Hydrogeological mapping as a basis for establishing site-specificgroundwater protection zones in Denmark

R. Thomsen · V. H. Søndergaard · K. I. Sørensen

Abstract The water supply in Denmark is based on high-quality groundwater, thus obviating the need for complexand expensive purification. Contamination from urbandevelopment and agricultural sources, however, increas-ingly threatens the groundwater resource. In 1995 theDanish Government thus launched a 10-point plan toimprove groundwater protection. In 1998 this was fol-lowed by a decision to instigate spatially dense hydro-geological mapping of the groundwater resource withinthe 37% of Denmark designated as particularly valuablewater-abstraction areas. The maps will be used to estab-lish site-specific groundwater protection zones and asso-ciated regulation of land use to prevent groundwatercontamination. Traditional mapping based solely on bore-hole data 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, con-sumers will pay a e 0.02 surcharge per m3 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.

R�sum� L’alimentation en eau au Danemark supposeune haute qualit� des eaux souterraines, en �liminant ainsi

le co�teux processus d’�puration. N�anmoins, la qualit�des sources souterraines est menac�e par la pollutionprovoqu�e par le d�veloppement urbain et agricole. En1995 le gouvernement danois a lanc� un plan en 10 pointspour am�liorer la protection des eaux souterraines. En1999 ce plan a �t� suivi par la d�cision de promouvoir unecartographie hydrog�ologique � grande densit� sur 37%du territoire du Danemark o� se trouvent des zones decaptages importantes. Les cartes seront utilis�es pour�tablir les zones de protection des eaux souterraines, entenant compte des conditions locales du site ainsi que desr�glements conjoints d’utilisation des territoires, afin depr�venir la pollution des eaux souterraines. La cartogra-phie traditionnelle, bas�e seulement sur les donn�es desforages, est trop impr�cise pour ce but. Les travaux vontdurer 10 ans avec un co�t estim� � 120 millions d’euros(e). Pour ces travaux les consommateurs vont payer unesurcharge de e 0.02 par m3 d’eau potable, ceci pendant10 ans. Cette r�vision de la strat�gie du Danemarkconcernant la protection des ressources en eaux souter-raines a d�montr� les raisons pour lesquelles on a accord�un r�le central aux nouvelles m�thodes g�ophysiquesdans la future cartographie hydrog�ologique de vaste r�-gions. On pr�sente un exemple de cartographie g�ophy-sique r�alis�e dans la r�gion d’Aarhus.

Resumen El abastecimiento de agua en Dinamarca est�basado en agua subterr�nea de alta calidad, evitando deesta manera la necesidad de una purificaci�n compleja ycara. Sin embargo, la contaminaci�n a trav�s del des-arrollo urbano y de fuentes agrcolas, ha incrementado laamenaza para el recurso de agua subterr�nea. Entonces en1995 el gobierno lanz� un plan de 10 puntos para mejorarla protecci�n del agua subterr�nea. Este fue seguido en1998 por la decisi�n para promover una cartografa hi-drogeol�gica espacialmente detallada, para el recursoagua subterr�nea dentro del 37% de las �reas de extrac-ci�n consideradas por Dinamarca con una importanciaespecial. Los mapas ser�n usados para establecer zonasespecficas de protecci�n para puntos de agua subterr�neay una regulaci�n asociada al uso del territorio, para pre-venir la contaminaci�n del agua subterr�nea. La carto-grafa tradicional basada exclusivamente en datos de laperforaci�n es muy inexacta para este prop�sito. Estetrabajo tomar� 10 aos y costar� aproximadamenteDKK 920 millones, equivalentes a 120 millones de Euros

Received: 18 October 2002 / Accepted: 1 April 2004Published online: 13 August 2004

� Springer-Verlag 2004

R. Thomsen ()) · V. H. SøndergaardEnvironmental Division, Groundwater Department,Aarhus County,Lyseng Alle 1, 8270 Højbjerg, Denmarke-mail: rt@ag.aaa.dkTel.: +45-8944-6680Fax: +45-8944-6982

