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Groundwater protection in fractured media: a vulnerability-basedapproach for delineating protection zones in Switzerland

Alain Pochon & Jean-Pierre Tripet & Ronald Kozel &Benjamin Meylan & Michael Sinreich &

François Zwahlen

Abstract A vulnerability-based approach for delineatinggroundwater protection zones around springs in fracturedmedia has been developed to implement Swiss water-protection regulations. It takes into consideration thediversity of hydrogeological conditions observed infractured aquifers and provides individual solutions foreach type of setting. A decision process allows forselecting one of three methods, depending on the springvulnerability and the heterogeneity of the aquifer. At thefirst stage, an evaluation of spring vulnerability isrequired, which is essentially based on spring hydrographsand groundwater quality monitoring. In case of a lowvulnerability of the spring, a simplified method using a

fixed radius approach (“distance method”) is applied. Forvulnerable springs, additional investigations must becompleted during a second stage to better characterizethe aquifer properties, especially in terms of heterogeneity.This second stage includes a detailed hydrogeologicalsurvey and tracer testing. If the aquifer is assessed asslightly heterogeneous, the delineation of protection zonesis performed using a calculated radius approach based ontracer test results (“isochrone method”). If the heteroge-neity is high, a groundwater vulnerability mappingmethod is applied (“DISCO method”), based on evaluat-ing discontinuities, protective cover and runoff parame-ters. Each method is illustrated by a case study.

Keywords Fractured rocks . Groundwater protectionzones . Springs . VulnerabilityMapping . Switzerland

Introduction

Fractured aquifers represent an essential groundwaterresource for large parts of the world. In Switzerland, wheremore than 80% of drinking water is provided by ground-water, fractured media cover 78% of the land surface andsupply approx. 35% of the exploited groundwater. Otherwater-bearing formations are porous, unconsolidated aqui-fers (6% of the land surface) and karst aquifers (16% of theland surface), which provide some 45 and 20% of theexploited groundwater respectively (Tripet 2005).

The delineation of protection zones in fracturedaquifers is a challenging task due to the heterogeneityand anisotropy of hydraulic conductivities, which makesprediction of groundwater flow organization and flowvelocities difficult (Berkowitz 2002). Important efforts arecurrently being made by researchers in various fields ofhydrogeology to gain better understanding of groundwaterflow in fractured rocks (Krasny et al. 2003), but significantimprovement is still needed in the field of groundwaterprotection (Bradbury 2002).

To date, several approaches have been proposed for thedelineation of protection zones in fractured rocks by

Received: 5 April 2006 /Accepted: 13 May 2008Published online: 22 August 2008

© Springer-Verlag 2008

A. Pochon ()) : F. ZwahlenCentre of Hydrogeology CHYN,University of Neuchâtel,CP 158, 2009 Neuchâtel, Switzerlande-mail: [email protected].: +41-32-7182602Fax: +41-32-7182603

F. Zwahlene-mail: [email protected]

J.-P. TripetHydrogeologist, 2022 Bevaix, Switzerlande-mail: [email protected]

R. Kozel :B. Meylan :M. SinreichFederal Office for the Environment FOEN, 3003 Bern, Switzerland

R. Kozele-mail: [email protected]

B. Meylane-mail: [email protected]

M. Sinreiche-mail: [email protected]

Hydrogeology Journal (2008) 16: 1267–1281 DOI 10.1007/s10040-008-0323-0

applying basic fixed-radius methods to groundwater flowmodels (Bradbury et al. 1991; Lipfert et al. 2004). Althoughthe application of numerical models may provide the mostaccurate results for well-documented sites, insufficient dataoften mean that such quantitative approaches are difficult toapply. Methodologies for delineating groundwater protec-tion zones must be adapted to different site characteristicsand to the type of data available. A multi-stage approach,beginning with a general characterization of the hydro-geological context, which then applies adequate fieldtechniques and finally selects the most appropriate methodfor protection zone dimensioning should be considered inorder to obtain reliable results in any type of setting (Heath1995; Barton et al. 1999).

The approach presented here sets out to delineateprotection zones for public drinking water supply infractured rocks in Switzerland. Groundwater is essentiallyextracted from springs linked to shallow aquifers oftenlocated in mountainous regions. They typically providemoderate to low yields of between 20 and 500 L/min. Thesprings are mainly located in areas where pollutionsources are related to cattle pasturing, mountain farmingand tourism, although in some areas, especially in theSwiss Plateau, pollution hazards related to intensivefarming and industry may also exist. Delineation ofprotection zones is required for a large number of watersupplies as each community generally relies on severalsprings. It is therefore imperative that the methodologyis affordable in terms of cost and is technically feasiblefor private consulting hydrogeologists. To promote thenational harmonization of protection zoning, the approachmust also be verifiable, reproducible and applicable to any

type of fractured aquifer exploited in Switzerland. Theapproach presented in this paper is assumed to achievethese objectives.

Characteristics of fractured aquifersin Switzerland

Fractured non-karst aquifers exploited in Switzerland aredistributed across the Alps, the Prealps and the Swiss Plateau(Fig. 1). The variety of sedimentary, igneous and metamor-phic rocks located in diverse tectonic contexts coincideswith different types of hydrogeological settings (Table 1):

– Weakly fractured sandstones, marls and conglomeratesrepresent the bedrock of the Swiss Plateau (PlateauMolasse). The hydraulic conductivity of these depositsmeasured on samples and boreholes is low to moderate(10−7–10−5 m/s) with a maximum of 10−4 m/s in poorlyconsolidated coarse sandstone (Keller 1992). Preferen-tial flow is observed along bedding planes andfractures. Enhanced hydraulic conductivity is alsolinked to the presence of high secondary interstitialporosity in the uppermost weathered zone, which mayreach a thickness of several metres (Parriaux 1981).Such aquifers are frequently associated with shallowwater-bearing Quaternary deposits (fluvio-glacial chan-nels, sandy or gravely moraine deposits), which mayconstitute the main storage capacity of these mixedsystems. Although many tracer experiments have beenperformed in this type of aquifer for the delineation ofprotection zones, little information has been published

Fig. 1 Hydrogeological sketch of Switzerland with location of the three test sites Wysseberg, Sarreyer and Ronco sopra Ascona

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to date. In general, tracer test results imply low massrecovery and velocities not exceeding a few metres perday (Wernli and Leibundgut 1993).

