Hydrogeol J DOI 10.1007/s10040-016-1395-x PAPER Review: Natural tracers in fractured hard-rock aquifers in the Austrian part of the Eastern Alps—previous approaches and future perspectives for hydrogeology in mountain regions Sylke Hilberg 1 Received: 31 August 2015 / Accepted: 7 March 2016 # The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Extensive in-depth research is required for the im- Introduction plementation of natural tracer approaches to hydrogeological investigation to be feasible in mountainous regions. This re- Approximately 25% of the Earth’s total land area is mountain- view considers the application of hydrochemical and biotic ous (Meybeck et al. 2001). This mountainous part of the parameters in mountain regions over the past few decades Earth’s land area plays a major role in water supply for the with particular reference to the Austrian Alps, as an example remaining nearly three-quarters, which is characterised as for alpine-type mountain belts. A brief introduction to subhorizontal or flat landscape below an elevation of 1,000 Austria’s hydrogeological arrangement is given to show the m above sea level (asl); thus, mountainous areas are some- significance of fractured hard-rock aquifers for hydrogeological times called Bwater towers^, which, from the hydrological science as well as for water supply purposes. A literature search perspective, indicates their importance for the global water showed that research concerning fractured hard-rock aquifers supply (Liniger et al. 1998; Viviroli et al. 2007). in Austria is clearly underrepresented to date, especially when Large mountain belts abound in North and South America taking the abundance of this aquifer type and the significance of (e.g. Andes, Rocky Mountains), in Africa (e.g. Grand Atlas this topic into consideration. The application of abiotic natural Mountains), Asia (e.g. Himalayas, Altai Mountains), tracers (hydrochemical and isotope parameters) is discussed Australia and New Zealand (Australian Alps, Southern generally and by means of examples from the Austrian Alps. Alps), and in Europe (Alps, Apennines, Carpathians, The potential of biotic tracers (microbiota and meiofauna) is Caucasus, Scandinavian orogenic belt). In terms of many elucidated. It is shown that the meiofauna approach to investi- hydrogeological aspects, these alpine type orogenic belts can gating fractured aquifers has not yet been applied in the be regarded as similar. The predominance of fractured (or reviewed region, nor worldwide. Two examples of new ap- karstic) hard rock aquifers, a pronounced relief with elevation proaches in mountainous fractured aquifers are introduced: variations of several thousands of meters, function as a conti- (1) use of CO2 partial pressure and calcite saturation of spring nental divide and the complex internal structure of folded, water to reconstruct catchments and flow dynamics (abiotic fractured and thrusted nappes of various lithology, are typical approach), and, (2) consideration of hard-rock aquifers as hab- characteristics of alpine type mountain regions, and they affect itats to reconstruct aquifer conditions (biotic approach). hydrogeological properties and processes. Thus, many of the findings and methods developed for one of these mountain Keywords Fracture rocks . Hydrochemistry . Ecology . belts are transferable to other regions. Additionally existing Natural tracers . Austria methodological shortcomings and knowledge gaps in alpine hydrogeology are more or less comparable for all alpine type mountain ranges. * Sylke Hilberg This paper focuses on the Eastern Alps (a Central [email protected] European mountain range) and particularly on the Austrian part. Aside from the aforementioned similari- 1 Department of Geography and Geology, University of Salzburg, ties with other alpine mountain belts, the Alps are Hellbrunner Str. 34, 5020 Salzburg, Austria characterised by some special features such as range Hydrogeol J of elevation (500 up to nearly 4,000 m asl), climate and Hydrogeological overview of the Austrian Alps high population density, and by its lithological distribu- tion. For instance, the up to 2,000-m thick and wide- With regard to the hydrogeological characterisation of the spread karst aquifer of the Northern Calcareous Alps is alpine range, the Austrian part can be divided into greater a special feature of the European Eastern Alps that does geological areas that are more or less arranged from north to not exist with comparable prevalence in other alpine south (Fig. 1;Table1). Whilst porous aquifers are dominant in type mountains; thus, many but presumably not all the the alpine forelands and in intra-alpine valleys, karstic and topics discussed here may be transferred to other