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Hydrology, Water Resources mid Ecology in Headwaters (Proceedings of the HeadWater'98 Conference held at Meran/Merano, Italy, April 1998). IAHS Publ. no. 248, 1998.

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Tracer-based assessment of vulnerability in mountainous headwaters CHRIS LEIBUNDGUT Institute of Hydrology, University of Freiburg, D-79085 Freiburg, Germany

Abstract Due to high precipitation and comparably low évapotranspiration, mountainous headwaters are regions abundant in water. However, they generally lack extensive underground reservoirs due to shallow soils and predominating hard rocks. Basic hydrological characteristics that cause high vulnerability are an uneven temporal and spatial distribution of water available, high susceptibility of mountainous ecosystems to atmospheric pollution and the often predominating fast-flow connections. The high vulnerability is often blurred, since at first sight the hydrological system works well with plenty of high quality water. Tracer techniques are a key tool in revealing the different aspects of vulnerability of mountainous headwaters. INTRODUCTION Nowadays there is a demand for high standards of drinking-water supply, both in quantity and quality. However, we feel in many places the increasing threat of pollution to our water resources. Vulnerability has to be assessed and effectoriented steps have to be taken. To facilitate far-sighted water resources management, the gap between science and policy must first be bridged. The main problem in this respect is establishing threshold values that are commonly accepted both ecologically and in terms of water management. The critical loads approach was proposed for acidification caused by atmospheric pollution (Nilsson, 1986), the critical load being the maximum deposition of atmospheric pollutants that will not cause long-term damage to ecosystems. Maps for acidity were compiled on the basis of this concept (e.g. CCE, 1993) and the first multinational agreements were reached. Problems still exist in applying the critical loads approach in practice (Bull, 1995). Moreover, pollutants other than sulphur or nitrate do not have the well-defined environmental threshold of an exhausted buffering capacity. In hydrology, a way to quantify vulnerability is to determine the hydroecological capacity (Leibundgut, 1987). This measure describes the capability of a system to compensate disturbances, both in quantity and quality. Different determinable limits expressing the state of a system exist (Fig. 1), in which the natural characteristics with their variability control the intrinsic vulnerability and are quantified by the ecological limit. The actual stress caused by an already existing human impact is added to the natural background as specific vulnerability resulting in the actual limit. The hydroecological capacity of the system occupies the space that is left until the political-economic limit is reached. Examples of this uppermost boundary are legal limits for the quality of drinking water or the minimum discharge within a river reach.

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political-economic limit hydfoccological capacity actual limit actual stress specific vulnerability ecological limit

natural characteristics intrinsic vulnerability

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Fig. 1 Different limits to quantify vulnerability.

In lowland areas and in population centres all over the world the drinking-water supply is the key aspect of water use, where groundwater is considered the most important source. The intrinsic vulnerability of deep aquifers is mostly low as these aquifers are covered and conserved by thick layers of soils and sediments. Still these water resources are endangered due to high and concentrated human impacts. Mapping procedures have been developed to locate dangerous areas and to help decision makers to protect the underground water (e.g. Foster, 1987). GIS techniques facilitate an easy combination of intrinsic and specific vulnerability (De Ketelaere et al, 1998). The spatial distribution of groundwater vulnerability can be analysed for specific pollutants and digitally presented using GIS tools (Sokol et al., 1993). On the other hand, the intrinsic vulnerability of the complete ecological system in mountainous headwaters is high. A GIS-based assessment of general vulnerability was made for a city in a mountainous region to include hydrological as well as geological hazards (Meja et al., 1994). The present paper aims at focusing on hydro-ecological aspects of the high vulnerability in mountainous headwaters. Here morphology is accentuated; low temperatures and low nutrient supplies are dominant characteristics. Water resources are unequally distributed over space and time. Most Alpine soils show low filtering capacities because of high hydraulic conductivity and low cation exchange capacities. Fast-flow connections prevail and ecosystems are susceptible to acidification. Special attention, in terms of vulnerability, is needed in karst regions, where it is extremely difficult to describe conduit-flow processes using hydrological models. Tracers often represent the only tool to obtain an insight into the hydrological system. Point-source pollution, especially by sanitary bacteria, threatens the water supply of mountainous karstic headwaters and makes them highly vulnerable (Leibundgut et al., 1998). In all mountainous headwaters the tracer technique offers additional insights to purely quantitative approaches and can be regarded as a starting point for a sustainable water-resources management. Delineation of catchment areas and vulnerable zones, separation of different runoff components, determination of flow regimes, storage behaviour and catchment altitudes, and the direct tracing of pollution sources are possible.

