Tracing ground water flow patterns into Heiðmörk
Vaiva Čypaitė, University of Iceland
Árný Sveinbjörnsdóttir, University of Iceland
Magnús Tumi Guðmundsson, University of Iceland
Bjarni Reyr Kristjánsson, Reykjavík Energy
Introduction
The project aims to trace ground water movement in the volcanic area
east of Reykjavík by analysing naturally occurring compounds and
isotope ratios in the water.
In the summer of 2014 it is planned to sample water from 24 different
locations distributed over the whole study area. In five selected
locations water samples are being collected every three weeks for one
year in order to establish the seasonal fluctuations in water composition.
Map of the study area showing water sample and monitoring locations
is shown on Figure 1.
The selected locations for monitoring are: Jaðar (Well V-1), Myllulækur
(Well V-13), Vatnsendakrikar (Well VK-5), Engidalskvísl (Well HU-01)
and Kaldárbotnar (Well L-1).
Sample analysis
The samples will be measured for the stable isotope ratios of oxygen
and hydrogen (18O and 2H), C13 and the anions Cl-, SO2-2 and F-.
Deuterium (2H) and 18O ratios can be used as natural tracers to
track ground water movement. The concentration of deuterium in local
precipitation varies depending on altitude and distance from a coastline
(Figure 2). Therefore by analysis of deuterium and 18O in groundwater
can be ascertained whether groundwater is of local or distant origin. In
general, deuterium and 18O content in precipitation is the highest by
the coast, varying from -50‰ to -64‰, and decreases inland with the
minimum concentration in the Northern part of Vatnajökull (-106‰)
(Arnason, 1976).
Carbon isotope C13
In Iceland there are three
main sources of ground water
carbon: atmospheric CO2,
CO2 from soil and CO2
dissolved from the medium
through which the water
seeps
Figure 1. Map of the study area showing water sample locations.
Figure 2. Deuterium concentration
precipitation in Iceland (Árnason, 1976)
in
Chloride can also be used as a naturally occurring tracer. It can
dissolve from bedrock through which water has percolated, or can occur
in groundwater as a marine component when blowing wind carries sea
spray and salt particles which, with precipitation, fall on the land and
infiltrate into the ground water. As a result, chloride content is greatest
close to the coast and significantly decreases inland (Sigurdsson,
1998).
Gvendarbrunnar and Jaðar areas are located at 80-90m a.s.l., one
kilometre eastwards from lake Elliðavatn. Ground water in this area
occurs just a few meters below surface, thus the aquifer can be easily
contaminated. To counter such a risk, the whole catchment area is
under strict protection (Figure 4).
Geological and hydrological features
Heiðmörk is a nature reserve eastwards of Reykjavik, which is located
on the flank of the active volcanic zone. Heiðmörk is mostly used for
outdoor recreation but it also contains the largest drinking water well
fields in Iceland in order to supply fresh drinking water for Reykjavik city.
Ground water is produced from few well fields, such as
Gvendarbrunnar, Jaðar, Myllulækur and Vatnsendakrikar (Figure 3).
Figure 5. The main ground water production areas for the capital (shown with red
circles). The areas are from north to south: Gvendarbrunnar-Jaðar, Myllulækur,
Vatnsendakrika and Kaldárbotnar. The arrows show the ground water flow as
calculated by a ground water model (Vatnaskil 2012)
Figure 4. The protection of water supply area.
According to previous studies, the ground water in Jaðar originates in
the southeast in the mountains of Bláfjöll, where annual precipitation
varies between 2000 and 3000mm/a (Kristján Sæmundsson, Freysteinn
Sigurðsson, 1995). Water in Jaðar is pumped from shallow boreholes
and also from an open fissure in which an 8m deep pump is installed
(Well V-1).
Myllulækur spring area is located southeast of lake Ellidavatn, just few
kilometers from Jaðar. Wells in this area are much deeper and produce
water from fractured interglacial lava (Kristjan Sæmundsson, Kristan H.
Sigurðson, Hrefna Kristmannsdottir, 1992).
Figure 3. The production wells of Reykjavík Energy in Heiðmörk are located in three
areas. Gvendarbrunnar-Jaðar, Myllulækur and Vatnsendakrikar. The arrows show the
ground water flow as calculated by a ground water model (Vatnaskil 2012)
Heiðmörk area – Gvendarbrunnar, Jaðar and Myllulækur springs
The bedrock of Heiðmörk consists of interglacial and postglacial lava
flows. The former are Eamian age lavas and are characterized as ‘fresh
olivine rich basalts consisting of thin flow units with scoriaceous
boundaries in between’ (Kristjan Sæmundsson, Freysteinn Sigurðsson,
1990, pg.. 1). Postglacial lavas cover the southeast part of Heiðmörk
and it assumed that youngest lavas erupted 1000 years ago. This strata
is covered by a thin layer of moss and soil (Kristjan Sæmundsson,
Freysteinn Sigurðsson, 1991). In general, the area is very permeable,
so most precipitation percolates to the bedrock and surface runoff
practically does not occur. Permeability is enhanced by the numerous
faults which transverse Heiðmörk to the northeast and southwest;
fissures with openings up to two metres are visible (Kristjan
Sæmundsson, Freysteinn Sigurðsson, 1991).
