Databank

POSIVA Report 1998-7

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Name:

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

Writer:

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

Language:

English

Page count:

141

ISBN:

951-652-045-6; 1239-3096

Summary:

Working report: POSIVA 98-07, Petteri Pitkänen, Ari Luukkonen, Paula Ruotsalainen, Hilkka Leino-
Forsman, Ulla Vuorinen, 139 pages, in English


GEOCHEMICAL MODELLING OF GROUNDWATER EVOLUTION AND
RESIDENCE TIME AT THE KIVETTY 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 bedrock at Kivetty has
been created and the significance of chemical reactions along different flowpaths calculated. Long term
hydrodynamics have also been evaluated.

The interpretation and modelling are based on groundwater samples (38 altogether) obtained from the
soil layer, shallow wells in the bedrock, and five deep multi-packered boreholes (KR1-KR5) in the
bedrock for which a comprehensive data set on dissolved chemical species and isotopes was available.
Some analyses of dissolved gases and their isotopic measurements were also utilised. The data covers
the bedrock at Kivetty to a depth of 850m. 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 C-14 age calculations of groundwater was given a
mass-balance approach (NETPATH). Reaction-path calculations (EQ3/6) were used to verify the
thermodynamic feasibility of the reaction models obtained.

The hydrogeochemistry of Kivetty is characterised by evolution from low-saline-carbonate-rich
recharge water towards Na-Ca-Cl-type water. The salinity remains low. The most important changes in
the chemistry of the groundwater are due to carbonate reactions: oxidising of organic carbon, and
dissolution and precipitation of calcite. The carbonate reactions and slight hydrolysis of silicates
stabilise the pH value at 8-9. In addition to aerobic oxidation of organic matter, oxidative dissolution of
biotite seems to be an important oxygen consumer at shallow depth during recharge. The most
important process controlling the redox state deeper in the bedrock was interpreted to be the
microbially mediated sulphate reduction with simultaneous anaerobic respiration of organic carbon.
This process buffers the redox level of about -200...-300 mV depending on the pH. Even though the
salinities of the groundwater samples and mass-transfer along flow paths remain low, the geochemical
evolution has fully developed and has reached quite a stable thermodynamic state. The residence times
of the groundwater samples cover the time span back to glaciation. Young ages seem to be limited to
the upper part of bedrock, and any really dynamic natural flowpath with deep observed recently
recharged water cannot be demonstrated. Deglacial or subglacial ages (over 9,700 years old at Kivetty)
are typical below the 150-300m level in the bedrock. Subglacial waters are interpreted to derive from
mixing of preglacial water and meltwater, the input of which is estimated to be about 20% at the most.
Indications of elevated oxygen intrusion cannot be observed in groundwater having glacial signals.


Keywords: groundwater chemistry, environmental isotopes, Proterozoic granitoids, nuclear waste
disposal, water-rock interaction, geochemical modelling, palaeohydrogeology
POSIVA 98-07, Petteri Pitkänen, Ari Luukkonen, Paula Ruotsalainen, Hilkka Leino-
Forsman, Ulla Vuorinen, 139 pages, in English


GEOCHEMICAL MODELLING OF GROUNDWATER EVOLUTION AND
RESIDENCE TIME AT THE KIVETTY 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 bedrock at Kivetty has
been created and the significance of chemical reactions along different flowpaths calculated. Long term
hydrodynamics have also been evaluated.

The interpretation and modelling are based on groundwater samples (38 altogether) obtained from the
soil layer, shallow wells in the bedrock, and five deep multi-packered boreholes (KR1-KR5) in the
bedrock for which a comprehensive data set on dissolved chemical species and isotopes was available.
Some analyses of dissolved gases and their isotopic measurements were also utilised. The data covers
the bedrock at Kivetty to a depth of 850m. 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 C-14 age calculations of groundwater was given a
mass-balance approach (NETPATH). Reaction-path calculations (EQ3/6) were used to verify the
thermodynamic feasibility of the reaction models obtained.

The hydrogeochemistry of Kivetty is characterised by evolution from low-saline-carbonate-rich
recharge water towards Na-Ca-Cl-type water. The salinity remains low. The most important changes in
the chemistry of the groundwater are due to carbonate reactions: oxidising of organic carbon, and
dissolution and precipitation of calcite. The carbonate reactions and slight hydrolysis of silicates
stabilise the pH value at 8-9. In addition to aerobic oxidation of organic matter, oxidative dissolution of
biotite seems to be an important oxygen consumer at shallow depth during recharge. The most
important process controlling the redox state deeper in the bedrock was interpreted to be the
microbially mediated sulphate reduction with simultaneous anaerobic respiration of organic carbon.
This process buffers the redox level of about -200...-300 mV depending on the pH. Even though the
salinities of the groundwater samples and mass-transfer along flow paths remain low, the geochemical
evolution has fully developed and has reached quite a stable thermodynamic state. The residence times
of the groundwater samples cover the time span back to glaciation. Young ages seem to be limited to
the upper part of bedrock, and any really dynamic natural flowpath with deep observed recently
recharged water cannot be demonstrated. Deglacial or subglacial ages (over 9,700 years old at Kivetty)
are typical below the 150-300m level in the bedrock. Subglacial waters are interpreted to derive from
mixing of preglacial water and meltwater, the input of which is estimated to be about 20% at the most.
Indications of elevated oxygen intrusion cannot be observed in groundwater having glacial signals.


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

Keywords:

groundwater chemistry; environmental isotopes; Proterozoic granitoids; nuclear waste disposal; water-rock interaction; geochemical modelling; palaeohydrogeology

File(s):

Geochemical Modelling of Groundwater Evolution and Residence Time at the Kivetty Site (pdf) (8.9 MB)


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