Introduction

The development of agro-tourism in the area of Gheţar–Ocoale in the Bihor Mountains, encouraged by the surrounding karstic landscape and by the presence of the largest ice cave of Romania—the Scărişoara Glacier, will give rise to the development of a network of roads and guesthouses and the building of a central water supply network that roughly overlaps the karst system drained by the source of Coteţul Dobreştilor. The lack of adequate protection measures for the aquifer, associated with the development of agro-tourism, will result in severe pollution of this important water resource.

In the scope of the Apuseni Project that was deployed in the watershed Gârda Seacă–Ordâncuşa during the interval 2001–2003, multi-disciplinary research was performed to propose a sustainable development of the Ţara Moţilor settlements. This research was financed by the German Ministry for Science and Education under no. 0339720/5. The detailed hydrogeological and paleontologic data obtained are used in this paper for assessing the vulnerability of the groundwaters to the impact of the future agro-touristic development of the area (Table 1).

Table 1 Result of tracer labeling performed in Gârda Seacă–Ordâncuşa watershed
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Morphological setting

The considered area lies at the eastern limit of the karst area of Bihor Mountains, in the drainage area of the Arieş River. It includes the southern part of the Gârda Seacă–Ordâncuşa watershed developed between the villages of Ocoalele and Mununa, an area that is known by the name of Gheţar Plateau.

The Ghetar Plateau is elevated 300–600 m above the streambeds of the adjoining streams and consists of isolated ridges separated by saddles and chaotically disposed karst depressions, without displaying a single dominating feature. Immediately northward from the junction of Gârda Seacă and Ordâncuşa streams (730 m altitude), the absolute elevations rapidly increase up to Mununa (1,100 m), then climb slowly up to Ocoale (1,325.3 m) and Rânjeşti (1,366.0 m) summits. The topography assumes the appearance of a karst plateau strewn with the entire range of both above-ground and below-ground karst landforms characteristic to these areas.

Ocoale stream is the only stream appearing on the carbonate surface of the plateau (Fig. 1). It has its headwater on the Early Jurassic terrains in the northwestern part of the Ocoale–Gheţar depression. The original course, tributary to Gârda Seacă stream, has been affected over time by a multitude of karst events, marked by the caves network that includes Groapa cu Apă a lui Miron ponor (Fig. 1, no. 1), the pothole in Şesuri (Fig. 1, no. 2), Gheţarul de la Scărişoara (Fig. 1, no. 3) and by a succession of fossil swallet holes which extend northward up to the present day permanent sinking point, situated at the contact of the limestone with the Early Jurassic sandstone and shales. On the plateau are also several springs that generally have an impermanent character and small flow rates. Their presence is connected with the discharge of the aquifer accumulation from the eojurassic and alluvial–deluvial (Vuiaga) deposits or with the existence of epi-karst aquifers: Bărâcia (Fig. 1, no. 4), Apa din Cale (Fig. 1, no. 5), Troaca (Fig. 1, no. 6), Rădăcini (Fig. 1, no. 7).

Fig. 1
figure 1

Hydrogeological map of the Gârda Seacă–Ordâncuşa water divide territory. Bihor Autochthon: 1 undivided Urgonian limestones (br + ap1); 2 black-bedded oolithic limestones (th); 3 reef limestones (ox-th1); 4 red oolithic limestones, yellowish spotted limestones, reddish and gray encrinitic limestones (J2); 5 reddish and gray encrinitic limestones, marls (si2-to); 6 quartzitic sandstones and conglomerates, argillaceous shales, black limestones (he + si1); 7 white reef limestones—Wetterstein limestone (ld + cr1); 8 gray dolomites (an). Bihor Autochthon and Gârda Nappe: 9 quartzitic sandstones and conglomerates, red argillaceous shales (w); 10 crystalline schists; 11 limit of endoreic (inland) areas; 12 limit between endoreic areas: 13 proven groundwater flow direction; 14 temporary shallow hole; 15 dug well; 16 karstic depression

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The annual average rainfall recorded at Gheţar is 1,315 mm/year and the annual average air temperature is 5.2°C (Orăşeanu and Varga 2003, 2004).

Geological and structural setting of the area

The Gârda–Ocoale area almost entirely consists of sedimentary deposits ascribed to the Bihor Unit. Only in the lower reaches of Gârda Seacă stream, in the southwestern part of the area, are there outcrops of Werfenian and Permian sandstone and conglomerate deposits which in structural terms are ascribed to Gârda Nappe of the Codru Nappes System.

