th
The 12 International Conference of
International Association for Computer Methods and Advances in Geomechanics (IACMAG)
1-6 October, 2008
Goa, India
In-situ Lysimetric Studies for Radionuclide Migration in
Undisturbed Unsaturated Soil under Geoenvironmental Conditions
R. R. Rakesh, P. K. Narayan, P. K. Wattal
Back End Technology Development Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India
S. Anil Kumar
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India
B. Hanumantha Rao, V. Sridhar, D. N. Singh
Dept. of Civil Engineering, Indian Institute of Technology Bombay, Mumbai, India
3
Keywords: Vadose zone, In-situ experiment, Lysimeter, Pore water samplers, Radionuclide ( H,
137
60
Cs, Co) migration.
3
137
60
ABSTRACT: Migration of radionuclides H,
Cs and Co in undisturbed vadose zone was studied using
lysimeters at near surface radioactive waste disposal facility, RSMS Trombay site. The soil type of the site was
clayey sand with appreciable quantity of montmorillonite. The studies were performed in two lysimeters separated
by a distance of 50m. Radionuclides 3H (20 MBq), 137Cs (0.55MBq) and 60Co (0.40MBq) were doped as a tracer
in sand layer placed at 100-200 mm from top of the surface of lysimeters along with the inactive salt solution of
CsNO3 and Co(NO3).6H2O. Pore water samplers at different depths and Time Domain Reflectometer (TDR) were
installed in the lysimeter. Periodically pore water samples were extracted using hand operated vacuum pump and
the samples were analysed for radionuclides present. After rainy season the soil was cored up to a depth of
55cm and analysed for radionuclides migration depth wards. Liquid Scintillation Spectrometer and high resolution
Gamma Ray Spectrometer were used for isotopic activity analysis. The study revealed that under normal
3
137
environmental condition, H migrated nearly 40 cm in 14 days of simulated rainfall condition; however Cs and
60
Co did not show any significant migration even in nearly 500 days.
1
Introduction
Low and Intermediate Level radioactive Waste (L & ILW) are generated at various stages of nuclear fuel cycle,
which includes the mining and milling of uranium ore, fuel fabrication, reactor operation and spent fuel
reprocessing. Besides these sources, radioactive wastes are produced as a result of the ever-increasing use of
radioisotopes in medicine, industry and agriculture. After proper treatment and conditioning of these wastes the
solid/solidified waste is disposed in the various disposal modules viz., Earth Trench (ET)/Stone Lined Trench
(SLT), Reinforced Concrete Trench (RCT) and Tile Holes (TH) of Near Surface Disposal Facility (NSDF). The
safety assessment and health monitoring of these disposal modules are carried out periodically. The safety
assessment of operating NSDF sites has proved that the site is providing adequate safety in terms of radiological
dose to the public and is below the regulatory limit (Rakesh et al., 2005, Rakesh et al., 2007). However, a
temporal and spatial variation in site-specific parameter of the site introduces uncertainty in exact prediction of
migration behavior. The issue is further complicated in case of radionuclide transport through the vadose zone
which is lying between the bottom of disposal modules and the water table at least for a part of the year. The
simulation of evaporation/evapotranspiration further complicates the modelling. Hence, to have a confidence
building in the radionuclide migration studies using mathematical modeling, the results of modeling studies need
to be supplemented with the in-situ experiment. With this in view, and to generate reliable database on
radionuclide migration under geo-environmental conditions, in-situ experiment on radionuclide migration using
Lysimeter has been initiated in 2005 and is in progress at one of the operating NSDF site, Radioactive waste
Storage and Management Site (RSMS), Trombay.
Lysimeter, a cylindrical metal container made of material such as fiberglass, MS, SS or aluminum and open at
both ends have become important tools and is in practice by many researchers to study the contaminant
transport in agriculture (Parizek and Lane, 1970; Swistock et al., 1990; Litaor, 1988), groundwater hydrology
(Parizek and Lane, 1970; Rogers and McConnel, 1991; Litaor, 1988) and for calibrating and validating theoretical
models of solute transport (Schoen et al., 1999; Eriksson et al., 1997). The contaminant migration is studied by
2320
exposing Lysimeter to natural physical, chemical and biological conditions, typical of disposal sites. This is
followed by temporal sampling of the pore water from different depths of the controlled matrix and its analyses to
know the radionuclide concentration in pore water and its temporal and spatial variation.
