Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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RESEARCH

Open AccessDistribution of U and REE on colloids in granitic
groundwater and quality-controlled sampling at
the Mizunami underground research laboratory
Takashi Munemoto*, Kazuaki Ohmori and Teruki Iwatsuki

Abstract
Colloids and their association with analogue elements, uranium, and rare earth elements (REEs), in deep granitic
groundwater were investigated at the Mizunami Underground Research Laboratory (MIU). Groundwater was
sampled from underground boreholes and gallery walls, and the colloids were separated by size-fractionatedultrafiltration (pore sizes, 0.2 μm, 10 kDa, and 1 kDa). For the groundwater sampled from fractures in excavation walls,
the size-fractionated concentrations of the colloid-forming elements were approximately constant relative to different
size fractions (0.2 μm, 200 kDa, 50 kDa, and 10 kDa). The contamination of Fe- and Al-bearing materials was insignificant
in the filtered groundwater from fracture seepages. Changes in the concentrations of U in the groundwater sampledfrom boreholes and excavation walls were associated with the Al-bearing colloids, Fe-bearing colloids, and organic
matter. The REE-bearing material(s) that were >0.2 μm in size were mobile in the deep granitic groundwater, rather
than occurring in association with Al-bearing, Fe-bearing colloids, and organic matter. It is suggested that samplingfrom water-conducting fractures in host rock and colloid elimination in borehole are important components of water
quality control in geochemical investigations.
Keywords: Mizunami Underground Research Laboratory; Colloid-facilitated transport; Quality-controlled sampling;
Deep granitic groundwater; Rare earth elements; Uranium

Background
Colloid sampling procedures in groundwater are an important aspect of groundwater research. Sampling ofgroundwater and colloids is often conducted in vertical ornear vertical boreholes or wells drilled from the groundsurface. However, the artesian pressure in groundwater
may decrease to atmospheric pressure, and the in situ
oxidation-reduction condition of the groundwater may
change during sampling. Furthermore, the subsurface
environment may become contaminated by drilling
fluid, artifacts such as steel drill bits and rods containing Fe, and meteoric water during drilling (Dideriksen
et al. 2007). Changes in the chemical composition andphysicochemical condition of groundwater may induce
the formation of anthropogenic colloids and secondary* Correspondence: munemoto.takashi@jaea.go.jp
Mizunami Underground Research Laboratory, Geoscientific Research
Department, Sector of Decommissioning and Radioactive Waste
Management, Japan Atomic Energy Agency, 1-64 Yamanouchi, Akeyo,
Mizunami, Gifu 509-6132, Japan

minerals, especially Fe(III)-(oxy)hydroxides. The chemical composition of groundwater may be affected by coprecipitation and sorption of elements onto newly
formed colloids and secondary minerals.
Groundwater sampling in an underground research laboratory (URL) is advantageous for in situ sample collection. In situ groundwater and colloid sampling techniquesthat are capable of maintaining artesian pressures and anaerobic conditions have been developed at the Mizunami
Underground Research Laboratory (MIU) in Mizunami,
Gifu Prefecture, Japan (Aosai et al. 2010). However, theeffect of the presence of anthropogenic colloids andsecondary minerals on groundwater chemistry remains
unclear due to sampling difficulties. We have observed
anthropogenic secondary minerals on borehole walls
using a borehole televiewer (BTV) in boreholes drilled
into granitic rock. Although the granite is fresh immediately after drilling, BTV photographs taken in different
boreholes [09MI20 (Figure 1a) and 10MI26 (Figure 1b)
details in the Sampling from boreholes section] show that© 2014 Munemoto et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, andreproduction in any medium, provided the original work is properly credited.

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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ab

Figure 1 BTV photographs of the boreholes (a) immediately
after drilling the 10MI26 borehole and (b) several days after
drilling the 09MI20 borehole.

an anthropogenic blackish precipitate has developed on
the borehole wall several days after drilling (Figure 1b).
We have conducted groundwater and colloid samplingby ultrafiltration using different pore size membrane filters. The distributions of analogue elements, especially
U and rare earth elements (REEs), onto colloidal material in deep granitic groundwater are presented herein.
Based on the results of ultrafiltration, the present study
discusses the effect of colloids on groundwater chemistry
and proposes technical methods for quality-controlled
groundwater and colloid sampling.
Colloids and nanoparticles

Colloids and nanoparticles (generally defined as particlesof <1 μm in diameter) are widespread throughout the
Earth's surface, in sediments, soil, rivers, oceans, andgroundwater (Sholkovitz et al. 1994; Degueldre et al.
2000; Hochella et al. 2008). Mineral colloidal particles,
Al/Fe-(oxy)hydroxides, clays, and carbonates, and biological organic colloids have been observed in natural
systems (Filella et al. 2009). The predominant sources ofPage 2 of 19

the colloids and nanoparticles are (1) the mobilization ofcolloidal materials from source rock and sediments and(2) the precipitation of supersaturated mineral phases
depending on the solubility and solution conditions(Ryan and Elimelech 1996).
The mobilization of colloids and nanoparticles is triggered by chemical disturbance such as changing solution
pH, ionic strength, and surface charge of colloids andnanoparticles, and physical disturbances, such as rapid
infiltration and groundwater flows. The mobility of theparticles in water is very complicated depending on theirgenesis, dispersion, and aggregation (McCarthy andZachara 1989). The removal of colloids and suspended
particles from solution has been conducted by ultrafiltration by using a 0.45 or 0.2 μm membrane (Buddemeierand Hunt 1988). However, the colloidal materials often
passed through the membrane (Pokrovsky et al. 2010),
while aggregates of very fine nanoparticles (5 to 7 nm)
can be trapped on the membrane (Tsubaki et al. 2012).
The sedimentation velocity of particles in solution isdependent on their size (Tsukimura et al. 2010). Largeparticles settle rapidly, whereas nanoparticles scarcely
settle. In natural systems, aggregation of nanoparticles
makes estimating sedimentation velocity difficult because the particle size changes with aggregation (Flury
and Qiu 2008). When colloids are strongly attached tosource rock and/or settle by aggregation, they are immobile. The primary physical disturbance causing colloid mobility is an increase in the water flow velocity bypumping and/or fluctuation of hydraulic pressure. Due
to such physical disturbances, naturally immobile colloids are mobilized in aqueous systems.
The interaction between minerals and water induces
the dissolution of primary minerals and the precipitationof secondary minerals and nanoparticles. The formationof secondary minerals and colloids is dependent on solubility. For example, aqueous Fe easily precipitates fromsolution under oxidizing conditions and a wide range ofpH conditions because of the extremely low solubilityof Fe-(oxy)hydroxides (Stefánsson 2007). In addition to
Fe-bearing colloids, the solubility of toxic elements andradioactive nuclides is very low under reducing conditions (Neck and Kim 2001). Intrinsic colloids consistingof radionuclides have been observed in groundwater, inwhich they occur as oxides and hydroxides (Kersting
et al. 1999). It is important to understand the mobilityof colloids and nanoparticles associated with the mobility of pollutants and radionuclides, especially for theevaluation of safe storage and disposal of nuclear waste.
Interaction between the liquid phase and colloid particles

Colloids and nanoparticles have large surface areas, highsorption capacity, and transport ability for sorbing
aquatic components (Kersting et al. 1999; Novikov et al.