The Bihor Unit formations in the considered area include Werfenian detritic deposits consisting of conglomerates and quartzite sandstones and red shales, which are overlain by a thick series of carbonate deposits with gray Anisian dolomites, followed by white Ladinian–Early Carnian reef limestone (the Wetterstein limestone). The carbonate deposits are transgressively overlain by the prevalently detritic Early Jurassic deposits, which consist of quartzite sandstone and conglomerates, shales and black limestones of 200–300 m overall series thickness (Hettangian–Early Sinemurian), reddish and gray encrinitic limestone, marls and marly limestone 6–80 m thick (Late Sinemurian–Toarcian). The series ends with reef limestone (Oxfordian–Early Tithonic) and black oncolithic Tithonic limestone (Dumitrescu et al. 1977; Bleahu et al. 1980).

The entire sedimentary series of the Bihor Unit forms a homoclinal structure which generally strikes NE–SE. The ensemble dips from NE to SW in the northern half of the structure and from east to west in the south. In general, neither a recurrence of the succession due to strike-slip faulting on reverse faults nor any folding can be identified. The structural continuity of the Triassic deposits along the NW–SE direction is broken by agraben with Jurassic deposits in the axis, a structure that deeply penetrates toward the northeast into the homoclinal structure of the Triassic deposits, from Brustur Brook up to the Ocoale area. In the southern part, the sequence of carbonate deposits is longitudinally dissected by the Hănăşeşti–Mununa fault system, with its western compartments uplifted. In this compartment along the fault, Anisian dolomite outcrops are frequent.

The geological background for the hydrogeological map is provided by sheets Poiana Horia and Avram Iancu of the Geological Map of Romania, scale 1:50.000, devised by Bleahu et al. (1980) and Dumitrescu et al. (1977).

Hydrogeological issues

The investigation of groundwater circulation in the Gârda Seacă–Ordâncuşa watershed was initiated by fluorescein tracer tests performed in the underground stream course of the pothole in Şesuri originally by Şerban et al. (1957). Rusu and Racoviţă in April 1964 (Rusu et al. 1970) later used 1.5 kg of fluorescein to also study the Ocoale sinking stream.

The karst system Coteţul Dobreştilor includes the mountainside catchment of the Gârda Seacă stream eastward of the sources of Coteţul Dobreştilor (0.9 km2), most of the internal drainage area of the Gheţar Plateau (8.2 km2) and the diffluence surface of Ordâncuşa–Coteţul Dobreştilor (10.3 km2) area that overlaps the upper catchment of the Ordâncuşa stream upstream from the Moara lui Ivan. The underground flow rate transiting from the diffluence surface to the resurgent sources is determined by hydrogeological balance (Orăşeanu and Jurkiewicz 1982; Orăşeanu 1985).

The karst system discharges through the sources from Coteţul Dobreştilor located at the contact between the limestone and quartzite sandstones of the Gârda Nappe, sources that are hydrologically interconnected, having a cumulative flow rate of about 350 l/s and an average temperature of 7.6°C. Such sources are: the Coteţul Dobreştilor spring, a temporary source having an annual average flow rate of ca. 270 l/s, the Morii spring and submersed sources from the valley floor of the Gârda Seacă stream.

Coteţul Dobreştilor karst system displays large values of the discharge time series variation coefficient, Cv, indicating a well-developed underground flow organization, quite probably along large cavities. Cv, the discharge time series variation coefficient, is the ratio between the average deviation of a hydrological annual series of mean daily discharge values (Oct–Sept) and the annual average of this series. It ranges between 0 and 1, the large values indicating outlets with large variations of flow rate, typically associated with karst systems subject to strong karst development with well-organized underground flow (Orăşeanu 2005, in press).

To obtain additional information concerning the degree of structuring of the main karst systems, electric conductivity of the water of the spring Coteţul Dobreştilor was measured every 2 days during May 2001–July 2003. The water is derived from at least two distinct populations, each one having its own geochemical evolution and hydrogeological history (Fig. 2).

Fig. 2
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The distribution diagrams of frequency values of the water conductivity of Coteţul Dobreştilor karst spring

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The aquifer discharging through the outlets at Coteţul Dobreştilor is well structured and organized, with a functional main flow axis that facilitates fast arrival to the spring without significant mixing with water stored in the annex systems of the aquifer.

Intrinsic vulnerability of aquifer and source

In this exercise to assess the intrinsic vulnerability of the aquifer from Coteţul Dobreştilor, the internal drainage area and the mountainside karstic catchment were considered (Fig. 3). The impact produced by waters seeping through the Ordâncuşa–Coteţul Dobreştilor diffluence area that is also part of the karst system has not been assessed here for lack of pedological and hydrological data of their specific contribution to the supply.

