1 Copenhagen
1.1
EXECUTIVE SUMMARY
through the abstraction well, is the main reason for
oxidation of Ni-bearing iron sulphides.
It is concluded that the major parts of the groundwater
bodies around Copenhagen are affected by modern,
potentially contaminated groundwater. The impact is
locally so profound that it is necessary to abandon wells or
well sites. More than 10%, or around 100 wells, of the
Copenhagen Energy water supply wells, have been
abandoned due to contamination mainly by pesticides and
chlorinated solvents during the past decade. However, trace
elements constitute a significant and possibly increasing
problem of potentially the same magnitude as the orgarnic
contaminants.
The number of wells where guideline values have been
exceeded seem to be comparable for organic contaminants
and trace elements according to the National Groundwater
Monitoring Program.
The Upper Cretaceous and Lower Tertiary carbonate
aquifers around Copenhagen generally contain modern
groundwater modified by human impact. This is
documented by the groundwater chemistry and
environmental tracer and dating tools. Agriculture, industry,
urban areas, and exploitation pose a threat to these
important aquifer systems that supply most of the greater
Copenhagen area with freshwater. Anthropogenic effects
are found in nearly all parts of the groundwater bodies and
definition of the baseline groundwater composition is
therefore difficult.
One way to estimate the baseline hydrochemistry of
groundwater is to analyse historical data from groundwater
hydrochemistry archives, another is to assume baseline
waters without human impact when one or more of the
environmental tracers for young groundwater (e.g. 3H, 85Kr,
CFC-12, SF 6) are below detection limits. All of the wells
sampled and analysed in this study, however, showed a
human impact as demonstrated for example by the presence
of post bomb values of the radioactive isotopes 3H and 14C
or measurable concentrations of the environmental tracer
CFC-12. Elevated concentrations due to human impact of
major “natural” ions (e.g. NO3, SO4 and Ca) and trace
elements (e.g. As, Ni), as well as organic micro
contaminants (e.g. pesticides and chlorinated solvents) are
frequently found in the investigated aquifers. No organic
contaminants, however, have been found in the wells
investigated.
Nitrate which occur naturally, but which is also the most
common contaminant in Danish groundwaters is measured
in only small concentrations (< 1 mg l–1 NO3–N) in the
investigated wells. This is for wells with elevated sulphate
concentrations probably partly a result of nitrate reduction
by iron sulphides in and above the aquifers.
The selected Baseline trace element indicators (Al, As,
Cd, Cr, Cu, Hg, Ni and Zn) are generally below guideline
values, however, Ni and Zn exceed the guideline value in
around 15 % and 10 % of the analysed wells, respectively.
This is similar to the number of Ni and Zn analyses above
the guideline value in the National Groundwater
Monitoring Program. Arsenic and Aluminium, which are
the trace elements most frequently found above the
guideline value in the Danish groundwater monitoring
program, do not exceed the guideline value in any of the
wells sampled in this study.
The Ni content is partly correlated to sulphate,
indicating that it is released by the oxidation of iron
sulphides in the carbonate aquifer. Ni concentrations above
guideline values are mainly located in areas where the
groundwater table has been lowered to below the base of
the confining clay tills. A recent study has documented, that
in such areas, advective oxygen transport governed by
atmospheric pressure fluctuations (barometric pumping)
Summary of conclusions
•
•
•
•
•
•
•
1
The important carbonate aquifers, that supply most of
the greater Copenhagen area with freshwater, generally
contain groundwater with a human impact. Severe
groundwater quality problems arise locally from
surface contamination and from natural processes
inside the aquifer itself.
Arsenic and nickel are among the most severe
groundwater quality problems. Ni-problems originate
from natural processes, but are enhanced by human
activities (lowering of the water table). Arsenic has
only recently been recognised as a problem in the area,
and its origin is as yet uncertain.
The most problematic organic contaminants are
pesticides and their degradation products (e.g. BAM –
a degradation product of the pesticide dichlobenil) and
chlorinated solvents.
Reducing environments sometimes offer the possibility
for management of high nitrate groundwaters by using
natural attenuation. However, reduction of nitrate by
e.g. iron sulphides may cause other problems that
deteriorate the water quality severely (e.g. increase
sulphate, hardness and Ni concentrations).
Well sites may have to be abandoned solely due to
natural processes enhanced by groundwater
exploitation and e.g. lowering of the water table.
The study provides information of a strategic value for
more than 100 inorganic and organic parameters, as an
aid to future water quality and quantity management in
the area.
Monitoring
networks,
groundwater
dating,
hydrological and hydrochemical modelling are
important tools for evaluation of the baseline
groundwater quality.
1.2
PERSPECTIVE
considerably (Boulton et al., 1996; Edmunds and Milne,
2001).
The investigated limestones were deposited in the
easternmost part of the Danish Basin, close to landmasses
in Scania (southern Sweden). Due to arid, climatic
conditions and low relief of the nearby landmasses the input
of non-carbonate, siliclastic components were very limited
(Haakansson, 1974; Thomsen, 1995; Surlyk 1997).
Palaeogene and Neogene, mainly fine-grained siliclastic
deposits, later covered the area of chalk deposition.
However, due to Miocene and Pliocene inversion tectonics
along the fault-zone, bordering the area against Scania
towards the east, and due to Quaternary glacial erosion,
these deposits were later eroded (Japsen et al., 2002).
During the Pleistocene, the area was transgressed by a
number of glaciers and ice sheets leaving behind a complex
sequence of glacial tills and meltwater deposits that
presently cover the limestone aquifer (Houmark-Nielsen,
1987), and to some degree protect it against pollution. Total
thicknesses of the Pleistocene deposits are typically in the
order of 20 to 50 meters, but range from a few meters to
about 100 m. This is indicated in Figure 1.2, which shows
the elevation of the Pre-Quaternary surface and the location
of the investigated wells in the study area. Locally, coarsegrained meltwater deposits in the Pleistocene sequence may
serve as groundwater aquifers above the Pre-Quaternary
limestones within the investigated area.
A large amount of groundwater quality data from
boreholes is available in different databases as the starting
point for evaluating the baseline groundwater quality of the
limestone aquifers. However, groundwater quality is in the
majority of the area is to some extent influenced by
contamination and the intense groundwater abstraction that
characterise the area, and it is difficult to find wells with
pristine baseline. Hence, baseline quality had to be
evaluated from the few time series available, that cover the
period back to the early 20th century, and must otherwise be
interpreted from present day quality and evaluation of the
The selected study area is roughly defined geographically
as the area where groundwater abstraction from limestone
rocks takes place for the supply of water for the Greater
Copenhagen area. In this text “limestone” is used as a broad
term to cover any type of carbonate rocks present in the
Maastrichtian and Danian sequence in the investigated area.
A more detailed classification is given in the text.
The Upper Cretaceous (Maastrichtian) and Lower
Tertiary (Danian) limestone sequence, from which most
groundwater in the investigated area is abstracted, has a
much wider distribution in Denmark and it serves as an
aquifer in several other regions of the country (e.g.
Nygaard, 1993). The limestone aquifers around
Copenhagen and in Denmark in general, are furthermore all
part of the important chalk reservoir system in North-West
Europe (Figure 1.1, Downing et al., 1993), that for example
supply water to some 40 million people in the London and
Paris Basins (Edmunds et al., 1992), and produces
considerable amounts of hydrocarbons in the Central North
Sea (D’Heur, 1993, Zink-Jørgensen & Hinsby, 2001).
The limestone aquifer is characterised by a high yield
and fairly good groundwater quality that generally only
needs simple treatment for the production of potable water.
However, the quality of the water is threatened from several
different anthropogenic and natural sources in and around
the aquifers.
Regional setting.
The limestone aquifers around
Copenhagen consist of carbonate rocks varying between
marine, pelagic, very pure, fine grained, biogenic carbonate
ooze (chalks), bryozoan mounds and near coastal skeletal
carbonate sands and oyster banks (Surlyk 1997). This
carbonate sequence was deposited during the Upper
Cretaceous (Maastrichtian) and lowermost Tertiary
(Danian). Generally, the limestone aquifers only constitute
the uppermost approximately 50 m of the total limestone
sequence with a total thickness of up to around 2000 meters
in the study area. The most important reason for this is that
complete flushing of the original marine pore waters,
probably primarily during the Pleistocene, generally did not
extend deeper than this, although the Pleistocene
glaciations had a tremendous effect on the groundwater
flow systems and increased the freshwater circulation
Figure 1.1 Spatial distribution of chalk reservoirs in
North-West Europe. The Chalk constitutes important
aquifers especially around London, Paris and
Copenhagen. The Chalk is also an important
hydrocarbon reservoir in the central North Sea, which
is currently exploited by Norway, Denmark and the
United Kingdom
Figure 1.2 Location of the wells of Copenhagen
Energy investigated in BASELINE and topography of
the pre-quaternary surface of North-Eastern Sjælland.