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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2006; Wang et al. 2013). Various laboratory experiments
involving the sorption of elements onto colloids andnanoparticles have been conducted (Waite et al. 1994;
Mahoney et al. 2009). Fe (oxy)hydroxide (ferrihydrite) is
one of the most important colloids in the natural environment for the transport of elements (Raiswell 2011).
Experiments to examine sorption onto ferrihydrite havebeen extensively investigated with regard to both cations
and anions (Dzombak and Morel 1990).
In natural environments, it has been reported that hydrous ferric oxide nanoparticles sorbing U and Pu havebeen transported for great distances in groundwater at
the Mayak Production Association in Russia (Novikov
et al. 2006). Kersting et al. (1999) have reported that Pu
and fission products in groundwater have been observed
1.3 km from the location of explosions at the Nevada
Test Site in the United States. The mobility of Pu andfission products could be facilitated by colloidal materials. The variety of the colloidal material at the site has
been characterized as including uranate, phosphates, silicate, and metallic aggregates (Utsunomiya et al. 2009).
The sorption capacity of colloids is dependent on surface properties, which can vary with solution conditions,such as pH and ionic strength (Davis and Kent 1990).
Clay minerals, such as illite and smectite, are also important sorbents in the natural environment, which canscavenge several toxic metals by adsorption and ion exchange (Opiso et al. 2009). Abdel-Fattah et al. (2013)
investigated the stability of intrinsic Pu colloids withsmectite clay colloid in solution and showed that the intrinsic Pu colloids can be transported in association withstable smectite clay colloids. Aqueous components canmove a considerable distance in the subsurface if they
are sorbed and attached onto mobile colloids and nanoparticles (Kersting et al. 1999; Buddemeier and Hunt 1988).
This phenomenon is referred to as colloid-facilitated transport by McCarthy and Zachara (1989). Thus, colloids andnanoparticles can significantly affect the subsurface transport of elements.
U and REEs in groundwater

Uranium is one of the most important elements of concern with regard to the safe storage and disposal of nuclear waste. In natural conditions, the redox states of U
are insoluble tetravalent [U(IV)] and soluble hexavalent[U(VI)]. The mobility of dissolved [U(VI)] associatedwith colloids has been observed in the natural environment (Claveranne-Lamolère et al. 2011). The adsorptionof U onto colloids is highly dependent on water chemistry.
For example, the adsorption of U onto Fe-(oxy)hydroxide
is dependent on dissolved U species (Waite et al. 1994).
In contrast to the hexavalent state, tetravalent U oxide(uraninite, UO2) is highly stable and insoluble, and its
formation results in the immobilization of U under

Page 3 of 19

reducing conditions (Metcalfe et al. 2006). Most recently, Wang et al. (2013) investigated mobile U(IV) associated with Fe and organic colloids in reducing porewater.
The chemical properties of trivalent REEs, especiallythose for light REEs (LREEs) are comparable to transuranic elements in high-level radioactive waste, americium (Am), and curium (Cm) due to similarities such asionic radius, oxidation state, and complexation of anions(Krauskopf 1986). In addition, REEs are used in industrial processes, and these are anthropogenically released
to river water (Kulaksız and Bau 2013). The mobility of
REEs is affected by rock-water interactions, such as thedissolution of source minerals and the adsorption andincorporation on/into secondary minerals (Bau 1999;
Coppin et al. 2002; Willis and Johannesson 2011). In natural environments, relative abundances of REEs in minerals and water are widely used as a geochemical tracer ofwater-rock interaction. For example, in the Tono area,
Japan (detailed in the site description section), the REEs ingroundwater are leached from granitic source rock deep
underground and transported to shallow sedimentary rockby groundwater flow (Takahashi et al. 2002). Many studies
have focused on the mobility of REEs associated with colloidal materials (Sholkovitz 1992; Dia et al. 2000; Willis
and Johannesson 2011). The distribution of REEs and U
onto colloids is complex because it depends on the colloidal and aqueous species, which vary with aquatic conditions such as pH, Eh, and ionic strength. The quantitative
estimation of the distribution of U and REEs on colloids
in natural groundwater is analogous to colloid-facilitatedtransport in the safe storage and disposal of waste.
Site description of the MIU

Around the MIU construction site in the Tono area,
sedimentary rocks of the Seto Group (12 to 1.5 Ma) andthe Mizunami Group (20 to 15 Ma) unconformably
overlie the basement rock, i.e., the 70 Ma Toki Granite(Sasao et al. 2006). The Toki granite has been classified
into three zones by Iwatsuki and Yoshida (1999) basedon the intensity of fracturing and fracture widths: an intact zone, a moderately fractured zone, and an intensely
fractured zone. The intact zone is characterized by relatively few fractures, which have kaolinite-, calcite-, andchlorite-filling minerals. The moderately fractured zone
has relatively thin fractures with minor alteration of therock matrix, and the dominant fracture-filling minerals
are smectite and chlorite. The intensely fractured zone isstrongly altered, and matrices are replaced by kaolinite
and smectite. In fractured geologic media, groundwaterflow and transport are affected by the density and properties of water conducting fractures (Tsang et al. 1990).
In the Toki granite, groundwater moves through thefractures along water flow paths (Illman et al. 2009). The

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baseline groundwater chemistry was determined throughsurface borehole investigations prior to MIU construction. The groundwater is Na-Ca-HCO3 type in the uppersedimentary rock formations, which is a result of waterrock interaction. In the deeper sedimentary rock and thebasement granite, Na-(Ca)-Cl type groundwater occurs,
which may be derived from a paleo-hydrothermal wateror from fossil sea water recharge in the Miocene (Iwatsuki
et al. 2005).
Construction of the MIU is being conduced by theJapan Atomic Energy Agency (JAEA). By 2013, the MIU
facilities had been excavated to 500 m below groundlevel (mbgl). The underground facilities consist primarilyof the main shaft and a ventilation shaft, There are substages at 100 m intervals connecting the two shafts,
and the two research galleries at 300 and 500 mbgl
(Figure 2a). Three horizontal boreholes, 07MI07, 09MI20,
and 10MI26, were drilled into granite at the −200, −300,
and −400 sub-stages, respectively. Each borehole has
been divided into six sampling sections using hydraulicpackers. Hydrochemical monitoring systems are installed in the boreholes (Figure 2b). These enable sampling of groundwater under anaerobic and in situ
pressure conditions and are used to measure physicochemical parameters (temperature, pH, Eh, and waterpressure).
A preliminary study by Saito et al. (2013) shows thatthe major colloids in the MIU groundwater are organic
matter, Fe-bearing colloids, and Al-bearing colloids. The
analogue elements (REEs and U) in the groundwater
were associated with organic matter and Al-bearing colloids. We focus on the effect of the colloid-forming elements, Fe, Al, and total organic carbon (TOC), on the

Page 4 of 19

groundwater chemistry. The colloid-facilitated transportof analogue elements by the aquatic colloids is also
evaluated.

Methods
Sampling from boreholes

The boreholes used in the present study are separated into
six isolated sampling intervals by impermeable packers(Figure 2d). Groundwater was sampled from two samplingintervals, 34.8 to 57.8 m in borehole 09MI20 and 37.9 to
49.6 m in borehole 10MI26, using the installed hydrochemical monitoring system (Figures 2b,d). The samplinginterval volumes are 31 and 24 L in 09MI20 and 10MI26,
respectively, and the groundwater pressures are 1.7 and
2.7 MPa, respectively. Before opening the groundwater
sampling valves in the monitoring system, the groundwater was resident in the sampling intervals. Upon opening the valve, groundwater flowed through a Teflon
tube to the outlet with an outflow rate of approximately
1 L/min in both boreholes. The groundwater and colloids were sampled and filtered on site by batch ultrafiltration using 0.2 μm, 10 kDa, and 1 kDa membranes in astainless steel, pressure-tight filter unit (Millipore, Billerica,
MA, USA). The ultrafiltration unit was connected directly
to the groundwater sampling outlet tube of the hydrochemical monitoring systems for each membrane pore size.
The groundwater passed through the membranes in the filter unit under differential hydraulic pressures between thesampling intervals and the pressure regulators in order toavoid abrupt changes in the hydraulic pressure in eachborehole: 1.7 and 1.4 MPa in 09MI20 and 2.7 and 2.4 MPa
in10MI26. The groundwater passed through the membranes under inert conditions: almost equal hydraulicFigure 2 Description of the MIU construction site. (a) Schematic representation of the MIU facilities and sampling sites. (b) Groundwater
sampling apparatus (photograph taken at the 10MI26 borehole). (c) Groundwater seepage from an excavated gallery wall. (d) Schematic representationof groundwater sampling from a borehole.