K. I. SørensenDepartment of Earth Sciences,University of Aarhus,Aarhus, Denmark

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

(e). Para financiar esto los consumidores pagar�n unsobreprecio de e 0.02 por m3 de agua potable durante unperiodo de 10 aos. Este an�lisis de la estrategia Danesapara proteger el recurso agua subterr�nea, demuestraporque la cartografa detallada, hecha con nuevos avancesen m�todos de medici�n geofsica, aplicados a grandes�reas aledaas, otorgan a la geofsica un papel altamenteimportante en el futuro de la cartografa hidrogeol�gica.Esto est� ilustrado con ejemplos de cartografa geofsica agran escala y espacialmente detallada, llevados a cabo enel �rea de Aarhus.

Keywords General hydrogeology · Geophysicalmethods · Groundwater management · Groundwaterprotection · Legislation

Introduction

In 1995, increasing problems with water quality in Den-mark due to urban development and contamination fromagricultural sources led the Minister for the Environmentto approve a 10-point plan to improve groundwater pro-tection. One of the major initiatives was to ban the use ofpesticides which can contaminate the groundwater. An-other was 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 water-abstraction areas (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 am-bitious plan 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 re-sources as a basis for establishing site-specific ground-water protection zones (Danish EPA 2000a).

The mapping and planning work is to be carried outover a 10-year period and encompasses all parts of Den-mark classified as particularly valuable water-abstractionareas. Together, these cover almost 16,000 km2 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 km2 for a programme en-compassing geophysical profiling every 250 m, surveydrilling every 4 km2, water sampling and hydrologicalmodelling. During the 10-year mapping and planningperiod, Danish consumers have to pay the County Coun-cils a e 0.02 surcharge per m3 of water consumed, i.e.about e 4 per family per year.

Public Water Supply Administration in Denmark

Planning and public administration in Denmark is carriedout at three governmental levels: State, county and mu-nicipal. Legislation is passed by the State. The 14 CountyCouncils are responsible for overall administration ofwater-abstraction permits and protection of water re-sources against 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 Plan

Groundwater protection in Denmark is based on the as-sumption that the physical environment provides some

Fig. 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-handcorner

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degree of protection against anthropogenic pressures, es-pecially as 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 strat-egy is based on three steps.

1. Spatially dense hydrogeological mapping based onnew geophysical surveys, survey drilling, water sam-pling, hydrological modelling, etc., aimed at facilitat-ing the 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 hy-drological and chemical conditions.

2. Mapping and assessment of all past, present and pos-sible, future sources of contamination—both point-source and diffuse.

3. Preparation and evaluation of an action plan stipulatingpolitically determined regulations for future land use

within 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 protec-tion zones and guidelines will be used to preventgroundwater contamination from urban developmentand agricultural activities and for planning the reme-diation of 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 protec-tion zones directed at contamination from point sources.The protection zones encompass a 10-m-diameter pro-tection zone 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 par-ticular. In Denmark, the new site-specific protection

Fig. 2 Example of zonedgroundwater protection at Bederabstraction site in Aarhus Mu-nicipality

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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 calcula-tions of the degradation of the contamination from pointsources and diffuse sources, taking into account knowl-edge of 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 infor-mation from 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 desig-nated as vulnerable, agreements have to be entered intowith farmers to limit the use of fertiliser and pesticides.

Broad-Based Support

Active 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 im-plementation of 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 coopera-tion are accorded high priority throughout the wholeprocess. Successful implementation of the plans necessi-tates that 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 set-aside. 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 is

increasing the confidence of the water-supply utilities thatthey will be able to continue to furnish the Danish pop-ulation with 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 re-strictions. This is very important, as spatially densemapping entails considerable traffic on farmland, and theconsent of the farmers is necessary for access to the16,000 km2 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 Denmark

The initial decision to initiate detailed hydrogeologicalmapping of much of Denmark to facilitate the establish-ment of site-specific groundwater protection zones datesback to 1998, to some extent in preparation for Danishimplementation of the new EU Water Framework Di-rective. The steps in the process are outlined in the Ap-pendix.