– The same rock types located at the southern border ofthe Swiss Plateau (Subalpine Molasse) show contrast-ing hydrogeological properties compared to the PlateauMolasse. The influence of the Alpine orogenesisresulted in the exhumation, folding and faulting ofmore consolidated rocks, due to diagenetic processes(Keller 1992). Interstitial porosity is low, while fractureporosity is globally moderate but may be enhanced bydissolution of carbonate minerals, especially alongdiscontinuities, with a resulting enhancement of theaquifer heterogeneity. In such contexts, flow velocitiesof up to several hundreds of metres per day have beendetermined by tracer experiments (compiled in Pochonand Zwahlen 2003). To the south of the SubalpineMolasse, flysch consisting mainly of sandstones, sandylimestone and shales show similar hydrogeologicalproperties, with dominant fracture porosity enhancedlocally by dissolution processes (Basabe-Rodriguez1982). Slope instabilities and landslides are frequentin these regions (Lateltin et al. 1997) and groundwaterresources may occur in both displaced fractured rockmass and Quaternary deposits.

– Igneous and metamorphic rocks represent the backboneof the central and southern Swiss Alps. In such rocktypes, groundwater essentially flows along fractures asmatrix permeability is negligible. Dissolution processesare not significant and flow organization is predomi-nantly dependent on fracture networks generated bytectonics (NRC 1996). Hydraulic conductivities deter-mined in the Swiss Alps are generally low, for examplebetween 10−7 and 10−5 m/s according to slug testsperformed in shallow depth boreholes (Beatrizotti1996; Ofterdinger 2001) and between 10−11 and10−6 m/s based on methods which take inflows alonggallery sections (Maréchal 1998; Ofterdinger 2001)into account. However, very permeable zones linked tointensely fractured zones are observed along tunnels(Jamier 1975; Dubois 1991), confirming the impor-tance of heterogeneity and the existence of locally high

flow velocities in such aquifers. In addition, adecompression zone, linked to post glacial relaxationand gravity forces, is often described in the Alps(Cruchet 1985; Maréchal 1998). It accounts forenhanced fracture aperture and permeability in the firsttens of metres below the surface. Crystalline Alpinerocks do not exhibit a shallow zone of high porositydue to chemical weathering as observed in otherclimatic regions (Taylor and Howard 1998). This canbe explained by intense glacial ablation processesacross the region which were active until less than10,000 years ago (Jäckli 1970).

Groundwater protection zoning in Switzerland

Regulations and requirementsAccording to the Swiss Federal Water Protection Ordinance(WPO 1998), the strategy in force for the protection ofgroundwater quantity and quality is based on prevention.Groundwater protection zones must be defined in the watercatchment area of each groundwater supply used for publicdrinking water. The aim of the protection zones (S1, S2 andS3) is mainly to protect the drinking water againstcontaminants, including pathogenic micro-organisms suchas protozoa, bacteria and viruses. Moreover, a code ofpractice regulates all potentially polluting activities oradverse influences threatening groundwater quality in eachzone (Table 2). In unconsolidated porous media, thedelineation of protection zones is based on groundwaterresidence time. The limits of the different zones areapproximately concentric around the groundwater well,with the degree of land use restriction decreasing withincreasing distance from the source (Fig. 2a). In karst andfractured media, flow velocities are often highly variable intime and space. Under these circumstances, the delineationof protection zones based on the hypothesis of uniformflow velocities may or may not be justified (Fig. 2b, 2c).For this reason, the WPO requires the degree of ground-water vulnerability to be considered as a basic criterion forthe delineation of protection zones in these aquifer types.Specific methodological approaches and guidelines have

Table 1 Groundwater resources in fractured media: geological and hydrogeological conditions in Switzerland

Lithology Porosity Flow type GeographicdistributionInterstitial Fracture

Sandstones, conglomerates, marls (PlateauMolasse)a

Low to moderate, primary andsecondary (alteration,dissolution)

Low Dual porosity(interstitialand/orfractures)

Swiss Plateau,synclines ofthe JuraMountains

Sandstones, conglomerates, marls, schists,siliceous limestones, non-karstic dolomites(Among others: Subalpine Molasse,flysch)a

Low to insignificant, mostlysecondary; may besignificant near the groundsurface (alteration)

Low to moderate,locally high inthe presence ofdissolution

Mostly infractures

Southernborder of thePlateau,Prealps, Alps

Granites, gneiss, schists, basic to ultra-basicigneous rocks, sandstones (Among others:crystalline units, permo-carboniferousseries of the penninic nappes)a

Insignificant Low to moderate In fractures Alps

a Structural units or examples

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thus been developed for delineating protection zones inkarst (Doerfliger et al 1999; SAEFL 2000) and in fracturedmedia (Pochon and Zwahlen 2003; Pochon et al. 2003). Asthe concept of protection zones is based on groundwaterresidence time, referring to the attenuation of bacteriolog-ical contaminant, an additional approach must be applied inthe case of observed contamination linked to persistentsubstances (OFEFP 2005; Bussard et al. 2006).