Tracer-based assessment of vulnerability in mountainous headwaters

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Fig. 2 Cumulative representation of precipitation (P) and runoff (R) within a mountainous catchment in the Swiss Alps. During winter the snow reservoir is recharged (hatched section), while runoff originates from the groundwater reservoir.

BASIC VULNERABILITY CHARACTERISTICS OF MOUNTAINOUS HEADWATERS In most Alpine headwaters soils are thin or even missing and no hydrologically relevant underground storage exists that could serve as a basis for water-resources management. The hard rocks underneath are less permeable or even impermeable and groundwater storage is limited. In mountainous karst areas the infiltrating water quickly percolates to great depths. Throughout the year the quantity of available water within mountainous headwaters is often highly variable and characterized by principal mechanisms (Fig. 2). During winter the snow reservoir is filled. Runoff is low as it is mainly fed by the recession of the groundwater reservoir. As the snowmelt begins, the groundwater reservoir is quickly recharged and overfilled, which leads to runoff. The runoff peak caused by snowmelt can be clearly recognized in typical Alpine runoff regimes. Although mountainous headwaters are generally characterized by an abundance in water, the unequal time distribution of the water causes problems (Leibundgut, 1986). On the one hand, the highest intrinsic vulnerability within mountainous regions occurs in late winter when the discharge of springs and rivers reaches its minimum. Problems of low water and hydrological drought are widely encountered. This can be shown in master recession curves of Alpine rivers, where discharges fall below the mean annual value during two-thirds of the year. The situation even worsens when the specific vulnerability shows a contemporaneous maximum due to pronounced tourism. On the other hand, disasters like big floods or mudflows are pronounced in times of snowmelt with overfilled hydrological reservoirs. Accentuated morphology

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and limited storage then lead to a quick runoff response. High flow velocities and very fast flow connections result. In general there is a tendency to pronounced atmospheric deposition with increasing altitude. Additionally, low temperatures and long winters retard biological and chemical processes. This means headwater soils are underdeveloped and generally characterized by a weak capacity to buffer or to regenerate. Especially on bedrock of low alkali content mountainous headwaters are susceptible to acidification, which is driven by the input of atmospheric sulphur and nitrate. In low mountain range forests, losses of base cations (especially Mg) are further increased by monocultures of superficially rooting spruce (Feger et al., 1991). All over the world the influence of acid deposition can already be felt in the surface waters of headwater basins. Flora and fauna have been severely damaged in many parts of northern Europe by harmful surges during snowmelt (e.g. Bjaernborg, 1983). Nitrate and particularly toxic aluminium are already endangering the drinking-water supply. Headwater lakes are particularly susceptible to acidification. Acidic shocks during snowmelt or after heavy rains were measured (Stoddard, 1995). Only in a few mountainous headwaters rapid geochemical processes in shallow groundwater matrixes can still balance the quality of surface water (e.g. Campbell et al., 1995). TRACERS AS AN ASSESSMENT TOOL FOR VULNERABILITY Tracers facilitate decoding of hidden information on the whole contributing catchment at one point of sampling (Leibundgut, 1987). Using this concept of convergence, detailed studies on the state of a system become possible. Specific volumes of water may be traced and followed from the input to the output. Insights into complex hydrological systems that are not reachable by conventional, purely quantitative methods become possible. For this reason, tracers must be seen as a key tool for sustainable water-resources management in mountainous headwaters. Here, an additional source of heterogeneity is created by significant altitude differences within a basin (Morel-Seytoux, 1990). Moreover, aquifers and underground passages are mainly complex. Although fissured aquifers in the bedrock, debris covers, alluvial fans and valley fills are of a limited extent, they have a strong influence on the runoff generation and behave completely differently. Tracers facilitate detection of their relative importance by the hydrograph separation method. Using a combination of different natural tracers, especially silica and 18 0, the dominating influence of the debris cover on runoff generation in the southern Black Forest was determined (Lindenlaub et al., 1997). The above-mentioned heterogeneity caused by significant altitude differences can be used directly to determine the mean altitude of headwater basins with the help of tracers. As their input significantly differs with altitude, stable isotopes are commonly used for this purpose. Natural tracers reveal directly the vulnerability of an ecosystem. In general, mineral concentrations show their maximum in periods of low flow and therefore document the time aspect of vulnerability in mountainous headwaters. If nitrogen levels are elevated, excess leaching of nitrate from the root zone can be detected through increased nitrate concentrations in surface and underground waters.


