Complex Hydrogeological Investigations in a Sensitive Karst Environment: the Koreničko

Complex hydrogeological investigations in a sensitive karst environment: the Koreničko Vrelo case study in the National Park "Plitvička jezera" in Croatia

Josip TERZIĆ1, Ante PAVIČIĆ1, & Tamara MARKOVIĆ1 1Croatian Geological Survey, Sachsova 2, HR 10.000 Zagreb

Keywords: karst hydrogeology, hydrochemistry, pumping test

Introduction, geological and hydrogeological setting The investigated area of the Koreničko Vrelo spring catchment area is situated in the Lika district, in central Croatia. This is part of the well-known Dinaric karst region, characterized by a strike of geological structures NW-SE, deep karstification as a consequence of compression of structures, and reverse faulting. Triassic, Jurassic and Cretaceous rocks, predominantly limestones and dolomites, show high tectonic disturbance and destruction; they are karstified and partially covered with Quaternary deposits. The Koreničko Vrelo (Fig. 1) is capped for the water supply of the small town of Korenica and adjacent villages. It is a typical karst spring with a marked difference in discharge quantity from the minimum (a few tens of l/s) to the maximum (above 1 m3). It is part of a wider hydrogeological system represented by karst springs: Korenička Rijeka, Mlinac, Kameniti Vrelac, Stipinovac and Koreničko Vrelo, besides a few springs of lesser importance. From the Korenička rijeka spring begins a stream (small river) called Matica. The stream is situated in the middle of a valley covered with Quaternary deposits, and the above-mentioned karst springs are situated on the right (western) side of the Matica. The majority of the rocks that form their catchment area and aquifer are composed of karstified limestones. All these springs appear at almost the same elevation of about 700 m a.s.l., a fact connected with the regional fault in whose zone the springs occur. On this fault, limestones from the hinterland are in contact with dolomites under quaternary deposits in the valley. On the eastern side of the Matica there is small dolomitic hill, with many springs of low discharge on its flank. Since the Korničko Vrelo is situated within an inhabited area, there is a high danger of micro- biological pollution. Therefore it is important to remove the water extraction site outside the village, and to reach the groundwater in the karst underground by boreholes before it wells out. Bulk hydrogeological investigations have been performed for that purpose, and are still continuing. Up to now, structural geological analysis of the terrain has been performed, as well as detailed hydrogeological mapping, hydrochemical research, prospecting of 27 springs in the area, geophysical explorations, core drillings, and monitoring of water quality and quantity (Andrić et al., 2006; Pavičić et al., 2006). The whole investigated area is situated inside the boundaries of the well-known "Plitvička jezera" national park. That circumstance is very important, because every kind of research procedure needs to be conducted with minimal impact on the environment. That particularly applies to bore-hole drilling. Although the Koreničko vrelo catchment area has no direct hydrological impact on the Plitvička jezera lakes, it is important to protect that part of the National park environment as well, and that includes groundwater use. Special permission needs to be obtained from the Croatian Ministry of Culture and the National Park management. Every exploratory procedure or step needs to be conducted with the knowledge of the park rangers who have the task of monitoring them. Figure 1. Schematized hydrogeological map of the Koreničko vrelo catchment area. 1-Quaternary deposits; 2- limestone, mostly Jurassic; 3-Jurrasic limestones and dolomites; 4-dolomites; 5- geologic boundary; 6-presumed geologic boundary; 7-main fault; 8-fault; 9-presumed fault; 10- groundwater connection (tracer test); 11-general groundwater flow direction; 12-groundwater divide; 13-spring; 14-borehole.

Hydrochemical properties In situ measurements were taken of: EC, TDS, T. pH and O2 saturation. Basic anions were determined on the LabAlliance's ion chromatograph, orthophosphates and ammonia were measured by colorimetric method on a Hach's spectrophotometer DL/2010. Basic cations were measured by Perkin Elmer's atomic absorber. The 3H isotope was measured in the laboratory of the "Ruđer Bošković" Institute in Zagreb, while isotope 2H, D and 18O were determined in the Joanneum research institute in Graz, Austria. The quantity of dissolved minerals in groundwater depends primarily on the partial pressure, as well as on the dissolution of CO2. With the pCO2 increase in groundwater, the quantity of carbonic acid also increases (Stumm & Morgan, 1996; Dimitrijević, 1988). That leads to better dissolution of calcite and dolomite minerals, according to the equation:

