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Hydrogeological mapping as a basis for establishing site-specificgroundwater protection zones in DenmarkR. Thomsen · V. H. Søndergaard · K. I. SørensenAbstract The water supply in Denmark is based on highqualitygroundwater, thus obviating the need for complexand expensive purification. Contamination from urbandevelopment and agricultural sources, however, increasinglythreatens the groundwater resource. In 1995 theDanish Government thus launched a 10-point plan toimprove groundwater protection. In 1998 this was followedby a decision to instigate spatially dense hydrogeologicalmapping of the groundwater resource withinthe 37% of Denmark designated as particularly valuablewater-abstraction areas. The maps will be used to establishsite-specific groundwater protection zones and associatedregulation of land use to prevent groundwatercontamination. Traditional mapping based solely on boreholedata is too inaccurate for this purpose. The work willtake 10 years and cost an estimated DKK 920 million,equivalent to 120 million euro (e). To fund this, consumerswill pay a e 0.02 surcharge per m 3 of drinkingwater during the 10-year period. This review of theDanish strategy to protect the groundwater resourcedemonstrates why dense mapping with newly developedgeophysical measurement methods in large contiguousareas accords geophysics a highly central role in theforthcoming hydrogeological mapping. It is illustrated byexamples of spatially dense, large-scale geophysicalmapping carried out in the Aarhus area.RØsumØ L’alimentation en eau au Danemark supposeune haute qualitØ des eaux souterraines, en Øliminant ainsiReceived: 18 October 2002 / Accepted: 1 April 2004Published online: 13 August 2004 Springer-Verlag 2004R. Thomsen ()) · V. H. SøndergaardEnvironmental Division, Groundwater Department,Aarhus County,Lyseng Alle 1, 8270 Højbjerg, Denmarke-mail: rt@ag.aaa.dkTel.: +45-8944-6680Fax: +45-8944-6982K. I. SørensenDepartment of Earth Sciences,University of Aarhus,Aarhus, DenmarkHydrogeology Journal (2004) 12:550–562le coßteux processus d’Øpuration. NØanmoins, la qualitØdes sources souterraines est menacØe par la pollutionprovoquØe par le dØveloppement urbain et agricole. En1995 le gouvernement danois a lancØ un plan en 10 pointspour amØliorer la protection des eaux souterraines. En1999 ce plan a ØtØ suivi par la dØcision de promouvoir unecartographie hydrogØologique à grande densitØ sur 37%du territoire du Danemark oœ se trouvent des zones decaptages importantes. Les cartes seront utilisØes pourØtablir les zones de protection des eaux souterraines, entenant compte des conditions locales du site ainsi que desr›glements conjoints d’utilisation des territoires, afin deprØvenir la pollution des eaux souterraines. La cartographietraditionnelle, basØe seulement sur les donnØes desforages, est trop imprØcise pour ce but. Les travaux vontdurer 10 ans avec un coßt estimØ à 120 millions d’euros(e). Pour ces travaux les consommateurs vont payer unesurcharge de e 0.02 par m 3 d’eau potable, ceci pendant10 ans. Cette rØvision de la stratØgie du Danemarkconcernant la protection des ressources en eaux souterrainesa dØmontrØ les raisons pour lesquelles on a accordØun rôle central aux nouvelles mØthodes gØophysiquesdans la future cartographie hydrogØologique de vaste rØgions.On prØsente un exemple de cartographie gØophysiquerØalisØe dans la rØgion d’Aarhus.Resumen El abastecimiento de agua en Dinamarca estµbasado en agua subterrµnea de alta calidad, evitando deesta manera la necesidad de una purificación compleja ycara. Sin embargo, la contaminación a travØs del desarrollourbano y de fuentes agrícolas, ha incrementado laamenaza para el recurso de agua subterrµnea. Entonces en1995 el gobierno lanzó un plan de 10 puntos para mejorarla protección del agua subterrµnea. Este fue seguido en1998 por la decisión para promover una cartografía hidrogeológicaespacialmente detallada, para el recursoagua subterrµnea dentro del 37% de las µreas de extracciónconsideradas por Dinamarca con una importanciaespecial. Los mapas serµn usados para establecer zonasespecíficas de protección para puntos de agua subterrµneay una regulación asociada al uso del territorio, para prevenirla contaminación del agua subterrµnea. La cartografíatradicional basada exclusivamente en datos de laperforación es muy inexacta para este propósito. Estetrabajo tomarµ 10 aæos y costarµ aproximadamenteDKK 920 millones, equivalentes a 120 millones de EurosDOI 10.1007/s10040-004-0345-1


(e). Para financiar esto los consumidores pagarµn unsobreprecio de e 0.02 por m 3 de agua potable durante unperiodo de 10 aæos. Este anµlisis de la estrategia Danesapara proteger el recurso agua subterrµnea, demuestraporque la cartografía detallada, hecha con nuevos avancesen mØtodos de medición geofísica, aplicados a grandesµreas aledaæas, otorgan a la geofísica un papel altamenteimportante en el futuro de la cartografía hidrogeológica.Esto estµ ilustrado con ejemplos de cartografía geofísica agran escala y espacialmente detallada, llevados a cabo enel µrea de Aarhus.551Keywords General hydrogeology · Geophysicalmethods · Groundwater management · Groundwaterprotection · LegislationIntroductionIn 1995, increasing problems with water quality in Denmarkdue to urban development and contamination fromagricultural sources led the Minister for the Environmentto approve a 10-point plan to improve groundwater protection.One of the major initiatives was to ban the use ofpesticides which can contaminate the groundwater. Anotherwas that the Counties (the regional authorities)should draw up new water-resource protection plans. Bythe end of 1997, the 14 County Councils had classified thecountry into three types of groundwater-abstraction areas:particularly valuable, valuable, and less-valuable waterabstractionareas (Fig. 1). This classification is based onan evaluation of the size and quality