J. Hydrol. Hydromech., 66, 2018, 2, 202–210
DOI: 10.1515/johh-2017-0047

Reassessment of the Goiânia radioactive waste repository in Brazil using
HYDRUS-1D
Elizabeth M. Pontedeiro1, 2*, Paulo F. Heilbron3, Jesus Perez-Guerrero3, Jian Su1,
Martinus Th. van Genuchten2, 4
1

Department of Nuclear Engineering, Federal University of Rio de Janeiro, UFRJ, Rio de Janeiro, RJ, Brazil.
Department of Earth Sciences, Utrecht University, Utrecht, Netherlands.
3
Brazilian Nuclear Energy Commission, CNEN, Rio de Janeiro, RJ, Brazil.
4
Center for Environmental Studies, CEA, São Paulo State University, UNESP, Rio Claro, SP, Brazil.
*
Corresponding author. E-mail: bettinadulley@hotmail.com
2

Abstract: In September 1987 an accident occurred with a cesium chloride (CsCl) teletherapy source taken from a cancer
therapy institute in Goiânia, Brazil. Misuse of the abandoned source caused widespread contamination of radioactive material (about 50 TBq of 137Cs) in the town of Goiânia. Decontamination of affected areas did lead to about 3,500 m3 ofsolid radioactive wastes, which were disposed in two near-surface repositories built in concrete in 1995. This paper documents a safety assessment of one of the low-level radioactive waste deposits containing 137Cs over a time period of 600
years. Using HYDRUS-1D, we first obtained estimates of water infiltrating through the soil cover on top of the repository into and through the waste and its concrete liners and the underlying vadose zone towards groundwater. Calculations
accounted for local precipitation and evapotranspiration rates, including root water uptake by the grass cover, as well as
for the effects of concrete degradation on the hydraulic properties of the concrete liners. We next simulated long-termwater fluxes and 137Cs transport from the repository towards groundwater. Simulations accounted for the effects of 137Cs
sorption and radioactive decay on radionuclide transport from the waste to groundwater, thus permitting an evaluation ofpotential consequences to the environment and long-term exposure to the public. Consistent with previous assessments,
our calculations indicate that very little if any radioactive material will reach the water table during the lifespan of the repository, also when accounting for preferential flow through the waste.
Keywords: Cs-137 transport; Goiânia radioactive waste repository; HYDRUS-1D; Safety assessment.
INTRODUCTION
In September 1987 a major accident occurred with an old
Cesium teletherapy source taken from an abandoned cancer
therapy institute (Instituto Goiano de Radioterapia) in the Brazilian town of Goiânia. Misuse of the source caused widespreadcontamination of radioactive material (about 1375 Ci of 137Cs,
or 5 x 1013 Bq), leading to four deaths and about 250 people
showing significant levels of internal or external contamination(Heilbron et al., 2002; IAEA, 1988; Paschoa et al., 1993; Roberts, 1987). Subsequent cleanup of 137Cs contaminated areas
generated about 3,500 m3 of solid radioactive waste totalingsome 6,000 tons of material. The waste was disposed in two
near-surface low-level radioactive waste repositories built in
reinforced concrete. Previously, two safety assessments werecarried out, in 1995 and 2002, to evaluate the performance ofthe repositories over a 400-year time period (Heilbron et al.,2002). The studies were mandated by the Brazilian Nuclear
Regulatory Agency (CNEN) as part of their regular assessmentsof the long-term effects of the Goiânia waste repositories on theenvironment and human health.
Numerical models have long been recognized to be important tools for radioactive waste repository design, operationand environmental regulation (e.g., Pontedeiro et al., 2010;
Robinson et al., 2012; Tsang (Ed.), 1987). They are importantto understand the long-term subsurface fate and transport ofcontaminants leached from solid waste disposal sites as a function of climatologic conditions, local hydrogeologic conditions,and repository design. Performance assessments are especially
important to evaluate the future safety of installations containing radioactive materials with long-lived radionuclides. This202