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Furthermore, the appearance of ecotoxic aluminium species in the water cycle documents an advanced state of soil acidification (e.g. Feger et al., 1991). Both natural and artificial tracers are suitable in describing underground reservoirs and determining fast flow connections. This means that a very important aspect of the increased vulnerability in mountainous headwaters may be described. In springs, which are common in mountainous headwaters, the response of the hydrological system can be analysed most effectively. As a result of the complex geology of most mountainous regions, several kinds of springs exist. Although they may have completely different characteristics, a quick reaction to hydrological changes can be found almost anywhere. A typical example can be found in a Black Forest headwater basin, Germany, where different springs have been analysed in detail. A combination of water characteristics and natural tracers (pH-value, total mineralization, 180 and silica) facilitated the detection of the source areas of the springs and a classification into three groups. Chloride could not be used in general because it was influenced by human inputs. Four springs were studied more closely in a wet period after snowmelt. A hydrograph separation was made using dissolved silica. A fast component could be separated in all the springs. To obtain a more detailed insight into these fast flow paths, one spring was analysed by artificial tracers. Fluorescence dyes (uranine and naphtionat) were injected into the unsaturated zone of an active talus (Mehlhorn et al., 1998). A very quick response was measured 88 m downslope, resulting in a very short residence time (Fig. 3). Direct runoff components and discharge fluctuations were most significant in springs originating from debris covered slopes. Springs originating from deep fissured aquifers showed a more constant yield. Nevertheless, environmental tracers showed also this type of spring to be influenced by short circuits of a direct component during extremely wet periods (Lindenlaub et al, 1997). Another important field for the application of artificial tracers is the delineation of vulnerable zones within mountainous headwater basins. The key role of the tracer technique became clear in two extensive three-year studies on the vulnerability of two adjoining mountainous karst systems: the Alpstein region and the ChurfirstenAlvier region in northeastern Switzerland. Different physical and chemical parameters were used as tracers. In particular, the simple tracers temperature and electric conductivity made it possible to determine the time lag and the intensity of 6 -,

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Time since injection (hours) Fig. 3 Measured and fitted breakthrough curve of naphtionat in a spring after lateral passage through 88 m of debris slope. The SFDM-model (Maloszewski, 1994) is used for fitting, resulting parameters are given.