of all groundwaterresources in the country.In July 1998, the Danish Parliament adopted an ambitiousplan to significantly intensify hydrogeologicalinvestigation to facilitate protection of the groundwaterresource in order to meet future water-supply challenges.Parliament decided that, in addition to being responsiblefor water-resource planning, the 14 County Councilsshould also be responsible for ensuring spatially densemapping and hydrological modelling of the water resourcesas a basis for establishing site-specific groundwaterprotection zones (Danish EPA 2000a).The mapping and planning work is to be carried outover a 10-year period and encompasses all parts of Denmarkclassified as particularly valuable water-abstractionareas. Together, these cover almost 16,000 km 2 or 37% ofthe total area of the country. The total cost of this spatiallydense mapping and planning work is estimated at aroundDKK 920 million, equivalent to 120 million euro (e,2002 prices) or e 7,500 per km 2 for a programme encompassinggeophysical profiling every 250 m, surveydrilling every 4 km 2 , water sampling and hydrologicalmodelling. During the 10-year mapping and planningperiod, Danish consumers have to pay the County Councilsa e 0.02 surcharge per m 3 of water consumed, i.e.about e 4 per family per year.Hydrogeology Journal (2004) 12:550–562Fig. 1 Groundwater classification map showing subdivision ofDenmark into particularly valuable, valuable and less-valuablegroundwater-abstraction areas in 2001. The Danish island Countyof Bornholm, which is located far from the mainland in the BalticSea south of Sweden, is shown as an inset in the upper right-handcornerPublic Water Supply Administration in DenmarkPlanning and public administration in Denmark is carriedout at three governmental levels: State, county and municipal.Legislation is passed by the State. The 14 CountyCouncils are responsible for overall administration ofwater-abstraction permits and protection of water resourcesagainst contamination. The Water Supply Act andthe Environmental Protection Act require the use ofgroundwater and surface water to be regulated throughintegrated planning and comprehensive assessment andprotection of the water resources while concomitantlyensuring water-supply needs and protection of nature andthe environment. The administration of water-abstractionpermits is regulated by the Water Resource Plan drawn upby each County Council pursuant to the Water Supply Act(Danish EPA 1999). The Water Resource Plan is theframework for the Water Supply Plan drawn up by eachof the 275 Municipal Councils (Thomsen 1997).Site-Specific Groundwater Protection Strategyand Action PlanGroundwater protection in Denmark is based on the assumptionthat the physical environment provides someDOI 10.1007/s10040-004-0345-1


552Fig. 2 Example of zonedgroundwater protection at Bederabstraction site in Aarhus Municipalitydegree of protection against anthropogenic pressures, especiallyas regards contaminants entering the subsurfaceenvironment. The fundamental concept of site-specificgroundwater protection zones is that some areas are morevulnerable to groundwater contamination than others. Thegoal is thus the subdivision of a given area according tothe different potential of the various sub-areas as regardsspecific purposes and uses. Assessment of groundwatervulnerability is described very comprehensively in theGuidebook on mapping groundwater vulnerability (Vrbaand Zaporozec 1994). The guidebook specifies the usesand limitations of groundwater vulnerability maps.The Danish site-specific groundwater protection strategyis based on three steps.1. Spatially dense hydrogeological mapping based onnew geophysical surveys, survey drilling, water sampling,hydrological modelling, etc., aimed at facilitatingthe establishment of site-specific protection zones.These zones are directed at both point sources anddiffuse sources of contamination within the wholegroundwater recharge area, and shall supplement thetraditional protection zones around the wells. Thevulnerability is interpreted in relation to the local hydrologicaland chemical conditions.2. Mapping and assessment of all past, present and possible,future sources of contamination—both pointsourceand diffuse.3. Preparation and evaluation of an action plan stipulatingpolitically determined regulations for future land useHydrogeology Journal (2004) 12:550–562within the site-specific groundwater protection zones.The action plan has to be evaluated through a publicplanning process with a high degree of transparencyand public participation. Moreover, it must include atimetable for implementation, and a description of whois responsible for implementing the plan. The protectionzones and guidelines will be used to preventgroundwater contamination from urban developmentand agricultural activities and for planning the remediationof contaminated sites.This three-step protection strategy is based on previousexperience with groundwater protection in Denmark andabroad. In some parts of Denmark, water-supply wellshave been successfully protected using two-level protectionzones directed at contamination from point sources.The protection zones encompass a 10-m-diameter protectionzone around the well indicated by a fence, and a300-m protection zone directed at point sources such asleaching of wastewater (Fig. 2). The well-site protectionzones correspond to the protection zones which have beenused for several years in countries such as Germany(DVGW 1995). In Germany, the well-site protectionzones are established as concentric circles with a radiusranging from 10 m to 2–3 km around the well. Thesezones are based on experience with the dilution anddegradation of contaminants from point sources, and theradius is fixed on the basis of the transport time and thehydraulic properties of the water-bearing layers in particular.In Denmark, the new site-specific protectionDOI 10.1007/s10040-004-0345-1