Hellisheiði
Water samples were taken in the vicinity of Hellisheiði geothermal
power plant located on the active volcanic ridge (Figure 3). At least
three volcanic eruptions occurred in the last 11 000 years. The surface
of the area consists of volcanic rocks. Subglacially formed hyaloclastites
and pillow lavas are predominant in the area and are found up to 1000m
b.s.l. (Mesfin, 2010). Ground water was collected about 1km west of the
power plant from a depth of 70m. The ground water from this well is
heated at Hellisheiði power plant and used in the capital area for district
heating.
Figure 4. Panoramic view from Engidalskvísl to Hellisheiði and Bláfjöll areas.
Kaldárbotnar, Hafnafjörður
Kaldárbotnar, located southeast of Reykjavik, provides the public
freshwater supply to the municipality of Hafnarfjörður. The geology of
the Kaldárbotnar region contains two main types of formations: pillow
basalts and hyaloclastites. The older bedrock which underlies these
formations is highly fractured. Due to the glacial history of the region
and the environment, there has been very little time for soil
development. The limited soil in the area means that contaminates can
percolate into the ground water system relatively quickly (Jakobsson,
2008).
This zone has young lavas and is heavily fractured and faulted, with
the overall trend of faults and fractures running parallel to the Western
volcanic zone with a NE-SW orientation. The fractures in the area also
tend to disappear under the young lavas into the underlying
formations. Water can enter the system through faulting and fissures
guiding the water towards the Kaldárbotnar area (Schopka, 2006).
The water table at Kaldárbotnar is 80 m above sea level. In general,
ground water flow is determined by relief, fractures and permeability of
the area. Kaldarbotnar is located by the side of the hill and southwards
relief increases by 150m. As long as water follows the laws of gravity,
ground water flows from higher towards lower relief. Therefore ground
water in the area moves towards the sea, where discharge occurs
(Figure 5).
Referances
Arnason, B. (1976). Ground water systems in Iceland traced by deuterium. Reykjavik.
Arny Erla Sveinbjörnsdottir, Jan Heinemeier, Stefan Arnorsson. (1995). Origin of 14C in Icelandic ground water.
Arny Erla Sveinbjörnsdottir, Sigfus Johnsen. (1992). Stable isotope study of the Thingvallavatn area. Ground water origin, age and
evaporation methods.
Arny Erla Sveinbjörnsdottir, Sigfus Johnsen, Stefan Arnorsson. (n.d.). The use od stable isotopes of oxygen and hydrogen in geothermal
studies in Iceland. Reykjavik.
Jakobsson, S. &. (2008). Subglacial and interglacial volcanic formations in Iceland. Jökull.
Kristjan Sæmundsson, Freysteinn Sigurðsson. (1990). Geology and hydrogeology of Heiðmörk area with regard to export of drinking water .
Kristjan Sæmundsson, Freysteinn Sigurðsson. (1991). Geology and hydrogeology of Heiðmörk area with speial regard to the Greinkriki well.
Kristjan Sæmundsson, Freysteinn Sigurðsson. (1995). Geology and hydrogeology oj Jaðar spring area with regard to export drinking water.
Reykjavik.
Kristjan Sæmundsson, Kristan H. Sigurðson, Hrefna Kristmannsdottir. (1992). Geology and hydrogelogy of Gvendarbrunnar and Myllulækur
east of Reykjavik, Iceland. Reykjavik.
Mesfin, K. G. (2010). Subsurface geology, hydrothermal alteration and geothermal model of Northern Skarðsmýrarfjall, Hellisheiði geothermal
field, SW Iceland . Reykjavik: University of Iceland.
Schopka, H. (2006). ). The formation of Helgafell, southwest Iceland, a monogenetic subglacial hyaloclastite ridge: Sedimentology, hydrology
and volcano-ice interaction. Journal of Volcanology and Geothermal research.
Sigurdson, F. (1998). Ground water resources in Iceland. Aspects of quality and quantity. 285 - 305.
Vatnaskil (2012). Höfuðborgarsvæði. Grunnvatns- og rennslislíkan. Árleg endurskoðun fyrir árið 2012. Verkfræðistofan Vatnaskil.