Existing methods and need for a novel approachThe topic of delineation of protection zones, specifically forsprings but regardless of the aquifer type, has beenaddressed by general guidelines (Jensen et al. 1997) andby the study of selected aspects such as the evaluation ofthe protective function of soil (Mania et al. 1998). Theimportance of establishing a hydrogeological conceptualmodel based on geological and hydrogeological data, aswell as delineating the spring catchment area, has beenemphasized (Jensen et al. 1997; Barton et al. 1999). In thecase of heterogeneous aquifer properties, hydrogeologicalmapping or groundwater vulnerability mapping approachesmay be indispensable in order to delineate protection zones(Jensen et al. 1997; Hemmer and Beach 1997).

Several methods for delineating protection zones infractured media have been proposed in specific guidelines(Bradbury et al. 1991) or have been included in guidelines

covering all aquifer types (Burgess and Fletcher 1998).However, they are generally focused on well protection andtherefore are not directly applicable to springs. Varioustechniques and methods including tracer experiments(Robinson and Barker 2000) and vulnerability mappingapproaches (Lipponen 2007) have been applied togroundwater protection zone delineation in several frac-tured aquifer case studies. However, criteria for choosingone technique over another, and for selecting the adequatemethod for protection zoning, are generally not specified.

In Switzerland, drinking water supplies linked tofractured aquifers essentially consist of small springslocated in mountainous regions, and pumping test orpiezometric data are therefore not available. Methodsbased on aquifer testing or groundwater modelling areconsequently not applicable. Accordingly, the delineationof protection zones requires that other types of data aretaken into account and the development of specificapproaches. Although some of the methods and conceptsmentioned in the literature can be applied in Switzerland,a novel approach was developed to meet the requirementsof Swiss regulations. Due to the great diversity ofgeological and hydrogeological conditions observed infractured media in Switzerland (Table 1; Fig. 2b and c), itis not possible to delineate protection zones for suchaquifers using a single method. The presented approachuses consistent criteria firstly for selecting the appropriate

Fig. 2 Explanatory diagram of three aquifers with increasing heterogeneity. a Homogeneous porous aquifer; b fractured aquifer, slightlyheterogeneous; c fractured aquifer, highly heterogeneous. Groundwater protection zones (S1, S2, S3) around the groundwater supply(spring) are represented for each case

Table 2 Groundwater protection zones according to the Swiss water regulations (WPO 1998)

S1 zone: wellheadprotection zone

This zone must prevent damage to the groundwater supply or artificial recharge facilities, as well as preventpollution in its immediate surroundings

S2 zone: inner protectionzone

This zone defines an area suitable for preventing germs and viruses reaching the groundwater supplies. TheS2 zone must also prevent drinking water supplies from being polluted by excavation and subsurfaceworks. Moreover, the flow of water towards the source should not be disturbed by subsurface works

S3 zone: outer protectionzone

This zone must provide sufficient space and time for remediation when accidental pollution threatens agroundwater supply

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method and secondly for proposing a reliable procedurefor the delineation of protection zones in each case. Forthis purpose, diverse existing techniques and conceptsapplicable to fractured rocks have been considered.

Main concepts and definitions

Aquifer global characterization: spring monitoringWithin the framework of protection zone delineation, it isessential to obtain data concerning the hydraulic proper-ties of the aquifer. When considering springs, interpretingthe natural response to recharge events at the dischargearea is the most appropriate way of globally evaluating theaquifer properties and compensating for the lack ofpumping test data.

Hydrograph analysis was initially applied to rivers andpermitted the separation of fast flow components linked torunoff from retarded flow components linked to soil, aswell as baseflow components linked to groundwaterexfiltration (Barnes 1939). The same method, using thefitting of an exponential decay function to the observeddata, was applied to karst springs to evaluate the volumeand the recession of groundwater reservoirs with differentpermeabilities (Ford and Williams 2007; Baedke andKrothe 2001).

More comprehensive approaches, including the con-sideration of physico–chemical parameters (temperature,electrical conductivity, isotopes, chemical elements, tur-bidity) proved useful in better characterizing karst hydro-geological systems (Dreiss 1989; Sauter 1992). Springhydrographs in non-karst fractured rocks have receivedless attention, but the same methods can be applied tocharacterize and compare crystalline aquifers (Gentry andBurbey 2004; Rossier and Sandmeier 1989).

Although these “global methods” are based on verysimplified conceptual models, they allow the characteriza-tion and comparison of hydrogeological systems. Springsrelated to different aquifer types show clearly differenthydrographs (Mangin 1982; Amit et al. 2002), i.e. on onehand the rapid evacuation of a high proportion of water

infiltrated during a rain event in the presence of welldeveloped karst or highly permeable fracture systems (e.g.a few days) and on the other the slow evacuation offreshly infiltrated water in the presence of less heteroge-neous and less permeable systems (e.g. several months).Within the scope of the present approach, hydrographsand the variability of groundwater quality are used as atool to evaluate, in a global manner, the vulnerability ofthe springs (Fig. 3; for definition see section Springvulnerability versus vulnerability mapping). Specifically,the vulnerability of the spring is defined as low if noevidence of a significant amount of freshly infiltratedwater is observed after an intense recharge event.

Aquifer spatial characterization: heterogeneityand vulnerability mappingThe spatial distribution of hydraulic properties is a crucialissue for the delineation of protection zones in fracturedmedia (Bradbury et al. 1991). If no piezometric data areavailable, the main tool for determining the spatialcharacteristics of the aquifer is hydrogeological mappingand field testing (Jensen et al. 1997). Most fieldtechniques include a qualitative interpretation of surfaceand subsurface features (geological, pedological, geomor-phological and geophysical survey), which are interpretedin terms of hydraulic properties and which allow somegroundwater flow hypotheses to be proposed.