zones shall encompass the whole recharge area, withparticular emphasis on protection of the capture zones.The protection zones will be established on the basis ofmodel calculations of groundwater flow, and calculationsof the degradation of the contamination from pointsources and diffuse sources, taking into account knowledgeof the local geochemical conditions. The new type ofprotection zone ensures that the protection is now alsodirected at contamination from diffuse sources such asagricultural use of fertiliser and pesticides, which cancause extensive loading from large areas.The establishment of protection zones of this typeimposes demanding requirements as to mapping of thewater resources, because the restrictions associated withthe zones have to be set at property level. The Quaternarygeology in Denmark is very complex, and the existinggeological maps are largely based on geological informationfrom wells. As a consequence, the maps are notsufficiently detailed and precise to enable delineation ofthe new protection zones. The new protection zonesdesignated as a result of the new investigations are shownin Fig. 2. As a result of the delineation of the vulnerableareas in the town of Beder, no further expansion of thetown will be permitted. At the same time, it was decidedthat future urban growth in the area should take placesouth of the town Malling in an area situated outside theparticularly valuable groundwater recharge area. Withinthe Beder town boundaries, regular campaigns are madeto curtail the use of pesticides. It is completely prohibitedto use them on publicly owned land. In all areas designatedas vulnerable, agreements have to be entered intowith farmers to limit the use of fertiliser and pesticides.Broad-Based SupportActive protection of the groundwater is accorded strongpublic support in Denmark, where there is a long traditionfor public participation in the decision-making process.The public participates in the planning process and implementationof the groundwater protection strategy atseveral phases. The interest organisations are incorporatedin the work in the drafting phase and, before the plans canbe adopted, they have to be presented at public meetingsaccording to a fixed timetable, with the possibility tosubmit comments before their final adoption by theCounty Council. Information, transparency and cooperationare accorded high priority throughout the wholeprocess. Successful implementation of the plans necessitatesthat broad acceptance is attained during the processfrom among the most important actors affected by theplans, especially the agricultural sector, the waterworksand the Municipalities. Certain types of restriction onagriculture can be compensated for economically—thisapplies, for example, to reduced use of fertiliser and setaside.This compensation is paid by the EU or by thewaterworks which directly benefit from the protection.At the government level, the new strategy is supportedby all the major political parties. Moreover, the strategy isHydrogeology Journal (2004) 12:550–562increasing the confidence of the water-supply utilities thatthey will be able to continue to furnish the Danish populationwith an ample supply of pure, minimally treateddrinking water.The agricultural organisations also endorse the new,spatially dense mapping initiative as a major condition foraccepting protection zones and associated land-use restrictions.This is very important, as spatially densemapping entails considerable traffic on farmland, and theconsent of the farmers is necessary for access to the16,000 km 2 which will be surveyed using boreholes andground-based geophysical methods over a 10-year period.The forest area in Denmark is to be doubled from 10 to20% over the next 80 years. State subsidies will primarilybe granted for forestation within the new groundwaterprotection zones. It is expected that the protection zoneswill substantially influence future urban development andland use in Denmark.Initiation of Hydrogeological Mapping as a Basisfor Establishing Site-Specific Groundwater ProtectionZones in DenmarkThe initial decision to initiate detailed hydrogeologicalmapping of much of Denmark to facilitate the establishmentof site-specific groundwater protection zones datesback to 1998, to some extent in preparation for Danishimplementation of the new EU Water Framework Directive.The steps in the process are outlined in the Appendix.The Danish Hydrological Setting553Denmark occupies a total area of some 43,000 km 2 .The country consists of mainland Jutland (30,000 km 2 ),which is contiguous with Europe, and nearly 500 islands,of which more than 200 are inhabited. Denmarkhas been continuously populated and cultivated for over3,500 years. Agriculture is one of the most importantindustries, and dominates the landscape. Most of thecountry consists of Quaternary deposits overlying Cretaceouschalk, limestone and Tertiary sand and clay. Thetopography is low-lying, reaching a maximum of 172 mabove sea level. The combination of low topography andwidespread, consolidated and unconsolidated aquifersensures a plentiful and easily accessible water resource.Groundwater recharge averages 100 mm per year, butcan vary in the range 50–350 mm. Currently, approx.800”10 6 m 3 of water is abstracted annually. Householdconsumption by the 5.35 million inhabitants amounts toapprox. 250 ”10 6 m 3 per year. Of this, 99% derives fromgroundwater.Groundwater quality in Denmark is generally good,thus obviating the need for complex and expensive waterpurification. Moreover, the Danish water supply isdecentralised, thus rendering expensive, lengthy pipelinesserviced by large, central water plants unnecessary.DOI 10.1007/s10040-004-0345-1