The legend indicates elevation in intervals of 25 m
2
investigated limestone aquifers. A sound understanding of
the natural variation of the baseline quality and the
controlling processes is crucial, in order to be able to detect
groundwater quality deterioration due to abstraction or
pollution as early as possible. Early warning of on-going or
advancing contamination and deterioration is important as
remedial measures generally are slow and inefficient, and
large volumes of high quality groundwater may deteriorate
quite fast – especially in dual porosity aquifer systems such
as the Chalk aquifers. Furthermore, the natural
biogeochemical environments control the degradability of
the xenobiotics in the subsurface and some contaminants
may for example degrade slowly only under certain
reducing conditions, while others may not degrade at all
under the same conditions. In line with this, emphasis is put
on a description of the natural baseline quality of the
groundwater resources and the natural geochemical
processes controlling the evolution of the natural
groundwater chemistry – as these control and determine the
conditions for contaminant transport in the subsurface
together with the physical characteristics of the aquifers and
confining layers.
water/rock interaction and on-going geochemical processes
(e.g. by geochemical modelling).
Background to the limestone aquifer. The limestone
aquifers around Copenhagen are of crucial importance for
the water supply in the region as virtually 100% of the
potable water demand is covered by groundwater
abstraction from these aquifers. The major regional water
supplier is Copenhagen Energy that annually produces
around 60 million m³ drinking water, which practically all
is abstracted from the limestone aquifers. Only small
amounts of surface water and groundwater from sand
aquifers are also exploited.
The counties of Frederiksborg, Roskilde and
Copenhagen and the two municipalities Frederiksberg and
Copenhagen administer groundwater abstraction permits in
the region. Due to intense exploitation of the groundwater
resources for drinking water purposes, and public focus on
maintaining flows in streams and water level in lakes and
moors, licences for irrigation, or purposes other than for
industrial or drinking water purposes, are almost impossible
to obtain. This policy seems very reasonable considering
that a new assessment of the available groundwater
resources in Denmark estimates that the freshwater resource
in the Copenhagen area currently is overexploited by 250%
(Figure 1.3) when a holistic integrated approach is applied,
and contaminated groundwaters and effects on surface
water ecosystems are taken into account (Henriksen and
Sonnenborg, 2003). This new estimate also reduces the
sustainable water abstraction in Denmark on a national
scale to nearly half of earlier estimates and it puts Denmark
between the 10% of countries worldwide with the lowest
available water resource per capita, according to the UN
estimate of national renewable water resources for c. 180
different nations in the world (UNESCO, 2003). It will also
move Denmark from the group of European countries
which are considered to be under low water stress to the
group considered to be under the most severe water stress,
together with e.g. Spain and Italy (EEA 2003).
The focus of this paper is on the documentation and
interpretation of the natural baseline quality and natural
processes controlling the geochemical evolution in the
1.2.1
Factors which may influence the groundwater
quality
The major concerns about groundwater quality in the
limestone aquifers are coupled to:
• Anthropogenic sources of pollutants; either point
sources or diffuse rural or urban sources.
• Quality deterioration caused by over-abstraction of
groundwater.
The metropolis of Copenhagen with its suburbs and its
new and old industrial areas dominate the land use in the
eastern and central parts of the study area. Within these
urban areas there are numerous old landfills, gasoline spills
and other industrial point sources of pollutants. By far the
majority of the water wells shut down due to pollution, are
closed due to contamination by pesticides and chlorinated
solvents originating from various sources (urban areas,
industry,
agriculture,
private
households
etc.).
Approximately 10% of the water wells or around 100 wells
owned by Copenhagen Energy have so far been closed due
to pollution.
The countless smaller and larger oil and gasoline spills
in contrast have virtually no impact on groundwater quality
since these pollutants seem to be quite readily degraded in
the natural environment. Exceptions are newer spills of
mid- or high-grade gasoline containing MTBE (methyl
tertiary-butyl ether).
Further away from Copenhagen, particularly in westerly
and south-westerly directions, the land-use changes
gradually towards agricultural exploitation (Figure 1.4).
Still, the farming intensity in these areas close to
Copenhagen (less than c. 40 km) only in rare cases reaches
a level comparable to the rest of rural Denmark. Typical
threats to groundwater quality in rural areas originate from
the agricultural use of manure, fertilisers and pesticides.
However, nitrate is only present in the groundwater in a few
areas, where the limestone aquifer is vulnerable due to the
absence of a protecting cover of clayey till, or where the till
cover is thin. Few water wells have been closed due to an
increase in nitrate content beyond the guideline value. The
groundwater quality in rural areas is threatened mainly by
Figure 1.3 Degree of (over)exploitation in different
regions of Denmark (Henriksen and Sonnenborg, 2003)
3
Scientific approach. Data evaluated and used in this
report are collected from varies sources including GEUS
(the National Groundwater Monitoring Database), Danish
EPA, Copenhagen Energy, Carlsberg Breweries and the
counties/cities of Roskilde, Copenhagen, Frederiksborg and
Frederiksberg. These data are supplemented by data on
groundwater sampled and analysed specifically for the
project.
1.3
BACKGROUND TO UNDERSTANDING OF
BASELINE
1.3.1
The evolution of the Cretaceous-Danian Chalk
Group
The depositional environments for the various carbonate
units that constitute the limestone aquifers around
Copenhagen have recently been outlined in Thomsen
(1995) and Surlyk (1997).
During the late Upper Cretaceous sea level high stand
(Maastrichtian), mainly pelagic ooze deposition took place
in the study area, whereas during the Danian, the whole
sequence of facies developed close to the basin margin.
Hence, during the Danian (Lowermost Tertiary) Oyster
banks were developed producing skeletal sands and gravel,
that gradually pass into sorted skeletal sands and silt
(packstones and wackestones (Dunham, 1962). These
sediments were deposited in subaqueous dune fields at the
basin margin – and part of these constitutes the København
Kalk Formation (The Copenhagen Limestone Formation).
Further offshore large bryzoan mounds with a varying
content of ooze were deposited. These deposits can be
classified as wackestone, packstone and grainstone –
(Dunham, 1962) depending of the content of ooze. Finally,
in the deeper parts of the basin, in places inter-fingering
with the bryzoan mounds, pelagic coccolithic ooze was
deposited (chalk – Scholle, 1977, or mudstone and
wackestone – Dunham, 1962).
The skeletal sand and silt facies both constitute the
formal København Kalk Formation (Stenestad, 1976). In
the city of Copenhagen skeletal sand dominates, whereas
skeletal silt is the dominant facies in the København Kalk
Formation elsewhere in the study area (Knudsen, 1996).
The Upper Cretaceous Danian Chalk Group forms a
coherent body in the North Sea region. The average
thickness is of about 500 m. The thickness varies in the
investigated area between ca. 500 m on Stevns south of
Copenhagen to more than 2 km in front of the Sorgenfrei
Tornquist Zone northeast of Copenhagen. The clastic influx
into the North Sea Basin was in general low during the
deposition of the Chalk. The Chalk Group does not contain
any significant clastic units in the wells on Sjælland, but
closer to the margins of the basin in Sweden, clastic units
appear in the chalk. The chalk was uplifted during Neogene
time at both margins of the basin. The uplift in the
Copenhagen area is in the order of 500 m increasing
towards the north to ca. 750 m (Japsen, 1998, Japsen and
Bidstrup, 1999). The Chalk Group in most of the
investigation area is overlain directly by Quaternary
sediments due to uplift and subsequent erosion.
The interval from The Base Upper Cretaceous (BUC) to
Top Cretaceous has been evaluated in this study based on
Figure 1.4 Land use map and the location of
investigated wells. Yellow (~white) colours are
agricultural areas, medium grey (rose) and dark grey
(green) colours mainly urban areas and woods,
respectively
the agricultural use of pesticides. Several water wells have
been closed due to pesticide pollution from agricultural
areas.
The overall geological and hydrogeological framework
of the limestone aquifers may seem to be quite
homogeneous within the study area. Nevertheless, it is
obvious that there are important differences in baseline
quality, which reflects variations in:
• The geochemical and physical characteristics of the
aquifer rocks.
• Type and thickness of aquitards confining the
limestone aquifers.
• Annual recharge to the aquifer.
• Residence time in the aquifer reflecting amount of
recharge and length of particle flow paths before
reaching discharge areas.
• Variations
in
confined/unconfined
and
saturated/unsaturated conditions.
