Databank

POSIVA Report 1996-21

Back

Name:

Hydrogeochemistry of Deep Groundwaters of Mafic and Ultramafic Rocks in Finland

Writer:

Timo Ruskeeniemi; Runar Blomqvist; Antero Lindberg; Lasse Ahonen; Shaun Frape

Language:

English

Page count:

122

ISBN:

951-652-020-0; 1239-3096

Summary:

Working report: HYDROGEOCHEMISTRY OF DEEP GROUNDWATERS OF MAFIC AND ULTRAMAFIC
ROCKS IN FINLAND




The present work reports and interprets the hydrogeochemical and hydrogeological data obtained
from deep groundwaters in various mafic-ultramafic formations in Finland. The work is mainly
based on the results of the research project "Geochemistry of deep groundwaters" financed by the
Ministry of Trade and Industry and the Geological Survey of Finland. Five sites were selected for
this study: (1) Juuka, (2) Keminmaa, (3) Mäntsälä, (4) Ranua, and (5) Ylivieska. Keminmaa and
Ranua are located in Early Proterozoic layered intrusions dated at 2.44 Ga. The Juuka site lies
within the massive Miihkali serpentinite, which is thought to represent the ultramafic part of a
Proterozoic (1.97 Ga) ophiolite complex. The Mäntsälä gabbro represents the deep parts of the
Svecofennian volcanic sequence, while the Ylivieska mafic-ultramafic intrusion is one of a group
of Svecokarelian Ni-potential intrusions 1.9 Ga in age.

For reference, groundwaters from four other sites are also briefly described. Three of these sites
are located within the nickel mining regions of Enonkoski, Kotalahti and Vammala, while the
fourth is a small Ni mineralization at Hyvelä, Noormarkku. The four reference sites are all of
Svecokarelian age.

The groundwaters at all sites, with the exception of Enonkoski, were sampled with a tube
sampling technique, which yields a drill hole sample extending from surface to bottom, in extreme
cases down to 1000 m. At Enonkoski a small electronically controlled Ruttner-type sampler was
used. During sampling, the pH, electric conductivity, and usually also the Eh of groundwater were
measured. Extensive chemical analyses were made at the Geological Survey of Finland.

The analytical results from the sampling sites are compared with each other, and the results from
the study sites proper are compared with those from the reference sites. Finally, groundwaters in
mafic and ultramafic rocks are compared with groundwaters in other rock types, mainly in felsic
rocks. The fracture-filling and fracture-surface-coating minerals of the drill-core samples were
studied, and the effects of fracture mineralogy on the pH values of the groundwaters are discussed.
Finally, aspects of the evolution of groundwater chemistry of the various sites are considered.

The study sites were grouped into three categories based on site hydrogeology. At Juuka the study
site is interpreted as a discharge area, where the sampled saline low-tritium groundwaters
represent the groundwaters of the upper and middle parts of the serpentinite body. At Keminmaa
the dilute bicarbonate groundwaters and the high tritium contents are suggestive of recharge. As
the groundwaters seem to be undersaturated with respect to calcite, a reasonably fast downward
flow is evident. At Ranua, Mäntsälä, and Ylivieska dilute bicarbonate waters prevail in the upper
part of the bedrock and saline groundwaters deeper down. According to the results of repeated
sampling surveys, the interface between the dilute and saline aquifers remain at a constant level at
Ranua and Ylivieska. The tritium values suggest recharge of surficial waters in the upper part of
bedrock at the Mäntsälä and Ylivieska sites, whereas discharge is likely at Ranua, based on low
tritium values.