The experiment is designed for developing a generalized in-situ experimental methodology that can be employed
for studying the migration behavior of different radionuclides under natural geo-environmental condition.
2
Experimental investigations
2.1
Soil characterization
Soil samples were collected from RSMS Trombay from a depth of 1.0 and 2.0m at two different lysimeter (L1, L2)
experimental locations which are separated by a distance of 50m each. However, to create a representative
sample from two different locations soils of same location of different depths were mixed together and
representative samples of two different locations were generated. These representative samples were
characterized for its physical characterization (specific gravity, ASTM D 5550-94; grain size, ASTM D 422-63;
consistency limits, ASTM D 4318-93; ASTM D 427-93), geotechnical characterization (compaction
characteristics, ASTM D 698-07; saturated soil hydraulic conductivity, ASTM D 5896-95) and geochemical
characterization of the soil samples was performed as per IS: 2720 (Part XXIV)–1976. Soil classification has
been done based on these results using ASTM D 2487-93. The geotechnical characterization was carried out on
the two representative samples. The physical, geotechnical and geochemical characteristics of the soil samples
are presented in Table 1. The grain size analysis of the soil samples are presented in Figure 1. The chemical
composition of these soils have been analysed (Vogel, 1986) for major oxides and is presented in Table 2.
Table 1. Soil characteristics
Soil samples
L1
L2
Property
Physical characteristics:
Specific Gravity
Particle size distribution characteristics:
2.2
Sand (%)
2.3
Silt (%)
2.4
Clay (%)
Consistency limits:
L.L. (%)
P.L. (%)
S.L. (%)
P.I. (%)
Geotechnical characteristics:
Compaction characteristics:
γdmax (g/cc)
OMC (%)
Saturated soil hydraulic conductivity (m/s)
Geochemical characteristics:
Cation Exchange Capacity (meq/100 g)
USCS
2.73
2.65
51
37
12
48
39
13
43
26
26
17
46
29
24
17
1.66
16.31
1.680E-04
1.70
19.45
1.075E-04
56.8
SM
48.8
SM
Table 2. Chemical composition of the soil samples
Oxide
L1
L2
SiO2
40.25
41.42
Fe2O3
31.30
30.08
Al2O3
20.46
20.90
CaO
2.92
2.79
TiO2
1.86
1.80
MgO
1.27
1.25
Na2O
1.26
1.09
K2O
0.56
0.54
V205
0.06
0.08
P2O5
0.06
0.05
2.5
Radionuclides
3
137
60
The radionuclides used in this study are H, Cs (CsCl) and Co [CoCl2]. These radionuclides were collected
from Board of Radiation and Isotope Technology (BRIT), Mumbai.
As the contaminants used in this study are radioactive, limited activity of radionuclides 3H (20MBq), 137Cs
(0.55MBq) and 60Co (0.40MBq) have been used in each lysimeter. These activity has been used as a tracer
along with inactive salt solution of CsNO3 and Co(NO3)26H2O.
2321
100
% finer
80
60
40
20
L1
L2
0
-3
10
10
-2
-1
10
0
D (mm)
10
10
1
2
10
Figure 1. Grain size distribution curves of the soil samples
2.3 Lysimetric experiment
The experiment has been performed in duplicate to have a reproducibility of data. For this two lysimeters, made
of MS of diameter 1200 mm and height 1500 mm have been installed at the RSMS Trombay site as depicted in
Figure 2. The top 200 mm of soil in the Lysimeter was excavated out and then part of the excavated portion was
backfilled with sand up to 100 mm as depicted in Figure 3. The two experimental setups were separated by an
approximate distance of 50m. These lysimeters have been secured by providing a steel caging to protect the
instruments installed in it from rodents and other small animals. The plan and sectional view of the lysimeter as
depicted in the Figure 3, presents the details of the instrumentation done for the study and the same has been
listed in Table 3.