The Danish Hydrological Setting

Denmark occupies a total area of some 43,000 km2.The country consists of mainland Jutland (30,000 km2),which is contiguous with Europe, and nearly 500 is-lands, 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 Creta-ceous chalk, 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�106 m3 of water is abstracted annually. Householdconsumption by the 5.35 million inhabitants amounts toapprox. 250 �106 m3 per year. Of this, 99% derives fromgroundwater.

Groundwater quality in Denmark is generally good,thus obviating the need for complex and expensive wa-ter purification. Moreover, the Danish water supply isdecentralised, thus rendering expensive, lengthy pipe-lines serviced by large, central water plants unnecessary.

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The drinking water is not chlorinated and is of bottle-water quality at the tap. Altogether, there are around2,900 public water supplies and some 70,000 small, pri-vate water supplies (boreholes or dug wells). Two thirdsof the water abstracted for the national water supply de-rives from 200 municipal waterworks, and one third fromprivate cooperative waterworks.

At the national level, the most important source ofinformation about the condition of groundwater andaquifers is borehole data. The national archive of bore-hole data at the Geological Survey of Denmark andGreenland contains information dating back to 1926 andhas hitherto been the primary source of geological andhydrological information about aquifers. In AarhusCounty, there are some 11,400 registered boreholes ofrelevance for groundwater mapping, some of which arepoorly documented. The corresponding value for thecountry as a whole is more than 100,000 boreholes,corresponding to an average of approx. 2.5 boreholes perkm2. This data density, however, allows for only a verygeneral description of the complex geological composi-tion of Danish aquifers.

Spatially dense geophysical mapping in and aroundAarhus County during the period 1994–1997 has revealedthat traditional mapping based solely on borehole data istoo inaccurate for establishing site-specific groundwaterprotection zones and regulating land use within them.According to the Danish EPA guidelines (Danish EPA2000b), geophysical methods are expected to play animportant role in hydrogeological mapping in Denmark.Experience with the application of geophysical methodsto the mapping of large-scale geological structures aroundthe city of Aarhus (see below) has been of great value. Inrecent years, new geophysical mapping methods havebeen developed through a collaborative effort by AarhusCounty, the Public Utilities of Aarhus and the Universityof Aarhus. The aim was to upgrade and rationalisefieldwork as well as to standardise the use of the mappingmethods in a region. These new methods are very im-portant tools for carrying out the spatially dense mappingneeded to determine the extent, vulnerability and waterquality of Danish aquifers as the basis for delineatingprotection zones. The geophysical mapping methods aredescribed below.

Mapping of Aquifer Vulnerability

Aquifers are recharged by water seeping down throughoverlying, covering layers. The complexity of the pathfollowed by the water on its way to the deeper-lyingaquifers used for abstraction varies. How fast the waterseeps down through the earth and what changes take placein the composition of the water very much depend on thenature of the soil layers through which the water passes(Vrba and Zaporozec 1994). Generally speaking, the bi-ological/chemical/physical processes which take placewhen water passes through clay purify water better thanwhen water passes through sand. Danish experience is

that the degradation of substances which can contaminatethe groundwater is considerable in the plough layer, butthat the thickness and the composition of the clay layersoverlying the aquifers are the most important geologicalfactors as regards protection of the groundwater againstthe types of contaminant which cause most problems inDenmark. It is therefore important to carry out spatiallydense mapping of clay thickness and of the distributionand variation of sandy areas (windows) in the soil layersoverlying the aquifers. The dense geophysical mapping isnecessary because the complex geology makes it impos-sible to produce sufficiently detailed maps, as there areusually only very few boreholes per km2 and the majorityof the boreholes are not particularly deep.

As regards vulnerability mapping, the aim of thegeophysical mapping is to determine the total claythickness within the upper approximately 30 m of thesubsurface, to delimit any sandy windows present in theupper clay layers, and to correlate the data with the in-formation from the boreholes. Figure 3 illustrates the oldproblem that the interpretation of the surface geophysicsreveals a simplified version of reality, while the boreholesreveal a wealth of detail which, however, represents onlythe actual site of the borehole. In Denmark it has longbeen tradition to primarily base mapping of water re-sources on geological interpretation of boreholes, andgeophysics has only been used when it was in accordancewith a previously established geological description.