quality of the borehole samples which can be collectedand, secondly, borehole drillers have only been requiredto have an education encompassing at least a minimum ofgeological knowledge since 2002. This means that a largenumber of borehole drillers are really good at drillingboreholes rapidly and cheaply but, unfortunately, also thatthey are less good at collecting geological information ofreasonable quality. The borehole driller now has to sendborehole samples to the national archive of borehole dataat the Geological Survey of Denmark and Greenland forsample description, but many samples were previouslyonly described by the borehole driller. It is now acceptedin Denmark, however, that borehole data have to beviewed critically. In the future, therefore, geological interpretationmust be based to a greater extent on all theavailable information, with the weight accorded to it beingto a greater extent determined by its quality.By carrying out measurements with pulled-array, continuouselectrical profiling, it has been possible to mapcontiguous variations within the upper approximately30 m of the subsurface which it would not be possible tomap on the basis of borehole data alone. Based on combinedinterpretation of the geophysical measurements andthe borehole information, it is possible to produce accuratemaps of the total clay thickness within the upperapproximately 30 m of the subsurface. Experience frominvestigations of water quality in particular has shownthat if the total thickness of the clay layer within the upper30 m of the subsurface comprises more than half of thetotal layer thickness at that depth interval, the groundwateris likely to be well protected against leaching ofnitrate and most of the other contaminants which typicallypose problems in Denmark. When the total clay layerthickness exceeds 15 m, it obviates the problem associatedwith fissures in the clay layer, as such fissures havenot been observed to penetrate to depths greater than7–8 m. The clay thickness maps correlate well withgroundwater quality measurements, in that the compositionof the groundwater in aquifers covered by more than15 m of clay confirms that the clay layer offers goodprotection against contaminants such as nitrate leachingfrom the root zone.The geophysical surface mapping was previously carriedout using resistivity data in the Wenner configurationfor 10-, 20- and 30-m electrode separations with manualelectrode placement. That method is very expensive andtime-consuming, and hence was only used for investigatingsmall areas. In the early 1990s, Aarhus Universitydeveloped the pulled-array, continuous electrical profilingmethod (PACEP) for obtaining electrical resistivity datain the Wenner configuration for 10-, 20- and 30-m electrodeseparations. The array pulled by a small caterpillarenables spatially dense measurements to be made alongprofiles (Fig. 4). The largest contiguous area in Denmarkin which apparent resistivity has been measured (Wenner,a=30 m) is shown in Fig. 4. The majority of the pilotmapping studies carried out since the early 1990s areincluded in this figure.Hydrogeology Journal (2004) 12:550–562Since the 1990s, the method has been improved toprovide measurements at eight different electrode separationsin the range 2–30 m, and is now known as thePACES method (pulled array continuous electrical sounding)with Pol–Pol configuration 2, 3, 4, 5 and 15 m as wellas Wenner configuration for 10-, 20- and 30-m electrodeseparations (Sørensen 1996). Having more current electrodes,the new method provides better possibilities forinterpreting in multiple layers. Fixed layer inversion for aselected depth of the top layer is used to determine theelectrical resistivity and, hence, the presence or absenceof protective clay covers. The acquisition of eight ratherthan three measurements at each location enables full,three-layer inversion of the data.Further interpretation and data analysis are carried outusing the programme package GGGWorkbench. Thethree Gs represent Geophysics, Geology and GIS. Geophysicalprocessing and interpretation is carried out ina GIS environment integrating geophysical, geologicaland geographical data. The GGGWorkbench representsthe newest generation of geophysical software from TheHydroGeophysics Group (see the website http://www.hgg.au.dk). The GGGWorkbench operates on GERDA asits internal database for geophysical data. As mentionedabove, GERDA is the national database for geophysicalmeasurements (see the website http://gerda.geus.dk).This new tool enables far more comprehensive, geologyrelatedinterpretation of large amounts of geophysicaldata to be performed than has previously been possible(Fig. 3).Aquifer Delineation555Aquifers in Denmark often occur as buried valleys erodedinto the Tertiary clay substratum, and are usually interconnectedto some degree. The buried valleys are oftenunrecognisable in the terrain. It is important to delineatethe regional structures and their interconnectivity in orderto be able to assess potential areas for water abstraction,to quantify regional resources, and to identify aquiferswhich are vulnerable due to the nature of their overlyingsoil layers. As the buried valleys are typically 200–300 mdeep and often up to 1 km wide, they are difficult todelineate using boreholes, even in areas with a highborehole density. In Fig. 5, a small inset in the upperright-hand corner shows a map in which the course ofvalleys in the pre-Quaternary clay substratum has beendetermined solely from borehole data. Figure 6 shows twomaps from the same small area illustrating the numberand distribution of the boreholes down to different depths.Below this is a map showing the number of TEM (transientelectromagnetic sounding) measurements in thesame area. Within the map area, the borehole density isrelatively high (three per km 2 ), and the quality of thedescription of many of the boreholes is reasonable (seeupper map in Fig. 6). However, only 100 of the boreholesreach down to depths exceeding 50 m, corresponding to0.6 boreholes per km 2 (see centre map in Fig. 6). Con-DOI 10.1007/s10040-004-0345-1