Artificial tracer tests are often the only tool available toprecisely determine groundwater velocity and to forecastcontaminant flow in aquifers (Käss 1998; Field 1999).Most tracer experiments performed in fractured rockspresented in the literature involve the injection of tracersin boreholes with sampling at wells (Robinson andBarker 2000), galleries (Bäumle et al. 2001) or springs(Maloszewski et al. 1999). Many experiments have beenperformed with a focus either on a single fracture(Novakowski et al. 2004) or at the scale of a block includingseveral fractures (Abelin et al 1991). Such experimentspermit precise characterization of groundwater flow andsolute transport at the small scale, but little informationis available regarding tracer experiments including

Fig. 3 Hypothetical example of spring hydrograph. a Low spring vulnerability; b high spring vulnerability

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injection on soil or in sinking streams while sampling atsprings. Tracer testing has been widely used in karstenvironments to explore conduit flow at the scale ofspring water catchment areas (Smart 1988; Ford andWilliams 2007) but such experiments are less common forthe characterization of the fracture network around non-karst springs. Even if flow velocities and recovery ratesare generally lower in fractured aquifers than in karstaquifers, some experiments performed in mountainousregions revealed flow velocities in excess of 100 m/dayover distances of several hundreds of metres in non-karstified aquifers (Maloszewski et al. 1999; Besson1990). By comparing the results of different tracer tests,it is possible to evaluate aquifer heterogeneity and tocorrelate surface or subsurface features with observedgroundwater flow velocities towards the spring. Thus,groundwater flow velocity hypotheses and hydrogeolog-ical maps can be locally verified and validated byquantitative data.

In the presence of highly heterogeneous aquifers andvery high local flow velocities along fractures revealed bytracer experiments, groundwater vulnerability mappingappears to be the only method that can be reasonablyapplied to delineate groundwater protection zones. In suchcontexts, methods based on the determination of iso-chrones in the saturated zone would result in very wideprotection zones, involving unnecessarily restrictive landuse across extended areas. In contrast, vulnerabilitymapping permits the identification of areas less sensitiveto contamination, where more intensive activities could beaccepted without compromising groundwater quality.

Spring vulnerability versus vulnerability mappingThe term “vulnerability” refers to an intrinsic property of anaquifer which is dependent on its sensitivity to natural andhuman impacts, irrespective of the nature of the contami-nant (Daly et al. 2002; Zwahlen 2004). This concept ofgroundwater vulnerability, defined either globally for aspring or spatially, is central for the approach developed fordelineating protection zones in fractured media (see sectionDecision process for method selection). The term “springvulnerability” is defined herein with reference to a globalcharacterization of the aquifer and allows the attribution ofa unique degree of vulnerability to the whole catchmentarea supplying the spring. The aim is to determine whethera spring is vulnerable to pollution without considering therange of vulnerabilities within its catchment area. Specif-ically, the question is whether a significant proportion offreshly infiltrated water quickly reaches the spring (highspring vulnerability) or not (low spring vulnerability)during a recharge event. This is achieved by performingdischarge and water quality measurements.

A low spring vulnerability is determined if thefollowing conditions are met (Fig. 3a):

– Spring discharge only varies with marked inertia andlow amplitude to recharge events without significant

relative variation of physico–chemical parameters (notclearly exceeding the analytical error or fluctuatingaccording to seasonal trends).

– No significant turbidity is observed even after intenserecharge events and no sediment deposits accumulatein the spring pool.

– Bacterial contamination is never detected.

A high spring vulnerability is determined if any of theabove conditions is not met (Fig. 3b).

On the other hand, when considering “vulnerabilitymapping methods”, vulnerability is determined spatiallyby evaluation and mapping of relevant parameters. In thiscase, the vulnerability is determined separately for eachsurface element of the catchment area. The concept ofgroundwater vulnerability mapping has been extensivelyused for different types of aquifers using various evalu-ation criteria (Vrba and Zaporozec 1994). Parameterstaken into account may include: soil thickness andhydraulic conductivity, aquifer type and hydraulic con-ductivity, depth to groundwater level and net recharge,among others (Aller et al. 1987; Adams and Foster 1992).These maps were initially useful in providing a decisiontool for land use planning and community information(NRC 1993). Recently, some groundwater vulnerabilitymapping methods have been developed to delineategroundwater protection zones around springs or wells,especially in karst settings (SAEFL 2000; DoELG/EPA/GSI 1999; Zwahlen 2004).

Unlike other methods which set out to delineateprotection zones, vulnerability mapping methods take theprotective effect of the unsaturated zone into account andassign less significance to transit times (which areincluded qualitatively in parameter evaluation, but arenot directly evaluated). Ignoring transit times in thepresence of highly permeable fractures is justified due tothe limited filtration and sorption capacities along frac-tures and conduits, which suggest a significantly lowerattenuation of bacteriological contamination compared toother media for the same residence time (Becker et al.2003). Higher significance is therefore given to soil coverin view of the filtration and degradation properties ofthese layers (Golwer 1983). Interest in groundwatervulnerability mapping as a tool to delineate protectionzones is particularly evident in highly heterogeneousmedia such as karst or highly permeable fracturedaquifers, where the delineation of protection zones basedexclusively on transit time in the saturated zone wouldresult in the determination of excessively restrictiveprotection zones over large distances (e.g. several kilo-metres). In such cases, the consideration of the protectivefunction of layers covering the aquifers (e.g. thick soils,moraine deposits) and/or the determination of lessfractured areas, may permit the designation of locally lessrestrictive protection zones within the spring catchmentarea, thus allowing the coexistence of viable economicactivities (agriculture, tourism) and the exploitation ofgroundwater.

