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POSIVA Report 1998-10

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Name:

Geochemical Modelling of Groundwater Evolution and Residence Time at the Olkiluoto Site

Writer:

Petteri Pitkänen; Ari Luukkonen; Paula Ruotsalainen; Hilkka Leino-Forsman; Ulla Vuorinen

Language:

English

Page count:

169

ISBN:

951-652-048-0; 1239-3096

Summary:

Working report: Posiva Report 98-10, Petteri Pitkänen, Ari Luukkonen, Paula Ruotsalainen,
Hilkka Leino-Forsman & Ulla Vuorinen, 184 pages, in English, ISBN 951-652-048-
0

GEOCHEMICAL MODELLING OF GROUNDWATER EVOLUTION AND
RESIDENCE TIME AT THE OLKILUOTO SITE

ABSTRACT

An understanding of the geochemical evolution of groundwater is an essential part of the
performance assessment and safety analysis of the final disposal of radioactive waste into the
bedrock. The performance of technical barriers and migration of possibly released
radionuclides depend on chemical conditions. A prerequisite for understanding these factors is
the ability to specify the water-rock interactions which control chemical conditions in
groundwater. The objective of this study is to interpret the processes and factors which control
the hydrogeochemistry, such as pH and redox conditions. A model of the hydrogeochemical
progress in different parts of the crystalline bedrock at Olkiluoto has been created and the
significance of chemical reactions and groundwater mixing along different flowpaths
calculated. Long term hydrodynamics have also been evaluated.

The interpretation and modelling are based on water samples (63 altogether) obtained from
precipitation, Baltic Sea, soil layer, shallow wells in the bedrock, and eight deep boreholes in
the bedrock for which a comprehensive data set on dissolved chemical species and isotopes was
available. Some analyses of dissolved gases and fracture calcite and their isotopic
measurements were also utilised. The data covers the bedrock at Olkiluoto to a depth of 1000m.
The results from groundwater chemistry, isotopes, petrography, hydrogeology of the site,
geomicrobial studies, and PCA and speciation calculations were used in the evaluation of
evolutionary processes at the site. The geochemical interpretation of water-rock interaction,
isotope-chemical evolution and mixing of palaeo water types were approached by mass-balance
calculations (NETPATH). Reaction-path calculations (EQ3/6) were used to verify the
thermodynamic feasibility of the reaction models obtained.

The interpretation and calculation of hydrochemical data from Olkiluoto reveals the complex
nature of hydrogeochemical evolution at the site. Changes in external conditions such as
glaciation, palaeo Baltic stages, land uplift and ancient hydrothermal events, have had a
significant effect on local palaeohydrogeological conditions. They have caused great
variability, which is observable in the chemical data notably in salinity (up to 70 g/l), water
type and contents of conservative parameters, such as Cl, Br and stable isotopes of water ("roo"H-2
and "roo"O-18). However, their influence is also significant on the water-rock interaction that
principally controls the pH and redox conditions - varying 7.5 to 8 and -200 to -300 mV,
respectively - in the groundwater, although the calculated mass transfer in the reactions is
minor compared with conservative mixing at the site. Calcite in fractures is interpreted to
principally control pH level in groundwater. Sulphidic redox conditions dominate in the upper
500m in brackish and slightly saline groundwater. Deeper sulphur species are absent and
methanic processes are obtained. The water types can be connected to certain palaeo stages.
This enables to estimate mean residence time of groundwaters. Current meteoric recharge stage
(< 2500 a) mainly dominates in the upper 150m. Groundwater from Litorina stage (7500-2500 a
ago) forms the bulk at 100 - 250 m. Glacial melt water (about 10 000 a old) is an important
component of groundwater between 100 - 500 m. However, any remarks of oxygen intrusion
cannot be interpreted neither from mineralogy nor from groundwater. Deeper, subglacial and
older saline groundwater predominates. Despite the current locations of different groundwater
bodies it seems according to hydrogeochemical interpretation that dynamic flow conditions has
been limited to upper 150 - 200 m.