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pressures and anaerobic conditions. In addition to the filtered groundwater, unfiltered groundwater was also collected from the boreholes. The size-fractionated samples of
09MI20 and 10MI26 groundwaters were collected in July2010 and November 2011, respectively.
Blackish precipitation on borehole walls (Figure 1b)
may have been randomly flushed from the borehole wall
due to water pressure fluctuations during sampling. In
order to evaluate the temporal effects of anthropogenic
colloids on the chemical composition of the groundwatersfrom the two boreholes, the initial fluids in the samplingintervals were purged by flushing with fresh groundwater
under hydrostatic pressure conditions. Groundwater was
sampled after 5, 10, and 20 sampling interval volumes had
flowed through in the intervals. The groundwater passed
through a 0.2 μm membrane (ADVANTECH, Tokyo,
Japan) placed in the filter unit. The groundwater and colloids were sampled on site in October 2012 using theultrafiltration procedure for size fractionation describedabove.

Sampling seepage from fractures undergroundIn addition to groundwater sampling from boreholes,
groundwater seepage from fractures in granite was also
sampled. Water conducting fractures occur in the −300 m
Access/Research Gallery at 79.7 and 44.8 m from the main
shaft. The fractures are designated A-SP-198 and A-SP199 (Figure 2c), respectively. The usual groundwater flow
rates from A-SP-198 and A-SP-199 are 1.1 and 15 L/min,
respectively. The groundwater seepages from these fractures are at atmospheric pressure and are slightly oxidized.
However, groundwater devoid of anthropogenic colloids
is expected because man-made disturbances by borehole drilling did not occur. The groundwater seepage
from fractures in granite was collected into 5 L stainless steel reservoirs in the gallery. Immediately after
sampling, the groundwater was portioned into five
Teflon-coated 1 L vessels in the laboratory. Four of thegroundwater samples were filtered through 0.2 μm,
200 kDa, 50 kDa, and 10 kDa membranes, and theremaining sample was not filtered and used for estimating the interaction between ‘natural colloids’ and analogous elements. The molecular weight cutoffs (MWCO)at 200 kDa, 50 kDa, and 10 kDa (ADVANTEC) correspond to pore sizes of approximately 100, 50, and
6.3 nm, respectively (Kanai 2011). The contaminationof stainless steel components with Fe, As, and Cr is almost negligible, and the oxidation-reduction potentialof the collected groundwater was maintained throughultrafiltration (Aosai et al. 2010). In addition to the filtering of groundwater, we also collected unfiltered groundwater. The concentrations in unfiltered fractions refer tothe total content in groundwater, including aqueousPage 5 of 19

components and colloidal material. The groundwater
sampling was conducted in September 2012.
Analytical procedure

The physicochemical parameters (temperature, pH, andEh) of borehole groundwater were measured using a conductivity temperature depth profiler (OceanSeven305,
Idronaut, Brugherio, Italy) equipped with hydrochemicalmonitoring systems maintaining hydraulic pressure andanaerobic conditions. For fracture seepages, the physicochemical parameters were measured on-site using an automated pH meter (D-54 pH meter, HORIBA Ltd.,
Kyoto, Japan) with a glass electrode (9621-10D) and a
pH/ORP meter (D-55 pH meter, HORIBA) with a Ag/
AgCl combination Pt electrode (9300-10D). Before themeasurements, the pH meter was calibrated with
pH 4.01, 6.86, and 9.18 buffers at 25°C, and the ORP
meter was checked its function property using saturated
quinhydron solution. Immediately after the groundwater
samplings, the samples were transferred to a laboratory.
The chemical composition of the major elements (Na+, K+, NH4+, Ca2+, Mg2+, Cl−, F−, Br−, NO3−, and SO42−) was
measured by ion chromatography (ICS-1000, Dionex,
Sunnyvale, CA, USA). The concentrations of Si and B
were measured by inductively coupled plasma opticalemission spectrometry (ICP-OES, CIROS-Mark II, Rigaku,
Tokyo, Japan). The TOC analysis was performed using
an infrared gas analyzer (NDIR, Analytik Jena, multi N/C
2100S, Analytik Jena, Thuringia, Germany).
Both the filtered and unfiltered groundwaters were acidified with ultrapure HNO3 and were refrigerated until
chemical analyses were performed. The chemical compositions were measured by inductively coupled plasma mass
spectrometry (ICP-MS, PerkinElmer, ELAN DRC II,
PerkinElmer, Waltham, MA, USA) for Al, Fe, U, andREEs. The concentrations of U and REEs in the groundwater are very low. Both the REEs and U were preconcentrated using a chelating resin disk (Empore Disk
Cartridge P/N4371, 3M, MN, USA). The chelating disk
was conditioned by sequential injection of 5 mL of 2 M
HNO3, ultrapure water, and a 0.1 M ammonium acetate
solution. After conditioning, 40 mL of the groundwater
sample was injected onto the chelating disk, and the REEs
and U were recovered using 2 mL of 2 M HNO3. Thus,
102 ± 4% of the concentrated REEs and U were recoveredin the preliminary analysis tests. The excess recovery rate
may arise from uncertainty in the concentrating technique but it can also be accounted for by the analyticalerror of ICP-MS (±10%). The morphology and compositions of colloid specimens on the 0.2 μm membranefilter were analyzed by scanning electron microscopy(SEM, HITACHI TD-1000, Hitachi Ltd., Tokyo, Japan) withenergy-dispersive X-ray spectrometry (EDS, HORIBA
EMAX ENERGY EX-250).

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Intensity (Counts)

200

a

b
Si
Pt

100
Al

0

b

S

K Ca

4

2

Fe

6

8

Energy (keV)c
1 µm

Intensity (Counts)

500

c

Si

400
300
200 Al
K

Pt

100

Fe

S

0

2

4

6

8

Energy (keV)

Figure 3 SEM image of colloid collected on a membrane in the 09MI20 borehole. (a) BSE image of the colloid. (b) and (c) EDS spectra ofareas b and c indicated in (a).

Results
SEM observations of colloids in borehole groundwater

The backscattered electron image of the colloids collected in the 09MI20 borehole indicates that they arespherical in shape and have a diameter of approximately
100 nm (Figure 3a). The SEM-EDS examination showedthat the particles mainly consist of Si, Al, S, K, Ca, andFe (Figure 3b,c). Mineralogically, the particles are most
likely Fe-bearing aluminosilicates. In the 10MI26 borehole, Fe sulfide, probably pyrite of approximately 1 μm
in diameter, and aluminosilicates of several micrometers
in diameter were observed (Figure 4).
Groundwater chemistry of major constituentsThe major chemical composition of groundwater is provided in Table 1. The chemical compositions of major

elements in the filtered groundwater from the 09MI20
and the 10MI26 boreholes obtained by size-fractionatedultrafiltration and groundwater replacement in the borehole were approximately the same as that of the periodically sampled groundwater (although not shown, thedata was similar to that presented in Table 1).
The chemical compositions of filtered groundwater
should be similar to smaller pore size fractions because
colloids and suspended particles in groundwater werebasically removed by ultrafiltration. However, the sizefractionated concentrations of Fe in the 09MI20 borehole
increased for the smaller membrane pore sizes (Figure 5a).
The concentrations of Fe in the 09MI20 groundwater
were 52, 16, 40, and 97 μg/L in unfiltered and 0.2 μm,
10 kDa, and 1 kDa filtered groundwaters, respectively.
The concentrations of Al in the 09MI20 groundwater

Intensity (Counts)

1000

a

600
400
200

Si

2

5 µm

Intensity (Counts)

c

4

6

8

Energy (keV)

1000
800

Fe

Al

0

b

b

S

800

c

Si

600
400

Al

200
0

2

4

6

8

Energy (keV)

Figure 4 SEM image of colloids collected on a membrane in the 10MI26 borehole. (a) BSE image of the colloid. (b) and (c) EDS spectra ofareas b and c indicated in (a).