Table 3. Instrumentation details of Lysimeter 1 (L1)
Designation
S1
S2
S3
S4
SS
T11 and T12
GWT
U
Description
Pore-water sampler
MS steel ring
(Lysimeter)
Purpose
For sampling @ 285 mm depth
For sampling @ 355 mm depth
For sampling @ 455 mm depth
For sampling @ 565 mm depth
For creating a control volume
For determining in situ moisture
Moisture probe (TDR) content for the entire depth of the
control volume
Ground water Table Below the zone of study
Undisturbed
Vadose zone (control volume)
unsaturated soil
Four numbers of pore water samplers (Sreedeep and Singh, 2005a,b) designated as S1 to S4, have been
installed to extract pore solution from the soil mass. These samplers consist of Perspex tube of length 450 mm
and diameter of 20 mm. The bottom end of the Perspex tube is provided with a U shaped ceramic thimble of 45
mm in length and of 100 kPa air entry value. The top end of the Perspex tube is provided with an air-tight screw
cap through which a flexible rubber tube, of sufficient length, can be inserted. The other end of this flexible tube is
connected to a Perspex sampling bottle. Two tubular access tubes for insertion of Time Domain Refelctometer
(TDR) tube probe, designated as T11 and T12, have been employed for determination of the volumetric moisture
content of the soil, θ, as a function of depth, Z. These moisture content measurements shall be used as an input
parameter in the mathematical modeling of radionuclide transport through vadose zone. The details of the
location of the pore water samplers and access tube of the TDR probes have been depicted in the Figure 3.
2322
Suction samplers
TDR Access tubes
Figure 2. Lysimeter along with instruments installed at
RSMS Trombay
Fig.3. Instrumentation details in the experiemtnal set
up Lysimeter 1 (L1)
In both the experiments radioisotopes (3H, 137Cs and 60Co) were used as tracers with the inactive salt solution of
CsNO3 and Co(NO3)26H2O. A total 10 l active solution of specific activity concentration 2000 Bq/ml (3H), 55 Bq/ml
137
60
( Cs) and 40 Bq/ml ( Co) was sprinkled over this sand layer as depicted in Figure 3. The inactive Cs and Co
concentrations in the solution were of 17 and 45 (mg/ml) respectively. This was followed by backfilling of
remaining excavated portion (0-100 mm) of Lysimeter with native soil by providing gentle compaction.
In due course of time, under the influence of various hydro-geologic conditions (viz., precipitation, runoff,
infiltration, evapo-transpiration and cyclic wetting-drying cycles) the contaminant would migrate through the
vadose zone first followed by migration through saturated aquifer system. To expedite the experiment equivalent
rainfall was simulated once a day for 15 minutes at an average flux rate of about 2000 ml min-1 (0.1768cm min-1)
for 45 days duration in non monsoon period and then it was left for natural environmental condition. The flux rate
was finalized based on the average annual rainfall and was distributed over a rainfall period of 90 days.
Hand operated vacuum pump (of capacity 100 kPa) have been used to extract the soil water through pore water
samplers installed at different depths and collected it in bottles attached with the pore water samplers. These
water samples were collected at regular intervals and analysed for traces of contaminant migration up to different
depths. Out of the four pore-water samplers installed in each lysimeter only one (S4) installed at the maximum
depth, yielded significant amount of pore-water at all times, one yielded (S3) intermittently where as the during
rainy season all the samplers performed satisfactorily. It must be noted that choking of samplers does not seems
to be a possibility for no yielding of the pore-water, but it may be the volumetric moisture content less than a
certain value (35%) in top soil as supplemented by the TDR readings at corresponding depths.
2.4 Pore solution analysis
3
137
60
Base line data on H, Cs and Co concentration in soil water of the experimental facility was generated by
extracting the pore water samples using the samplers before doping the activity in the lysimeter and analyzing it
for the activity of above-mentioned radionuclides. After doping the activity in experimental set-up, pore water
samples were extracted from the experimental facility at different times and were analyzed for different
radioisotopes. These pore water samples were first analysed for 137Cs and 60Co concentration in the water
2323
samples using HPGe based high resolution gamma ray Spectrometer. The detector is co-axial P-Type HPGe
(make: EURISYS MESURES, FRANCE) with a relative efficiency of 50% and energy resolution of 2 keV for 1332
keV gamma energy of 60Co. Detector output is analyzed using PC based multi channel analyzer and associated
software. The pore solution samples were counted in standard vial geometry for a period of 10000 sec. After
137
60
3
counting for Cs and Co the water samples were distilled and the distillate was counted for H activity using
Liquid Scintillation Spectrometry (LSS) System (make: LKB WALLAC QUANTULUS 1220). The distillate samples
were counted for 500 min in the standard LSS geometry.
