Bentonite is a key component in many designs for
radioactive waste repositories. The plasticity, swelling capacity,
colloid filtration, low hydraulic conductivity, high retardation
of key radionuclides and stability in relevant geological
environments all make bentonite an ideal barrier/buffer
material. However, bentonite is chemically unstable under\ud
higher pH conditions and this is a potential problem for
repository designs which mix cement and concrete with
bentonite barriers. The hyperalkaline (pH~13) leachates from
the cement are expected to cause alteration of the bentonite.
Low alkali cements produce lower pH (around 10-11) leachates
and it is expected that this will slow bentonite reaction (or even
stop it altogether) over the timespan of relevance to repository
safety. Unfortunately, it has proven extremely difficult to study
these very slow reactions in the laboratory so an alternative
approach, that of studying natural analogues of the reaction
process, has begun in Cyprus. In this paper, preliminary details
of this new investigation of long-term bentonite reaction in the
natural hyperalkaline groundwaters of the Troodos ophiolite in
Cyprus are presented. Here, groundwater pH values of 10.0 to
11.9 have been reported, falling into the range typical of lowalkali
cements that are presently being developed for use in
radioactive waste disposal. The aims of this stage of the project
were to identify likely sites of hyperalkaline
groundwater/bentonite reaction and assess the relevance of the
current site conceptual model. Preliminary groundwater and
petrographic data for one group of related sites where
hyperalkaline groundwaters are present are also discussed