Surface mapping with the new geophysical methods,combined with better interpretation programmes, hasshown that it is time to do away with the old way of usinggeophysics. The quality with which water boreholes aredescribed in Denmark varies considerably. Firstly, thechoice of drilling method considerably influences the

Fig. 3 The picture of the world is your own

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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 in-terpretation must be based to a greater extent on all theavailable information, with the weight accorded to it be-ing to a greater extent determined by its quality.

By carrying out measurements with pulled-array, con-tinuous electrical 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 com-bined interpretation of the geophysical measurements andthe borehole information, it is possible to produce accu-rate maps 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 ground-water is 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 associ-ated with 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 composi-tion of 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 car-ried out 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 investi-gating small 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 elec-trode separations. 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.

Since the 1990s, the method has been improved toprovide measurements at eight different electrode sepa-rations in the range 2–30 m, and is now known as thePACES method (pulled array continuous electrical sound-ing) 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 elec-trodes, 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. Geo-physical processing 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 geophysi-cal measurements (see the website http://gerda.geus.dk).This new tool enables far more comprehensive, geology-related interpretation of large amounts of geophysicaldata to be performed than has previously been possible(Fig. 3).

Aquifer Delineation

Aquifers in Denmark often occur as buried valleys erodedinto the Tertiary clay substratum, and are usually inter-connected to 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 (tran-sient electromagnetic sounding) measurements in thesame area. Within the map area, the borehole density isrelatively high (three per km2), 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 km2 (see centre map in Fig. 6). Con-

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Fig. 4 Aquifer vulnerability.The large map (area 1,073 km2)shows the apparent resistivity(Wenner, a=30 m) of the uppersoil layers measured by pulled-array continuous electrical pro-filing (PACEP) or sounding(PACES). The individual pro-files along which the measure-ments were made are indicatedas black lines. Variations in thethickness of the clay cover arereflected as variation in resis-tivity. The clay content of thenear-surface sediments is ofgreat importance with respect tothe vulnerability of the aquifers.These vulnerable areas are de-lineated much more accuratelythan can be determined fromborehole data or other geo-physical methods

Fig. 5 Aquifer delineation. The large map (area 1,073 km2), whichis based on transient electromagnetic sounding (TEM) and pulled-array TEM (PATEM), shows the buried valleys in the pre-Qua-ternary clay 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 clay

surface 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 val-leys were unknown before the TEM/PATEM studies, but have nowbeen confirmed by new drilling surveys

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siderable interpretation has therefore gone into the prep-aration of 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 trans-mitter coil 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 resis-tivity of the soil is low, e.g. as in clayey soil, the currentremains longer, resulting in a slowly dissipating mea-surement response. 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 continu-ous recording 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 con-tiguous area in Denmark hitherto mapped using the TEMmethod. In recent years, transient electromagnetic sound-ing (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 map-ping of 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-Qua-ternary clay surface in particular, which is apparent in theTEM interpretations as a low-lying, particularly well-conducting layer. In Aarhus County alone, the TEM datahave been confirmed using 150 new, deep, dedicatedmonitoring boreholes in addition to the existing bore-holes.

High-quality interpretation of TEM data can reveal themajor heterogeneities of most aquifers. However, high-density data 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 reveal

Fig. 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 km2. 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 km2, Incomparison, the lower panel shows the density of TEM soundingsin the area, which corresponds to 19 TEM soundings per km2

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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 impor-tance to 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. Measure-ments are 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 map-ping with TEM reveals numerous structures of majorhydrogeological importance, and delineates these struc-tures and 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 remark-able level 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 devel-oped the 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).

Mapping of Groundwater Quality

The 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 tradi-tional drilling techniques.

The Ellog auger drilling method (Sørensen and Larsen1999) was developed to rapidly, inexpensively and rela-tively unobtrusively collect water samples and measurethe electrical resistivity of the subsurface. With this meth-od, electrical resistivity and gamma logs are recordedcontinuously during drilling with an auger drill which also

Fig. 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 high-resistivity area shown on the PACEP map (centre panel) is docu-mented in the lower panel, which shows the nitrate concentrationdetermined from Ellog data for a vertical section along the profileindicated on the PACEP map

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enables 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 geo-logical conditions. Because the water is sampled at thesame level as the geophysical measurements, the Ellogmethod enables the variation in water quality to be cor-related with the geophysically interpreted lithology.