Quality decline due to abstraction or over-abstraction of
the groundwater resource alters the natural quality patterns
in the aquifer, as defined by the above mentioned factors.
The concentrations of a number of naturally occurring
substances have changed due to abstraction. Major quality
problems of this type in the region are increases in nickel
content and in some areas chloride and sodium. Minor, but
still important, quality problems are increases in the content
of sulphate and hardness. In addition to these problems,
there are natural quality problems that are no necessarily
caused by abstraction e.g. methane, hydrogen sulphide,
fluoride and nitrate.
In addition to measures protecting the groundwater
quality, great efforts are made to minimise the effect of
groundwater abstraction on the impact on fresh surface
water ecosystems (Henriksen and Sonnenborg, 2003).
However, this is not a groundwater quality issue and will
not be dealt with further.
4
large number of small faults correlate with the eastern flank
of the Höllviken Graben, and in this area they might have a
connection to the top of the chalk.
The uppermost reflector that can be mapped on the new
data is close to the Top Cretaceous. A map combining this
seismic reflector and Top Cretaceous from the well
database at the Geological Survey of Denmark and
Greenland has been constructed. For the depth conversion
of the seismic interpretation a velocity of 2 km sec-1 has
been used for the layers above the Top Chalk (“Top Upper
Cretaceous” / “TUC”) , the result can be seen on (Figure
1.8). The map is machine contoured and no faults have
been used. The map shows some deviation between seismic
data and well data, which is probably due to the rough
depth conversion. The velocity of 2 km s-1 is a compromise
between lower velocities in the quaternary sediments and
higher in the Danian chalk, therefore a deviation will occur
as the overburden changes. The map shows that the surface
in general is deeper in the north, but it also shows local
variations, which are most likely due to quaternary erosion.
Figure 1.5 Map showing location of evaluated seismic
lines and faults in the study area. The location of the
Carlsberg Fault and well 200.27g at the Carlsberg
Breweries is also indicated
all available seismic data including very recent surveys
(Figure 1.5). The BUC is a well-defined reflector, which is
easy to map in most of the area. The BUC horizon is in
general a smooth surface only interrupted by major faults in
two cases. The faults are related to the beginning salt
tectonics in the Slagelse and Stenlille area (southwest of the
investigated area) and to deeper structural elements mainly
at the boundary of the Höllviken Graben (northeast of
investigated area).
In the area of interest, only a few faults can be traced to
the surface on conventional seismic data. Due to the
relative low resolution by conventional seismic data, the
faults look more like a flexure than an actual fault when
they get close to the surface. However, a high-resolution
seismic line has been shot across the Carlsberg fault
(Fallesen, 1995) on the Isle of Amager to the east of central
Copenhagen, and it shows that the fault actually reaches the
top of the Chalk. This is in accordance with
hydrogeological investigations and groundwater heads in
Copenhagen, which show that the Carlsberg fault has a
significant influence on the groundwater quantity and
quality of wells in the central part of Copenhagen (see later
section on time series). An example of a conventional
seismic line showing the Carlsberg Fault can be seen on
(Figure 1.6).
On the high-resolution marine seismic data small faults
in the top of the section can be identified in part of the area
(Figure 1.7). The faults die out at depths corresponding to
between 200 and 300 ms (approximately 100 – 150 m
below surface). These small faults are probably
glaciotectonic features which locally probably influence the
groundwater flow systems significantly, and may have been
important for meltwater circulation during the Pleistocene
glaciations (Boulton et al. 1996). This type of small fault
can not be identified on the land-lines, this is most likely
due to the lower resolution rather than the absence of that
type of faults.
All faults observed on BUC have been mapped, larger
faults have been correlated, whilst the minor faults have
been marked on the lines where they have been identified,
and the interpreted results are illustrated in Figure 1.5. The
larger faults most likely have connection to the top of the
chalk, whilst minor faults probably do not have connection
to the top of the chalk. In Northern Sjælland, however, a
1.3.2
Basin inversion, Pleistocene events and fresh/saltwater boundaries
According to Japsen et al. (2002), younger Tertiary clastic
sediments with a thickness of around 500 to 750 meters
Figure 1.6 The eastern part of the seismic line
HGS_001 showing the Carlsberg Fault. The fault looks
more like a flexure closer to the surface, this is probably
an effect of the low resolution of conventional seismic.
The location of the line is indicated on Figure 1.5
Figure 1.7 The high-resolution marine seismic line
HGS_8n13 shows small faults in the top of the section.
These faults die out between 200 and 300 ms (roughly
100-150 m below surface). The small faults are probably
related to glaciotectonics. The location of the line is
indicated in Figure 1.5
5
Holocene, formerly marine transgressed areas, there may
locally be saltwater problems associated with residual
saltwater which has not been flushed from low permeable
layers of clayey till or postglacial marine fine grained
deposits.
Stratigraphy. No complete formal lithostratigraphic
scheme covering the various units in the Danish onshore
limestone sequence exists. The various lithologic units that
constitute the limestone sequence are therefore named with
a mixture of formal and informal lithostratigraphic and
lithological names, the generally used names and selected
data for the different units are listed in Table 1.1.
The chalk sequence has just recently (2002) been
completely penetrated by a deep geothermal well situated in
Copenhagen. In this well, “Margretheholm-1”, the Upper
Cretaceous and Danien limestones reach a thickness of
approximately 1600 meters. An evaluation of the resistivity
log run in the well, which starts at ca. 700 m below surface,
indicates that the formation water in the chalk is connate
water of approximately marine salinity from this level to
the bottom of the Chalk at ca. 1600 m depth.
Only the uppermost ca. 50–100 meters of this sequence
appear to be more or less completely freshened. Below this
level a diffusion profile is developed which slowly increase
in salinity towards sea water salinity found in the connate
waters at depth.
The Danian sequence is absent due to erosion in parts of
the area, but where the sequence is most complete it may
reach thicknesses of 100 to 120 meters.
Base of the freshwater aquifer. The base of the
freshwater bearing aquifer may be defined by the typical
chloride concentration range of freshwaters in an area
(10-50 mg l–1 Cl in this study), or in some cases for
management reasons the maximum permissible chloride
concentration for drinking water, which in Denmark is
250 mg l–1 (EU and WHO guideline value). The first level
can be easily found by hydro-geophysical logging (on e.g.
induction, resistivity, electrical conductivity logs) as the
level where a steady increase or decrease of the
conductivity or resistivity, respectively, sets in (c. –70 m in
Figure 1.9). The second level is not as easily detected but it
will in the selected study area typically be found a few 10’s
of meters below the onset of the diffusion profile.
The nature of the saltwater/freshwater boundary is not
well investigated in Denmark, however geophysical logging
suggest that the boundary is developed quite similar to the
boundaries observed in e.g. Southern England in some
regions. That is the boundary is developed as a diffusion
profile (Edmunds et al., 1992; Buckley et al., 2001),
between connate marine formation waters at depth and
circulating freshwater above. The top 50 meters of this
diffusion profile can be observed in e.g. well no. 207.3633
south of Copenhagen (e.g. Figure 1.9). The depth at which
the connate formation waters of marine salinity is reached
is uncertain – it may be at a depth in the order of 500
meters as observed in wells in the UK (Edmunds et al.,
1992), this is partly supported by the new geothermal well
mentioned above indicating marine waters from at least
700 m below the surface. Unfortunately the resistivity log
started at this depth and no information is available from
shallower depths.
The maximum depth of freshwater circulation is
controlled by the regional geology, the base level during the
glacial maxima (the sea level was up to c. 130 m below the
Figure 1.8 Map showing the depth to the Top
Cretaceous in metres below sea level. The map combines
the seismic reflector believed to be at or close to the top
Chalk and well information. For the depth conversion of
the seismic interpretation a velocity of 2 km sec–1 has
been used for the layers on top of the horizon
have covered the limestone aquifers around Copenhagen.
Erosion of these deposits took place during the Neogene
during two phases of basin inversion and during the
Pleistocene. The thickness of the missing section is
important, since this corresponds to burial depth. Burial
depths control diagenesis and compaction, and hence to
what extent porosity and permeability are preserved in the
aquifer carbonates.
Another important period of time is when the land
emerged above sea-level. This moment defines the earliest
onset of infiltration of freshwater and flushing of the
saltwater in the aquifer i.e. the initial generation of the
freshwater resource.
According to data in Japsen et al., (2002), it can be
inferred that freshwater flushing of the aquifer probably
started during the Pliocene, corresponding to not later than
2 to 5 million years before present.
