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As the drinking water supply in Denmark is totally based on
groundwater, monitoring of the groundwater quality is ex-
tremely important to the Danish community. With more
than 62% of the total land area under agricultural use the
Danish Government has determined that the entire area is
vulnerable to nitrate polution, and therefore the groundwa-
ter monitoring programme should cover the entire country.
The Danish groundwater monitoring programme com-
prises water supply well monitoring, the groundwater moni-
toring network and agricultural watershed monitoring (Figs
1, 2) and is described on www.groundwater.dk. The pro-
gramme is part of the National Action Programme for the
Water Environment and Nature, NOVANA (Svendsen &
Norup 2004; Svendsen
Groundwater quality monitoring is carried out on the
basis of data from approximately 6200 public water supply
wells. Furthermore, a detailed analytical programme is car-
ried out on 1415 well screens from the monitoring network
comprising 70 catchment areas, and on 100 shallow screens
from the five agricultural watersheds (Svendsen
The detailed quality monitoring includes analyses for 97
chemical elements, comprising 26 main elements, 14 heavy
metals, 23 organic micro-pollutants and 34 pesticides and
The water supply wells generally have long screens and are
intended to provide representative information on the distri-
bution of the nitrate content in primary groundwater reser-
voirs. Data from water extraction wells are, however, biased
since the wells are intended to ensure production of drinking
water with nitrate concentrations below the maximum
admissible concentration (MAC) of 50 mg nitrate per litre.
The groundwater monitoring wells give a more accurate
picture of the general nitrate pollution in the Danish ground-
water. In 19982004, mean nitrate concentrations were
above the MAC limit for drinking water in 16.9% of the
wells, whereas about 60% had no nitrate (
The spatial distribution of nitrate in the groundwater
aquifers varies. West of the Weichselian glaciation borderline
(Fig. 1), the outwash plains are dominated by upper, uncon-
fined aquifers overlying deeper, confined Quaternary and
Miocene sands. East of the glaciation borderline the sandy
meltwater deposits and the pre-Quaternary limestone aqui-
fers are generally covered by clayey till which reduces or pre-
vents nitrate pollution (Fig. 3).
Over the past half century the use of fertilisers in farming
has intensified dramatically (Fig. 4). Groundwater from the
monitoring screens has been dated using the CFC (chloro-
fluorocarbon) content (GEUS 2004), and demonstrates that
the highest nitrate values reflect the increase in the use of fer-
tilisers (Fig. 4). Continued monitoring will show whether the
decrease in the use of fertilisers during the last decade will
result in a decrease in the nitrate content, or whether the
increasing use of manure will maintain high nitrate levels.
Preliminary data suggest, however, that since 1979 farmers
have adapted their spreading practice for fertilisers and
manure such that nitrate pollution is now following a decli-
Groundwater quality monitoring in Denmark
Geological Survey of Denmark and Greenland Bulletin
7, 3336 (2005) © GEUS, 2005
Fig. 1. Extraction of water for domestic consumption from approxi-
mately 3000 Danish public waterworks in 2001. All waterworks (except
one) are based on groundwater extraction.
Red dashed line
the position of the Weichselian glaciation borderline. The island of Born-
holm is shown in the inset map. Modified from Fraters
Nickel, arsenic and other inorganic trace
The most widespread impact from inorganic trace elements
is due to nickel derived from oxidation of pyrite, bravoite and
other heavy metal bearing sulphides (Fig. 5). Massive disso-
lution of sulphides occurs in areas where large-scale extrac-
tion of groundwater has caused lowering of the groundwater
table, followed by an influx of oxic groundwater. During dis-
solution, some of the nickel is adsorbed by contemporane-
ously precipitated manganese oxide. The trapped nickel is,
however, released when the groundwater table rises again,
since this leads to reducing conditions and dissolution of the
Implementation in 2001 of the EU Drinking Water Di-
rective in Danish legislation has led to increased concern with
respect to arsenic, for which the MAC value was decreased
from 50 to 5 µg/l. Nine per cent of the monitoring well
screens in the groundwater monitoring programme currently
exceed 5 µg/l in all samples, while 14% exceed 5 µg/l in at
least one sample (Fig. 6). The distribution of arsenic is to a
large degree controlled by redox conditions, as the solubility
of arsenic is about ten times higher in reducing environments
compared to oxidising conditions. High arsenic concentra-
tions are mainly found in aquifers underlying clayey sedi-
As a result of seven years of groundwater monitoring, the
background levels of the 23 inorganic trace elements analysed
are well known and are illustrated by the cumulative curves
in Fig. 7. Most curves show a regular distribution, e.g. stron-
tium (Sr) and mercury (Hg), indicating little or no pollution,
whereas the skewed curves for aluminium (Al) and nickel
(Ni) reflect pollution.
Pesticides and metabolites
Analytical results from the water supply wells, the groundwa-
ter monitoring areas and the agricultural watersheds differ
markedly (Fig. 8). Only agricultural pesticides were detected
in water samples collected from young groundwater in the
agricultural watersheds, whereas in water samples collected in
the groundwater monitoring network other pesticides, such
as those used in consolidated areas like urban areas, roads or
farmyards have also been found. Pesticides and their metabo-
lites are found in more than 25% of all wells (Table 1; GEUS
The most frequently found pesticide group consists of tria-
zines and their metabolites. These compounds are commonly
found in both farming and urban areas. In the agricultural
watersheds the triazines and their metabolites make up about
half of all recorded pesticides and metabolites.