Location

Depth

PackerSampling(mbgl)

intervalterm

pHTemperature

Eh

(°C)

(vs. SHE)(mabh)
07MI07200

38.7 to 47.2

2010.4.21 to

Na+

K+

Ca2+

Mg2+

TC

TIC

TOC

Alkalinity

SO42−

F−

Cl−

NO3−

Br−

NH4+

PO43−

Si

B

(mV)

[mg/L][mg/L][mg/L][mg/L][mg/L][mg/L][mg/L][meq/L][mg/L][mg/L][mg/L][mg/L][mg/L][mg/L][mg/L][mg/L][mg/L]

8.76 ± 0.15

15.8 ± 5.7−180 ± 30

82 ± 40.4 ± 0.1

12 ± 1<0.1

14 ± 114 ± 10.6 ± 0.1

1.2 ± 0.1

13 ± 18.9 ± 0.3

77 ± 10<0.05

0.16 ± 0.05

0.15 ± 0.05

<0.5

6.8 ± 0.1

1.1 ± 0.1

8.44 ± 0.12

19.7 ± 3.9−110 ± 20

110 ± 70.7 ± 0.1

16 ± 30.9 ± 0.1

15 ± 115 ± 10.6 ± 0.1

1.2 ± 0.1

10 ± 18.8 ± 0.3

130 ± 20<0.05

0.23 ± 0.06

0.12 ± 0.04

<0.5

6.7 ± 0.2

1.3 ± 0.1

8.40 ± 0.10

24.1 ± 2.9−210 ± 30

120 ± 50.8 ± 0.1

17 ± 20.8 ± 0.1

14 ± 113 ± 10.6 ± 0.2

1.1 ± 0.17±1

8.6 ± 0.3

160 ± 20<0.05

0.29 ± 0.04

0.12 ± 0.04

<0.5

6.8 ± 0.2

1.5 ± 0.1

8.66

22.9

−71

130 ± 70.7 ± 0.1

36 ± 4<0.1

5.1 ± 0.4

4.8 ± 0.4

<0.5

0.41 ± 0.03

0.9 ± 0.2

8.7 ± 0.4

240 ± 20<0.05

0.48 ± 0.05

0.21 ± 0.03

<0.5

6.8 ± 0.2

1.4 ± 0.1

7.99

22.3

−110

130 ± 70.7 ± 0.1

37 ± 3<0.1

5.5 ± 0.5

5.2 ± 0.5

0.8 ± 0.2

0.43 ± 0.03

0.7 ± 0.2

8.7 ± 0.4

240 ± 20<0.05

0.49 ± 0.06

0.2

<0.5

6.6 ± 0.2

1.4 ± 0.1

2012.12.10
09MI20300

34.8 to 57.8

2010.4.27 toMunemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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Table 1 Average chemical concentration of major elements in the MIU groundwaters

2012.12.4
10MI26400

37.9 to 49.6

2011.1.15 to2012.12.5

ASP-198

300-

2010.4.2 to2012.12.18

ASP-199

300-

2010.4.2 to2012.12.18

On-site measurements of pH, temperature, and Eh for the seepage groundwater were single-point measurements. Groundwater samples were collected from the 07MI07, 09MI20, and 10MI26 boreholes, and from the
A-SP-198 and the A-SP-199 water conducting fractures in excavation walls. mabh, meters along borehole (from collar).

Page 7 of 19

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Page 8 of 19

Figure 5 Size-fractionated concentrations of U and colloid-forming elements from the boreholes. Concentrations of U (inverted triangles),Fe (squares), and Al (circles) during groundwater exchange in the (a) 09MI20 and (b) 10MI26 boreholes.

were 9.5, 5.5, 6.6, and 7.7 μg/L in unfiltered and 0.2 μm,
10 kDa, and 1 kDa filtered groundwaters, respectively.
In the case of groundwater sampled from the 10MI26
borehole, Fe and Al were significantly concentrated inunfiltered and 0.2 μm fractions (Figure 5b). The concentrations of Fe were 85, 110, 3.1, and 2.0 μg/L in unfiltered and 0.2 μm, 10 kDa, and 1 kDa filtered groundwaters,
respectively, and the concentrations of Al were 40, 48, 14,
and 19 μg/L in unfiltered and 0.2 μm, 10 kDa, and 1 kDa
filtered groundwaters, respectively.
Table 2 shows the chemical compositions of the filtered groundwater in each sample interval volume fromthe 09MI20 and 10MI26 boreholes during purging ofgroundwater by allowing flow-through in the boreholes.
Changes in the concentrations of Fe and Al were different
in the two boreholes 09MI20 and 10MI26 (Figures 6c,d).
For the 09MI20 borehole, the concentrations of Fe and
TOC were highest immediately after groundwater purging
began (11 μg/L and 1.8 mg/L for Fe and TOC, respectively)
and remained approximately constant (3.4 to 4.0 μg/L and
0.8 to 1.3 mg/L for Fe and TOC, respectively) during thepurging of 5, 10, and 20 sampling interval volumes ofgroundwater. Despite the changes in the concentrations ofFe and TOC, the Al concentrations were approximately
constant (6.1 ± 0.8 μg/L) during groundwater flow-throughin the 09MI20 borehole (Figure 6d). Compared with
the 09MI20 borehole, the trend in borehole 10MI26
was different. Although the concentrations of Fe and
TOC changed during groundwater flow-through in the
09MI20 borehole, they were approximately constant(2.9 ± 0.4 μg/L and 1.3 ± 0.1 mg/L, respectively) during
the flow-through of the groundwater in the 10MI26
borehole (Figure 6b,c). On the other hand, the Al concentration was highest immediately after the replacement of the groundwater began (13 μg/L). The Alconcentrations remained approximately constant (5.3
to 7.5 μg/L) during the flow-through of 5, 10, and 20
sampling interval volumes of groundwater.

The chemical compositions of the size-fractionatedcolloids sampled from fracture seepages on gallery walls(A-SP-198 and A-SP-199) are listed in Table 3. The concentrations of Fe and Al were approximately constant,regardless of filter pore size. The size-fractionated concentrations of Fe were 3.2 to 4.2 μg/L and 3.2 to 3.6 μg/L
for fractures A-SP-198 and the A-SP-199, respectively(Figure 7). The concentrations of Fe were almost within
the experimental error, i.e., 3.7 ± 0.4 μg/L (1 σ) and 3.5 ±0.2 μg/L (1 σ) for A-SP-198 and the A-SP-199, respectively. Similar to Fe, the size-fractionated concentrations of
Al were approximately constant, but differed significantly
between fractures A-SP-198 (18 ± 1 μg/L) and A-SP-199(9.2 ± 0.3 μg/L) (Figure 7). Although TOC in A-SP-199
was observed in periodic groundwater sampling (Table 1),
the size-fractionated concentrations of TOC could not be
detected for either of the groundwater seepages, even in
the unfiltered fraction (Table 3).

Groundwater chemistry of U and REEs

The size-fractionated concentration of U in unfiltered
09MI20 groundwater (16 ng/L) was slightly higher thanthose of 0.2 μm and 10 kDa filtered groundwaters at 14
and 11 ng/L, respectively (Figure 5a). The U concentration of 1 kDa filtered groundwater was inconsistent withthe size fractionation trend, having the highest value
among the sizes (24 ng/L). In the 10MI26 borehole, as
well as the changes in Fe and Al concentrations, U wasconcentrated in unfiltered and 0.2 μm filtered groundwaters (60 and 65 ng/L for unfiltered and 0.2 μm, respectively). The concentrations of U in 10 and 1 kDa
filtered groundwaters were 3.9 and 7.5 ng/L, respectively,
which were significantly lower compared to those of unfiltered and 0.2 μm filtered groundwaters (Figure 5b).
The concentrations of U were highest immediately
after the flow-through of groundwaters began in thesampling intervals. The concentrations were 12 andBorehole
09MI20

10MI26

Sampling
dateInterval
volumes

V/V0

Al

Fe

TOC

La

Ce

Pr

Nd

[μg/L][μg/L][mg/L][ng/L][ng/L][ng/L][ng/L] [ng/L]

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

[ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L] [ng/L]

Lu

U[ng/L]

2012.10.9

0

0

5.3

11

1.8

4.2

7.6

1.1

4.8

1.3

n.d.