It follows from the above discussion that it is assumed
that the sea did not cover the area during the Pleistocene. In
accordance with this, no marine deposits have been found
within the sequence in the study area. However, the
Pleistocene ice sheets of all the major glaciations covered
the area (Houmark-Nielsen, 1987) and these had a
tremendous effect on freshwater circulation and
consequently freshening of the aquifer systems (Boulton
et al., 1996).
The sea has, however, covered marginal low-lying parts
of the area during Holocene time. Marine transgression
started c. 7500 years BP – the Litorina transgression At that
time, deglaciation and the accompanying sea level rise
happened faster than land rise due to buoyant rise of the
land, caused by the release of weight from the glaciers. Old
Litorina shorelines and thin marine deposits are
recognizable along the coasts and lower parts of stream
valleys at elevations ranging from 2 meters towards the
south up to 5 – 7 meters towards the north. In these
6
Table 1.1
Selected data from the different limestone units and formations compiled from various sources. All units
may serve as aquifers depending on locality
Age
Formation
Seelandian
Lellinge Grønsand
(Lellinge Greensand)
Facies1)
Glauconitic, marly
carbonate sands and
ooze with clay seams
and clay layers
Primarily glauconitic
and quartzitic
grainstone
82 wt.%
Rock classification*
(after Dunham, 1962)
Danian
N.N.
Danian
Danian
København Kalk Bryozoan
(Copenhagen
Limestone*
Limestone)
Carbonate ooze Carbonate sands Bryozoan
and silt (~70 %
Mounds
in sand fraction) (typically 20-45
% Bryozoa)
Mudstone
Wackestone to
Wackestone to
(chalk/micrite) packstone w.
grainstonew.
w. chert
chert
chert
98-99 wt.%
95 wt.%
Carbonate content2)
Non-carbonate
Chert content of bulk rock
10-20 vol.%
Silica2); wt%
10,5
Clay minerals3); wt%
2,5
Pyrite; wt%
0,002 (0,0075)
Selected elements2); mean values (selected peak values in brackets)
Calcium; wt%
32,8
Silicon; wt%
4,9
Aluminum; wt%
0,5
Magnesium; wt%
0,6
Strontium; wt%
0,1
Iron; wt%
0,8
Sulphur; wt%
Phosphorus; wt%
0,3
Fluorid; wt%
Trace elements 2); mean values (selected peak values in brackets)
Zink; ppm
25
Nickel; ppm
5-10
Copper; ppm
5
Cadmium; ppm
Lead; ppm
Arsenic; ppm
1)
Maastrichtian
"Skrivekridt" (”White
Chalk”)
Carbonate ooze w. rare
marly layers
Mudstone
(chalk/micrite) w. chert
10-20 vol.%
0,8
0,15
20-30 vol.%
0,5-1
0,5
0,05-0,1
98 wt.%
0,5-10 wt.%
5-10 vol. %
0,5
smectite and illite; 0,25
0,05
39,0
0,37
0,03
0,3
0,07
0,04-0,11
0,035
0,1 (0,5)
0,02-0,06
38,0
0,60
0,10
0,50
0,07
0,07
0,025-0,04
0,03 (1,2)
0,04 (0,6)
39,0
0,30
0,05 (1,5)
0,1-0,2
0,09
0,06
0,02-0,03
0,04
0,05 - 0,06
7-13
5-10
3-10
0,8
5 (124)
0,4
35 (55)
6-10 (300)
4
1
2
0,5-1
20 (60)
2-5
2-3 (6)
1
1-2
0,2-0,5
Facies from Thomsen (1995) and Surlyk (1997), 2) Outside chert layers (data from Knudsen and Nygaard 1996),
Informal name
3)
Calculated from content of Al assuming a smectite-like mineralogy. * =
the saltwater/freshwater boundary in different parts of the
region, which is funded by Copenhagen Energy and the
present day sea level during the last glaciation) and the high
hydrostatic pressures below the ice sheets (e.g. Boulton et
al., 1996; Edmunds 2001). The present freshwater
circulation is still to a great extent controlled by fracture
and fault systems developed during the glaciations partly
for
meltwater
drainage.
Hence
the
present
saltwater/freshwater transition zone is developed as a
combined effect of diffusion and advection partly under
high hydrostatic pressures during the Pleistocene
glaciations (Boulton et al., 1996).
In some places, however, the diffusion profiles are
interrupted or non-existing due to impermeable layers, and
in these areas more abrupt boundaries develop.
Furthermore, if the impermeable layers are cut by faults or
buried valleys, saline waters may migrate upwards to
abstraction wells above. Basically the type of boundary is
controlled by the local and regional geology and hence may
be developed differently even within short distances as a
result of the geological heterogeneity.
The salt-/freshwater boundary in the investigated inland
areas is typically encountered at a depth of between 50 to
100 meters below the top of the limestone aquifer or
roughly 70 to 150 meters below the ground surface (as
illustrated in Figure 1.9). In near coastal parts of the region,
the boundary is generally located at shallower depth, while
in other areas it may be located deeper depending on
geology. A project investigating the nature and location of
Figure 1.9 Hydrogeophysical logs and selected tracer
concentrations for 7 depths sampled in well 207.3633,
Torslunde
7
Søndersø and Alnarp valleys towards the north in
Copenhagen and Frederiksborg County (Figure 1.2). In
other places these sand deposits directly overlie and are in
direct hydraulic contact with the limestone aquifer and are
in such places part of the aquifer.
The thickness of the confining Quaternary beds varies
between typically 20 and 50 meters. In the Pre-Quaternary
valleys it may reach thicknesses of 50 to 100 meters.
Roughly speaking tills constitute 50 to 75% of the
Quaternary thickness and c. 50% of the area (Figure 1.10).
The water table is in parts of the area lowered
considerably below the top of the limestone aquifer.
The hydraulic characteristics of the limestone aquifers
around Copenhagen are summarised in Table 2. Ranges
given in the table represent typical ranges. They may in
places vary beyond the limits given. Water wells are
typically exploited by 15 to 50 m³ pr. hour.
Generally, 75 to 90% of the aquifer transmissivity is
typically located in fractured zones within the uppermost 5
to 20 meters of the aquifer (e.g. Figure 1.9). The data in
Table 1.2 also indicate that flow in the matrix is almost
negligible compared to flow in the fractures. The bulk
hydraulic conductivity is several orders of magnitude
greater than in the matrix. Hence, the yield of water supply
wells may vary considerably, reflecting to what degree
glacial stress or tectonic movements have fractured the
limestones (Figure 1.6 and Figure 1.7).
A typical flow log documenting that flow takes place
within a restricted zone near the top of the aquifer is shown
in Figure 1.9. However, exceptions do exist, where the
main inflow to the well takes place well below the top of
the aquifer. Fractures related to fault activity may extend to
greater depths.
The nature and origin of fractures in Danish onshore
limestone aquifers has been the subject of several
investigations during the last years. Based on detailed
studies, it has been possible to relate fracture generation in
the limestones to the various tectonic stresses the aquifer
rocks has been influenced by. Results are summarised in
Figure 1.10 Quaternary geology map of north-east
Sjælland and the location of wells investigated in
Baseline. The dominating dark (brown) areas are clayey
tills confining the limestone aquifers in most of the area.
Wells which are discussed specifically in the text are
highlighted in blue (dark grey) colour
counties of Greater Copenhagen, is currently conducted in
cooperation between the Geological Survey of Denmark
and Greenland and the Technical University of Denmark
(GEUS 2003a).
1.3.3
Hydrogeology
Confining aquitards and covering layers. Quaternary tills
confine the limestone aquifer in most places (Figure 1.10).
There do exist, though, minor areas, where the limestone
aquifer is unconfined and therefore more vulnerable to
pollution from the surface. Such areas are found locally
where limestone rocks outcrop, or are covered by meltwater
sands or gravel or along some streams where the glacial
tills have been removed by erosion.
In the southwestern and central part of Roskilde County,
where the Neogene and Quaternary erosion is least, the
limestone aquifer is confined by younger Palaeocene
deposits consisting of the marly, glauconitic Lellinge
Greensand Formation (Table 1.1). The boundary between
the Danian limestones and Selandian Greensand is marked
by a hiatus. The Greensand serves as an independent
aquifer further towards the west.
Relatively coarse grained glaciofluvial deposits in the
covering Quaternary sequence, may in places constitute
important local aquifers, especially in the Pre-Quaternary
Table 1.2
Unit
Danian
Limestones
Table 1.3
Timing of faulting and fracturing with
importance for limestone bulk permeability (compiled
from Jakobsen and Klitten, 1999; Jakobsen &
Rosenbom, 2002; Japsen et al., 2002 and this study)
Quaternary
Miocene and
Pliocene
Summary of aquifer hydraulic parameters
TransMatrix
Porosit Storage
missivity; hydraulic
coefficient
m²/sec.
conductivit y %
y m/sec.