Nine groundwater types are encountered within the study sites. At Juuka only saline (TDS: 9.2-
9.5 g/l) Na-Cl groundwaters have been detected from the ground surface down to a depth of 660
m. At Keminmaa there is dilute groundwater of the Ca-HCO3 type throughout the drill hole length
(375 m). At Mäntsälä Ca-Na-Mg-HCO3 waters prevail in the upper part of the drill holes down to
depths of 200 to 600 m. The saline waters in drill hole MHa-1 are of the Ca-Na-Cl type (TDS 8-
10 g/l), which in turn is underlain by less saline Na-Ca-Cl water (TDS 4-5). The deeper
groundwaters of drill hole MHa-2 are strongly saline Ca-Cl waters (TDS 20-50 g/l). The
groundwater at Ylivieska is a dilute Mg-Na-Ca-HCO3 water down to a depth of 320 m. The
underlying saline waters (TDS 43-83 g/l) are of the Na-Mg-Ca-Cl type. The dilute groundwater
at Ranua is of the Ca-Mg-HCO3 type. From a depth of 690 m downwards a slightly saline (TDS
0.4-0.5 g/l) Na-Ca-Cl water is encountered.

Based on the results, a correlation between pH of groundwater and lithology is evident. In
gabbroic environments (Keminmaa, the upper part of Ranua and the lower part of Ylivieska), pH
varies from 6.9 to 7.5. The groundwaters in the ultramafic upper part of the Ylivieska drill hole
have an average pH of 8.5, whereas the deep groundwaters in the ultramafic part of the Ranua
formation have a pH of 9.8. The pH of the groundwaters in the Juuka serpentinite are still higher,
around pH 10. The observed pH values of the groundwaters are generally in good agreement with
the abrasion pH values of the dominant minerals in contact with the groundwaters; accordingly the
mineralogy has a distinct influence on the recorded pH values of the groundwaters.

When comparing the bicarbonate groundwaters of mafic and ultramafic intrusions with the
corresponding waters in schist belts, higher Ca/Na ratios are generally observed in the former,
evidently due to the higher Ca/Na ratios in the rocks and to the greater abundance of calcite in
mafic rocks than in felsic lithologies. The increase in the Ca/Na ratio with decreasing pH probably
reflects the instability of calcite under low pH conditions. Generally, a high abundance of Mg is
typical of dilute groundwaters in mafic and ultramafic rocks.

Saline deep groundwaters from the various mafic and ultramafic study sites plot in distinct fields
in the Ca/Na vs. Br/Cl diagram. The ophiolitic groundwaters, e.g. in the Juuka serpentinite, have
similar Br/Cl ratios and only slightly higher Ca/Na ratios than seawater. The Alpine-type often Ni-
critical mafic-ultramafic intrusions, e.g. Ylivieska and Ranua, differ from the former in having
2—3 times higher Br/Cl ratios in saline groundwaters. On the other hand the groundwaters of the
gabbroic formations plot within the same group as most of the groundwaters from the schist belts.
Based on these criteria it is difficult to distinguish the gabbroic waters proper from the average
felsic groundwaters, probably as both have plagioclase in common. Chemically the ophiolitic
groundwaters are sodium dominant and the groundwaters in the Alpine-type mafic-ultramafic
intrusions are magnesium and sodium dominant. The gabbroic groundwaters, on the other hand,
are mainly calcium dominant.

In conclusion, each mafic-ultramafic site has its own characteristic groundwaters with respect to
dissolved solids and elemental ratios. Mixing of groundwaters and differences in the prevailing
hydrogeological conditions account for most of the major variability concerning groundwater
salinity. Nevertheless, the groundwaters of the various sites correspond distinctly to the
compositional variability of their host rocks, providing a convincing evidence for pervasive water-
rock interaction in the bedrock. More generally, the existence of formation-characteristic
groundwaters is also indicative of relatively limited interaction between groundwaters of adjacent
geological formations.

Avainsanat - Keywords
ground water, fresh water, salt water, hydrochemistry, hydrogeology, ultramafics, gabbros, water-rock interaction
HYDROGEOCHEMISTRY OF DEEP GROUNDWATERS OF MAFIC AND ULTRAMAFIC
ROCKS IN FINLAND




The present work reports and interprets the hydrogeochemical and hydrogeological data obtained
from deep groundwaters in various mafic-ultramafic formations in Finland. The work is mainly
based on the results of the research project "Geochemistry of deep groundwaters" financed by the
Ministry of Trade and Industry and the Geological Survey of Finland. Five sites were selected for
this study: (1) Juuka, (2) Keminmaa, (3) Mäntsälä, (4) Ranua, and (5) Ylivieska. Keminmaa and
Ranua are located in Early Proterozoic layered intrusions dated at 2.44 Ga. The Juuka site lies
within the massive Miihkali serpentinite, which is thought to represent the ultramafic part of a
Proterozoic (1.97 Ga) ophiolite complex. The Mäntsälä gabbro represents the deep parts of the
Svecofennian volcanic sequence, while the Ylivieska mafic-ultramafic intrusion is one of a group
of Svecokarelian Ni-potential intrusions 1.9 Ga in age.