moun- fractured aquifers and hard rock aquitards are characteristic tain ranges in the world. for mountain hydrogeology. Hydrogeology in mountain areas is mainly concerned with hard rock aquifers with conduit or diffuse flow Molasse zone conditions. Fractured rock environments are important as a field of study because of their abundance, the dif- The northern foreland of the Alps reaches a maximum height ficulties in predicting their behaviour, and their increas- of 400 m asl in a smooth hilly topography and is characterised ing importance for water supply (Glynn and Plummer by mean precipitation amounts of 800–1,000 mm/year 2005). For a comprehensive hydrogeological model of (Austrian Ministry of Life 2012). The surface area is covered a fractured hard-rock aquifer, aside from geology (e.g. by deep soils and rich vegetation, and is characterised by Neumann 2005), abiotic and biotic processes must be regionally intensive agricultural use. considered. Facies and structural history of the Molasse zone are de- Some important information about the aquifer charac- scribed by Berge and Veal (2005). The aquifer consists of teristics is reflected in the groundwater components such sands and conglomerates in alternation with marl or clay; in as dissolved main ions and trace elements, mineral par- deeper sections the alternation is in the form of consolidated ticles, stable or radioactive isotope composition, organic marl- or claystone. As well as crystalline components, material, and organisms. Understanding the processes carbonatic conglomerates dominate in large areas of the leading to the groundwater composition is an important Molasse zone (Goeppert et al. 2011). Aside from porous aqui- premise to understanding flow systems (as introduced fers in unconsolidated upper layers with hydraulic conductiv- by Toth 1999) in hard rock aquifers. Thus, physio- ities in the range of 10−3 to 10−5 m/s (Gattinger 1980), frac- chemical and biological natural tracers have a high po- tured as well as karst aquifers are present within this northern- tential to determine catchments and infiltration areas, most part of the Eastern Alps. aquifer lithology, flow dynamics, and mean residence The most important process of groundwater recharge is times in all kinds of aquifers. laminar infiltration of rain water from the ground surface. This paper gives an overview of different types of Alternating sequences of medium-grained sand (aquifer) and fractured hard rock aquifers and their characteristics fine-grained clay or marl (aquitard) create a multi-aquifer for- with the example of the Austrian Eastern Alps and mation and local artesian basins (Andrews et al. 1985; some brief glances at studies in other mountain regions, Schubert 2003). and shows how hydrochemical and isotope data as well Due to the share of carbonates in the sedimentary fill, the as biological data are currently used as hydrogeological groundwater quality in the Molasse zone is characterised by indicators. In addition, two approaches to improve qual- the Ca–Mg–HCO3 water type (Kilchmann et al. 2004), alka- itative methods in hard rock hydrogeology are line pH-values and, compared to other alpine hard rock envi- discussed: (1) the assessment of hydrochemical data, ronments, high electrical conductivity values (around 600 μS/ particularly considering the carbon acid system and pro- cm, Austrian Geological Survey 2004; Kralik et al. 2005). cesses in the unsaturated zone (Hilberg 2011; Hilberg Three groups of groundwater were defined within the et al. 2013), and (2) using groundwater inhabitants as Molasse basin of Upper Austria (Andrews et al. 1985): (1) hydrogeological agents (Eisendle-Flöckner and Hilberg shallow local groundwater systems with oxygen-rich water, 2015). calcite saturation and recent tritium values, (2) and (3) both The article is organised into chapters dealing with the fol- deep and regional circulating groundwater with residence lowing topics: times of several hundreds and thousands of years, respectively under conditions of oxygen scarcity. & Hydrogeological overview of the Austrian Alps & The use of abiotic natural tracers in hard rock aquifers Rhenodanubian Flysch zone (RFZ) & Biotic natural tracer use in hydrogeology & Some thoughts about future perspectives for hard rock The RFZ north of the Northern Calcareous Alps (NCA) is hydrogeology in alpine environments characterised by a hilly landscape with the highest elevations Hydrogeol J 0 20 60 0 km 500 km 1.000 km 13° Germany h Czec Republic Slovakia 7 Switzerland Austria Hungary Slovenia (b) Italy Linz Vienna River Danube 40 (a) Salzburg 9 River Enns River Inn ur 6 Innsbruck 5 River Salzach 1 3 River M 2 47° 10 Graz 8 River Drau Legend 4 Quatenary - porous aquifers Klagenfurt Molasse - porous