grou ndwater levels in the region surrounding the Rjec ina spring, and at th e north-wes tern edge of the Grobni cko Polje, and have been compared with the ra tes of discharge from the Rjccina spring (Fig. 7). Data from the observa ti on boreholes an ound the Rjecina spring and those at the north-western edge of the Grob­ nicko Polje suppo rt the theory o f full aquifer ac tivit y even during a tota l cessat ion of acti vity from the Rjecina spri ng. To defin e the aqu ifcr dynamics, good results were obtained by measudng groundwater tcmperature along the aqu ifer depth in dilTerent hydraulic conditions (Fig. S) . Th e boreho le SP-3 showcd a constant decrease of water temperature downwards approaching the temper­ ature of the Rjecina spring water (7.5 °C) and thi s a lso mean s the approach to the depth from where wa ter LITHOLOGY f--.-+-,---+=~ '" " • '" -'\- 149 III 196 m 51'-3 DESCRIPTION LOW PERMEABLE DOLOMITIZED BRECCIAS LOW PERMEABLE DOLOMITIZED LIMESTONE HIGH PERMEABLE CALCAREOUS BRECCIA HIGH PERMEABLE LIMESTONE 1.,----='. -"'=-1 -.NQm Fig. 6 Exploratory borehole SP-3. f lows toward the spring. Thi s is also an indi cator of an active aquifer, open toward deep structures, that is also possible geologically. A different situati on ex ists in the observation well at the north-western edge or the Grob­ ni cko Polje where the lower temperatu re values were lJeo!Ogla LrOallCa )U/L _----'-___ '-_J ____ -'-~. ~ 100 _ Rjt'<"inJ Q - s r-2 t 10 0 - SI'-3 -SI'-4 ~ E" " ~ :;; , 0 Ci - SI' -S - SP·6 so - RJSTinJfk" Luh( Rjctina Ic\'ei o I, 1 ~1j,f1994 06,'0111995 2810511995 19!IQlI995 D • - -Autumn w I 264 I ..... . Winter I 244 I I I 224 I Fi g. oS Diagram of the g roundwater temperature variations ill lhe borehole SP-3. 02!08t199& 2411 2119% Fig. 7 Diagram of th e observed grollndwater level s and Rje­ tina spring capacity. explored karst aquifer with groundwater at the NW edge of the Grobnicko Polje, and also with the Rjccina spring, where the traser appeared. This shows a lateral openness of the aquifer toward the Grobnicko Polje, and also toward the Rjecina spring. 4, CONCLUSION The Rjecina river spring, due to its position and water quality, is an outstandingly suitable sourcc for the water supply of Rijeka. It enables a gravitational inflow of high-quality water into the whole water-supply area. However, the cessation of act ivity for up to three months in the dry summers is a problem as during these dry periods the entire urban water supply has to be con­ verted LO the permanent coastal springs. Th is conside­ rably increase production cost and creates problcms in the protecting of spring water quality (BIONDle & GOA TIL, 1986). Detailed hydrogeological mapping , geophysical explorat ion, exploratory boreholes, groundwater flow tracing and detailed measurements of hydrodynamic and geohydrochemical parame ters identified karst aquifers which arc active during Rjecina spring inacti ­ vity. They indicated that the best aquifer conditions exist in a folded struc ture in thc second overthrust above the RjeCina spring. This structure is composcd of Lower Cretaceous calcareous breccias and limestones. The measurements in the borehole SP-3 show a high water-level amplitude betwecn 352 and 475 m above sea level, water temperature lowering with depth, a considerable exchangc of groundwater under different hydrological conditions and connection with a zone of springs at the north-westcrn edge of the Grobnicko Po­ lje. This was a good indicator of the aquifer activity and justifies elaboration of the first phase of the project for water extraction and further aquifer exploration. It has been proposed that the explored karst aquife r can be tapped by a gal1elY from the RjeCina river cany- BiondiC. Dllkaric. Kllhla & l3iondic: Hydrogeological bplor;)tioll of the Rjecilla River Spring ... 2<7 ~oundwater temperature CC) , 7.7 7.8 7.9 265 -i--- i 255 245 235 ~ ;, '0 225 0- o o .~ 215 " E o .5 205 - ~ a § 195 ." ~ w 185 175 165 --Spring --Summer - - Autumn .. . ---- . Winter 8.0 \ \t ". 