Fig. 3
figure 3

The Coteţul Dobreştilor karst system. 1 extension of the karst system: a karstic slope basin, b internal drainage area, c diffluence surface; 2 boundary of internal drainage area; 3 boundary inside internal drainage areas; 4 proven groundwater flow connection; 5 inferred groundwater flow connection

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Detailed pedological studies performed in the boundaries of the Gârda Seacă–Ordâncuşa watershed (Parichi and Stănică 2003; Stănilă et al. 2003) resulted in a detailed pedological study and specific pedological maps, out of which the soil thickness map, the vegetation map, the field capacity map, the map describing hydraulic conductivity of the soil and the slope map were chosen for this research.

Protective cover

The soil developed in the studied area is relatively thin, less than a meter. For assessing the protective capacity of the soil cover, the field capacity was multiplied by its thickness. The field capacity of the soil cover was ranked in four classes: very low (under 10%), medium (21–25%), medium-high (21–30%) and high (26–30%), while its thickness (Fig. 5) was ranked in five classes (0–10, 11–20, 21–50, 51–75, 76–100 cm).

The product obtained by integrating the two maps (n = 4 × 5 = 20 values) has been distributed in four vulnerability classes: P = 1, very low protection degree (n = 1–5); P = 2, low protection degree (n = 6–10); P = 3, moderate protection degree (n = 11–15) and P = 4, medium protection degree (n = 16–20).

There is no non-karst rock layer between soil and unsaturated karst rocks. The development of the crack systems is assumed to be high and evenly distributed over the entire area. The eojurassic deposits (sandstones, shales) from the north of the area occur in sunken tectonic blocks separated from the carbonate deposits by vertical faults. Their lithological structure displays a high protection degree. The areas covered by eojurassic deposits were allotted a medium-high protection rating, P = 5. The P map is shown in Fig. 4.

Fig. 4
figure 4

P map

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Determination of the I parameter

The I parameter shows the degree to which the protective cover is being bypassed by the water and has two components: I′ or seepage and the surface catchment.

The I′ parameter estimates the occurring seepage and is controlled by the permeability of the soil, the land slope and the vegetation. The integration of these factors is shown in Table 2. Permeability of the soil has been estimated on the basis of saturated hydraulic conductivity.

Table 2 Determination of the I′ factor
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The surface catchment map shows components which bypass the protective cover as a result of lateral surface and subsurface flow in shallow holes and sinking streams. The surface catchment map has been drawn on the basis of hydrogeological mappings indicating the presence of shallow holes and sinking streams. Buffer zones of 10 and 100 m around such features are also considered (Table 3).

Table 3 Determination of the I factor
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The I map (Fig. 5) is obtained by intersecting the I′ map and the surface catchment map according to the scheme presented in Table 3. A value I = 1.0 indicates that the protective cover is not bypassed. On the other hand, where the protective cover is completely bypassed by a shallow hole through which surface water directly enters the karst aquifer, the I factor is 0.0.

Fig. 5
figure 5

I map

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The PI vulnerability map (Fig. 6, Table 4), is obtained by intersecting the P map with the I map.

Fig. 6
figure 6

Vulnerability map of the Gheţar plateau area. The legend of the symbols is shown in Table 4

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Table 4 Legend of the P map, I map and vulnerability map
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The hydrogeological data indicate that in the saturated area of Coteţul Dobreştilor karst aquifer, the karst network is well developed (K factor in the EPIK method and the European Approach method), a conclusion also sustained by the large number of caves and potholes and by their significant lengths (Pothole of Şesuri: 3,840 m length and 220 m denivelation, Coteţul Dobreştilor cave: 294 m, Pojarul Poliţei cave: 400 m, Scărişoara ice cave: 700 m, etc.). The karst network of the aquifer facilitates a fast transit of the water reaching the aquifer surface towards the sources from Coteţul Dobreştilor, and thus of polluting substances. There is little opportunity to develop physical or chemical processes that might facilitate the degradation of polluting substances.

The protective capacity of the overlying soil is low to very low because of reduced thickness. The area with very high protective function of soil reflects the distribution of Jurassic rock outcrops (P = 5). According to the I map, the degree of runoff bypassing the protective cover is low to very low for most of the area. This is because the Ocoale sinking stream and shallow holes make up only a portion of the area. The PI map shows many areas of extreme and high groundwater vulnerability.

Conclusions

On the basis of available geological, hydrogeological and hydrological data (tracer tests, spring hydrograph, water electro-conductivity time series), as well as pedological data (thickness, field capacity and saturated hydraulic conductivity soil maps), vegetation and slope maps were drawn showing the effectiveness of protective cover (Fig. 4), bypassing of protective cover (Fig. 5) and finally the intrinsic vulnerability of the Coteţul Dobreştilor karst aquifer (Fig. 6).

The soil provides a weak protection to the karst aquifer, being easily and frequently bypassed by the permanent or impermanent superficial flows, while the karstic network is very well developed. Over most of the considered area, the vulnerability of the karst aquifer is extremely high. The source discharging the karst aquifer is also very vulnerable.