2+ 2+ - ( Ca1-x Mgx) CO3 (s) + CO2(g) + H2O  (1-x)Ca + xMg + 2HCO3 By chemical composition the water from all of investigated springs belongs to the Ca- HCO3 hydrochemical type. Water from analyzed samples from Kameniti vrelac, Koreničko vrelo, Stipinovac, and Korenička river spring is saturated with calcite and unsaturated with dolomite; and at Makarevo vrelo and Mlinac spring vice versa. Saturation indexes were calculated by the mass balance geochemical model NETPATH (Plummer et al., 1994). The water is saturated with oxygen and generally of very good quality. Isotopic composition analyses are used to determine the catchment area of each karst spring. An understanding of each separate catchment area will serve to improve the placement of new water extraction points outside the influence of the nearby highway. Based on the deuterium/oxygen-18 ratio it was determined that the mean altitude of the catchment area of Korenička river spring, Mlinac and Makarevo vrelo is higher than that of the Koreničko vrelo, Stipinovac and Kameniti vrelac catchment.

Table 1. Results of hydrochemical analysis: in situ, ionic and isotopic composition. September 2006. EC TDS T pH O O TOC Na+ K+ Ca2+ Mg2+ HCO - 2 2 3 (S/cm (mg/l (mg/l) (oC) (%) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) ) C) Makarevo vrelo/09/06 496 348 8 7.1 90 9.7 1.328 1.1 0.7 71 14.8 279 Korenička rijeka/09/06 464 325 7.9 7.44 90 9.7 1.3 0.8 0.8 73.3 10.8 268 Mlinac/09/06 447 313 7.8 7.38 97 10.4 0.398 1.4 0.8 83.3 4.1 248 Kameniti vrelac/09/06 510 357 8.5 7.68 99 10.6 0.463 12.4 0.7 88.7 10.3 248 Koreničko vrelo/09/06 437 306 8.2 7.57 105 11.4 0.492 1.1 0.6 83 4.4 248 Stipinovac/09/06 465 326 8.1 7.62 104 11.4 0.501 5 3.2 84.4 5.2 275 MAC (N.N. 182/04) 150 12 3-- 3- 2- + PO4 - 2- - 3 18 NO NO NH4 Cl SO4 Br H dD d O P o o (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) Bq/l T.U. /oo /oo 0.88+- 7.4+- - - Makarevo vrelo/09/06 6.3 <0.001 0.01 <0.01 3.3 7.9 0.7 0.25 2.1 71.18 10.48 0.78+- 6.5+- - - Korenička rijeka/09/06 5.6 <0.001 <0.01 <0.01 3.4 5.6 0.8 0.24 2 71.89 10.85 0.89+- 7.4+- - - Mlinac/09/06 6.6 <0.001 <0.01 <0.01 4.7 6 1.2 0.24 2 69.56 10.46 0.68+- 5.7+- - - Kameniti vrelac/09/06 5 <0.001 0.01 <0.01 58 6.5 1.1 0.14 1.1 67.91 10.22 0.44+- 3.7+- - Koreničko vrelo/09/06 5.9 <0.001 0.01 <0.01 4.6 6.1 0.8 -10.3 0.24 2 67.08 0.58+- 4.9+- - - Stipinovac/09/06 6.2 <0.001 <0.01 0.04 10.4 6.6 1.2 0.23 1.9 67.73 10.21 MAC (N.N. 182/04) 50 0.1 0.5 0.3 250 250