2.1

0.6

4.8

1.6

6.2

0.9

5.6

1.0

12

2012.10.9

155

5

5.8

4.0

1.3

4.0

8.6

1.2

5.9

1.7

n.d.

2.8

0.8

6.9

2.2

8.5

1.2

7.5

1.4

4.2

2012.10.10

310

10

6.1

3.6

0.8

3.9

8.3

1.2

5.5

1.7

n.d.

2.6

0.6

5.9

1.8

7.2

0.9

6.4

1.1

2.9

2012.10.11

620

20

7.3

3.4

0.8

4.3

8.9

1.3

6.2

1.8

n.d.

2.7

0.7

6.2

1.9

7.8

1.1

7.0

1.3

3.7

2012.10.9

0

0

13

3.3

1.3

3.5

6.3

0.9

4.2

1.1

n.d.

1.9

0.5

4.8

1.5

5.7

0.8

5.3

0.8

11

2012.10.9

120

5

7.5

3.1

1.2

3.7

6.6

1.0

4.6

1.4

n.d.

2.0

0.6

5.2

1.6

6.2

0.9

5.5

0.9

4.0

2012.10.10

240

10

5.7

2.8

1.3

4.0

7.6

1.1

5.2

1.4

n.d.

2.4

0.7

5.6

1.8

6.7

1.0

5.7

0.9

4.2

2012.10.11

480

20

5.3

2.4

1.3

3.8

7.9

1.2

5.0

1.5

n.d.

2.4

0.6

5.8

1.8

7.0

1.0

6.0

1.0

5.3

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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Table 2 Changes in the chemical concentrations of colloid-forming elements (Al, Fe, and TOC), REE, and U in the groundwater

V/V0 indicates the ratio of the total groundwater flow-through volume to the packer volume of the sampling intervals in the boreholes. Ultrafiltration was conducted using 0.2 μm membranes. Groundwater samples
were collected during replacement (purging) of the groundwater in the 09MI20 and 10MI26 boreholes.

Page 9 of 19

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Page 10 of 19

3

15

10

5

0

b
TOC (mg/L)

U (ng/L)a

09MI20
10MI26

0

200

2

1

0

600

400

200

0

Replaced volume (L)

600

20

c

d

09MI20
10MI2610

Al (µg/L)

Fe (µg/L)

400

Replaced volume (L)

15

5

0

09MI20
10MI26

0

200

400

600

Replaced volume (L)09MI20
10MI26

10

0

0

200

400

600

Replaced volume (L)Figure 6 Changes in concentrations of U and colloid-forming elements from the boreholes. The concentrations of (a) U, (b) TOC, (c) Fe,and (d) Al during groundwater exchange in the 09MI20 (triangles) and the 10MI26 (circles) boreholes.

11 ng/L in the 09MI20 and 10MI26 boreholes, respectively (Figure 6a). The U concentrations decreased as thegroundwater flow-through proceeded in the boreholes,
becoming approximately constant (3.7 to 4.2 and 4.0 to
5.3 ng/L for the 09MI20 and the 10MI26 boreholes, respectively) as 5, 10, and 20 sampling interval volumes ofgroundwater had been replaced. The changes in the Uconcentrations were similar to those in Fe and TOC in
09MI20 and Al in 10MI26 (Figure 6).
The variations in the size-fractionated chemical concentration of U for the A-SP-198 and the A-SP-199
groundwaters are listed in Table 3. The concentrationsof U in the fracture seepages were approximately constant for the various filter pore sizes. The U concentrations in the fracture seepages were comparable to thatin the boreholes after 5, 10, and 20 volumes of sampling interval space of groundwater had been replaced(Figures 6 and 7).
Figure 8 shows size-fractionated concentrations of
REEs in unfiltered and filtered groundwater sampledfrom 09MI20 and 10 MI26 boreholes. The concentrations of REEs were higher in the unfiltered fractions ofgroundwaters sampled from boreholes. The REE concentrations were decreased by ultrafiltration. In contrast to thesize-fractionated groundwater and colloid sampling, thegroundwater was passed through a 0.2 μm membrane during groundwater flow-through in the sampling interval
space in the boreholes. The concentrations of REEs wereapproximately constant and within analytical error (±10%)during groundwater flow-through in the sampling intervals
in the 09MI20 and 10MI26 boreholes (Figure 9).

Changes in the concentrations of REEs in the unfiltered and filtered groundwater sampled from fracture
seeps are shown in Figure 10. Similar to the changes in
the REE concentrations in the size-fractionated groundwater sampled from boreholes, the above describedchanges were significantly elevated in the unfiltered fractions of the groundwater seepages (Figures 8 and 10).
The REE concentrations decreased with smaller poresize fractions and were approximately constant in 200 to
10 kDa. The higher concentration of REEs in unfiltered
groundwaters from boreholes and fracture seepages suggests that REE-bearing material(s) were suspended in
the groundwater.

Discussion
Groundwater chemistry in boreholes

Saito et al. (2013) studied the size distribution and elemental compositions of colloids in the MIU groundwater
sampled from the 09MI21 borehole located in the −300 m
Access/Research Gallery (Figure 2a) using the flow fieldflow fractionation (Fl-FFF) technique coupled with an online UV/VIS fluorescence detector and ICP-MS analysis.
The Fl-FFF analyzed size-fractionated chemical compositions in colloidal materials trapped on the 1 kDa membrane. The size-fractionated compositions of colloids inthe 09MI21 groundwater varied among the samples andsizes as follows: (1) the size distributions of Fe overlappedwith samples and sizes with a range of 0.5 to 20 nm; (2) Al
and Mg had similar size distributions; (3) changes in REE
concentrations were correlated with those in Al, Mg, andLocation
A-SP-198

A-SP-199

Sampling
date

Pore size

2012.9.12

Unfiltered

Al

Fe

TOC

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

U

[μg/L][μg/L][mg/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L][ng/L]

18

3.2

n.d.

9.1

18

2.2

8.8

2.2

0.1

2.8

0.5

3.5

0.9

2.8

0.3

1.9

0.3

4.5

2012.9.12

0.2 μm

18

4.2

n.d.

3.7

6.2

0.9

3.7

1.0

0.1

1.4

0.3

2.2

0.7

2.0

0.3

1.5

0.3

1.5

2012.9.12200 kDa

18

3.5

n.d.

1.7

3.3

0.5

1.8

0.6

n.d.

0.6

0.1

0.8

0.3

0.8

0.1

0.6

0.2

2.8

2012.9.12

50 kDa

18

4.1

n.d.

1.1

1.9

0.3

1.1

0.4

n.d.

0.5

0.1

0.8

0.3

0.9

0.2

0.6

0.2

1.3

2012.9.12

10 kDa

16

3.5

n.d.

1.1

1.7

0.3

1.1

0.3

n.d.

0.5

n.d.

0.8

0.3

0.8

0.2

0.6

0.2

4.9

2012.9.12

Unfiltered

9.2

3.6

n.d.

3.4

4.5

0.6

2.8

0.9

n.d.

1.5

0.3

2.4

0.6

2.1

0.2

1.3

0.2

2.3

2012.9.12

0.2 μm

9.4

3.6

n.d.

1.6

2.4

0.4

1.4

0.5

n.d.

0.6

0.2

1.1

0.4

1.1

0.2

0.7

0.2

2.0

2012.9.12200 kDa

9.7

3.6

n.d.

1.2

1.9

0.4

1.1

0.4

n.d.

0.5

0.1

0.9

0.3

1.0

0.2

0.7

0.2

2.1

2012.9.12

50 kDa

9.1

3.7

n.d.

1.5

2.3

0.4

1.4

0.4

n.d.

0.5

0.1

1.1

0.4

1.1

0.2

0.8

0.2

1.1

2012.9.12

10 kDa

9.0

3.2

n.d.

0.7

1.0

0.2

0.7

0.2

n.d.