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reactions for single rain- or snowmelt events. In most springs the chemical parameters chloride and nitrate, indicating surface inputs, showed concentrations comparable to those of the precipitation background. In a few springs, however, small peaks at higher discharge could be determined. These peaks cast suspicion on quick direct flow paths between surface layers and distant springs. This assumption was confirmed by the fact that in some springs problems with sanitary bacteria had already been reported. A final proof of paths and velocities of flow was only possible through multiple-tracer tests using fluorescence dyes. The time of the highest discharge after snowmelt was chosen for the experiments. Predominant drainage directions were determined and catchment areas were localized (Fig. 4). Maximum velocities were between 12 and 408 m h"1, typical for mountainous karst regions. Moreover, it could be shown that most of the occurring lakes were not hydrologically inactive: direct flow connections between lakes and springs existed. The most important result formed the vulnerable zones that could be clearly localized within the two mountainous headwaters; this helped decision makers greatly to safeguard water resources. Bacteria themselves were used as tracers to show vulnerability caused by sanitary pollution, thereby making a perfect understanding of transport and degradation processes possible. The movement of sanitary bacteria within a karst system was also analysed on a large scale (Gunn et al., 1998). CONCLUSIONS Due to high precipitation and comparably low évapotranspiration, mountainous headwaters are regions abundant in water. They are characterized by a large storage of ice and snow but generally lack extensive underground reservoirs due to shallow soils and predominating hard rocks. In the special case of mountainous karst, flow velocities are comparable to those of surface drainage. Thus all mountainous headwaters react extremely and quickly to changes in the input, and their overall vulnerability is high. This high vulnerability is often blurred, since at first sight the hydrological system works well with plenty of high quality water. In some Alpine headwaters, quality-oriented water conservation has shown reasonable success in past decades. However, human-induced changes in flow regime (hydropower) and structural morphology (flood protection) still severely affect the biota of headwater streams (Blôsch, 1997). Other catchment-wide processes further enhance these effects. On alkaline-poor bedrock acidification threatens more and more the weakly buffered ecosystems. In most basins containing carbonate rocks, the hydrochemical quality of the water resources is still favourable. Also in these regions increasing concentrations of chloride and nitrate are the first to show reactions of the fragile mountainous headwaters to human-induced changes. Rapid flow connections exist and there is limited ability for self-purification due to short soil and underground passage. As a result, comparably modest human impacts (e.g. sanitary pollution) have already left their mark on springs and rivers. This fact can only be proven comprehensively by applying artificial tracers. Springs are widespread in mountainous headwaters and are the obvious output of underground reservoirs, where future efforts should therefore be concentrated. Generally, a combined

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approach including natural and artificial tracers should guide decisions on measures to be taken to safeguard water resources. The fragile mountainous headwaters are the first link within a hydrological chain, generating huge river systems. A change within a river's hydrological system is transferred to distant places further downstream. There some natural or human-induced changes might cancel each other out, e.g. those within the runoff regime (Leibundgut, 1986). Others, for instance large floods or pollutants, add up and might reinforce each other on their way downstream. Headwater regions therefore need special protection and a spatially distributed catchment-wide approach in the discussion on water quality and conservation is necessary (Leibundgut, 1996). Only a well-functioning fluvial ecosystem will combine the interests of those who want to use and those who want to conserve lakes and rivers (Blôsch, 1997). Multipurpose water-management planning is required to obtain a most natural state of the surface waters in headwater basins while respecting the interests of flood protection, agriculture, water-power use, industry and tourism. Existing conflicts cannot be avoided but have to be minimized to guarantee enough water for future generations.

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Meran/Merano, April 1998). IAHS Publ. no. 248, this volume. Meja, N. M., Wohl, E. E. & Oaks, S. D. (1994) Geological hazards, vulnerability and risk assessment using GIS; model for Glenwood Springs, Colorado. Geomorphology 10, 331-354. Morel-Seytoux, H. J. (1990) Quantitative and qualitative water cycle aspects in heterogeneous basins. In: Hydrology in Mountainous Regions. I—Hydrological Measurements; The Water Cycle (ed. by H. Lang & A. Musy) (Proc. IAHIAHS Symp., August 1990), 803-810. IAHS Publ. no. 193. Nilsson, J. (1986) Critical Loads for S and N. Nordic Council of Ministers, Copenhagen. Sokol, G., Leibundgut, Ch., Schulz, K. P. & Weinzierl, W. (1993) Mapping procedures for assessing groundwater vulnerability to nitrate and pesticides. In: Application of Geographic Information Systems in Hydrology and Water Resources Management (ed. by K. Kovar & H. P. Nachtnebel) (Proc. HydroGIS 93 Conf., Vienna, April 1993), 631-639. IAHSPubl.no. 211. Stoddard, J. L. (1995) Episodic acidification during snowmelt of high elevation lakes in the Sierra Nevada Mountains of California. Wat., Air and Soil Pollut. 85, 353-358.