may need to include evaluations of how gradual processes ofdegradation of engineered and natural barriers of the repositories after their closure would affect the environment (Jacques etal., 2013, 2014; Pabalan et al., 2009). Other events or processes
could also compromise the integrity of a repository, such as
intrusion by humans and animals (Naveira-Cotta et al., 2013;
Yu et al., 2007).
The current study was carried out to provide a mandatory
five-year reassessment of the possible future impacts of theGoiânia radioactive waste repositories on the environment andbiosphere, as requested by the Brazilian Nuclear Regulatory
Agency. While many models potentially could be used for suchan analysis (e.g., as inventoried by Mallants et al., 2011), we
used in our study the HYDRUS-1D software of Šimůnek et al.
(2016). Below we first give a brief description of the Goiânia
waste disposal site, including a synopsis of previous assessments, then discuss HYDRUS-1D as used to evaluate 137Cs
transport from the waste site to the underlying vadose zone andgroundwater, and provide results of the calculations.
THE GOIÂNIA RADIOACTIVE WASTE REPOSITORY
To facilitate the safe disposal of radioactive waste generated
during decontamination in Goiânia, two near-surface repositories were constructed. The first one was a near-surface repository for disposal of very low-activity wastes (40% in volume,with specific 137Cs activities less than 87 Bq/g), while the second repository (also a near-surface type) was constructed forhigher-activity wastes (60% in volume, with specific activities
above 87 Bq/g). This second repository, being the focus of thisstudy, was built in concrete, having dimensions of 19.6 m

Reassessment of the Goiânia radioactive waste repository in Brazil using HYDRUS-1D

Fig. 1. View of the Goiânia near-surface repositories. The repository in the front contains relatively low-activity 137Cs waste (specific activities less than 87 Bq/g), and the repository in the back higher-activity waste (specific activities above 87 Bq/g).
SECTION A-A
3.0 m

backfill
4.00 m3.20 m9.80 mA

19.60 mA
60.00 mFig. 2. Schematic internal cross-section (a) and areal view (b) of the Goiânia near-surface higher-activity radioactive waste repository(Heilbron et al., 2002).

length, 60 m width and 6.20 m height, and with 20-cm thickconcrete walls along all sides of the facility, including the top
and bottom. A 50-cm thick uniform soil cover was placed on
top of the repository and vegetated with grass. Figure 1 gives anabove-ground view of the two constructed repositories.
Waste inside the second repository considered here consisted
mostly of compacted soil contaminated with 137Cs, covering aheight of about 4.38 m. A schematic of the design of the facility
is shown in Figure 2. The concrete deposit contained insidevarious types of containers filled with waste having diverse
activity levels. Void spaces between the containers were filled
and compacted with natural material, composed of sand andclay. While macroscopically quite heterogeneous, for the purpose of this study we considered the resulting waste material to
be distributed in a homogeneous manner, with the hydrauliccharacteristics of the waste being the same as those of the compacted soil. The repository is located at 16° 45′ 32″ S, 49° 26′ 16″W, about 770 m above sea level. Depth to groundwater below thelower concrete liner during the year fluctuated between 4 and 5m, with 4.5 m being a good average value.
PREVIOUS ASSESSMENTS
In 1995 the first environmental safety assessment of theGoiânia radioactive waste disposal site was conducted by theBrazilian Nuclear Energy Commission (CNEN) using a screening model (Heilbron et al., 1995, 1996). The assessment considered one of the main scenarios of the possible impact of therepository to be the water pathway, mostly involving the ingestion of water or vegetable and animal products contaminated byirrigation water from a nearby contaminated well. Also considered was an agricultural pathway in which an inadvertent intruder mixes contaminated soil with native garden soil, as well
as a scenario where the intruder during digging encounters anintact engineered barrier that is not further penetrated. Anotherconservative scenario considered rapid natural degradation of203