2.5 Soil sample analysis
Base line data on 137Cs and 60Co concentration in the experimental setup soil was generated by coring of the soil
samples and its analysis for the specific activity of above-mentioned radioisotopes, before doping the activity in
the experimental facility. Once the activity was doped, after rainy season, soil samples from the experimental
facility were cored out up to a depth of 55 cm and the cored soil samples were analyzed for specific activity of
137
60
Cs and Co using HPGe based high resolution gamma ray Spectrometer.
3
Results and Discussions
In first 45 days of simulated rainfall condition, the pore water samples from different depths were extracted daily.
The analysis of pore water samples extracted from different depths at different time revealed that only 3H (non3
reactive) was present in the pore water samples. This is due to fact that H is non-reactive tracer and so it doesn’t
3
get sorbed in the migrating media. The results of H concentration with time at a depth of 40 cm is presented in
3
Figure 4. The movement of H indicates that as time passes, the wet front moves downward through the soil and
reaches sequentially at different monitored levels. All other radionuclides have not shown any activity in the
extracted pore water samples, during the simulated rainfall period of 45 days.
1300
L1
L2
1200
1100
1000
3
H Activity (Bq/ml)
900
800
700
600
500
400
300
200
100
0
-100
-20
0
20
40
60
80
100
120
140
160
180
200
Time (days)
3
Figure 4. H activity concentration vs. time in different Lysimeters
137
60
In case of other radionuclides, (i.e. reactive solutes, Cs and Co) due to high CEC values of RSMS soil, the
radionuclides are sorbed in the migrating media and so the radionuclides concentration in the pore solution was
137
60
Cs and Co specific activity (radionuclide concentration per unit dry weight of soil) was
not traced. The
determined from the cored soil samples and the results are depicted in Figure 5. The results clearly indicate that
maximum specific activity of the doped radionuclides are traced at the depth of 15-20 cm, which indicates that
maximum activity is still present there and very little migration has occurred depth wise in nearly 500 days. The
results also indicate that only traces of radionuclides are expected when the depth exceeds 55 cm. Also due to
high sorption value of the soil (Rakesh, 2005), whatever small contaminant migrates it gets sorbed in the soil
matrix and it doesn’t available in free water.
2324
Figure 5. 137Cs and 60Co migration pattern with depth
To have a reproducibility of results, 3H doping was repeated in the following year again and the pore solution
3
samples were collected and are being analysed for H concentration. Soil coring in lysimeter has also been
performed after second year rainfall and the cored soil samples are also being analysed for specific activity of
137
60
Cs and Co.
4
Conclusions
This paper dealt with methodology and associated instrumentation development for in-situ radionuclide migration
experiment at the NSDF sites, and to analyse the reactive solute transport through the vadose zone. The
experiment is of long term (few years) and the results are intermediate in nature. The study revealed that only
few water samplers yielded significant pore water at all times, whereas during rainy season all performed
satisfactorily. This indicates that choking of samplers does not seem to be a possibility for no yielding of the porewater. The study concluded that under normal environmental condition, 3H migrated nearly 40 cm in 14 days of
simulated rainfall condition; however 137Cs and 60Co did not show significant migration even after nearly 500
days. Although, this is an interim result of the in-situ experiment but it concludes with fair degree of confidence
137
60
that in RSMS Trombay, migration of Cs and Co through vadose zone is very limited and maximum activity is
still retained near the doped zone. The migration results obtained are promising and they have demonstrated the
suitability of the method for studying the in-situ radionuclide migration at NSDF sites. The results have also
proven the great importance of sorption, responsible for retardation of reactive radionuclides while migrating
through the geo-environment. Such results can be useful for selecting the NSDF sites and it can also be used for
estimating the waste load capacity of a particular site.
5
Acknowledgements
Authors acknowledge sincere gratitude to Atomic Energy Regulatory Board for providing financial support for this
project. Authors are extremely thankful to Shri S. D. Misra, Director NRG/BARC, for his keen interest in this
project and for granting the permission to publish this study. Authors also acknowledge Shri M. R. Joshi, Dr. D. N.