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Decision process for method selection

OverviewThe development of new guidelines for the delineation ofprotection zones in fractured media requires considerationof the varying properties of fractured aquifers exploited inSwitzerland whilst applying a scientifically justifiedapproach which is also user-friendly, financially affordableand avoids unnecessary land use restrictions. The recom-mended approach involves more or less detailed investi-gation depending on the complexity of the case. A suitablemethod for delineating groundwater protection zones isselected according to spring vulnerability and aquiferheterogeneity (Fig. 4).

Evaluation of the spring vulnerability: basic dataacquisitionThe first stage includes the collection and the interpretationof basic data about the aquifer. Monitoring of dischargerate, physico–chemical and microbiological parameters,turbidity, including observations in various hydrologicalconditions, allows the global vulnerability of the spring tobe evaluated. Although continuous monitoring of dischargeand physico–chemical parameters is becoming increasingly

feasible in technical terms and economically affordable,and is therefore highly advisable, only a basic evaluation ofspring hydrographs is required in the initial phase. This“qualitative” evaluation of spring hydrographs includesmonthly measurements of discharge, temperature andelectrical conductivity, complemented by more detailedmonitoring (e.g. one measurement per day over threesuccessive days) of the same parameters during an intenseflood event (e.g. > 20 mm of daily precipitation in a periodwith limited soil water deficit). Additionally, several moredetailed analyses (e.g. three samplings) including bacterio-logical, chemical, and turbidity measurements in adverseconditions are required. At least one detailed analysis mustbe performed after a high recharge event occurring during aperiod of intense potentially polluting activities (e. g.fertilizer or manure application).

Spring vulnerability is assessed as low if no groundwaterwith short residence time contributes to spring discharge atall, even during flood events. This situation is characterizedby a non-significant temporal variability in spring discharge,temperature and electrical conductivity (Fig. 3a) and by anundisputable water quality (bacteriology, turbidity). Springvulnerability is assessed as high once a significant propor-tion of groundwater with short residence time contributes tospring discharge during flood events (even in the range of

Fig. 4 Decision process allow-ing the selection of one of threespecific methods to delineategroundwater protection zones infractured media (after OFEFP2004). The examples are pre-sented in the text

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some percents). This situation is characterized by detectablevariations of groundwater discharge, temperature andelectrical conductivity implying the presence of locally highflow velocities within the aquifer (Fig. 3b). If the springvulnerability is low, protection zones can be defined with aminimum size according to the Swiss regulations (extent ofthe S2 zone: 100 m), without any additional investigation(distance method, Fig. 4, see section Low vulnerabilitysprings). For vulnerable springs, a second investigationstage is required.

Evaluation of aquifer heterogeneity: additionalinvestigationsOnly in the case of vulnerable springs must a second stageof field survey be carried out. The following additionalinvestigations may be required: detailed analysis of rockjointing in the groundwater catchment area, identificationof concentrated infiltration zones, continuous springmonitoring and detailed analysis of groundwater dis-charge, temperature and electrical conductivity time series,as well as tracer testing. These data allow more preciseinformation concerning groundwater flow organizationand the degree of aquifer heterogeneity to be obtained. Ifthe spring is vulnerable and the degree of heterogeneity isfound to be low, the residence time of groundwater in thenetwork of connected joints is expected to increase inrelation to distance from the spring. In this case, theprotection zones are delineated by means of isochrones(isochrone method, Fig. 4, see section Vulnerable springsin slightly heterogeneous aquifers) on the basis of tracertesting. If the groundwater supply is vulnerable and theaquifer is highly heterogeneous, rapid hydraulic connec-tions between vulnerable points located in any part of thecatchment area and the spring may be possible; theresidence time of groundwater in a highly permeablefracture network does not significantly increase withdistance from the spring. In this case, a multi-parametergroundwater vulnerability mapping method is applied(“DISCO” method, Fig. 4, see section Vulnerable springsin slightly heterogeneous aquifers). The three methods arepresented into more detail and illustrated by examples inthe following chapter.

Methods and examples

Low vulnerability springs

Distance methodThe distance method (fixed radius) is applied when the lowvulnerability of the spring is demonstrated. In this case,groundwater residence time is assumed to be sufficient toallow natural purification processes to take place. Suchsprings can be related both to homogeneous and heteroge-neous aquifers (e.g. confined in the discharge area;associated with a deep flow system drained by regionalfractures). In these types of hydrogeological setting, tracertests are often unsuccessful (no tracer recovery occurs even

after several weeks) and do not represent an adequateinvestigation method. Only the collection of basic data isneeded and small protection zones are sufficient toguarantee the quality of the exploited water.

In a simplified manner, this method considers thefractured aquifer as a continuous medium at the scale ofthe spring groundwater catchment area. The delineation ofthe protection zones according to the minimum distancedefined for non-consolidated porous media by the Swisswater regulations is adequate to efficiently protect thegroundwater supply (WPO 1998):

S1 zone. This zone must extend for a minimum at 10 maround or upstream of a spring and mustincorporate drains, draining trenches or galleries.

S2 zone. The distance between the external limits of theS1 and the S2 zones must be at least 100 mupstream, considering the general groundwaterflow direction.

S3 zone. The distance between the external limits of the S2and the S3 zones must be at least equal to thedistance between the outer limits of the S1 and S2zones.

Test site applicationThe Sarreyer test site is located in the western part of theSwiss Alps (Canton Valais). The aquifer consists ofmetamorphic gneiss and schists, and groundwater iscollected from four subhorizontal drains measuring 20–30 m. Water quality is good throughout the year and nosignificant variation of discharge, electrical conductivityor quality has been observed even during heavy rainfall.This suggests a low spring vulnerability.

Due to the subhorizontal drains, the extension definedfor S1 is approx. 40 m. The S2 zone extends between40 m (external limit of S1) and 140 m from the spring andthe S3 zone is defined between 140 m (external limit ofS2) and 240 m from the spring (Fig. 5).