0.3

n.d.

0.6

0.2

0.6

0.1

0.5

0.2

0.92

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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Table 3 Size-fractionated chemical concentrations of colloid-forming elements (Al, Fe, and TOC), REE, and U in the groundwater

Groundwater samples were collected from A-SP-198 and A-SP-199 fractures.

Page 11 of 19

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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Page 12 of 19

Figure 7 Size-fractionated concentrations of colloid-forming elements from underground fracture seepages. Fe (squares) and Al (circles),and U (inverted triangle) for the (a) A-SP-198 and (b) A-SP-199 fractures.

organic colloids; and (4) size distributions of U corresponded to those of REEs.
In the present study, size fractionations of groundwater samples with colloids were conducted by ultrafiltration. In general, suspended particles in groundwater
were removed by ultrafiltration. However, colloids having particle sizes smaller than the membrane pore size
can pass through the filter and mix in the filtrate. The
concentrations of colloid-forming elements in the filtrate
apparently change according to the colloid penetration.

The size-fractionated concentrations of Fe in the
09MI20 borehole varied with the pore size (Figure 5).
The Fe concentration of the 1 kDa filtered groundwater
was higher than that of the unfiltered sample. The possible reason for the higher concentration in the filtered
groundwater compared to the unfiltered groundwater
can be explained by the natural variation of groundwaterchemistry during sampling and/or experimental contamination by colloids in the filtrate. For the groundwater
sampling from boreholes, anthropogenic precipitation(s)

60

a

La
Nd
Ce
Sm
Pr

40
30
20

20

HREE (ng/L)

LREE (ng/L)

50

b

Gd
Dy
Er
Yb
Lu

Tb
Ho
Tm

10

1 kDa

10 kDa0.2 µm0

1 kDa

10 kDa0.2 µmunfiltered0

unfiltered

10

Pore size

Pore size
50

Pore size

d

Gd
Dy
Er
Yb

40

Tb
Ho
Tm
Lu

30
20

1 kDa

10 kDa

0

0.2µm

10

unfiltered

10 kDa0.2 µ m10 kDa00.2 µm0200

unfiltered

LREE (ng/L)

10

400

1 kDa

La
Nd
Ce
Sm
Pr

HREE (ng/L)

c

1 kDa

600

Pore size

Figure 8 Size-fractionated concentrations of REEs from boreholes. (a) LREE and (b) HREE in the 09MI20 borehole and (c) LREE and (d) HREE
in the 10MI26 borehole.

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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LREE (ng/L)

La
Ce
Pr

10

5

0

0

200

400

b

15

Nd
Sm

HREE (ng/L)a15

Page 13 of 19

10

0

200

Replaced volume (L)LREE (ng/L)

La
Ce
Pr

5

0

100

200

300

400

d

15

Nd
Sm

10

0

400

600

Replaced volume (L)HREE (ng/L)

c

15

Tb
Ho
Tm
Lu

5

0

600

Gd
Dy
Er
Yb

500

Gd
Dy
Er
Yb

10

Tb
Ho
Tm
Lu

5

0

0

100

200

300

400

500

Replaced volume (L)Replaced volume (L)Figure 9 Changes in REE concentration during replacement of the groundwater from the boreholes. (a) LREE and (b) HREE in the 09MI20
borehole, and (c) LREE and (d) HREE in the 10MI26 borehole.

5
6
4

4

d

Ce
Nd

Pore size

Tb
Ho
Tm
Lu

0

200 kDa1

0.2 µm0

2

unfiltered

10 kDa50 kDa200 kDa00.2 µm2

3

HREE (ng/L)

1

Pr and Sm (ng/L)

La
Pr
Sm

4

unfiltered

La, Ce and Nd (ng/L)

c

Gd
Dy
Er
Yb

Pore size

Pore size

6

10 kDa2

10 kDa0
50 kDa0

50 kDa2

unfiltered

10 kDa50 kDa200 kDa00.2 µm

1

Tb
Ho
Tm
Lu

4

200 kDa

2
10

Gd
Dy
Er
Yb

b

0.2 µm3

20

HREE (ng/L)

Ce
Nd

Pr and Sm (ng/L)aunfiltered

La, Ce and Nd (ng/L)

30

La
Pr
Sm

Pore size

Figure 10 Size-fractionated concentrations of REEs from water-conducting fractures in excavation walls. (a) LREE and (b) HREE in the
ASP-198 fracture, and (c) LREE and (d) HREE in the ASP-199 fracture.

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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was observed by BTV (Figure 1b). Since the major
concern with the mobilization of colloids is rapid
groundwater pumping, it is recommended that thepumping rate be kept low, to a maximum of approximately 100 mL/min (Ryan and Elimelech 1996). In thepresent case of groundwater sampling using hydrochemical monitoring systems, the groundwater flow rateattained was approximately 1 L/min. Consequently, thedeviation in the fractionated concentration of Fe canbe attributed to the mobilization of colloids in theborehole.
The concentration of Fe in the filtered groundwaterchanged with the replacement of groundwater in thesampling-intervals (Figure 6). Moreover, size fractionation by ultrafiltration may cause a change in the quantity of the colloidal material that passed through the
0.2 μm membrane and/or the aqueous concentration. In
general, the source of aqueous Fe can be the dissolutionof biotite, chlorite, and smectite (Acker and Bricker
1992). The deeper groundwater is significantly reduced(Table 1). However, Fe hydroxides such as goethite are
precipitated on fractures and mineral surfaces in the granite (Iwatsuki and Yoshida 1999). Sugimori et al. (2008)
described the weathering processes of biotite and chlorite in the Toki granite. Iron (oxy-)hydroxides such as ferrihydrite and goethite can precipitate depending on thepO2 or Eh.
We calculated saturation indices (SI = log IAP/Ksp;[IAP, ion activity product; Ksp, solubility product]) for
Fe-bearing minerals using the program REACT from
‘The Geochemist's Workbench’ (Bethke 1992) with thethermodynamic database ‘thermo.com.v8.r6 + .dat’. Immediately after groundwater replacement began, thegroundwater was supersaturated with respect to highFe-bearing smectite (SI = 0.03) and low Fe-bearing
smectite (SI = 0.03 and 0.52), while they became undersaturated (SI = −0.03 and −0.80) after 20 sampling intervalvolumes of groundwater were replaced. If Fe-bearing
phase(s) passed through the membrane, the filtrate should
be in equilibrium and/or oversaturated with respect tocorresponding solid phase(s). The high Fe concentration
immediately after groundwater purging was the result of
Fe-bearing colloids being flushed out and contaminating
the filtrate. Based on SEM observations, Fe-bearing aluminosilicates with a diameter of 100 nm were collected on
the membrane in the 09MI20 groundwater (Figure 3).
Tsubaki et al. (2012) reported that aggregations of
nanoparticles can be trapped by a membrane. Accordingly, the aggregation of colloids explains the trappingof Fe-bearing aluminosilicates on the membrane. Febearing aluminosilicate in the 09MI20 groundwater(Figure 3) was identified by thermodynamic calculation
as Fe-bearing smectite. Such precipitation may be released to groundwater and mobilized by water pressure