For reference, groundwaters from four other sites are also briefly described. Three of these sites
are located within the nickel mining regions of Enonkoski, Kotalahti and Vammala, while the
fourth is a small Ni mineralization at Hyvelä, Noormarkku. The four reference sites are all of
Svecokarelian age.

The groundwaters at all sites, with the exception of Enonkoski, were sampled with a tube
sampling technique, which yields a drill hole sample extending from surface to bottom, in extreme
cases down to 1000 m. At Enonkoski a small electronically controlled Ruttner-type sampler was
used. During sampling, the pH, electric conductivity, and usually also the Eh of groundwater were
measured. Extensive chemical analyses were made at the Geological Survey of Finland.

The analytical results from the sampling sites are compared with each other, and the results from
the study sites proper are compared with those from the reference sites. Finally, groundwaters in
mafic and ultramafic rocks are compared with groundwaters in other rock types, mainly in felsic
rocks. The fracture-filling and fracture-surface-coating minerals of the drill-core samples were
studied, and the effects of fracture mineralogy on the pH values of the groundwaters are discussed.
Finally, aspects of the evolution of groundwater chemistry of the various sites are considered.

The study sites were grouped into three categories based on site hydrogeology. At Juuka the study
site is interpreted as a discharge area, where the sampled saline low-tritium groundwaters
represent the groundwaters of the upper and middle parts of the serpentinite body. At Keminmaa
the dilute bicarbonate groundwaters and the high tritium contents are suggestive of recharge. As
the groundwaters seem to be undersaturated with respect to calcite, a reasonably fast downward
flow is evident. At Ranua, Mäntsälä, and Ylivieska dilute bicarbonate waters prevail in the upper
part of the bedrock and saline groundwaters deeper down. According to the results of repeated
sampling surveys, the interface between the dilute and saline aquifers remain at a constant level at
Ranua and Ylivieska. The tritium values suggest recharge of surficial waters in the upper part of
bedrock at the Mäntsälä and Ylivieska sites, whereas discharge is likely at Ranua, based on low
tritium values.


Nine groundwater types are encountered within the study sites. At Juuka only saline (TDS: 9.2-
9.5 g/l) Na-Cl groundwaters have been detected from the ground surface down to a depth of 660
m. At Keminmaa there is dilute groundwater of the Ca-HCO3 type throughout the drill hole length
(375 m). At Mäntsälä Ca-Na-Mg-HCO3 waters prevail in the upper part of the drill holes down to
depths of 200 to 600 m. The saline waters in drill hole MHa-1 are of the Ca-Na-Cl type (TDS 8-
10 g/l), which in turn is underlain by less saline Na-Ca-Cl water (TDS 4-5). The deeper
groundwaters of drill hole MHa-2 are strongly saline Ca-Cl waters (TDS 20-50 g/l). The
groundwater at Ylivieska is a dilute Mg-Na-Ca-HCO3 water down to a depth of 320 m. The
underlying saline waters (TDS 43-83 g/l) are of the Na-Mg-Ca-Cl type. The dilute groundwater
at Ranua is of the Ca-Mg-HCO3 type. From a depth of 690 m downwards a slightly saline (TDS
0.4-0.5 g/l) Na-Ca-Cl water is encountered.

Based on the results, a correlation between pH of groundwater and lithology is evident. In
gabbroic environments (Keminmaa, the upper part of Ranua and the lower part of Ylivieska), pH
varies from 6.9 to 7.5. The groundwaters in the ultramafic upper part of the Ylivieska drill hole
have an average pH of 8.5, whereas the deep groundwaters in the ultramafic part of the Ranua
formation have a pH of 9.8. The pH of the groundwaters in the Juuka serpentinite are still higher,
around pH 10. The observed pH values of the groundwaters are generally in good agreement with
the abrasion pH values of the dominant minerals in contact with the groundwaters; accordingly the
mineralogy has a distinct influence on the recorded pH values of the groundwaters.