Keywords: groundwater chemistry, environmental isotopes, nuclear waste disposal, palaeohydrogeology,
water-rock interaction, mixing, geochemical modelling
Posiva Report 98-10, Petteri Pitkänen, Ari Luukkonen, Paula Ruotsalainen,
Hilkka Leino-Forsman & Ulla Vuorinen, 184 pages, in English, ISBN 951-652-048-
0

GEOCHEMICAL MODELLING OF GROUNDWATER EVOLUTION AND
RESIDENCE TIME AT THE OLKILUOTO SITE

ABSTRACT

An understanding of the geochemical evolution of groundwater is an essential part of the
performance assessment and safety analysis of the final disposal of radioactive waste into the
bedrock. The performance of technical barriers and migration of possibly released
radionuclides depend on chemical conditions. A prerequisite for understanding these factors is
the ability to specify the water-rock interactions which control chemical conditions in
groundwater. The objective of this study is to interpret the processes and factors which control
the hydrogeochemistry, such as pH and redox conditions. A model of the hydrogeochemical
progress in different parts of the crystalline bedrock at Olkiluoto has been created and the
significance of chemical reactions and groundwater mixing along different flowpaths
calculated. Long term hydrodynamics have also been evaluated.

The interpretation and modelling are based on water samples (63 altogether) obtained from
precipitation, Baltic Sea, soil layer, shallow wells in the bedrock, and eight deep boreholes in
the bedrock for which a comprehensive data set on dissolved chemical species and isotopes was
available. Some analyses of dissolved gases and fracture calcite and their isotopic
measurements were also utilised. The data covers the bedrock at Olkiluoto to a depth of 1000m.
The results from groundwater chemistry, isotopes, petrography, hydrogeology of the site,
geomicrobial studies, and PCA and speciation calculations were used in the evaluation of
evolutionary processes at the site. The geochemical interpretation of water-rock interaction,
isotope-chemical evolution and mixing of palaeo water types were approached by mass-balance
calculations (NETPATH). Reaction-path calculations (EQ3/6) were used to verify the
thermodynamic feasibility of the reaction models obtained.

The interpretation and calculation of hydrochemical data from Olkiluoto reveals the complex
nature of hydrogeochemical evolution at the site. Changes in external conditions such as
glaciation, palaeo Baltic stages, land uplift and ancient hydrothermal events, have had a
significant effect on local palaeohydrogeological conditions. They have caused great
variability, which is observable in the chemical data notably in salinity (up to 70 g/l), water
type and contents of conservative parameters, such as Cl, Br and stable isotopes of water ("roo"H-2
and "roo"O-18). However, their influence is also significant on the water-rock interaction that
principally controls the pH and redox conditions - varying 7.5 to 8 and -200 to -300 mV,
respectively - in the groundwater, although the calculated mass transfer in the reactions is
minor compared with conservative mixing at the site. Calcite in fractures is interpreted to
principally control pH level in groundwater. Sulphidic redox conditions dominate in the upper
500m in brackish and slightly saline groundwater. Deeper sulphur species are absent and
methanic processes are obtained. The water types can be connected to certain palaeo stages.
This enables to estimate mean residence time of groundwaters. Current meteoric recharge stage
(< 2500 a) mainly dominates in the upper 150m. Groundwater from Litorina stage (7500-2500 a
ago) forms the bulk at 100 - 250 m. Glacial melt water (about 10 000 a old) is an important
component of groundwater between 100 - 500 m. However, any remarks of oxygen intrusion
cannot be interpreted neither from mineralogy nor from groundwater. Deeper, subglacial and
older saline groundwater predominates. Despite the current locations of different groundwater
bodies it seems according to hydrogeochemical interpretation that dynamic flow conditions has
been limited to upper 150 - 200 m.

Keywords: groundwater chemistry, environmental isotopes, nuclear waste disposal, palaeohydrogeology,
water-rock interaction, mixing, geochemical modelling

Keywords:

groundwater chemistry; environmental isotopes; palaeohydrogeology; water-rock interaction; mixing; geochemical modelling

File(s):

Geochemical Modelling of Groundwater Evolution and Residence Time at the Olkiluoto Site (pdf) (11.9 MB)


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