Vulnerable springs in slightly heterogeneous aquifers

Isochrone methodThe isochrone method (calculated radius) is applied to thecase of vulnerable springs linked to slightly heterogeneousaquifers. In such contexts, some water may discharge atthe spring after short residence time without sufficientnatural purification processes in the aquifer (even aproportion of some percent may alter the spring waterquality). Additional investigations are therefore needed tobetter characterize the aquifer. As fractured media aregenerally heterogeneous at the local scale, tracer injectionmust be performed on the most permeable areas based ona detailed hydrogeological survey (geological mapping,geophysics, infiltration tests). The isochrone method isapplied when groundwater flow velocities along fracturesare typically moderate to high (maximum: a few tens ofmetres per day) and when flows are relatively uniform at the

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scale of the spring catchment area. The results of tracerinjections performed at several favourable points duringhigh water conditions allow determination of the isochrones.

When applying this method, the fractured aquifer isconsidered as a continuous medium at the scale of thegroundwater catchment area. Therefore, the delineation ofthe protection zones is achieved according to the criteriarecommended by the Swiss water regulations for uncon-solidated porous media (WPO 1998):

S1 zone. This zone must extend to a minimum of 10 maround or upstream of a spring and mustincorporate drains, draining trenches or galleries.

S2 zone. This zone is based on the evaluation of themaximum flow velocity in the aquifer. The 10-day isochrone must be defined and correspondsto the outer limit of the S2 zone. Moreover, thedistance between the external limit of the S1zone and the external limit of the S2 zone mustbe at least 100 m upstream, taking into accountthe general groundwater flow direction (Fig. 6).

S3 zone. The distance between the external limits of theS2 and the S3 zones must be at least equal to thedistance between the outer limits of the S1 andS2 zones.

Test site applicationThe Ronco sopra Ascona test site is situated in thesouthern part of the Swiss Alps (Canton Ticino). The

aquifer consists of metamorphic amphibolites and gabbrosand the groundwater supply collects water dischargingfrom a fracture. Significant variations in discharge,electrical conductivity and temperature are observedduring the year and suggest that the spring is potentiallyvulnerable to contamination. Consequently additionalinvestigations were carried out.

A field survey including structural geology, geomor-phology observations, geophysics and a multi-tracerexperiment led to the conclusion that the degree ofaquifer heterogeneity is low. Fracturing is distributedevenly across the catchment area with a spacing ofapprox. 10 to 20 m. However, rock permeability andflow velocities were assumed to be variable at the smallscale. Therefore, tracer testing was performed on twopotentially highly permeable areas (fractured zones).The fluorescent dyes tinopal and uranine (Käss 1998)were injected respectively 20 and 90 m from the spring.To account for a probable increase of flow velocitiesduring high water conditions, the experiment wasconducted during an intense recharge event. Moderateflow velocities of 12 and 15 m/day respectively weredetermined. Moreover continuous monitoring of dis-charge, electrical conductivity and temperature over a

Fig. 6 Principle of the delineation of groundwater protectionzones with the isochrone method for relatively homogeneousfractured aquifers

Fig. 5 Delineation of groundwater protection zones with thedistance method for Champi Spring at Sarreyer

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period of several months suggested a significant inertiaof the spring to recharge events, illustrated by slowrecession curves (1 month to return to the base flowafter a significant recharge period of approx. 250 mm ofprecipitation over 5 days). It was also determined thatthere was no significant variation in temperature andelectrical conductivity during moderate recharge events(10 to 20 mm).

As water is collected from a small fracture discharginginto the catchment facility, an S1 zone extending 10 mfrom the spring was considered to be sufficient. Thedistance between the external limits of the S1 and theS2 zones was set at 150 m based on the maximum flowvelocity determined by tracer tests (15 m/day). Accord-ingly, the S3 zone extends from 160 to 310 m from thespring (Fig. 7).

Vulnerable springs in highly heterogeneous aquifers

DISCO methodThe DISCO method (groundwater vulnerability mappingmethod) is applied in the case of vulnerable springslinked to a highly heterogeneous aquifer. In suchcontexts, a significant proportion of water dischargesat the spring after a short residence time in the aquifer,preventing sufficient natural purification processes totake place. Additional investigations are needed andtracer testing typically reveals locally very high flowvelocities (up to several hundreds of metres per day). Inthis case, a groundwater vulnerability mapping methodis applied to the whole catchment area of the spring.The parameters taken into account in the DISCOmethod include characterization of hydrogeologicalproperties of the fractured aquifer (DIScontinuitiesparameter) and evaluation of the thickness and perme-ability of protective cover (protective COver parameter).Moreover, taking runoff processes into account may berequired for some areas, where surface movement ofpolluted water towards preferential infiltration zones isexpected. A protection factor map is determined bycombining the different parameter maps and theprotection zones are finally determined by convertingthe protection factor map into protection zones. Conse-quently, the multi-parameter vulnerability mappingmethod includes four steps as follows:

Step 1 Assessment of the parameters “discontinuities” and“protective cover”.