Page 14 of 19

fluctuations during sampling. The ‘mobilized’ Fe-bearing
colloids affect the concentration of Fe by passing throughthe membrane. The concentrations of Fe in the borehole
and fracture seeps were comparable (Figures 6c and 7).
Consequently, the Fe-bearing colloids were flushed out
with the groundwater from inside the borehole during removal of the groundwater in the borehole. Thus, the variation in the size-fractionated concentration of Fe in
09MI20 can be explained by the contamination by secondary Fe-bearing colloids formed in the borehole after
drilling.
For the size-fractionated groundwater sampled fromthe 10MI26 borehole, Fe and Al were concentrated inunfiltered and 0.2 μm filtered groundwater, whereas
the concentrations were very low in 10 and 1 kDa filtered groundwater. The 10 kDa MWCO corresponds
to a pore size of 6.3 nm. Consequently, the Fe-bearing
and Al-bearing colloids of 0.2 μm to 6.3 nm in size
were present in the borehole groundwater. The concentration of Al changed during removal of groundwater from the 10MI26 borehole. Immediately aftergroundwater removal began, the filtered groundwater
was supersaturated with respect to several tens of Albearing minerals, kaolinite, low Fe-bearing smectite,
pyrophyllite, etc. After purging 20 borehole volumes,
the groundwater became undersaturated with respect
to low Fe-bearing smectite and pyrophyllite. The highAl concentration immediately after groundwater purging was the result of Al-bearing colloids being flushed
out and contaminating the filtrate. Accordingly, the filtered groundwater was supersaturated with respect tothe low Fe-bearing smectite and pyrophyllite in the initial sampling. After elimination of the mobilized Albearing phase(s), the groundwater became undersaturated with respect to the Al-bearing phase(s) and large
portions of the Al-bearing phase(s) were flushed out
during the groundwater purging. The colloids and secondary minerals adhering to the borehole walls were
mobilized by fluctuating water pressures during borehole sampling. These were unavoidable processes insome uncertainties in the research on colloids ingroundwater.
Nevertheless, groundwater sampling from boreholes
has the significant advantage of in situ sample collection. At least, the concentrations of Fe, Al, and TOCchanged during purging of groundwater from boreholes. The colloids were likely mobilized by fluctuations
in water pressure during sampling. For quality controlof groundwater and colloid sampling from boreholes,
the presence of anthropogenic colloids can be mitigatedto some extent by pumping out or flushing the groundwater from a borehole by removing several volumes ofgroundwater from the packer isolated sampling intervals in the borehole.

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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Groundwater chemistry of size-fractionated fracture
seepage

In the Toki granite, groundwater flow is fracturecontrolled (joints and faults) (Illman et al. 2009). Hence,groundwater from both boreholes and seepage passed
through the granitic fractures and/or fracture networks.
The concentrations of major constituents in the groundwater seepages in the −300 m gallery were different fromthe concentrations in the 09MI20 borehole groundwater,even though both sample locations are at the same
depth (Table 1). Around the study area, the groundwaterchemistry is Na-Ca-HCO3 type at shallow depths,
whereas it is Na-(Ca)-Cl type at deep levels (Iwatsuki
et al. 2005). Although the concentration of Cl− in borehole groundwater increased with depth, that in the
groundwater seepages were higher than in boreholes atthe same depth (−300 m) (Table 1). The sampling locations of boreholes are sandwiched between the main
shaft and the ventilation shaft (Figure 2), and thus theunderground conditions were depressurized vertically as
shaft construction progressed (Mizuno et al. 2013). In
contrast, the fracture seepages are located in the horizontal −300 m access/research gallery (Figure 2). Consequently, the groundwater sources for the boreholes andthe fracture seepages on the gallery wall were likely
different.
For the groundwater seepage from fractures in galleries, the colloid-forming elements, Fe and Al, are likely tobe present as aqueous (complex) ions in the groundwater flowing in fractures (Figure 7). In the Toki granite,montmorillonite and chlorite occur as fracture-fillingminerals (Iwatsuki and Yoshida 1999). The differences in
the Al concentrations can be accounted for by the variation of fracture-filling minerals. The groundwater seepage was supersaturated with respect to beidellite in the
A-SP-199 fracture, but was undersaturated in the A-SP198 fracture. The saturation index for Al-bearing minerals in the fractures, the Mg-bearing smectite group,
and illite could not be calculated because the Mg concentration is too low, i.e., below the detection limit(Table 3). Since the mobile Fe-bearing and Al-bearing
colloids were uncommon in the groundwater from thefracture seepages, the natural Fe-bearing and Al-bearing
colloids may be negligible in the groundwater in fractures.
The groundwater sampled from seepages was free of contamination by anthropogenic materials. The groundwater
sampling from the water-conducting fractures is suitablefor water quality control of colloid issues.
Distribution of U on colloids

Changes in the size-fractionated concentrations of U ingroundwater from the 09MI20 and 10MI26 boreholes
are associated with similar changes in concentration of
Fe and Al (Figure 5). For the 09MI21 borehole, the size

Page 15 of 19

distributions of U correspond to those of Al and organic
matter (Saito et al. 2013). In the present study, the U association with Al for the 10MI26 is consistent with thatfor borehole 09MI21. In addition, the U concentration isassociated with the Al and Fe in the 10MI26 groundwater.
The U concentrations decreased at an approximately constant rate as groundwater replacement proceeded in theboreholes (Figures 5 and 6). Although aqueous U forms
strong carbonate complexes (Waite et al. 1994), alkalinity
was constant during the groundwater flushing in the boreholes (1.24 and 1.15 meq/L for the 09MI20 and 10MI26
boreholes, respectively). Saturation indices for U-bearing
minerals, such as uraninite (UO2) during flushing of thegroundwaters were −1.0 to −1.5 and −1.0 to −1.3 in the
09MI20 and 10MI26 boreholes, respectively. The groundwaters were undersaturated with respect to the U-bearing
minerals. Accordingly, U-bearing intrinsic colloids cannot occur in the groundwater. The changes in the Uconcentrations were similar to those in Fe and TOC in
the 09MI20 borehole and Al in the 10MI26 borehole(Figure 6). The association of U with Fe-bearing, Albearing, and organic colloids resulted in the contamination of the filtered groundwater. The size distributionsof U-bearing colloids correspond to those of Al-bearing
colloids and organic colloids (Saito et al. 2013). Thechange in U concentration along with the concentrations of Al was consistent with the size fractionation by
Fl-FFF. In addition, Fe-bearing and/or organic colloids ofdiameter <0.2 μm affected the sampling quality of thegroundwater.
The size-fractionated chemical compositions of U for
the A-SP-198 and A-SP-199 groundwaters are approximately constant, irrespective of filter size. If the U moves
with colloids of various particle sizes, the U concentrations should decrease as the pore size decreases. However, the U concentrations were approximately constantregardless of pore size. Thus, the uranium migration bycolloidal phases in groundwater fracture flow was not
identified in the present study.
Distribution of REEs on colloids

The size-fractionated concentrations of REEs in borehole
groundwaters decreased significantly as membrane poresizes decreased (Figure 8). The REE concentrations of
unfiltered groundwater were approximately ten times
higher than that of 0.2 μm filtered groundwater. The
correlation between size-fractionated concentration of
REEs and Al and Fe were not observed in the boreholes(Figures 5 and 8). In contrast, the concentrations of
REEs were approximately constant during groundwater
replacement in the sampling intervals in the 09MI20
and 10MI26 boreholes (Figure 9). The transport of REEs
associated with Fe-bearing and Al-bearing colloids was
not observed (Figures 6 and 9). Ultrafiltration wasMunemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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conducted using a 0.2 μm membrane during groundwater flow-through in the sampling intervals in theboreholes. The independence of REE concentrations on
groundwater replacement suggested that the >0.2 μm
REE-bearing colloids in the groundwater were almost
completely trapped on the 0.2 μm membrane. In addition
to the borehole groundwater, the concentrations of REEs
are significantly higher in unfiltered groundwater fromfracture seepages (Figure 10). The REE concentrations decreased with decreasing pore size fractions, which wereapproximately constant from 200 to 10 kDa. The 200 kDa
of MWCO membrane corresponds to a pore size of
approximately 100 nm. The mobile Fe-bearing and Albearing colloids are rare in the groundwater from fracture seepages (Figure 7). Consequently, the mobility of
REEs was scarcely facilitated by Fe-bearing and Albearing colloidal materials. The >0.2 μm REE-bearing
particles were transported in the MIU groundwater.
Iida et al. (1998) investigated REE-bearing minerals in
the Toki granite, bastnaesite ((LREE)CO3F), and parasite (Ca(LREE)2(CO3)3 F2), which are several hundreds
to tens of micrometers in size. Thus, the large REEbearing mineral(s) were potentially removed from thegroundwater by the 0.2 μm membrane. In the present
study, REE-bearing mineral(s) suspended in the groundwater cannot be directly observed by SEM observation
because of the very low concentration of colloids ingroundwater. The REE-bearing particles are mobile ingroundwater sampled from both borehole and fracture
seepages (Figures 8 and 10).
The chemical properties of REEs, especially LREEs, arecomparable to those of transuranic elements due to thesimilarities in ionic radii, oxidation state, and complexation
with anions (Krauskopf 1986). Chondrite-normalized REE
patterns of size-fractionated groundwater sampled from
boreholes are shown in Figure 11. For the unfiltered
09MI20 groundwater, the REE pattern has a downward
convex shape in the middle REE (MREE), which wasPage 16 of 19