Elizabeth M. Pontedeiro, Paulo F. Heilbron, Jesus Perez-Guerrero, Jian Su, Martinus Th. van Genuchten

the repository cap until complete failure after only 30 years, atwhich time the contaminant is allowed to move freely to thewater table and then to migrate laterally by groundwater to awell only 2 m downgradient. This last scenario, which allowed
water from the well to be used in a vegetable garden, resulted ina transit time for 137Cs through the 4 to 5 m thick unsaturated
zone of at least 423 years, and concentrations that produced
potential doses that remained always below the lower limit of0.25 mSv allowed by CNEN. Since the calculated maximum
concentrations in groundwater below the repository always
remained below the allowed concentrations at any time, thegroundwater pathway was not considered important.
A reassessment of the Goiânia repository was carried out in2002 using a hybrid numerical-analytical approach based on thestandard advection-dispersion equation assuming steady-state
downward flow through the repository, with the contaminant
subject to equilibrium sorption and radioactive decay, and witha certain fraction of the waste being stable and not subject toleaching (Heilbron et al., 2002). The 2002 reassessment confirmed the 1995 results in that the 137Cs water pathways related
to a possible residential scenario near the site were found to be
negligible, mostly again because of the long residence time inthe unsaturated zone below the repository, which hence served
as a natural barrier. The maximum 137Cs concentration at any
time below the repository was again found to be below the
maximum value of 25,000 Bq/m3 (about 6.8 x10–7 Ci/m3),
which would result in a dose for an individual of 0.25 mSv/y.
The 2002 reassessment also found that of the three intrusion
scenarios considered in 1995, the potentially most hazardous
case would be the agriculture scenario. But since this would
occur only after the assumed complete degradation of the concrete (300 to 500 years), the results would still not be importantsince after 280 years the doses would be lower than the allowed
limit of 1 m Sv/y due to natural decay of 137Cs. If one were toneglect this possibility (i.e., degradation of concrete during aperiod of time less than 300 years), the most important scenario
would be the post-drilling scenario after an institutional control
period of 50 years. The calculations were consistent with a 7year environment monitoring plan (EMP) of the site, whichshowed that it was very unlikely to find in the future dangerous
concentrations of 137Cs in the aquifer for population living near
the site. All concentrations were found to be below the detection limit of 200 Bq/m3 (about 5.4x10–9 Ci/m3).
In this paper we provide another reassessment, as mandatedby CNEN, in which HYDRUS-1D is used to follow 137Cs
transport from the repository through the unsaturated zone tounderlying groundwater. An important part of the current study
is an assessment of the possible effects of degradation of theconcrete cap on transient infiltration into and through the repository. Modeling of fluid flow through the entire repository
assembly was done in two steps. The first step focused on fluid
flow through the cover over a period of 600 years, rather than
only 400 years as in the first two assessments, as well as partial
degradation of the upper concrete liner covering the waste. In asecond step, water flow and 137Cs transport through the waste,
the lower concrete liner and the vadose zone was followed
using the infiltration rates obtained from the first step. We also
briefly considered the effects of possible preferential flow
through the waste.
SIMULATION METHODOLOGY
Simulations using HYDRUS-1D assumed applicability of
the Richards equation for flow in variably-saturated media as
follows:204∂θ ( h ) ∂  ∂h  =  K ( h)  − 1   − S ( h)∂t∂z  ∂z  (1)

where θ is the volumetric water content, h is the pressure head,
K is the unsaturated hydraulic conductivity, S(h) is a water sinkterm (in our study associated with root water uptake), z is thevertical coordinate (positive downwards), and t is time. The soil
hydraulic functions θ(h) and K(h) in Eq. (1) were describedusing the original constitutive relationships of van Genuchten(1980):S e ( h) =θ ( h) − θ rn −m= 1+ α hθs − θr()(m = 1 − 1/ n)

K (h) = K s Se0.5 [1 − (1 − Se1/ m )m ]2

(2)

(3)

where Se is effective saturation, θr and θs are the residual andsaturated water contents, respectively, Ks is the saturated hydraulic conductivity, and α and n are quasi-empirical shapeparameters.
The root water uptake term S(h) in Eq. (1) was describedwith the equation (Feddes et al., 1978)