This first step consists of the evaluation and mappingof the parameters “discontinuities” and “protectivecover” over the whole catchment area, subdivided intoareas of uniform properties for each of the parameters.The parameter “discontinuities” referred to below as“D” takes into account the groundwater flow velocitywithin the fractured aquifer between an infiltration pointin the water catchment area and the spring underconsideration (e.g. highly permeable fractured zone withrapid connection to the spring versus weakly fracturedarea). The evaluation of the parameter “D” is essentiallybased on structural geology (field survey and remotesensing), geophysics and tracer experiments. The ratingvalues of “D” range from 0 to 3, with increasing valuescorresponding to higher residence time and attenuation

Fig. 7 Delineation of groundwater protection zones with theisochrone method, Livurcio Spring at Ronco sopra Ascona, basedon tracer experiments

Table 3 “Discontinuities” parameter evaluation

Class Rating Evaluation criterion

D0 0 Highly permeable discontinuities with preferential connection to the spring (maximum groundwater residence time of afew tens of hours) / no significant natural purification processes

D1 1 Discontinuities with a relatively rapid connection to the spring (residence time of a few days) / limited purificationprocesses

D2 2 Discontinuities with a relatively slow connection to the spring (residence time of approx. ten days) / significantpurification processes

D3 3 Low permeability zone or discontinuities with a slow connection to the spring (residence time of several tens of days) /efficient purification processes

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processes (Table 3). The parameter “protective cover”referred to below as “P” takes into account the protectiveeffect linked to the flow of water through the soil andgeological formations (e.g. moraine, slope deposits, marls,clay material) overlying the fissured aquifer. Data neces-sary for the evaluation of the parameter “P” may beassessed using hand drilling, on-site soil analysis, geo-morphological mapping (field survey and remote sensing),geophysics and infiltration tests. The rating values of “P”range from 0 to 4, with increasing values correspondingboth to higher protective cover thickness or/and lowerpermeability of the deposits (Tables 4 and 5). The ratingswere defined based on field studies at numerous test sites.It takes the feasibility and constraints of field methods(e.g. maximal depth of hand drilling) for each parameterinto account, as well as groundwater velocity estimate fordifferent types of soil and fractured rock.

Step 2 Determination of the intermediate protection factor“Fint”.

For each polygon presenting uniform values for theparameters D and P, the protection factor Fint is calculatedas follows:

Fint ¼ 2Dþ 1P

This empirical relationship is based on the assumption thatthe protective effect related to the fractured aquifer asdescribed in Table 3 is greater than the protective effect ofthe soil as described in Tables 4 and 5.

Step 3 Evaluation of the runoff parameter and calculationof the final protection factor “F”.

Surface or subsurface runoff can generate rapidcontaminant flow over several tens (diffuse runoff) or

hundreds of metres (e.g. presence of natural or artificialdrain, perennial or temporary stream, path or road). Sincethe majority of fractured aquifers exploited in Switzerlandare located in mountainous regions, it is essential to takethese processes into consideration. Unlike the parameters“discontinuities” and “protective cover”, which aremapped over the whole catchment area, the runoffparameter is only considered for areas where runoff mayinduce substantial pollutant movement toward vulnerablezones, i.e. where the intermediate protection factor valueis low or very low (0≤Fint≤4). The extent of these areas(“local surface watersheds”) is determined by estimatingthe influence of runoff, i.e. slope gradient and soilpermeability, etc. (Fig. 8). Evaluation of these factorsrequires field observation during intense rainfall events.

The final protection factor map is obtained bymodifying the intermediate protection factor map(obtained under step 2) according to the already men-tioned local surface watersheds. In these areas, the valueof the intermediate protection factor Fint for the vulnerablezones threatened by runoff is extended to cover the wholelocal surface watershed. Outside these local surfacewatersheds, the final protection factor F remains the sameas the intermediate protection factor Fint determined instep 2.

Step 4 Protection zone delineation. The final protectionfactor “F” map is converted into protection zonesS according to the relationship presented inTable 6.

In all cases, spring drains, draining trenches andgalleries as well as their immediate surroundings mustbe part of the S1 zone, with a buffer of 10–30 mdepending on the gradient of the slope. Additionally, asa precautionary measure, no S3 zone must be defined at adistance of less than 100 m from the outer limit of the S1

Table 4 “Protective cover” parameter evaluation taking into consideration pedological soil overlying the aquifer

Pedological soilsThickness (m) High permeability soil (sand, pebbles) Moderate permeability soil (silt, loam) Low permeability soil (loam, clay)

Class Rating Class Rating Class Rating

0.0–0.2 P0 0 P0 0 P0 0> 0.2–0.5 P0 0 P0 0 P1 1> 0.5–1.0 P0 0 P1 1 P2 2> 1.0 P1 1 P1 1 P3 3

Table 5 “Protective cover” parameter evaluation taking into consideration geological formations other than pedological soils overlying theaquifer

Additional presence of low permeability formations (e.g. clay, loam, marl)Thickness (m) Combined with P0 soil Combined with P1 soil Combined with P2 soil Combined with P3 soil

Class Rating Class Rating Class Rating Class Rating

< 1.0 m P1 1 P2 2 P3 3 P3 31.0–2.0 m P2 2 P3 3 P3 3 P4 4> 2.0 m P3 3 P3 3 P4 4 P4 4

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zone adjacent to the water supply, and no areacorresponding to the “rest of the catchment area” shouldbe defined closer than 200 m to this S1 zone boundary.

Test site applicationThe Wysseberg test site is located in the Swiss Prealps tothe south of Bern (flysch of the Niesen Nappe, Canton ofBern). The aquifer consists of folded and fractured flyschsandstones and schists (Fig. 9a). Groundwater protectionzones have been delineated for two groups of springs(three springs to the east, five springs to the west), asexplained below. The springs are aligned along majorfractures. They collect groundwater from the fracturedbedrock and from weathered flysch at shallow depths(approx. 2 m). The aquifer is covered by approx. 1 m ofbrown soil with high proportions of silt and clay in thebottom layer. Additionally, ground and lateral moraine aswell as slope deposits (scree) are present locally. Thelimits of the groundwater catchment area were definedaccording to topographical and geological criteria andhave been verified with a water balance calculation. Cattlerearing and manure spreading between June and Septem-ber represent the main pollution hazards upstream of the

springs. Significant fluctuations in spring discharge, watertemperature and electrical conductivity, as well as indica-tions of bacteriological contamination, even in winter,have been observed. Consequently, the springs appear tobe vulnerable to contamination and additional investiga-tions are needed.