When comparing the bicarbonate groundwaters of mafic and ultramafic intrusions with the
corresponding waters in schist belts, higher Ca/Na ratios are generally observed in the former,
evidently due to the higher Ca/Na ratios in the rocks and to the greater abundance of calcite in
mafic rocks than in felsic lithologies. The increase in the Ca/Na ratio with decreasing pH probably
reflects the instability of calcite under low pH conditions. Generally, a high abundance of Mg is
typical of dilute groundwaters in mafic and ultramafic rocks.

Saline deep groundwaters from the various mafic and ultramafic study sites plot in distinct fields
in the Ca/Na vs. Br/Cl diagram. The ophiolitic groundwaters, e.g. in the Juuka serpentinite, have
similar Br/Cl ratios and only slightly higher Ca/Na ratios than seawater. The Alpine-type often Ni-
critical mafic-ultramafic intrusions, e.g. Ylivieska and Ranua, differ from the former in having
2—3 times higher Br/Cl ratios in saline groundwaters. On the other hand the groundwaters of the
gabbroic formations plot within the same group as most of the groundwaters from the schist belts.
Based on these criteria it is difficult to distinguish the gabbroic waters proper from the average
felsic groundwaters, probably as both have plagioclase in common. Chemically the ophiolitic
groundwaters are sodium dominant and the groundwaters in the Alpine-type mafic-ultramafic
intrusions are magnesium and sodium dominant. The gabbroic groundwaters, on the other hand,
are mainly calcium dominant.

In conclusion, each mafic-ultramafic site has its own characteristic groundwaters with respect to
dissolved solids and elemental ratios. Mixing of groundwaters and differences in the prevailing
hydrogeological conditions account for most of the major variability concerning groundwater
salinity. Nevertheless, the groundwaters of the various sites correspond distinctly to the
compositional variability of their host rocks, providing a convincing evidence for pervasive water-
rock interaction in the bedrock. More generally, the existence of formation-characteristic
groundwaters is also indicative of relatively limited interaction between groundwaters of adjacent
geological formations.

Avainsanat - Keywords
ground water, fresh water, salt water, hydrochemistry, hydrogeology, ultramafics, gabbros, water-rock interaction

Keywords:

ground water; fresh water; salt water; hydrochemistry; hydrogeology; ultramafics; gabbros; water-rock interaction

File(s):

Hydrogeochemistry of Deep Groundwaters of Mafic and Ultramafic Rocks in Finland (pdf) (8.1 MB)


Back


Share article:
This website stores cookies on your computer. These cookies are used to improve our website and provide more personalised services to you.
Close

Cookies

To make this site work properly, we sometimes place small data files called cookies on your device. Most big websites do this too.

1. What are cookies?

A cookie is a small text file that a website saves on your computer or mobile device when you visit the site. It enables the website to remember your actions and preferences (such as login, language, font size and other display preferences) over a period of time, so you don’t have to keep re-entering them whenever you come back to the site or browse from one page to another.

2. How do we use cookies?

A number of our pages use cookies to remember your actions and preferences (such as login, language, font size and other display preferences.)

Also, some videos embedded in our pages use a cookie to anonymously gather statistics on how you got there and what videos you visited.

Enabling these cookies is not strictly necessary for the website to work but it will provide you with a better browsing experience. You can delete or block these cookies, but if you do that some features of this site may not work as intended.

The cookie-related information is not used to identify you personally and the pattern data is fully under our control. These cookies are not used for any purpose other than those described here.

3. How to control cookies

You can control and/or delete cookies as you wish – for details, see aboutcookies.org. You can delete all cookies that are already on your computer and you can set most browsers to prevent them from being placed. If you do this, however, you may have to manually adjust some preferences every time you visit a site and some services and functionalities may not work.

Close