556Fig. 4 Aquifer vulnerability.The large map (area 1,073 km 2 )shows the apparent resistivity(Wenner, a=30 m) of the uppersoil layers measured by pulledarraycontinuous electrical profiling(PACEP) or sounding(PACES). The individual profilesalong which the measurementswere made are indicatedas black lines. Variations in thethickness of the clay cover arereflected as variation in resistivity.The clay content of thenear-surface sediments is ofgreat importance with respect tothe vulnerability of the aquifers.These vulnerable areas are delineatedmuch more accuratelythan can be determined fromborehole data or other geophysicalmethodsFig. 5 Aquifer delineation. The large map (area 1,073 km 2 ), whichis based on transient electromagnetic sounding (TEM) and pulledarrayTEM (PATEM), shows the buried valleys in the pre-Quaternaryclay substratum as blue/green elongated areas on the map.These comprise some of the most important aquifers. The smallmap in the upper right-hand corner shows the pre-Quaternary clayHydrogeology Journal (2004) 12:550–562surface in the same area as the inset on the large map based only ondata from boreholes. Observe the differences in detail between thetwo maps. Many of these complex and interconnected buried valleyswere unknown before the TEM/PATEM studies, but have nowbeen confirmed by new drilling surveysDOI 10.1007/s10040-004-0345-1


557Hydrogeology Journal (2004) 12:550–562DOI 10.1007/s10040-004-0345-1


558siderable interpretation has therefore gone into the preparationof the contours of the valley shown in the upperinset in Fig. 5, where the valleys are more than 200 mdeep. In comparison, the TEM measurement coverage isconsiderably greater (Fig. 6, lower panel). TraditionalTEM requires a very large setup with a quadratic transmittercoil which transmits eddy currents down into theground. A receiver measures how quickly the currentdissipates. If the resistivity of the soil is high, e.g. as insandy soil, the current dissipates rapidly, resulting in arapidly dissipating measurement response. If the resistivityof the soil is low, e.g. as in clayey soil, the currentremains longer, resulting in a slowly dissipating measurementresponse. The measurements enable the bed ofthe buried valleys to be determined and provide a roughmeasure of the extent to which the valleys are filled withsand or clay. To facilitate acquisition of high-densitydata, the University of Aarhus developed a continuousrecording system known as the pulled array TEM(PATEM; Sørensen et al. 2000). The setup is illustratedin Fig. 5, which shows a TEM map of the largest contiguousarea in Denmark hitherto mapped using the TEMmethod. In recent years, transient electromagnetic sounding(TEM) has been used extensively and successfully todelineate the layers of electrically conductive clay whichborder the aquifers (Christensen and Sørensen 1998).Despite the fact that TEM has only been used inDenmark for a limited number of years, a large number ofinvestigations have been performed. As of October 2003,the national geophysics database GERDA contained themore than 45,000 TEM measurements hitherto carried outin connection with the nationwide hydrogeological mappingof the water resources. The relatively few boreholeslocated in the mapped areas compared with the number ofTEM soundings are in good agreement with the TEMinterpretations with respect to the depth to the pre-Quaternaryclay surface in particular, which is apparent in theTEM interpretations as a low-lying, particularly wellconductinglayer. In Aarhus County alone, the TEM datahave been confirmed using 150 new, deep, dedicatedmonitoring boreholes in addition to the existing boreholes.High-quality interpretation of TEM data can reveal themajor heterogeneities of most aquifers. However, highdensitydata are required to reveal coupling effects in thedata due to man-made conductors (Sørensen et al. 2000).As the measured coupling effects have a considerablyfaster lateral variation than the effects of the geologicalvariations in Denmark, spatially dense data will revealFig. 6 The density of various types of data within the small insetshown in the upper right-hand corner of Fig. 5. The upper panelshows borehole density, which is three boreholes per km 2 . Thecentre panel indicates those boreholes which are deeper than 50 m,which only applies to one in five of the boreholes in the area,corresponding to a density of only 0.6 boreholes per km 2 , Incomparison, the lower panel shows the density of TEM soundingsin the area, which corresponds to 19 TEM soundings per km 2Hydrogeology Journal (2004) 12:550–562DOI 10.1007/s10040-004-0345-1


whether the variations in these are attributable to couplingeffects.Heterogeneities of the order of magnitude which canbe mapped using the TEM method are of great importanceto groundwater flow patterns within aquifers and,hence, to interpretation of measured groundwater qualityat any given location. To ensure the quality of the dataand their interpretation, it is necessary to perform laterallydense measurements along profiles or in grids. Measurementsare acquired in profiles separated by an averagedistance of 250 m. Processing increases the data densitywithin the profile to one sounding every 25–50 m, incontrast to a typical single-site density of 250 m betweensoundings.As shown in Fig. 5, systematic spatially dense mappingwith TEM reveals numerous structures of majorhydrogeological importance, and delineates these structuresand their interconnections. The small map in theupper right-hand corner of Fig. 5 shows that even in anarea with a high borehole density (more than 500 in thearea shown), the structural complexity is only weaklydescribed by the borehole data compared to the remarkablelevel of detail revealed on the TEM map. The newstructures identified have now been verified by surveyboreholes. Note the very small valley in Fig. 5 which canbe seen just outside the left side of the inset indicated onthe large map. This valley is approx. 