S (h) = α w (h) β ( z )Tp(4)

in which αw(h) is the water stress response function (0 ≤ α ≤ 1),β(z) represents the normalized root density distribution in the
profile, and Tp is the potential transpiration rate. For αw(h) weused the piecewise linear reduction function by Feddes et al.
(1978) using the default parameter values of grass as embedded
within HYDRUS-1D.
Transport of the Cs radionuclide was modeled at first using
the equilibrium advection-dispersion equation accounting for
decay in both the liquid and solid phases:∂(θ RC ) ∂∂C ∂qC= (θ D ) −− μ RC
∂t
∂z
∂z
∂z

(5)

where C is the solution concentration, R is the solute retardation
factor, D is the solute dispersion coefficient, q is the volumetric
fluid flux, and μ is the radionuclide decay rate coefficient. The
retardation factor R for linear equilibrium sorption is given by

R = 1+

ρ Kd
θ

(6)

in which ρ is the dry soil bulk density and Kd the distribution
coefficient of 137Cs between the liquid and solid phases. For thedispersion coefficient D we used the standard equation

D = Doτ + λ

q

θ

(7)

in which Do is the diffusion coefficient of 137Cs in water, τ is the
tortuosity factor described here as a function of the water content using the equation of Millington and Quirk (1961), and λthe dispersivity.
Tables 1 and 2 summarize the main soil physical and chemical parameters, respectively, used in the simulations. For thesoil cover we used soil hydraulic parameters from a typical
Cerrado soil in the Goiânia area of Brazil (Batalha et al., 2010)
containing 53% of sand, 10% of silt and 37% clay. The initialReassessment of the Goiânia radioactive waste repository in Brazil using HYDRUS-1D
Table 1. Soil physical properties of the repository system used in the simulations.
Layer
Top soil
Upper concrete liner
Waste layer
Lower concrete liner
Vadose zone

Depth
(m)
0–0.500.50–0.70
0.70–5.08
5.08–5.28
5.28–10.28

θrθs(cm3 cm–3)
0.067
0.000
0.070
0.000
0.078

(cm3 cm–3)
0.386
0.204
0.365
0.306
0.409

α
(cm–1)
0.02670.001420.02810.001420.0232

n
(–)
1.31
1.52
1.18
1.52
1.29

Ks
(cm day–1)
13.2
0.000864*
5.99
0.0864
7.47

λ
(cm)
25.0
25.0
25.0

Kd
(cm3 g–1)
463
10.0
430

μw, μs(day–1)
6.33x10–5
6.33x10–5
6.33x10–5*Initial value before degradation

Table 2. Soil chemical properties of the repository system used in the simulations.
Layer
Waste layer
Lower concrete liner
Vadose zone

