94
The Swedish Nuclear Fuel (SKB) and Waste Management Co. managed an international project entitled “Poços de Caldas Project” with the participation of Brazil
(INB), Sweden (SKB), Switzerland (NAGRA), the United Kingdom (UK DECC),
and the USA (US DOE) (Chapman et al. 1992). The Osamu Utsumi open-pit uranium mine and the Morro do Ferro ore deposit were selected for natural analogue
studies, i.e., they were considered natural systems in which the chemical and physical processes were studied as the conditions were expected to occur in the vicinity
of radioactive waste repositories. The main key objectives of the project consisted
of (1) checking of equilibrium thermodynamic codes and databases, (2) determining the mobility of colloids and their role in the radionuclides transport, (3) modeling the geochemical evolution of redox fronts, and (4) evaluating the migration of
rare-earth elements and uranium/thorium series radionuclides during periods of
hydrothermal activity (Chapman et al. 1992).
Figure 1 shows the location of the Osamu Utsumi and Morro do Ferro sites.
Figure 2 illustrates a typical alpha spectrum obtained for uranium extracted from
one groundwater sample collected at Morro do Ferro, in which are evident the peaks
of the natural U isotopes
238
U and
234
U and
232
U spike added for checking the chemical yield and providing the U concentration value. Figure 3 shows the results
obtained for the dissolved U content in groundwater from bores drilled at Morro do
Ferro that were sampled in May 1983 and September 1983, as well as the rainfall in
the same period. May 1983 was a period of very low rainfall, and the dissolved U
contents were the lowest in all bores, except in SR-9. September 1983 was the
beginning of the rainy period immediately after the typical droughts affecting the
months of July and August. The dissolved U contents increased in all bores during
September 1983, except in SR-4. Thicker clayey packages implied on lower amounts
of U leached by infiltrating waters as indicated the inverse significant relationship
between these parameters for the waters sampled during the beginning of the wet
season (September 1983) (Fig. 4).
Several campaigns for sampling waters and fine sediments (particulate matter) of
South Stream at Morro do Ferro area were carried out by Grimaldi (1981) using a
manual sampler. The U concentration and
234
U/
238
U activity ratio data as reported in
Table 1 allowed the application of the U isotopes method developed by MoreiraNordemann (1980, 1984) in order to determine the weathering rate in the region.
The database for rock, water, and sediment samples permitted the estimation of a
solubility coefficient for U corresponding to k = 0.44 in the area, which implied that
44% of the available U in rocks is taken into solution during the weathering process
(Bonotto et al. 2007). Using all available data and applying the equations established by the U isotopes model, Bonotto et al. (2007) estimated a value of
0.015 mm yr
−1
for the weathering rate v at the Morro do Ferro region and concluded
that one vertical meter of rock needs about 67,000 years to be weathered,
demonstrating thus the chemical weathering rate of rocks under tropical climatic
conditions.
J. A. Galhardi et al.
The Swedish Nuclear Fuel (SKB) and Waste Management Co. managed an international project entitled “Poços de Caldas Project” with the participation of Brazil
(INB), Sweden (SKB), Switzerland (NAGRA), the United Kingdom (UK DECC),
and the USA (US DOE) (Chapman et al. 1992). The Osamu Utsumi open-pit uranium mine and the Morro do Ferro ore deposit were selected for natural analogue
studies, i.e., they were considered natural systems in which the chemical and physical processes were studied as the conditions were expected to occur in the vicinity
of radioactive waste repositories. The main key objectives of the project consisted
of (1) checking of equilibrium thermodynamic codes and databases, (2) determining the mobility of colloids and their role in the radionuclides transport, (3) modeling the geochemical evolution of redox fronts, and (4) evaluating the migration of
rare-earth elements and uranium/thorium series radionuclides during periods of
hydrothermal activity (Chapman et al. 1992).
Figure 1 shows the location of the Osamu Utsumi and Morro do Ferro sites.
Figure 2 illustrates a typical alpha spectrum obtained for uranium extracted from
one groundwater sample collected at Morro do Ferro, in which are evident the peaks
of the natural U isotopes
238
U and
234
U and
232
U spike added for checking the chemical yield and providing the U concentration value. Figure 3 shows the results
obtained for the dissolved U content in groundwater from bores drilled at Morro do
Ferro that were sampled in May 1983 and September 1983, as well as the rainfall in
the same period. May 1983 was a period of very low rainfall, and the dissolved U
contents were the lowest in all bores, except in SR-9. September 1983 was the
beginning of the rainy period immediately after the typical droughts affecting the
months of July and August. The dissolved U contents increased in all bores during
September 1983, except in SR-4. Thicker clayey packages implied on lower amounts
of U leached by infiltrating waters as indicated the inverse significant relationship
between these parameters for the waters sampled during the beginning of the wet
season (September 1983) (Fig. 4).
Several campaigns for sampling waters and fine sediments (particulate matter) of
South Stream at Morro do Ferro area were carried out by Grimaldi (1981) using a
manual sampler. The U concentration and
234
U/
238
U activity ratio data as reported in
Table 1 allowed the application of the U isotopes method developed by MoreiraNordemann (1980, 1984) in order to determine the weathering rate in the region.
The database for rock, water, and sediment samples permitted the estimation of a
solubility coefficient for U corresponding to k = 0.44 in the area, which implied that
44% of the available U in rocks is taken into solution during the weathering process
(Bonotto et al. 2007). Using all available data and applying the equations established by the U isotopes model, Bonotto et al. (2007) estimated a value of
0.015 mm yr
−1
for the weathering rate v at the Morro do Ferro region and concluded
that one vertical meter of rock needs about 67,000 years to be weathered,
demonstrating thus the chemical weathering rate of rocks under tropical climatic
conditions.
J. A. Galhardi et al.