(0,5–
–3
Maastrichtian 20)⋅10
Chalk
~5⋅10–8
~2 ⋅ 10
–7
25-40
3⋅10–4 to
c. c. 0,03
35-50
3⋅10–4 to
c. 0,01
Late Palaeocene
8
Glacial processes:
•
Crushing of uppermost 0 to 3 meters
of the aquifer limestones.
•
Horizontal fracturing within the
uppermost c. 10 meters due to
pressure release caused by melting of
ice cap.
•
Subvertical faulting and fracturing 0–
150 m ?*
Tectonic deformation:
•
Faulting due to basin inversion and
movements along the Fennoscandian
Border Zone.
•
Fault movements, basin inversion and
erosion of c. 500 m tertiary sediments
causes vertical fracturing and horizontal
fracturing due to pressure release.
Vertical fracturing due to dextral fault
movements along the Fennoscandian Border
Zone
full use of the available climate and geology data, together
with groundwater-level and river discharge observations.
The construction of the dynamic 3D integrated
groundwater / surface water model was completed in 2003.
The “DK-model” which in total covers 43,000 km2 is build
of 11 regional hydrological submodels (Figure 1.11). The
hydrological submodels, which are based on a 1 km2
computational grid, are composed of:
• a relatively simple root zone component for estimating
the net precipitation;
• a comprehensive three-dimensional groundwater
component for estimating hydraulic heads and recharge
to different geological layers;
• a river component for routing of flows in streams and
calculating the exchange of water between aquifers and
rivers.
The model was constructed on the basis of the MIKE
SHE code (Abbott et al., 1986; Refsgaard and Knudsen
1996; DHI, 2002; Henriksen and Sonnenborg,, 2003) and
by utilising comprehensive national databases on geology,
soil, topography, river systems, climate and hydrology.
Climate and exploitable freshwater resources. The
exploitable groundwater resource in Denmark is
continuously decreasing due to pollution by e.g. pesticides
and chlorinated solvents, up-coning of saline waters and
increasing trace metal mobilisation e.g. due to overexploitation and lowering of the water table. When taking
this into account a new assessment by the National Water
Resources Model estimates that the sustainable exploitable
freshwater resource in Denmark is in the order of just 1 Mia
m3 per year or 200 m3 per capita per year, which is c. 60%
of the earlier estimate and similar to what is currently
abstracted when permissions for irrigation are fully utilised.
For comparison, the total renewable freshwater
resources in French Guiana and Kuwait, the nations with
the highest and lowest freshwater resources per capita in the
world, are ca. 800000 and 10 m3/capita/year (UNESCO,
2003).
The new estimate for Denmark is a serious reduction,
and it put Denmark together with e.g. Yemen and Bahrain
as amongst the 10% of countries with the smallest available
resource per capita per year (UNESCO, 2003) if only the
estimated sustained resource is abstracted. The new lower
estimate is an effect of a more detailed approach and a more
thorough evaluation of the sustainability, which includes
consideration of climate variation, pollution and surface
water ecology. In the new assessment the sustained
exploitable groundwater resource is estimated to be only
6% of the actual recharge when the ecology of surface
waters in rivers and wetlands and the pollution of
groundwater are taken into account. This emphasises the
strong need for careful management and regulations of
water abstraction and land use.
The resource is considered severely overexploited,
primarily by abstraction for drinking water supply,
especially around large urban areas like Copenhagen
(Figure 1.3). The situation in these areas is governed by the
climate conditions in the winter period (precipitation from
October to March), which is most significant for the
groundwater recharge and the prime control on the summer
(low) flow conditions. Significant lowering of the water
table in these areas locally result in severe deterioration of
the groundwater quality due to for instance acceleration of
adverse natural geochemical processes in the unsaturated
Table 1.3.
Generally, the maximum depth of freshwater circulation
is located at depths of 70 to 150 m below ground surface.
Apparently most fractures are sealed where overburden
reaches such thicknesses. Exceptions are where faulting has
taken place. In accordance with this observation, it has
further been found that the limestone aquifer often
experiences pressure dependent hydraulic properties,
resulting in declining yield as a function of pressure draw
down.
During the past decade, several water works have tried
to locate deeper unpolluted water resources in order to
replace the shallow contaminated wells. Deeper wells
however face the threat of up-coning of saline waters as
demonstrated in a later section, where the monitoring of a
water supply well of the Carlsberg breweries illustrate this
phenomenon.
1.3.4
The water balance and freshwater resources
The National Water Resource Model. Denmark is located in
a humid Temperate Zone. The annual precipitation is in the
order of 700–1000 mm and the potential evaporation is
around 600 m. Theoretically this should leave plenty of
water to exploit, however, increasing pressure on the water
resource decreases and deteriorate the water quantity and
quality considerably.
In acknowledgement of this fact, the Geological Survey
of Denmark and Greenland (GEUS) initiated the
development of a National Hydrological Model (“The DKmodel”) in 1996 as a water resource management and
assessment tool. The overall goal was to conduct a more
accurate assessment of the exploitable resource by making
Figure 1.11 Net precipitation in Denmark and the
regional subdivision of the National Water Resources
Model (Henriksen and Stockmarr, 2000; Henriksen and
Sonnenborg, 2003)
9
bicarbonate and calcium contents and hence the hardness of
the groundwaters until equilibrium is reached.
The rainfall weighted rainfall chemistry, based on data
from Goffeng (1973) is compared to selected typical
groundwater compositions in Table 1.4. In Table 1.4, the
rainfall has been evaporated by a factor 3.5 (the ratio
between precipitation and net precipitation in the area) by
PhreeqC. Note that the pH decreases in the evaporated
rainwater as expected as the proton concentration increases,
and that ammonium is unstable and therefore completely
oxidised to nitrate.
zone e.g. nickel mobilisation by oxidation of iron sulphides.
The oxidation of iron sulphides has been demonstrated to
deteriorate the water quality in both limestone and sand
aquifers due to nickel mobilisation (Jensen et al., 2003;
Larsen and Postma, 1997) and due to increasing the
hardness (Hinsby et al., 2003 a, b).
Four different quantitative indicators are used in the
general assessment of the sustained exploitable water
resource in Denmark. The indicators are based on the
comparison of groundwater recharge to the deep aquifer
system, as well as the mean annual and minimum
(baseflow) runoff in situations with and without abstraction.
The results from the 11 areas show that these indicators are
highly dependent on the regional geology and
hydrogeology. In some of the areas in the eastern part of the
country recharge to the deeper aquifers is reduced due to
numerous clay layers. Climate change will influence the
deep water cycle with considerable delay in these areas. For
other areas, like the strongly overexploited area in
Northeast Sjælland around Copenhagen, and the western
part of Jutland, indicators based on recharge and flow, are
strongly dependent on the present climate and they will
respond quite fast to climate change.
1.3.5
1.4
DATA FOR GROUNDWATER QUALITY OF
LIMESTONE AQUIFERS IN THE COPENHAGEN
AREA
1.4.1
Typical chemical compositions of groundwater from
limestone aquifers collected from selected archives and in
the Baseline project are compared in Table 1.4.
The sample “Carlsberg 1897” is the first analysis of a
series of analyses from the well DGU nr. 200.27 g. at the
Carlsberg Breweries in Copenhagen collected in 1897 soon
after the well was established. “Carlsberg 1943” is an
analysis from the time series 1897–1944 with the highest
salinity. Well 200.27 g was an old abstraction well used in
production by the Carlsberg Breweries. The well was taken
out of beer production just before World War II because of
a sudden increase in salinity possibly due to saltwater
migrating upwards through the Carlsberg Fault (Figure
1.12). The well is located c. 200 m west of “the Carlsberg
Fault” (Figure 1.3), which is a fault of very high
transmissivity (Markussen, 2002) and salinity that may
originate from deeper formations (Andersen and Ødum
1930, Rosenkrantz 1964). The Carlsberg Fault can be
followed for about 30 km, but it was described for the first
time in 1925 at the Carlsberg Breweries (Rosenkrantz 1925,
Blem 2002). Figure 1.12 show a time series of major ions in
the groundwater pumped from the well for the period 1897–
1944. The first sample, Carlsberg 1897, is considered to
exhibit a typical baseline groundwater quality for the area,
while the following samples, although completely natural,
Background geochemistry
Geochemistry of the investigated limestones. Selected
geochemical data are listed in Table 1.1. The table shows
that the limestone rocks have a very high content of CaCO3
and only small amounts of other minerals (sulphides, clay
minerals etc.) although chert layers are quite abundant. The
Danian and Maastrichtian limestones are geochemically
quite similar.