600 m wide and hasbeen shown to be 110 m deep, possibly even deeper insome parts. This valley has now been verified by fourboreholes. The valley was previously unknown, and thereare no visible signs in the present terrain to indicate thepresence of a buried valley. It would have taken a largenumber of boreholes to reveal this buried valley with theaid of boreholes alone, as it is markedly cut down into anotherwise flat, pre-Quaternary clay plateau.In 2002, the HydroGeophysics Group further developedthe TEM method into the SkyTEM method, withgreater transmitter strength and a 12”12 m transmittercoil which can be mounted beneath a helicopter. The firstexperiments indicate that the airborne measurements areof the same quality as those made on the ground usingPATEM equipment (Sørensen and Auken 2003).559Mapping of Groundwater QualityThe Danish drinking-water strategy aims to obviate thenecessity for purification of raw water at waterworks.Detailed knowledge of the quality and the variation inquality of the water in aquifers is thus essential. Foreconomic and practical reasons, however, it is impossibleto acquire such detailed information by means of traditionaldrilling techniques.The Ellog auger drilling method (Sørensen and Larsen1999) was developed to rapidly, inexpensively and relativelyunobtrusively collect water samples and measurethe electrical resistivity of the subsurface. With this method,electrical resistivity and gamma logs are recordedcontinuously during drilling with an auger drill which alsoFig. 7 Aquifer water quality. Pulled-array continuous electricalprofiling (PACEP) reveals areas of high resistivity (and thus of lowclay content), indicating vulnerability to nitrate leaching. The highnitrate content in the deeper parts of the aquifer below the highresistivityarea shown on the PACEP map (centre panel) is documentedin the lower panel, which shows the nitrate concentrationdetermined from Ellog data for a vertical section along the profileindicated on the PACEP mapHydrogeology Journal (2004) 12:550–562DOI 10.1007/s10040-004-0345-1


560enables undisturbed groundwater samples to be collectedat selected levels (Fig. 7). Using the method it is possibleto drill to a depth of 60–80 m, depending on the geologicalconditions. Because the water is sampled at thesame level as the geophysical measurements, the Ellogmethod enables the variation in water quality to be correlatedwith the geophysically interpreted lithology.Ellog data can be correlated with the surface geophysicaldata to develop a complete picture of waterquality in an aquifer (see, for example, Fig. 7). ThePACEP map reveals the variation in the resistivity of theupper 30 m of the subsurface. Areas with an apparentresistivity greater than 50–60 ohm metres are predominantlysandy and comprise “windows” whereby contaminantssuch as nitrate can leach to the groundwater. Thelower part of Fig. 7 is a vertical section along the profileshown on the PACEP map. The nitrate content in thegroundwater was determined by analysis of the Ellogwater samples. Comparison of the two parts of the figurereveals that the nitrate concentration of the groundwaterwas much higher, and the nitrate contamination penetratedto much greater depths beneath the sandy areas thanbeneath those protected by clay. The Ellog measurementsare used to support interpretation of vulnerability tocontamination and to prepare the clay thickness maps.measurement methods and data interpretation. The extensivemeasurement campaign has shown that geophysicsis presently a cheap and effective method to fill inthe geological picture between boreholes. The practicalwork of undertaking the geophysical measurements andinterpreting the results has been carried out by privateconsultancy firms. The quality-assurance procedures forthe geophysical measurements and the programming ofinterpretation programmes are paid for by the Countiesand the University. The work is carried out by the HydroGeophysicsGroup and made available free of chargeto the consultants. Measurement data have to be submittedto the national geophysics database GERDA. The costof carrying out the geophysical measurements in Denmarkhas now reached a stable level. The followingprice examples encompass measurement, interpretationand comprehensive reporting with various theme maps.The costs are based on mapping tasks encompassing anarea of at least 10 km 2 , and include 25% Danish VAT.Providing these assumptions are met, a single TEMsounding will cost e 140, with 16 soundings normallybeing made per km 2 . PATEM will cost e 750 per linearkm, with four lines needed per km 2 , while PACES willcost e 550, with four lines needed per km 2 , and Ellogdrilling will cost approx. e 125 per drilled metre.Combined Interpretation of DataThe new mapping initiative includes setting up conceptualgeological models and hydrological models for use inanalysing the geological information and determining thewater balance, the effect of climatic change, the extent ofriver basins, the capture zone of water-supply wells andthe contamination risk. Particle path calculations in particularhave been used to delineate the capture zone. Forthe past 15 years, Aarhus County has used the MIKE-SHEmodel programme package to model major areas of upto 1,600 km 2 (see http://www.dhisoftware.com/mikeshe).Model resolution is normally 250”250 m (correspondingto the average distance between the geophysical measurementlines). In recent years, smaller areas have alsobeen modelled using the programme package GroundwaterVistas (see http://www.groundwatermodels.com),which is based on the MODFLOW code developed by theUS Geological Survey (see http://water.usgs.gov/nrp/gwsoftware/modflow.html) and, to a lesser extent, ProcessingModflow (see http://www.pmwin.net). The modelcalculations are in turn used to draw up site-specificgroundwater protection maps and to establish protectionzones.During the