Various systems of structural discontinuities at thescale of the groundwater catchment as well as at outcroplevel have been observed. They dictate the location of thesprings. Geomorphological depressions constitute poten-tial locations of concentrated infiltration and open frac-tures are apparent along stream beds. Tracer tests haveshown highly variable flow velocities for differentinjection points in the spring catchment (50–600 m/day).All these data indicate the presence of rapid flow alonginterconnected networks of highly permeable joints. Rapidconnections are possible between the springs and surfacesthat may be distributed across the whole catchment area ofthe spring. Consequently, the groundwater residence timedoes not increase globally or significantly with increasingdistance from the spring, and an assimilation of thefissured aquifer to a continuous medium is inappropriate.Under these conditions, only the use of a multi-parametergroundwater vulnerability mapping method over thewhole catchment area enables effective delineation ofprotection zones by taking into account the large degree ofheterogeneity within the aquifer.

With reference to the delineation of protection zones atWysseberg test site, strips of 20–30 m in width with thevalue D1 were assigned along the major faults for theparameter discontinuities (Fig. 9b). The value D2 wasassigned to the rest of the groundwater catchment. For theparameter protective cover (Fig. 9c), surfaces of aquiferoutcrops (P0), surfaces covered with soil only (P1, P2) andsurfaces with an overlay of moraine material (P3) were

Table 6 Conversion between the protection factor F and the ground-water protection zones

Protection factor F Vulnerability S zones

F very low (0, 1) Very high S1F low (2, 3, 4) High S2F moderate (5, 6, 7) Moderate S3F high (8, 9, 10) Low Rest of the catchment area

Fig. 8 Modification of the in-termediate protection factor totake runoff processes intoaccount

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mapped. Accordingly, the value determined for Fint rangesfrom 3 (low protective effect) to 7 (moderate protectiveeffect).

Due to the slope topography and the presence of lowpermeability cover over large parts of the groundwatercatchment area at the site, it was necessary to adjust theFint map, taking the runoff parameter into account. Finally,the protection zone map (Fig 9d) shows that S2 zoneswere assigned to the most important draining disconti-nuities, taking into account the aquifer heterogeneity. S3zones are only defined in areas characterized both bythe absence of important fractures and the presenceof efficient protective cover. Due to the presence ofcollecting drains of unknown extension and the rela-tively steep gradients of the slopes above the springs(10 to 20%), the S1 zones were extended to not lessthan 25 m.

Conclusions

In Switzerland, specific methodologies for the delineationof protection zones have been developed and are currentlyused for the main aquifer types (unconsolidated porousaquifers, karst and fractured media). For fractured media,

a new vulnerability-based approach, which takes intoaccount the great diversity of geological and hydro-geological conditions observed in this country, wasestablished. This approach includes a decision processthat alternatively selects one of three methods dependingon the hydrogeological settings considered. It is dedicatedto springs, which are the main water supply in fracturedenvironments in Switzerland.

This approach has the advantage of globally assessingthe spring vulnerability at an early stage of the investiga-tion, allowing the selection of a simplified method(distance method) for uncomplicated cases (low springvulnerability). In such hydrogeological settings character-ized by high residence times and low flow velocities,detailed hydrogeological investigation would not bringuseful additional data for the protection of the groundwa-ter supply. More detailed investigation including tracerexperiments and an evaluation of the aquifer’s heteroge-neity are required for more complicated cases (high springvulnerability). The additional data obtained can be used todetermine whether a method considering the aquifer ashomogeneous can be applied (isochrone method), orwhether vulnerability mapping is necessary to adequatelydelineate spring protection zones, i.e. for highly hetero-geneous media (DISCO method).

Fig. 9 Wysseberg test site. a Bloc diagram showing the geological setting; b discontinuities parameter map; c protective cover parametermap; d delineation of groundwater protection zones with the DISCO method

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Investigations carried out at various test sites haveshown the technical feasibility and the suitability of thevulnerability-based approach for fulfilling the require-ments of the Swiss regulations concerning groundwaterprotection zones. It is applicable to the variable conditionsencountered across the whole country, and gives repro-ducible results with a financial effort proportionate to thecomplexity of the site. The methodology should facilitatea countrywide harmonization of the groundwater protec-tion zone delineation in fractured media.

The Swiss environmental authorities recommend thismethodology in their current guidelines (OFEFP 2004).The delineation of protection zones according to theproposed methodology and the application of associatedland use restrictions is assumed to prevent the majority ofspring contamination in fractured aquifers in Switzerland.However, a detailed analysis of the effectiveness of theapproach will only be possible as more case studies arecarried out and data from long-term groundwater qualitymonitoring become available.

It is assumed that this approach is also applicable tofractured media outside Switzerland. Furthermore, thisconcept may also be adapted to the delineation ofprotection zones around wells and to other aquifer types(e.g. deep and/or confined aquifers). The implementationof this approach is believed to contribute to a valuableimprovement of spring drinking water quality in manyregions. It is expected that ongoing research on fracturedmedia hydrogeology and experience with additionalapplications of the approach and its associated methodswill give significant input for further refinement.

Acknowledgements The authors wish to thank all the persons thathave contributed to the completion of the present paper. Fruitfulcomments from reviewers were greatly appreciated and allowedsubsequent improvement of the paper structure. Many individualshave made possible the achievement of the approach and receiveour thanks: the experts group of the Swiss Society of Hydrogeologyfor supervising the methodological developments, staff at the Centreof Hydrogeology of the University of Neuchâtel for inspiring input,cantonal environmental and geological surveys for providing usefulinformation concerning regional hydrogeology and local authoritiesand employees at the test sites for logistical support. The assistanceof Raymond Flynn and James Morris for improving the English textis acknowledged.

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