1.3.6
Historical and recent data on water quality
Rainfall chemistry
The rainfall chemistry generally constitutes the major part
of the dissolved components in shallow groundwater in
uncontaminated areas. Evaluation of the relation between
the rainfall chemistry and groundwater chemistry in an area
provide valuable information about additional natural and
anthropogenic sources for the dissolved elements and
compounds in groundwater and surface water. In many
uncontaminated areas dissolution of sea salt aerosols in
rainwater is practically the only source for chloride in the
shallow aquifer systems. Some of the other elements,
however, have significant anthropogenic and/or natural
additional sources of general importance for the evolution
of the groundwater chemistry and quality. Sulphur, which
to some extent originates from global atmospheric
pollution, has a considerable dry deposition that contributes
significantly to the dissolved ions and in this case
acidification of the infiltrating rainwater and subsequently
the shallow groundwater (e.g. Paces, 1985). This is
however not the case in carbonate areas like the aquifers
investigated in this study. In such systems the carbonate
containing aquifer rocks buffer the infiltrating groundwater
to neutral pH values.
The effect of natural subsurface water/rock interaction
processes, as for instance carbonate dissolution may be
evaluated by the use of geochemical modelling tools such
as PhreeqC (Parkhurst and Appelo 1999, BASELINE,
2003). The dissolution of carbonates in the limestone
aquifers investigated in this study increases primarily the
Figure 1.12 Time series of major ions in an old water
supply well of the Carlsberg Breweries in Copenhagen,
Denmark. The sudden increase in e.g. chloride,
demonstrates the up-coning of saline waters through the
Carlsberg Fault shown in Figure 1.6
10
Table 1.4
Comparison of hydrochemical composition of groundwater from selected wells in Denmark and the UK
Recharge* Copenhagen
1897
Hårlev Carlsberg
Stevns
200.27g
Depth
pH
TDS
DO
Ca
Mg
Na
K
Cl
SO4
HCO3
NO3
NH4
Si
TOC
Fe
Mn
F
Br
Sr
As
Ni
Al
Copenhagen Værløse
1943
1943
Carlsberg Søndersø
200.27g
water
works
3.8
4
0.9
7
0.9
12
9
7.22
418
109
13
26
209
33
109
103
16
22
31
28
376
283
158
404
48
31
335
12
8
7.5
2.7**
12
6
Værløse
2000
Søndersø
well 200.263
Værløse
2000
Søndersø
well '200.3749
Copenhagen London***
”Baseline”
Median of 29 Brigthwalton
investigated unconfined
wells
London*** London***
Shalford
farm
confined
Mortimer
confined
70 m
7.04
120 m
7.09
7.11
7.05
7.23
7.49
0.01
104
12
25
3
48
39
299
<1
0.221
15
2.6
2.3
0.089
0.3
0.12
1.1
<1
1.5
3
0
112
17
58
5.6
100
45
332
<1
0.253
14
2.7
2.2
0.096
0.44
0.35
2.5
<1
0.6
4
0,01
114
19
20
4
47
82
336
<0.1
0.1
13
1.9
0.66
0.02
0.43
0.06
0.91
<1
4.8
2.5
10.3
124
1.6
5.6
0.6
11
2.5
348
22
5.8
0.3
87
15
13
3.4
16
13.5
323
< 0.1
0.01
9.7
<0.1
52
10
80
5.1
72
34
268
<2
0.64
9.8
<0.0003
< 0.00003
0.11
<0.6
0.25
0.18
0.005
1.40
0.07
4.5
0.072
0.003
1.25
0.28
2.1
*Rainfall chemistry evaporated 3.5 times, based on precipitation data from Goffeng (1973) ** Calculated from COD (KmnO4) assuming reduction only by
organic arbon *** data from Edmunds et al. (1992). All Danish samples are from confined anaerobic aquifers. Samples in data column 2 and 3 are from an
old abstraction well at the Carlsberg Breweries in central Copenhagen. The samples in column 4,5 and 6 are from one of the major abstraction sites of
Copenhagen Energy. The sample in column 7 is the median value of the 29 wells investigated in the ”BASELINE” project. The UK sample in column 8 is
from an unconfined aerobic aquifer, the samples in column 9 and 10 are from confined and anaerobic aquifers. All concentrations are in mg/l except As, Ni
and Al, which are in µg/l.
level or 80 m below the surface (Klitten, 1993). The same
picture was observed in the three northernmost wells
investigated in this study in the Æbelholt abstraction site
(Figure 1.10), except that the Chalk and saltwater boundary
is located approximately 23 m (well 187.1376) and 12 m
(wells 187.1354 and 193.1963) deeper than at well
200.3749 at Søndersø. At the Æbeltholt abstraction site the
salinity variations between different wells may however
also be partly controlled by a fault running through the area
(Figure 1.5, Hinsby et al., 2002), similar to what was
described above for the wells of the Carlsberg Breweries,
and faults may also influence saltwater migration at other
abstraction sites of Copenhagen Energy. For example,
evaluation of recently processed seismic and TEM
(Transient Electro-magnetic Method) data at another
abstraction site of Copenhagen Energy (Havelse
Kildeplads), also indicate that salt water problems in one
abstraction well are related to a fault at the site (pers.comm.
K.R. Hansen and L.Bennedsen, Copenhagen Energy).
However, the influence of faults on the migration of
saltwater in the Chalk in Denmark is generally not well
documented and needs a more detailed investigation e.g. in
order to estimate the permeability of the faults and quantify
the effects of the migrating saltwater. Faults, such as the
glaciotectonic faults illustrated on Figure 1.7, may have a
significant effect on the migration of saltwater but this has
not been investigated so far.
are increasingly affected by human activity (abstraction).
The chloride contents in well 200.27 g reach levels just
above the guideline value of 250 mg l-1, while shallower
wells situated in the Carlsberg Fault have levels of a few
thousand mg l-1 (Andersen and Ødum 1930). The saline
waters in the Carlsberg Fault have a higher salinity than the
seawater in coastal waters close by and hence it is not a
result of salt water intrusion of coastal waters (Rosenkrantz,
1964). The saline waters in the wells may originate from
residual marine waters in the Chalk, which is considered to
be the source for most of the salt water problems at Danish
abstraction sites (pers.comm. K. Klitten, GEUS), however,
a deeper origin in salt deposits was suggested in early
studies (Andersen and Ødum 1930).
The samples Søndersø 1943 and 2000 are analyses from
one of the oldest abstraction sites of Copenhagen Water
(now Copenhagen Energy), which was originally
established in the late 19th century. The sample Søndersø
1943 is from the archives of Copenhagen Energy. The
samples Søndersø 2000 (well 200.263 and 200.3749) were
sampled by GEUS in “Baseline” from an old well drilled in
1929 which is about 70 m deep (well 200.263) and a
relatively new well drilled in 1993, which is c. 120 m deep
(well 200.3749). The higher salinity observed in the new
deep well is interpreted to originate from residual saline
waters in the Chalk, which exhibit increasing salinity from
the boundary between the Danian Limestone and the
Maastrichtian Chalk at a depth of about 67 m below sea
11
Figure 1.13 Cumulative plots of major and minor elements and the Baseline trace element indicators for the wells
sampled and analysed in the project
1.4.2
New sampling programme
The median composition of the 29 wells sampled by
Copenhagen Energy and GEUS for the BASELINE project
are also listed in Table 4 for comparison with the historical
data and typical examples of groundwater composition
from British Chalk aquifers. The compositions show as
expected a large similarity, except where influenced by
saltwater mixing or redox processes. Selected elements
from the analyses of the 29 “BASELINE” groundwater
samples is furthermore presented in cumulative plots
(Figure 1.13) and a Piper diagram (Figure 1.14).
other
CFCunder
2002;
1.6
GEOCHEMICAL
CONTROLS
REGIONAL CHARACTERISTICS
AND
1.6.1
Major element controls
The natural evolution of the major elements in the
groundwaters of the limestone aquifers are controlled
primarily by rainfall chemistry, carbonate dissolution,
1.5
HYDROCHEMICAL CHARACTERISTICS
OF GROUNDWATER IN THE COPENHAGEN AREA
1.5.1
groundwater, however, under reducing conditions
tracers such as 85Kr and 3H/3He are preferred as the
gases in some cases have been observed to degrade
such conditions (Hinsby et al., 1997; Hinsby et al.,
Plummer and Busenberg, 2000).
Summary statistics
Selected statistical parameters for groundwater chemistry
measured on the samples collected for “Baseline” by
Copenhagen Energy and GEUS for major, minor and trace
elements are listed in Table 1.5 and Table 1.6, respectively.