past four years, all the major water utilitiesin Denmark have gained experience in all the aspects ofspatially dense hydrogeological mapping. The cooperationbetween the HydroGeophysics Group at AarhusUniversity and the Counties is very important for ensuringhigh quality in the collection and interpretation of geophysicaldata. The HydroGeophysics Group at AarhusUniversity works intensively to continually improve theHydrogeology Journal (2004) 12:550–562Experience with Groundwater Protectionin Aarhus CountyThe establishment of site-specific protection zones, withregulation of land use within them, has proven successfulat a waterworks on the island of Tunø within AarhusCounty (Aarhus County 2001; Thomsen 2003). About12 years ago, the only aquifer on the island was highlycontaminated with nitrate, and measures needed to betaken to rectify the situation. It was calculated that theestablishment of protection zones and associated regulationof land use would be cheaper than the introduction ofwater treatment. The effect of the site-specific protectionzones has been monitored since 1990, and the nitratecontent of the water in the upper layers of the aquifer hasdeclined to a level below the EU limit value. In a fewyears time, water quality is expected to be acceptable inthe whole aquifer.The success of this and other attempts to protectgroundwater is partly due to the fact that they were basedon spatially dense geological and hydrological mappingcombined with modelling of the hydrological system. Themodels have been used to delineate the capture zonesfeeding the aquifer and determine the effect of sources ofcontamination. The land-use regulations imposed werespecific to these protection zones and were accepted bythe landowners because of comprehensive documentation.In Aarhus Municipality, the total thickness of claylayers above the aquifers has been determined by combinedinterpretation of geophysical and geological data.Groundwater protection zones are now being establishedin the particularly valuable water-abstraction areas, on theDOI 10.1007/s10040-004-0345-1


asis of the spatially dense geophysical mapping andhydrological modelling. After negotiation with AarhusCounty, Aarhus Municipality has accepted the importanceof active protection of the groundwater resources as abasis for ensuring a future drinking-water supply of goodquality. All new urban development around the city ofAarhus must take groundwater interests into consideration,and urban development will not usually be permittedin areas where natural protection of the groundwater ispoor. Moreover, plans to develop 1,000 ha of land designatedfor urban development have been abandoned in2001 as a result of the new information. In 1997, AarhusCounty Council decided that no new areas could bedesignated for urban development before spatially densehydrogeological mapping of the areas in question hadbeen carried out. It is estimated that the groundwater isprotected by clay layers in 50% of Denmark.The Water Framework Directiveand the European CommunityEuropean Community waters are under increasing pressurefrom the continuous growth in demand for sufficientquantities of good-quality water for all purposes (Thomsen1993; European Environment Agency 1995, 1999,2000, 2001). An EU directive establishing a frameworkfor Community activity in the field of water policy wasthus enacted in 2000 (EU 2000). The main aspects of thisinnovative approach to protection of the aquatic environmentare outlined in the Appendix.With respect to groundwater, the Directive specificallyrequires Member States to implement the measures necessaryto prevent, limit and remediate contamination of allbodies of groundwater in order to reduce the level ofpurification required to ensure the drinking-water supply,among other means through the establishment of “safeguardzones” for those bodies of groundwater. The goodexperience with the new geophysical methods shows thatit is possible to carry out the task without imposing majoradditional costs on consumers.ConclusionIt is expected that the site-specific groundwater protectionzones currently being established to ensure the futurewater supply in Denmark will substantially influence futureurban development and land use. It is thus importantthat the protection zones are based on spatially densehydrogeological mapping encompassing geophysical andgeochemical data. The geophysical mapping will beperformed using PACES and PATEM (now SKYTEM)combined with Ellog auger drilling studies. The sitespecificprotection maps designating the protection zonesand associated regulation of land use will be used toprevent groundwater contamination from urban developmentand agricultural activities, and for planning remediationof contaminated sites. Mapping and establishmentHydrogeology Journal (2004) 12:550–562of the groundwater protection zones will take place over a10-year period at a total cost of around e 120 million.During the 10-year period, consumers will pay the CountyCouncils a e 0.02 surcharge per m 3 of water consumed,i.e. less than e 4 per family per year. This ongoingDanish initiative to draw up spatially dense hydrogeologicalmaps of the 37% of Denmark designated as particularlyvaluable water-abstraction areas will have to beadjusted to the new EU Water Framework Directive, asthe latter encompasses all water bodies.Acknowledgements We would like to thank Aarhus CountyCouncil for encouragement throughout the project, Ejgil Pedersenand Lars Schrøder of Public Utilities of Aarhus for financial supportand trust, and David Barry for substantial support duringpreparation of the manuscript.AppendixChronology of Dense Hydrogeological Mappingin Denmark561– December 1998: the Danish Parliament decided thatthe County Councils should be responsible for detailedmapping of the water resources within a combined areaequivalent to 37% of Denmark.