Pumping test on the BK-2 borehole The BK-2 borehole was drilled in Jurassic limestones some 75 m behind the Kameniti vrelac spring. The borehole is 50 m deep, the entrance is at 702.49 m a.s.l. The groundwater level in the steady state was 688.80 m a.s.l., which is in accordance with the Kameniti vrelac (687.63 m a.s.l.). Middle Jurassic limestones in the drilled core were fractured and karstified to a certain extent. The rock mass was homogeneous, and discontinuities were in uniform setting. No real karst conduits were determined. Since the casing diameter was quite narrow (7.5 cm), the pumping test could be done only with a relatively weak pump. The maximal pumping rate was 1.54 l/s. The step-drawdown pumping test was carried out, with three pumping rates: 0.56, 1.00 and 1.54 l/s. Drawdown was increasing, for specified pumping rates from 1.6 m to 3.14 m and to 4.8 m. The recovery of the water level was measured for 24 hours, and it was complete (Fig. 2). According to graphical/analytical solution (Driscoll, 1995) the parameters of laminar (B=2964.2) and turbulent losses (C=111504) were calculated. The total drawdown equation for BK-2 could be expressed: s=2964.2Q+111504Q2. That means that for a pumping rate of 3 l/s drawdown would be 9.90 m, for 5 l/s 17.61 and for 10 l/s 40.79 m. Laminar losses theoretically express losses in the aquifer, and turbulent losses are those on the well construction. In practice, they are mixed (Driscoll, 1995) and it is hard to distinguish one from the other. Calculation showed that the majority of groundwater flow near the BK-2 borehole is laminar. For 1 l/s laminar flow is 96.4% of total flow, for 5 l/s 84.2 and for 10 l/s 72.7. In the steady state most of the groundwater flux is laminar. Because of that the calculation of the aquifer parameters is justified, so the transmissivity is 3.7*10-4 and the hydraulic conductivity of the rock mass is 3.9*10-6. The outflow regime on the Kameniti vrelac spring supports the conclusions reached. It is not a typical karst spring, and the differences between minimal and maximal discharges are quite small. During the quantity monitoring (whole of 2006) the minimal discharge was 8.2 and maximal 13.7 l/s. Water level (m a.s.l.) Q (l/s) 690.00 1.8

689.00 1.6

688.00 1.4

687.00 1.2

686.00 1

685.00 0.8

684.00 0.6

683.00 GWL, BK-2 (m a.s.l.) 0.4 Kameniti vrelac (m a.s.l.) 682.00 Pumping rate, Q (l/s) 0.2

681.00 0

Figure 2. Step-drawdown pumping test on the BK-2 borehole.

Concluding discussion Discharge monitoring on springs showed several different types of springs. That was corroborated by the hydrogeological mapping, where different rock mass blocks could be distinguished (Fig. 1), and by geophysical researches, hydrochemical and isotopic results and even the pumping test on the BK-2 borehole. Investigations were carried out systematically, and some initial positive results were achieved. Water can be extracted at the location of the BK-2 borehole. That water is of much lower pollution risk than the one from the main present water supply source: Koreničko vrelo. At the BK-2 borehole the influence of the highway is still significant. That was proven by the high chloride concentration (Table 1). These chlorides probably originate from the salt that is sprinkled on the road during winter time. This requires to be proven by further hydrochemical monitoring. The researches are continuing, and the main goal is to capture the groundwater in the area distant from the road, deeper in the north, or at the Mrsinj mountain slopes in the south. It is still not clear if a part of the water comes to the Koreničko vrelo from that direction. Further research has to be done with minimal or no damage to nature and/or the landscape, because the greater part of the explored area is part of the National Park Plitvička jezera. This is a limiting factor for certain potential borehole locations. On the other hand from the National Park area there is almost no man-made pollution. With precise research planning it is possible to satisfy both sides: to create an unpolluted water source for the local community, and to cause no damage to the environment.

References Andrić, M., Padovan, B., Krsnik, M. & Grgec, D. (2006): 2nd phase of water research in the Koreničko vrelo area – Geophysical researches. Institut građevinarstva Hrvatske, Zagreb. (technical report, in Croatian) Dimitrijević, S. N. (1988): Hidrochemistry. Univerzitet u Beogradu. Rudarsko-geološki fakultet OOUR Grupa za hidrogeologiju. 312 str. Beograd. Driscoll, F. G. (1995): Groundwater and Wells. ISBN 0-9616456-0-1, Published by Johnson screens, St. Paul, Minnesota 55112. 1089 pg. Pavičić, A., Terzić, J. & Buljan, R. (2006): Hydrogeological research-the 1st phase of water research in the Koreničko vrelo area. Fond HGI 102/2005. (technical report, in Croatian) Plummer, L.N., Preston, E.C. & Parhurst, D.L. (1994): An interactive code (NETPATH) for modelling net geochemical reactions along flow path. Version 2.0 USGS Water-Res Inv Report, 94-4169, Reston, Virginia. Stumm, W. & Morgan, J. J. (1996): Aquatic chemistry. Chemical Equlibria and Rates in Natural Waters. John Wiley & Sons, INC. New York – Chichester – Brisbane – Toronto – Singapore.