Ellog data can be correlated with the surface geo-physical data 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 predomi-nantly sandy and comprise “windows” whereby contam-inants such 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 pene-trated to 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.

Combined Interpretation of Data

The new mapping initiative includes setting up concep-tual geological 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 par-ticular have 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 km2 (see http://www.dhisoftware.com/mikeshe).Model resolution is normally 250�250 m (correspondingto the average distance between the geophysical mea-surement lines). In recent years, smaller areas have alsobeen modelled using the programme package Ground-water Vistas (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, Pro-cessing Modflow (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 coopera-tion between the HydroGeophysics Group at AarhusUniversity and the Counties is very important for ensuringhigh quality in the collection and interpretation of geo-physical data. The HydroGeophysics Group at AarhusUniversity works intensively to continually improve the

measurement methods and data interpretation. The ex-tensive measurement campaign has shown that geo-physics is 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 Hy-droGeophysics Group and made available free of chargeto the consultants. Measurement data have to be submit-ted to the national geophysics database GERDA. The costof carrying out the geophysical measurements in Den-mark has 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 km2, and include 25% Danish VAT.Providing these assumptions are met, a single TEMsounding will cost e 140, with 16 soundings normallybeing made per km2. PATEM will cost e 750 per linearkm, with four lines needed per km2, while PACES willcost e 550, with four lines needed per km2, and Ellogdrilling will cost approx. e 125 per drilled metre.

Experience with Groundwater Protectionin Aarhus County

The 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 regula-tion of 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 com-bined interpretation of geophysical and geological data.Groundwater protection zones are now being establishedin the particularly valuable water-abstraction areas, on the

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basis 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 considera-tion, 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 des-ignated for 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 Community

European Community waters are under increasing pres-sure from the continuous growth in demand for sufficientquantities of good-quality water for all purposes (Thom-sen 1993; 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 envi-ronment are outlined in the Appendix.

With respect to groundwater, the Directive specificallyrequires Member States to implement the measures nec-essary to 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 “safe-guard zones” 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.

Conclusion

It is expected that the site-specific groundwater protectionzones currently being established to ensure the futurewater supply in Denmark will substantially influence fu-ture urban 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 site-specific protection maps designating the protection zonesand associated regulation of land use will be used toprevent groundwater contamination from urban develop-ment and agricultural activities, and for planning reme-diation of contaminated sites. Mapping and establishment

of 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 m3 of water consumed,i.e. less than e 4 per family per year. This ongoingDanish initiative to draw up spatially dense hydrogeo-logical maps of the 37% of Denmark designated as par-ticularly valuable 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 sup-port and trust, and David Barry for substantial support duringpreparation of the manuscript.

Appendix

Chronology of Dense Hydrogeological Mappingin Denmark

– 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 tech-nical advisory group responsible for coordinating thegroundwater protection scheme. Three subgroups wereestablished to solve practical problems within geo-physical mapping, hydrological modelling and geo-chemistry respectively. The County Councils signed afive-year contract with the Department of Geophysics,University of Aarhus, called HydroGeophysics Group,to enhance geophysical data acquisition and interpre-tation through workshops and the development of im-proved quality 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 geophysi-cal database. 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 sys-tem known as GERDA. See the website http://ger-da.geus.dk

– May 2000: the County Councils signed a contract withthe Geological Survey of Denmark and Greenland toprovide instruction in the use of hydrological mod-elling. 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).

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– 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 pro-tection plans 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 Aar-hus, to ensure development of new methods, amongother things for the identification of Quaternary melt-water sediments.

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 all

sources such as agriculture, industry, urban areas, etc.,– ensures active participation of all stakeholders, in-

cluding local communities, in water management ac-tivities, and

– requires water-pricing policies and ensures that thepolluter pays.

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