RD&D-Programme 2004 - SKB

Diffusion measurements in the laboratory on site-specific material are also being performed
within the site investigation programme. These investigations are described in greater detail in
/19-73/.
19.2.15 Reactions with the rock – groundwater and rock matrix
Conclusions in RD&D 2001 and its review
In the long term, the chemistry of the groundwater is determined by reactions with the primary
minerals in the rock, which weather and release dissolved substances into the groundwater.
The composition of the groundwater in fractures, a relatively well-studied area, is often
affected by both mixing and reactions. The composition of the matrix water, on the other hand,
is expected to depend mainly on reactions between rock minerals and groundwater. An experiment
aimed at studying the composition and age of the matrix water and determining the effect
of diffusion processes on this composition was started at the Äspö HRL. One interpretation of
the results is that so-called matrix water comprises a very small fraction of the water that has
been collected in the borehole. Most comes from fractures and microfractures that are connected
to larger structures. There is therefore no reason to suspect that a very saline water caused by the
matrix would enter the deposition holes at any point.
Newfound knowledge since RD&D 2001
In the Matrix Fluid Chemistry Experiment in the Äspö HRL /19-74/, analyses have been made
of in situ matrix fluid, i.e. water present in the pores in the rock that is accessible for interaction
with water circulating in the fractures. The results of the experiment show that contact between
matrix fluid and groundwater in the fractures takes place through flow in a network of microfractures
with small dimensions and low hydraulic conductivities. Solutes are transported within
the rock matrix by diffusion. The matrix pore water has preserved brackish and saline waters
that are old, dating back to at least the last glaciation, i.e. more than about 10,000 years old. This
relatively rapid turnover rate for the matrix fluid at Äspö is supported by observations of the fact
that the rock generally has high permeability. The matrix fluid therefore contains only a small
component that originates from long-term reactions between rock and water.
Experience from the Matrix Fluid Chemistry Experiment at Äspö has been applied to the site
investigations. Diffusion experiments on a laboratory scale have been conducted on samples
from different depths in Simpevarp. The experiments indicate different compositions with
salinities that increase with depth.
Programme
The Matrix Fluid Chemistry Experiment at Äspö may be supplemented by additional measurements.
Aside from these investigations, the field is not judged to require any further research,
development or demonstration today. New developments are being monitored and will be acted
on when appropriate.
19.2.16 Reactions with the rock – dissolution/precipitation of
fracture-filling minerals
Conclusions in RD&D 2001 and its review
Dissolution and precipitation of fracture-filling minerals is a constantly ongoing process.
Most of these minerals have been formed under hydrothermal conditions, but low-temperature
minerals are also present and can be utilized to understand the evolution of the groundwater
chemistry, for example with respect to redox conditions.
260 RD&D-Programme 2004