1.5.2
Indicators of pollution
The most common pollutants and indicators of pollution
found in the investigated area are organic micro
contaminants such as pesticides, pesticide metabolites and
chlorinated solvents. Nitrate is not a significant problem in
the investigation area, in contrast to many other regions of
the country.
Indicators of young, potentially contaminated
groundwater, tools for dating of young groundwater include
a whole range of environmental tracers including for
example 3H, 85Kr, CFCs and SF 6 (e.g. Cook and Herczeg,
2000; Hinsby et al., 2001). The CFC’s are widely used in
Denmark e.g. in the Groundwater Monitoring Program as a
dating tool and indicator of young potentially contaminated
Figure 1.14 Piper diagram of the hydrochemistry of
groundwaters sampled around Copenhagen in the
Baseline project
12
(Edmunds et al., 1987; Edmunds et al., 1992; Price et al.,
1993; GEUS, 2003). The groundwater evolves quite fast by
calcite dissolution to a state of calcite saturation (in
Denmark primarily under closed conditions). Likewise, in
up-gradient parts of the aquifer, sulphides and organic
carbon reduce the oxygen and nitrate, producing anaerobic
groundwaters with only very small concentrations of O2
and NO3, and increased concentrations of sulphate and
bicarbonate. This happens generally quite fast and by far
the largest part of the abstracted groundwaters from the
limestone aquifers, which are typically abstracted at depths
between 30 and 60 m below the surface, are anaerobic.
Especially, the oxidation of sulphides in both the saturated
and unsaturated zone frequently deteriorate the water
quality significantly by increasing the hardness and more
important the concentrations of the trace elements Ni and
possibly As. Both Ni and As create problems in the
investigated area but not under the same redox conditions.
Arsenic generally, but not always, seems to reach the
highest concentrations under very reducing conditions
where sulphate has been more or less completely reduced
(e.g. Figure 1.15). This is in agreement with many other
observations globally (Smedley and Kinniburgh, 2002).
Ion exchange processes are active along the complete
flowline, and play an important role in e.g. the mobilisation
of Ni (Jensen et al., 2003; Larsen and Postma, 1997) and As
(Smedley and Kinniburgh, 2002) but the effects are
generally most clearly seen further down-gradient where
the advancing groundwater encounters more saline
groundwaters at depth or towards coastal discharge areas.
The saline waters are more or less stagnant waters found in
the matrix (connate waters) or migrating waters in fissures
and faults. In these high salinity areas, Na predominates on
the ion exchange sites and Ca will exchange with Na
resulting in an increase of Na and a decrease of the Ca
concentrations in the groundwater. In rare cases this will
change the main groundwater type from a “hard” Cabicarbonate to a “softer” Na-Ca-bicarbonate type of water
(Figure 1.14). Increased concentrations of e.g. Mg, Sr, F, B
and Br are also frequently observed in groundwaters
influenced by salt water mixing and/or by ion exchange and
related dissolution/precipitation processes.
Further along the flow direction, sulphate reduction and
subsequently methanogenesis may affect the groundwater
chemistry in limestone aquifers due to the production of
H2S and CH4 (e.g. Price et al., 1993). However, the organic
carbon content is generally low and recalcitrant in the
Danish limestones, and these processes seem to be
relatively rare in the exploited limestone aquifers in
Denmark. Furthermore, it has been suggested that the
hydrogen sulphide and methane in the limestone aquifers in
Denmark mainly originate from the confining Pleistocene
sediments, which are known locally to contain these gases
(GEUS, 2003). Finally, in some areas, these gases may
migrate upwards from deeper lying units through fractures
and faults, similar to the saline waters migrating up through
the Carlsberg Fault (Figure 1.5 and Figure 1.12).
Table 1.5
Selected statistics for major and minor
elements of the groundwater sampled around
Copenhagen in the “BASELINE” project
T
pH
Eh
DO
SEC
Ca
Mg
Na
K
Cl
SO4
HCO3
TOC
Si
Fe
Mn
Sr
NO3-N
NO2-N
NH4-N
P
F
Br
I
n
13
16
15
16
16
35
35
35
35
35
35
28
28
35
35
35
35
9
9
16
35
16
15
8
min
8.9
6.90
-110
0.00
543
55
10
10
2
12
15
133
0.7
10
0.01
0.00
0.34
0.20
0.003
0.00
0.02
0.25
0.03
0.00
max
10.3
7.21
-41
0.53
878
283
43
188
10
260
433
409
4.6
15
7.32
0.19
6.48
0.84
0.005
0.29
0.14
0.79
0.35
0.03
mean
9.7
7.09
-75
0.05
677
128
19
29
4
53
103
334
1.9
13
1.22
0.04
1.56
0.27
0.003
0.13
0.04
0.46
0.10
0.01
median
9.8
7.11
-77
0.01
633
114
19
20
4
47
82
336
2.1
13
0.66
0.02
0.91
0.20
0.003
0.10
0.02
0.43
0.06
0.01
Table 1.6
Selected statistics for selected Baseline
trace indicators of the groundwater sampled around
Copenhagen in the “BASELINE” project
Al
As
Cd
Cr
Cu
Hg
Ni
Zn
n
35
35
35
35
35
35
35
35
Min*
<1
<1
<0.05
<0.5
<0.1
<0.1
<0.2
<0.5
Max*
13
<1
<0.05
3.7
4.5
2
41
199
Mean*
2
<1
<0.05
<0.5
0.99
0.44
9
32
Median*
<1
<1
<0.05
<0.5
0.60
<0.1
2
20
* All concentrations in ug/l
redox processes (mainly sulphide and organic carbon
oxidation), ion exchange (e.g. between Ca and Na) and
mixing with saline waters.
The limestone aquifers are marine deposits that
originally contained marine connate waters and where these
waters have not been flushed completely the freshening
processes (ion exchange and mixing) as for instance
described by Appelo (1994) is still on-going and relatively
easy to recognise. Similarly, if saline waters are drawn
towards a freshwater well, salinisation will occur. The
effects of these processes on the temporal and spatial
evolution of many of the major, minor and trace elements
can be recognised relatively easily.
1.6.2
Down-gradient evolution
1.6.3
It has not been possible to investigate a set of wells along a
flowline and hence the down-gradient evolution of the
groundwater chemistry directly in this study. However, the
down-gradient evolution is known to be very similar to
what is described for British Chalk aquifer systems
Quality changes with depth and time
1.6.3.1 DEPTH PROFILES
The groundwater chemistry and age in the limestone
aquifers locally exhibit significant variations with depth,
13
Figure 1.15 Crossplots of Ni, As, HCO3 and SO4 data from monitoring wells in Danish limestone aquifers. Data from
the Danish Groundwater Monitoring Program (Hinsby and Nyegaard, 2003)
representative of the formation waters at the sampled levels,
and that the separation pumping technique may be difficult
to control in open fissured boreholes where a few fissures
produce most of the waters (Hinsby et al., 2002).
which to a large extent is controlled by a complex system of
secondary fissures and chert layers and the resulting
complex flow pattern (e.g. Jakobsen, 1991). The evolution
of the groundwater quality e.g. the freshening of the former
marine connate waters in the limestone aquifers and the
development of the fissure and fracture system is closely
linked to increased hydraulic gradients and pressures
enforced on the aquifer systems during the Quaternary
glaciations (Boulton et al., 1996), and the sea-level changes
and general evolution of the aquifer systems during the
Pleistocene (Edmunds and Milne, 2001). Flushing and
fracturing of the limestone aquifer system by glacial melt
waters and advancing ice sheets, respectively, has been of
cardinal importance for the evolution of the present day
groundwater quality.
In order to investigate and illustrate the significance of
fissure and fracture systems hydrogeophysical logging and
depth specific sampling were performed in a 120 m deep
well with increasing salinity towards the bottom of the well
(Figure 1.9). The well was sampled at seven different levels
for inorganic hydrochemistry and selected isotope and
environmental tracers for evaluation of groundwater ages,
salinity and human impacts (Hinsby et al., 2001). The
samples were collected by the separation pumping
technique developed at GEUS (Nilsson et al., 1995), a
method that performed very well and was recommended for
sampling of open wells in a comparative study of methods
for depth specific groundwater sampling (Lerner and
Teutsch, 1995). Selected logs and analytical results for
chloride and environmental tracers from the investigations
in well 207.3633 (Figure 1.10) are shown in Figure 1.9.