notable for filtered groundwater (Figure 11a). The REE
pattern of unfiltered 10MI26 groundwater was significantly different from those of the filtered groundwaters.
The unfiltered 10MI26 groundwater was enriched withLREE (Figure 11b). The REE-bearing materials weresuspended in the groundwater and affected the REE
pattern of the unfiltered groundwater. In contrast, theREE patterns of filtered groundwater were enriched inheavy REE (HREE), which was comparable to the patterns during groundwater replacement.
Figure 12 shows chondrite-normalized REE patterns of
the filtered groundwaters during groundwater replacement in the borehole. Although the concentrations of
Fe, Al, and TOC for the filtered groundwater changed,
the REE patterns in the groundwaters did not change
during the groundwater flow-through in the boreholes
and were significantly enriched in HREEs. In CO2-richand pH-neutral groundwater, HREEs are enriched due tothe REE carbonate aqueous complex (Négrel et al. 2000).
In the present study, REE aqueous species were calculatedusing ‘The Geochemist's Workbench’ (Bethke 1992),
which revealed that the predominant REE species were
REE(CO)3+ in all filtered groundwater. The Takahashi
et al. (2002) study revealed that the REEs in the
groundwater (approximately 150 mbgl) are dissolved
carbonate complexes from the Toki granite. The HREE
enrichment in the groundwater can be explained by theformation of carbonate complexes.
With respect to borehole sampling, anthropogenic colloids and secondary mineral(s) precipitated on the borehole walls (Figure 1). The anthropogenic colloids formedin the borehole were flushed out during groundwaterflow-through (Figure 6). However, the mobility of REEs
in the borehole groundwater did not change with themobilized Fe-bearing, Al-bearing, and organic colloids.
Sholkovitz (1992) studied the mobility of REEs associated with colloids in river water. The REE patterns of
the river water and those of colloids are enriched in
10-2

10-3

10-4

10-5

10-6

La Ce Pr Nd

Sm EuGd Tb Dy Ho Er TmYb Lub
Sample / C1 Chondrite

Sample / C1 Chondriteaunfiltered
0.2 µm
10 kDa
1 kDa

10-3

unfiltered
0.2 µm
10 kDa
1 kDa

10-4

10-5

10-6

La Ce Pr Nd

Sm EuGd Tb Dy Ho Er TmYb Lu

Figure 11 Chondrite-normalized REE patterns of the unfiltered groundwater and filtered groundwater with different pore sizes.
Chondrite data are from Anders and Grevesse (1989). Unfiltered (circles), 0.2 μm (triangles), 10 kDa (squares), and 1 kDa (inverted triangles) in the(a) 09MI20 and (b) 10MI26 boreholes.

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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Page 17 of 19

10-4

10-4

b
Sample / C1 Chondrite

Sample / C1 Chondrite

a

10-5

10-6

La Ce Pr Nd

10-5

10-6

Sm EuGd Tb Dy Ho Er TmYb Lu

La Ce Pr Nd

Sm EuGd Tb Dy Ho Er TmYb Lu

Figure 12 Chondrite-normalized REE patterns of the groundwater during development of the (a) 09MI20 and (b) 10MI26 boreholes.
Chondrite data are from Anders and Grevesse (1989). V/V0 is the ratio of the replaced volume of the groundwater to the sampling intervalvolume in the boreholes.

HREE and LREE, respectively. The enrichment of LREE in
unfiltered groundwater and HREE in filtered groundwater isin agreement with the river colloid samples. In the present
study, REE carriers are the <0.2 μm REE bearing particles.
For the groundwater sampled from fractures, chondritenormalized REE patterns of unfiltered and size-fractionatedfiltered groundwaters are shown in Figure 13. The REE pattern of the unfiltered groundwater for A-SP-198 wasslightly enriched in LREE. In contrast, the REE pattern of
the filtrate was almost flat from the lightest REE (La) to theheaviest REE (Lu). For A-SP-199, because of a small change
in the concentrations of REEs in the unfiltered and filtered
groundwater (Figure 10), changes in the REE patterns by
REE-bearing particles were not observed (Figure 13b). The
REE-bearing materials are mainly >0.2 μm in size in the
groundwater sampled from underground fracture seeps(Figure 10). Accordingly, the REE patterns of unfiltered
groundwater probably exhibit a mixture of the groundwaterand REE-bearing solid materials. In addition to theborehole sampling, the enrichment of LREEs in the unfiltered groundwater may be accounted for by the contamination of LREE-enriched natural colloidal materials(>0.2 μm) in the groundwater. The mobility of REEs was
only slightly facilitated by the Fe-bearing and Al-bearing
colloids. Consequently, the REE-bearing materials thatoriginated from the source granite were transported in acontinuous groundwater inflow system.

Conclusions
We investigated colloids and their association with analogueelements at the Mizunami Underground Research Laboratory.
Groundwater was sampled from underground boreholes and gallery walls, and the colloids were separated
by size-fractionated ultrafiltration. The major findings
and conclusions are as follows:
1) U was associated with Al-bearing colloids, Fe-bearing
colloids, and organic matter.
2) REE-bearing material(s) >0.2 μm were mobile indeep granitic groundwater.
3) Groundwater sampling from water-conducting
fractures in host rock and colloid elimination in
boreholes are important components of water quality
control in geochemical investigations.

10-4

10-4

10-5

b
Sample / C1 Chondrite

Sample / C1 Chondriteaunfiltered
0.2 µm200 kDa
50 kDa10 kDaunfiltered
0.2 µm200 kDa
50 kDa10 kDa

10-5

10-6
-6

10

La Ce Pr Nd

Sm EuGd Tb Dy Ho Er TmYb Lu

La Ce Pr Nd

Sm EuGd Tb Dy Ho Er TmYb Lu

Figure 13 Chondrite-normalized REE patterns of the unfiltered groundwater and filtered groundwater with different pore sizes.
Chondrite data are from Anders and Grevesse (1989). (a) A-SP-198 and (b) A-SP-198 fracture seeps.

Munemoto et al. Progress in Earth and Planetary Science 2014, 1:28
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Abbreviations
BTV: borehole televiewer; EDS: energy-dispersive X-ray spectrometry; Fl-FFF: flow
field-flow fractionation; HREE: heavy REE; JAEA: Japan Atomic Energy Agency;
LREE: light REE; MIU: Mizunami Underground Research Laboratory; REE: rareearth elements; SEM: scanning electron microscopy; URL: underground researchlaboratory.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
TM carried out microscopic analysis, interpreted the data, drafted the paper. KO
carried out groundwater sampling and chemical analysis. TI designed anddirected the study and helped to draft the paper. All authors read andapproved the final manuscript.
Acknowledgements
We would like to thank Dr. K. Umeda, convener of the session ‘Geological
Disposal of Radioactive Waste and Earth Science’ at the Japan Geoscience
Union Meeting 2013 for recommending invited research articles based on apresentation at the annual meeting. The technical comments and English
review by Mr. G. McCrank, an ex-JAEA International Fellow, are appreciated.
The quality of our manuscript has been significantly improved by valuablecomments from three anonymous reviewers.
Received: 8 January 2014 Accepted: 12 November 2014

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Cite this article as: Munemoto et al.: Distribution of U and REE on
colloids in granitic groundwater and quality-controlled sampling at
the Mizunami underground research laboratory. Progress in Earth andPlanetary Science 2014 1:28.

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