– August 1999: the County Councils established a technicaladvisory group responsible for coordinating thegroundwater protection scheme. Three subgroups wereestablished to solve practical problems within geophysicalmapping, hydrological modelling and geochemistryrespectively. The County Councils signed afive-year contract with the Department of Geophysics,University of Aarhus, called HydroGeophysics Group,to enhance geophysical data acquisition and interpretationthrough workshops and the development of improvedquality assurance procedures for measurementof geophysical data, data processing, modelling andanalysis software. See the website http://www.hgg.au.dk– March 2000: the County Councils signed a contractwith the Geological Survey of Denmark and Greenlandfor constructing and managing the national geophysicaldatabase. The mapping data can be transferred toand from the database via Internet. Together with thesoftware company NetNord, Aarhus County Council,the University of Aarhus, the Geological Survey ofDenmark and Greenland and the Spatial PlanningDepartment developed the geophysical database systemknown as GERDA. See the website http://gerda.geus.dk– May 2000: the County Councils signed a contract withthe Geological Survey of Denmark and Greenland toprovide instruction in the use of hydrological modelling.See the website http://vandmodel.dk/– June 2000: the Minister for the Environment issued astatutory order stipulating how the County Councilsare to implement the groundwater protection scheme(Danish EPA 2000a).DOI 10.1007/s10040-004-0345-1


562– August 2000: the Danish EPA published guidelinesfor the detailed mapping work and establishment ofgroundwater protection zones (Danish EPA 2000b).– 2001: the County Councils established action plansincluding a timetable for implementing the measuresnecessary to protect the groundwater.– 2000–2010: detailed mapping and groundwater protectionplans to be developed for 37% of Denmark.– 2000–2015: EU Water Framework Directive to beimplemented.– 2001: the County Councils signed a five-year contractwith the Department of Geology, University of Aarhus,to ensure development of new methods, amongother things for the identification of Quaternary meltwatersediments.The EU Water Framework DirectiveIn summary, the new Directive– protects all waters—rivers, lakes, coastal waters andgroundwaters,– sets ambitious objectives to ensure that all waters meet“good status” by 2015,– sets up a system of management within river basins,– ensures reduction and control of pollution from allsources such as agriculture, industry, urban areas, etc.,– ensures active participation of all stakeholders, includinglocal communities, in water management activities,and– requires water-pricing policies and ensures that thepolluter pays.ReferencesAarhus County (2001) Tunø status report 1989–1999. EnvironmentalDivision, Aarhus County http://www.nm.aaa.dk/publikat/pdf/Tuno_eng.pdfChristensen NB, Sørensen KI (1998) Surface and borehole electricand electromagnetic methods for hydrogeophysical investigations.Eur J Environ Eng Geophys 3(1):75–90Danish EPA (1999) Consolidated Act No. 130 of 26 February 1999on Water Supply etc. as amended by section 10 of Act No. 355of 2 June 1999 and Act No. 374 of 2 June 1999. http://www.mst.dk/homepageDanish EPA (2000a) Statutory Order No. 494 of 28 May 2000 ongroundwater protection plans (in Danish). Danish EPADanish EPA (2000b) Zonation. Detailed mapping of zones forprotecting the groundwater resources (in Danish). Danish EPAGuideline no 3DVGW (1995) Guidelines for drinking water protection. Part 1.Groundwater protection zones. Technical Standard W 101 (inEnglish). German Technical and Scientific Association for Gasand Water, BonnEU (2000) Directive 2000/60/EC of the European Parliament andof the Council of 23 October 2000 establishing a framework forCommunity action in the field of water policy. Official JournalEuropean Communities L 327, 22.12.2000 http://europa.eu.int/European Environment Agency (1995) Europe’s environment.The Dobris Assessment. European Environment Agency http://www.eea.eu.intEuropean Environment Agency (1999) Sustainable water use inEurope. Part 1. Sectoral use of water. European EnvironmentAgency Environmental Issue Rep no 1European Environment Agency (2000) Sustainable water use inEurope. Part 2. Demand management. European EnvironmentAgency Environmental Issue Rep no 19European Environment Agency (2001) Sustainable water use inEurope. Part 3. Extreme hydrological events: floods anddroughts. European Environment Agency Environmental IssueRep no 21Sørensen KI (1996) Pulled array continuous electrical profiling.First Break 14(3):85–90Sørensen KI, Auken E (2003) New developments in high resolutionairborne TEM instrumentation. In: Proc ASEG 16th GeophysicalConf Exhibition, February 2003, Adelaide, Australia.ASEG, Everton Park, QueenslandSørensen KI, Larsen F (1999) Ellog auger drilling: three-in-onemethod for hydrogeological data collection. GroundwaterMonitoring Remediation Fall 1999:97–101Sørensen KI, Auken E, Thomsen P (2000) TDEM in groundwatermapping—a continuous approach. In: Proc Symp Applicationof Geophysics to Engineering and Environmental Problems,Arlington, VA. Environmental and Engineering GeophysicalSociety, Wheat Ridge, CO, pp 485–492Thomsen R (1993) Future droughts, water shortages in parts ofWestern Europe. EOS Trans Am Geophys Union 74(14):161,164–65Thomsen R (1997) Water administration and planning by theCounty Councils, water supply. Danish Water Supply Association,Water Supply Denmark 65(6):296–300Thomsen R (2003) Use of protection zones and land management,restore contaminated groundwater in Denmark. EOS Trans AmGeophys Union 84(7):63. Animated version http://www.aaa.dk/nm/undersoeg/tunoe_flash_eng.htmlVrba J, Zaporozec A (1994) Guidebook on mapping groundwatervulnerability. Int Contrib Hydrogeol 16Hydrogeology Journal (2004) 12:550–562DOI 10.1007/s10040-004-0345-1

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