155 .L----"----'--- 1 8.1 8.2 j' .. 8.3 Fig. 9 Diagram of the groundwater temperature var iations in the spring zone of Grobnicko polje. on, so that the active part of the aqu ifer is interested, bu t above the recorded minimal g round water levels. Global experience support that such "water mining" is no t to be recommended. Such an intake construction might causc permanent harm to the aqu ifer, negatively affecting the zone of permanent coasta l spri ngs. An underground entrance into the aquifer above the levels of minimal water stages does not mean the avoidance of problems associated with high water stages. In any case a gravitation al dra inage of water from such construc­ tions would have to be insured. On the basis of all these and earlier exp lorations, a 1200 m-Iong ga llery at an elevation of 355 m above sea lcvel can be proposed (Figs. 4 & 5) . Thi s woul d enter the massif from thc Rjccina river canyon above the level of the Rjecina spring, but needs to be further defined. Past experiences has proved tha t any water construction in such he tero­ geneous karst underground requires constant reinterpre­ tation and design right up un til the final construction and test pumping. The final aqui fer geometry, water dynamics and possible volumes of water extraction must be defined '" '" .§. ~ ~ " '0 Q, " 0 ." '" i': '" ~ .n 0 '" .c ~ .... 0 " 0 ." '" > '" UJ ['Conductivity (mS/cm) 1 0.265 0.270 0.275 0.280 0.285 0.290 0.295 424 414 404 394 384 374 364 354 344 / 334 324 f f V / V' !'fi / : f ./ / f f I f I /,' I r -- i ! - - I ./ - - ---_ ... I ;/1 I I I ' 1.1'-, . Spring Summer Autumn Winter Fig. 10 Diagram of the groundwater conductivity variation s in the borehole SP-3. by additional hydrogeological, geophysical and geo­ chemical explorat ion , and by the drilling and monito­ ring of observation boreholes, as well as by test pump­ ing of production wells. 5. REFERENCES BIONDle. B. (1988): Tapping and protection of gro­ undwater in the Adriat ic karst region related to the new conception of structures of Dinarides.- Proce­ edings of the 21 st JAH Congress. 187-193, Quil in, China. BrONDle. B. , GOA TTl , V. & VULle, Z. (1979): Hyd­ rogeological investigation of watershed Rjecina spring, Grobnicko polje, Zvi r and Martinscica.- Pro­ ceedings of the 1st Intern. Symp. Aboul Groundwa­ leI' - UNDP. 61-69, Zagreb. BIONDle. B. & GOA TTl , V. (1986): Protection of groundwater in karst area of Croatian littoral.- Pro­ ceedings of 19th IAH Congress, 112-120, Karlove Vary. mONDlC, B., SARIN, A., HERTELENDI, E., DUKA­ RIC, F. HINIC. V. HRVOJIC. E., GOATTI. V., IVICIC, D., KAPELJ, S., KOROLIJA, B., SIN­ GER, D., B[ONDlC, R. & MESIC, S. (1995): Nati­ onal report for Croatia.- Final report of EU COST 65 Project "Hydrogeological aspects of groundwater protection in karstic area", 65-87, Bruxellcs. mONDlC, B, mONDlC, R. & DUKARIC, F. (1998): Protection 01" karst aquifers in the Dinarides in Croa­ tia. - Environmental Geology, 34/4, 309-3 I 9. BOZ[CEV[C, S. (1973): Contribution to the hydrogeo­ logy of the Rjec ina spri ng. - Geol. vjesnik, 25, 277- 283. BOZICEVIC, S. (1974): Morpho[ogy of the Rjccina spring watcr channcls.- Geo!. vjesnik, 27, 273-281, Zagreb. LJeolOgla LrOaliCa ~UfL HERAK, M. (1986): A new concept of geotectonics of Dinarides.- Acta geo\., 16/1, 1-42, Zagreb. HERAK , M. (1991): Dinaridi. Mobilisticki osvrt na gcnczu i strukturu (Dinaridcs. Mobi li stic view of the genesis and structurcs).- Acta geol. , 21/2, 35 - 117, Zagreb. KUHTA , M. (1998): Speleo diving explorations of the Rjccina spring.- Speleolog, 44/45, in print , Zagreb. SEGOTA, T. (1968): Sea leve[ changes in ho[ocen and younger wtirm. - Geographical bulletin , 30, 15-39, Zagreb. SIKIC, D. & PLENICAR, M. (1975): Osnovna geo­ loska karta SFRJ 1:100.000. Tumac za list lIirska Bistriea L33-89 (Geology of Ili rska Bistrica sheet).­ Institut za geol. istr., Zagreb & Geo!. zavod Ljub­ ljana (1967), Sav. geol. zavod Beograd, 50 p. Manuscript received Apri l 14, 1997. Revised manuscript accepted November 10, 1997.