The 12th International Conference on Environmental ... - Events

Abstracts Session 63
6) FORMATION OF SECONDARY MINERALS AND UPTAKE OF VARIOUS ANIONS UNDER NATURALLY-
OCCURRING HYPERALKALINE CONDITIONS IN OMAN -16344
Sohtaro Anraku, Tsukuba Sato, Tetsuro Yoneda, Hokkaido University (Japan);
Kazuya MORIMOTO, National Institute for Materials Science (Japan)
In Japanese transuranic (TRU) waste disposal facilities, 129I is the most important key nuclide for the long-term safety assessment.
Thus, the Kd values of I to natural minerals are important factor in the safety assessment. However, the degradation of cement
materials in the repositories can produce high pH pore fluid which can affect the anion transport behavior. <strong>The</strong>refore, it is necessary
to understand the behavior of anions such as I- under the hyperalkaline conditions.
<strong>The</strong> natural hyperalkaline spring water (pH>11) in the Oman ophiolite is known to be generated from the partly serpentinized
peridotites. <strong>The</strong> spring water is characteristically hyperalkaline, reducing, low-Mg, Si and HCO3- , and high-Ca, while the river
water is moderately alkaline, oxidizing, high-Mg and HCO3-. <strong>The</strong> mixing of these spring and river water resulted in the formation
of secondary minerals. In the present study, the naturally occurring hyperalkaline conditions near the springs in Oman were used
as natural analogue for the interaction between cement pore fluid and natural Mg-HCO3- groundwater. <strong>The</strong> present aim of this paper
is to examine the conditions of secondary mineral formation and the anion uptake capacity of these mineral in this system.
Water and precipitate samples were collected from the different locations around the spring vent to identify the effect of mixing
ratios between spring and river water on mineral composition and water-mineral distribution coefficient of various anions. Onsite
synthesis was also carried out to support these data quantitatively.
Aragonite was observed in all precipitates, while calcite, brucite and Mg-Al hydrotalcite-like compounds (HTlc) were also
determined in some samples. Calcite was observed only closed to the springs. At locations far from the springs, calcite formation
was inhibited due to high-Mg fluid from river water. Brucite was observed from the springs with relatively low-Al concentration
and HTlc was the opposite.
During the formation of the minerals at the mixing points, HCO3- in the river water was fixed as carbonate minerals such as
in aragonite and calcite while H3SiO4- in the river water was dominantly fixed into interlayers and surfaces of HTlc. Iodine in spring
and river water was mainly fixed in aragonite. <strong>The</strong>refore, the uptake I- by secondary minerals can be expected at hyperalkaline conditions
as observed at Oman hyperalkaline springs.
7) REACTION OF BENTONITE IN LOW ALKALI CEMENT LEACHATES: PRELIMINARY RESULTS
FROM THE CYPRUS NATURAL ANALOGUE PROJECT (CNAP) - 16276
W. Russell Alexander, Bedrock Geosciences (Switzerland)
Bentonite is a key component in many designs for radioactive waste repositories. <strong>The</strong> 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 higher pH conditions and this is a potential
problem for repository designs which mix cement and concrete with bentonite barriers. <strong>The</strong> 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 low-alkali cements that are
presently being developed for use in radioactive waste disposal. <strong>The</strong> 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...
8) DEVELOPMENTS IN HYPERALKALINE NATURAL ANALOGUE STUDIES
AROUND THE ZAMBALES OPHIOLITE, PHILIPPINES - 16278
C. Pascua, M. L.L. Honrado, University of Philippines (Philippines);
W. Russell Alexander, Bedrock Geosciences (Switzerland); Yamakawa, N. Fujii, RWMC (Japan);
K. Namiki, K. Kawamurs, Obayashi Corpration (Japan); I.G. McKinley, McKinley Consulting (Switzerland)
Past studies have identified ophiolites as potential sources of hyperalkaline waters that can be considered analogues of the
leachates produced by cementitious materials in repositories for radioactive waste. <strong>The</strong> Zambales ophiolite in the Philippines has
been determined to undergo widespread active serpentinisation, resulting in hyperalkaline springs at several locations. Measured
pH values range up to 11.1, falling into the range typical of low alkali cements that are presently being developed to minimise geochemical
perturbations resulting from use of concrete. A key goal is to examine the alteration of natural bentonite in contact with
such waters to determine if, indeed, pH values in this lower range eliminate the problems seen at the high values (>12) associated
with conventional concretes.
Evidence has been accumulated to support the conceptual model of hyperalkaline groundwater flow in fractures underlying
bentonite deposits in the Mangatarem area of the Zambales ophiolite. Drilling and trenching has exposed a direct contact between
bentonite and pillow lavas of the ophiolite and several generations of hyperalkaline groundwater flow within fracture zones have
been identified. As yet, water samples have not been obtained from this contact, but analysis of reacted bentonite is ongoing. Preliminary
data are reported here for the first time.
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