In water supply wells the analytical data indicate a very
high frequency of 2,6-dichlorobenzamide (BAM) findings.
Twenty-five per cent of the wells contain BAM, and 10%
have concentrations above the MAC value of 0.1 mg/l. BAM
is a metabolite from dichlobenil and chlorothiamide that were
commonly used prior to 1997 as a total herbicide in urban
areas, along roads and in farmyards.
Fig. 2. Network of Danish national groundwater monitoring areas
(catchments) and selected agricultural watersheds covered by the ex-
tended monitoring programme. From Stockmarr & Nyegaard (2004).
Fig. 3. Distribution of nitrate versus depth to well screen in groundwa-
ter monitoring and water supply wells. The data cover the period 1998
2003 and are grouped into three classes according to mean nitrate. From
The monitoring network demonstrates high detection
rates for pesticides in the upper 4050 m of wells tested, and
a lower number of findings with increasing depth (Fig. 8).
Pollution in shallow groundwater
Drinking water from 628 dug wells and shallow drilled wells
used for single private supply and minor partnership supplies
(less than nine families) was investigated in a research project
focusing on the youngest groundwater (Brüsch
In 35% of the investigated wells, pesticides and their metabo-
lites were recorded with values above the MAC value for
drinking water (0.1 mg per litre), whereas 11% of wells had
more than ten times the MAC value.
The research project showed that nitrate pollution in 22%
of the wells was above 50 mg per litre. The MAC level for
bacteria was exceeded in 48% of the wells whereas 31% con-
tained coliforme bacteria. In total, 68% of the private supply
and minor partnership supply wells delivered undrinkable
Fig. 5. Occurrence of nickel in Danish water supply wells, 19982003.
Slightly modified from GEUS (2004).
Fig. 6. Occurrence of arsenic in Danish water supply wells, 19982003.
Slightly modified from GEUS (2004).
Fig. 4. Nitrate content versus CFC age of
groundwater. The annual use of nitrogen
fertilisers is shown for comparison.
Groundwater data from the anoxic zone
are corrected on the basis of sulphate con-
tent. Slightly modified from GEUS (2004).
During the last two decades many water supply wells have
been closed due to pollution; initially this was a result of
nitrate pollution, and subsequently also to pesticide pollu-
tion. Nickel contamination has also closed some wells, and it
is feared that arsenic will cause closure of more wells. In some
cases the water supply companies have extended their pro-
duction wells to greater depth instead of closing them, but
often this is not a viable solution.
Due to government policy that drinking-water produc-
tion should be based on pure groundwater requiring only
simple treatment (aeration and iron removal), only few water
supply companies have had permission to install advanced
water treatment for removal of pesticides or nickel. However,
more and more water supplies are expected to need advanced
water treatment in the future.
Nitrate pollution is still the most serious problem because
intensive agricultural practices cause leakage of nitrate into
groundwater and surface water. While a series of action plans
have been introduced to reduce nitrate pollution, much of
the young groundwater and most of the surface water in
Denmark are seriously polluted by nitrate.
Brüsch, W., Stockmarr, J., Kelstrup, N., von Platen-Hallermund, F. & Ro-
senberg, P. 2004: Pesticidforurenet vand i små vandforsyningsanlæg.
Danmarks og Grønlands Geologiske Undersøgelse Rapport
Fraters, B., Kovar, K., Willems, W.J., Stockmarr, J. & Grant, R. (eds) 2005:
Monitoring effectiveness of the EU Nitrates Directive Action Pro-
grammes. Results of the international MonNO
workshop, The Hague,
the Netherlands, 1112 June, 2003. RIVM Report
GEUS 2004: Grundvandsovervågning 19982003, 48 pp. København:
Danmarks og Grønlands Geologiske Undersøgelse.
Stockmarr, J. & Nyegaard, P. 2004: Nitrate in Danish groundwater. In:
Razowska-Jaworek, L. & Sadurski, A. (eds): Nitrates in groundwater.
International Association of Hydrogeologists, Hydrogeology, Selected
Stockmarr, J., Nyegaard, P., Larsen, C.L., Felding, G. & Brüsch, W. 2002:
Groundwater quality monitoring in Denmark. Third International Con-
ference on Water Resources and Environment Research (ICWRER),
Dresden, Germany 2226 July, 2002, Proceedings, 165169.
Svendsen, L. & Norup, B. (eds) 2004: NOVANA. Det nationale program
for overvågning af vandmiljøet og naturen. Programbeskrivelse del
1. Danmarks Miljøundersøgelser, Faglig rapport fra DMU
, 45 pp.
Svendsen, L.M., van der Bijl, L., Boutrup, S. & Norup, B. (eds) 2005:
NOVANA. Det nationale program for overvågning af vandmiljøet og
naturen. Programbeskrivelse del 2. Danmarks Miljøundersøgelser,
Faglig rapport fra DMU
, 128 pp.
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
Fig. 7. Inorganic trace elements found in Danish monitoring wells from
1993 to 1999. The skewness of the distribution pattern in the curves for
nickel (Ni) and aluminium (Al) reflects pollution. Regular distributions,
as e.g. for Hg and Sr, indicate little or no pollution. Slightly modified
Fig. 8. Occurrence of pesticides and metabolites in water supply and
groundwater monitoring wells versus depth to top of well screen, 1998
2003 as percentage of number of screens investigated. Slightly modified
from GEUS (2004).