Yadav, BETDD/BARC, and Dr. K. K. Narayan, RSSD/BARC for their help at several stages of experimentation.
2325
6
References
ASTM D 422-63. 1994. Standard Test Method for Particle Size Analysis of Soils, Annual Book of ASTM Standards, Vol. 04.08,
ASTM International, West Conshohocken, PA.
ASTM D 427-93. 1994. Test Method for Shrinkage Factors of Soils by Mercury Method, Annual Book of ASTM Standards, Vol.
04.08, ASTM International, West Conshohocken, PA.
ASTM D 2487-93. 1994. Standard Classification of Soils for Engineering Purposes (Unified Soil Classification System), Annual
Book of ASTM Standards, Vol. 04.08, ASTM International, West Conshohocken, PA.
ASTM D 4318-93. 1994. Standard Test Method for Liquid Limit, Plastic Limit and Plasticity Index of Soils, Annual Book of
ASTM Standards, Vol. 04.08, ASTM International, West Conshohocken, PA.
ASTM D 5550-94. 1994. Standard Test Method for Specific Gravity of Soil Solids by Gas Pycnometer, Annual Book of ASTM
Standards, Vol. 04.08, ASTM International, West Conshohocken, PA.
ASTM D 698-07. 2007. Test Methods for Laboratory Compaction Characteristics of Soil Using, Annual Book of ASTM
Standards, Vol. 04.08, ASTM International, West Conshohocken, PA.
ASTM D 5856-95. 2007. Standard Test Method for Measurement of Hydraulic Conductivity of Porous Material Using a RigidWall, Compaction-Mold Permeameter, Annual Book of ASTM Standards, Vol. 04.08, ASTM International, West
Conshohocken, PA.
Eriksson N., Gupta A., Destouni, G. 1997. Comparative Analysis of Laboratory and Field Tracer Tests for Investigating
Preferential Flow and Transport in Mining Waste Rock, Journal of Hydrology, 194, 143-163.
IS: 2720 Part XXIV. 1976. Determination of Base Exchange Capacity. Indian Standards, methods of tests for soils.
Litaor M.I. 1988. Review of Soil Solution Samplers, Water Resources Research, 24, 727-733.
Parizek R. R., Lane B. E. 1970. Soil-Water Sampling Using Pan and Deep Pressure-Vacuum Lysimeters, Journal of Hydrology,
11, 1-21.
Rakesh R.R. 2005. Simulation of radioactive contaminant transport in unsaturated soils, Ph. D. Thesis. Dept. of Civil
Engineering, Indian Institute of Technology, Bombay, India.
Rakesh R.R., Yadav, D.N., Narayan, P.K., Nair, R.N. 2005. Post Closure Safety Assessment of Radioactive Waste Storage
and Management Site, Trombay, BARC library internal report, BARC/2005/I/010.
Rakesh R.R., Yadav, D.N., Narayan, P.K., Wattal, P.K., Pal P., Vedamoorthy S. 2007. Post-closure Safety Assessment of
Solid Waste Management Plant, Kota, Rajasthan, BARC library internal report. BARC/2007/I/001.
Rogers R.D., McConnell Jr. J.W. 1991. Performance Verification of Solidified Radioactive Waste Using Lysimeters, Engineering
Geology, 30, 79-101.
Schoen R., Gaudet. J.P., Elrick D.E. 1999. Modelling of Solute Transport in a Large Undisturbed Lysimeter During Steady-State
Water Flux, Journal of Hydrology, 215, 82-93.
Sreedeep S., Singh D.N. 2005a. A Study to Investigate Influence of Soil Properties on Its Suction, Journal of Testing and
Evaluation, ASTM, 31(1), 579-584.
Sreedeep S., Singh D.N. 2005b. Estimating Unsaturated Hydraulic Conductivity of Fine-Grained Soils Using Electrical
Resistivity Measurements, Journal of ASTM International, 2(1), Published Online: 3 January 2005. 11 Pages.
Swistock B.R., Yamona J.J., Dewalle, D.R., Sharpe W.E. 1990. Comparison of Soil Water Chemistry and Sample Size
Requirements for Pan vs. Tension lysimeters, Water, Air and Soil Pollution, 50, 387-396.
Vogel A. 1986. Vogel’s Text book of Quantitative Inorganic Analysis, John Wiley & Sons Inc.
2326