The results show that all the sampled groundwaters are
affected by human impacts and that there is no general
decreasing trend in tracer or increase in chloride
concentrations (except for the lowermost sample) towards
the deeper parts of the aquifer. This result is ambiguous
and, together with a combined evaluation of the electrical
conductivity log and the measured chloride concentrations,
it indicates that the collected samples may not be
1.6.3.2 GROUNDWATER AGES
Absolute groundwater ages are very difficult to estimate in
dual porosity aquifers (e.g. Hinsby et al., 2001; Hinsby
et al., 2002) as investigated in this study. However, the
environmental tracers provide important information on
relative ages and human impacts when the evolution in the
groundwater quality is assessed. In the present study, the
radioactive isotopes and environmental tracers tritium (3H)
and 14C were analysed in all the investigated wells in order
to evaluate if the wells showed a human impact and
therefore potentially could be contaminated or if some
wells would have 3H below the detection limit and
relatively low 14C values indicating pre-industrial waters.
All analysed wells, however, showed post bomb values of
either 14C or 3H, and hence could all potentially be
contaminated. That is, none of the sampled wells has a
baseline water quality sensu strictu. Besides these
radioactive tracers and dating tools also the CFC-gases
were applied as environmental tracers (e.g. Plummer and
Busenberg 2000; Hinsby et al., 2002) and measured in a
few wells. The measured values of 3H, 14C and CFC–12 are
shown for the seven levels sampled in well 207.3633 in
Figure 1.9.
1.6.3.3 TRENDS IN WATER QUALITY PARAMETERS
The evolution of the groundwater chemistry over time is a
valuable tool for evaluating baseline groundwater
chemistry, changes in the geochemical environment, and
on-going processes in the aquifers. Long-term time series
from single wells are however difficult to find. Examples
do exist though as the Carlsberg time series illustrated in
Figure 1.12.
Figure 1.16 illustrates another example of a salinity
increase in groundwater at one of the abstraction sites of
14
contaminants (e.g. pesticides and chlorinated solvents) are
frequently found in the investigated aquifers. No organic
contaminants, however, have been found in the wells
investigated in “Baseline”. Nitrate, which occurs naturally,
but which is also the most common contaminant in Danish
groundwaters, is measured in only small concentrations in
the investigated wells (<1 mg l-1 NO3–N). In some wells
with elevated sulphate concentrations this is probably a
result of nitrate reduction by iron sulphides in and above
the aquifers. Chloride concentrations exceeding the
guideline are common and of concern in coastal areas, in
deeper parts of the limestone aquifers as well as in areas
where the saltwater from road salting may infiltrate to the
abstraction sites.
The selected Baseline trace element indicators (Al, As,
Cd, Cr, Cu, Hg, Ni and Zn) are generally below guideline
values, however, Ni and Zn exceed the guideline value in
around 15% and 10% of the analysed wells, respectively.
This is similar to the number of Ni and Zn analyses above
the guideline value in the National Groundwater
Monitoring Program. Arsenic and Aluminium, which are
the trace elements most frequently found above the
guideline value in the National Groundwater Monitoring
Program, do not exceed the guideline value in any of the
wells sampled in this study. Aluminium is generally not a
problem at the neutral pH values found in limestone
aquifers, and As is normally only a problem in very reduced
environments, which generally are not found in the Danish
limestone aquifers.
The observed high Ni contents are partly correlated to
high sulphate contents indicating that it is released by the
oxidation of iron sulphides in the limestone aquifer. These
processes are natural but catalysed directly by overabstraction. Ni concentrations above guideline values are
mainly located to areas in the south of Copenhagen, where
the groundwater table due to over-abstraction has been
lowered to below the base of the confining clay tills (Figure
1.17). This creates an unsaturated zone subject to advective
air (oxygen) transport in and out of abstraction wells during
increasing and decreasing atmospheric pressures,
Figure 1.16 Chloride time series from a Copenhagen
Energy abstraction site (Kilde XIII) in Copenhagen.
The increase in chloride is interpreted to be a result of
road salting (Pers.comm. Lars Bennedsen, Copenhagen
Energy)
Copenhagen Energy (Kilde 13) about 5 km north-west of
the Carlsberg Breweries. At this site, however, the increase
in salinity is interpreted, based on hydro-geophysical
logging and hydrochemistry, to be an effect of road salting
during the winter (pers.comm. Lars Bennedsen,
Copenhagen Energy).
In general, nitrate are chloride are the elements which
most commonly show concentration trends in Denmark.
However, trends in SO 4, Ni, Ca (hardness) are also
frequently observed. Chloride has both natural and
anthropogenic sources while nitrate generally is
anthropogenic. The three last mentioned solutes generally
have a natural source, but the trends are generally induced
by groundwater abstraction and the resulting lowering of
the water table.
1.7
SUMMARY OF THE BASELINE QUALITY
The Upper Cretaceous (Maastrichtian) and Lower Tertiary
(Danian) limestone aquifers around Copenhagen generally
contain modern groundwater with a human impact.
Groundwater chemistry and environmental tracer and
dating tools demonstrate this for all 29 wells investigated in
Baseline. Industry, urban areas, agriculture, saline waters
and exploitation itself pose a threat to these important
aquifer systems that supply most of the greater Copenhagen
area with freshwater. Anthropogenic effects are found in
nearly all parts of the groundwater bodies and definition of
the baseline groundwater composition is therefore difficult.
Baseline groundwater chemistry, however, can be
estimated from historical data in groundwater
hydrochemistry archives, by geochemical modelling – or by
assuming baseline groundwater chemistry when one or
more of the environmental tracers for young groundwater
(e.g. 3H, 85Kr, CFC-12, SF 6) are below detection limit. All
of the wells sampled and analysed in this study showed a
human impact as demonstrated for example by the presence
of post bomb values of the radioactive isotopes 3H and 14C
or measurable concentrations of the environmental gas
tracer CFC-12. Concentrations above guideline values of
major “natural” ions (e.g. Cl, NO3, SO4 and Ca), minor ions
(e.g. F), trace elements (e.g. As, Ni), and organic micro
Figure 1.17 Nickel concentrations in groundwater
monitoring wells of the National Groundwater
Monitoring Program. The EU guideline value of
20 µg l-1 is exceeded in many wells in the area south of
Copenhagen (red / dark grey dots)
15
1.8
respectively (“barometric pumping”). This process assures
continuous oxygen supply for the oxidation of sulphides in
the unsaturated zone, which under such conditions forms
the most important mechanism for oxidation of Ni-bearing
iron sulphides. Significant additional oxidation of sulphides
may take place by oxygen and especially nitrate in the
saturated zone, and this process is probably responsible for
the fact that all wells analysed in the project contain no
nitrate (<1 mg l–1). The processes responsible for
mobilisation of As and Zn in the Danish aquifers are not yet
investigated or understood. Both elements have possible
natural and anthropogenic sources and their origin in the
aquifer systems may therefore vary, however, the sources
and processes responsible for the observed concentrations
need to be further investigated.
It is concluded that the groundwater bodies around
Copenhagen generally contain modern, potentially
contaminated groundwater. The impact is locally so strong
that it is necessary to abandon wells or well sites. More
than 10%, or around 100 wells, of the Copenhagen Energy
water supply wells, have been abandoned due to
contamination mainly by pesticides and chlorinated
solvents during the past decade. However, trace elements
constitute a significant and possibly increasing problem of
potentially the same magnitude as the organic
contaminants. The number of wells where guideline values
have been exceeded seem to be comparable for organic
contaminants and trace elements according to the National
Groundwater Monitoring Program. Contaminants, trace
elements and chloride from above, inside and below the
main aquifer are all serious threats to the high quality fresh
groundwaters. That is, the groundwater quality in the
limestone aquifers around Copenhagen is virtually under
attack from all sides as well as from inside, and this is one
of the main reasons that these aquifer systems are
considered, in a new assessment, to be heavily overexploited. If remedial measures are not taken, the high
quality groundwater resource will continue to deteriorate
and decrease. Restoration of heavily deteriorated
groundwaters to baseline groundwater quality is considered
to be a long-term process probably in the order of decades
or maybe even hundreds of years for dual porosity systems
such as the limestone aquifers around Copenhagen. The
natural biogeochemical and physical environments define
and control the subsurface potential for contaminant
degradation, and different contaminants degrade in different
environments. Hence, a sound understanding of the baseline
groundwater chemistry and the on-going natural
biogeochemical processes in aquifers are crucial for taking
the correct remedial measures and for assuring good
groundwater quality in the future. The major concerns for
the water supply of Denmark primarily lie with the
deterioration of the groundwater quality due to
contamination, and the adverse effects of over-abstraction
on water quality and surface water ecosystems.
Monitoring
networks,
groundwater
dating,
hydrogeological and geochemical modelling are important
tools for evaluation of the baseline groundwater quality and
groundwater quality trends.
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