Depth below upperconcrete liner (m)0–4.38
4.38–4.58
4.58–9.58

Do(cm2 day–1)
1.0
1.0
1.0

hydraulic parameters for the upper reinforced concrete liner
were taken from literature values, mostly those by Schneider etal. (2012). We assumed that degradation of the concrete occurred slowly but steadily, starting after 10 years of operationof the repository. The main effect of concrete degradation, in
terms of the long-term flow rates through the repository system
and underlying vadose zone, is the change in the saturated
hydraulic conductivity, Ks, of the liner. We assumed that after
10 years of operation, Ks would increase slowly but continuously in a logarithm manner from the initial value of
10–10 m/s (0.000864 cm/day, or 0.315 cm/year) after 10 years to
10–9 m/s after 225 years and 10–8 m/s after 400 years, to approximately the same value as for the waste.
The contaminated waste layer in the repository was taken to
be 4.38 m thick, situated over a 20-cm thick concrete liner anda 5-m thick vadose zone to the average water table. Hydraulicproperties of the waste layer were measured (Pereira, 1996),
including the bulk density (average value of 1.70 g/cm3) and avalue of 463 cm3/g for the Kd of that layer. The relatively highbulk density of the waste reflected compaction of the 137Cs
contaminated soils. The concrete liner below the waste wasassumed to have the same unsaturated hydraulic properties as
the degraded upper concrete liner, except for a slightly higher
porosity and higher conductivity. Our reasoning here was thatthe flow rate into and through the repository was controlled
mostly only by the very low fluid flow rate through the upper
liner into the waste, with the conductivity of the lower liner nothaving not much of an effect anymore so that a constant value
could be uses (equal to Ks of the upper liner after degradation).
We further used a partitioning coefficient (Kd) of 10 cm3/g forthe concrete (Aguiar, 2006). Hydraulic properties and Kd of thevadose zone were measured (Pereira, 1996), with a bulk densityof 1.30 g/cm3 and a mean value of 430 cm3/g for the distribution coefficient, Kd.
Potential evapotranspiration (Ep) rates needed for the simulations were obtained with the equation of Hargreaves (1975)
using 10 years of temperature and precipitation data from a
local weather station. Results are shown in Figure 3. The Epdata were repeated several times as needed for longer-term
calculations.
WATER FLOW SIMULATIONS
Modeling of the repository was done in two steps (Fig. 4).
First flow rates through the tops soil cover and upper concreteρb(cm3 g–1)
1.72
1.60
1.30

8
7
6
5
4
3
2
0

500 1000 1500 2000 2500 3000 3500 4000

Time [days]

Fig. 3. Calculated potential evapotranspiration (Ep) rates during aten-year time period at Goiânia.

liner into the repository were studied, assuming some degradation of the concrete liner, during a time period of 600 years.
Fluxes through the upper liner into the repository obtained inthis manner were used next to estimate 137Cs transport rates
from the waste vertically down towards the water table.
A major challenge was to accurately estimate downward infiltration rates into the waste through the 70-cm cap of therepository, i.e., 50 cm topsoil covered by grass and the upperconcrete liner that presumably undergoes decay. Degradation of
the concrete would have mostly an effect on the saturated hydraulic conductivity (Ks), with the effects on porosity (or θs)
and the other hydraulic parameters (notably θr, α and n) being
far less important in terms of affecting fluid flow rates throughthe repository. Figure 5 shows how the Ks value of the upperconcrete liner affected infiltration rates into the waste. Infiltration rates up to about 15 cm/year were determined nearly exclusively by the permeability of the liner, while at higher Ks values
the local atmospheric conditions and moisture dynamics in the
top soil determined the infiltration rate. The final asymptotic
value for the flow rate was about 65 cm/year when the conductivity of the upper liner approached the value of the waste after
400 years (not further shown in Fig. 5).
The calculations for Fig. 5 were carried out using a seepage
face as the lower boundary of the cap, thus assuming limited
direct capillary contact between the waste and the upper liner.
The seepage face boundary at the bottom of the concrete liner
was set at zero pressure head, which hence allowed water205

Elizabeth M. Pontedeiro, Paulo F. Heilbron, Jesus Perez-Guerrero, Jian Su, Martinus Th. van Genuchten

Fig. 4. Assumed profiles for simulations of flow through the soil cover and upper concrete liner (left plot), and of flow and Cs transport
through the waste and underlying vadose zone (right plot), including the adopted finite element discretizations (please note that the left andright plot have different spatial scales).

hydraulic conductivity (Ks) of the upper liner before degradation, consistent with Fig. 4. Figure 6 shows that, as the upper
liner slowly degraded, more moisture would enter the repository and increase the water contents and fluid flow rates.
CESIUM TRANSPORT SIMULATIONS

Fig. 5. Effect of the saturated hydraulic conductivity (Ks) of theupper concrete liner on the rate of infiltration into the waste.

to slowly move (drip) from the liner into the waste (or allowing
water to evaporate from the liner). Using boundary pressure
head values of –10 or –60 cm for the seepage face did not havemuch effect on the infiltration rates, as did not any changes inthe values of θr, θs, α and n, generally leading to only a few
percentages higher (notably with higher θs values) or lower(with higher α values) flow rates. The results in Figure 4 wereobtained with a root distribution that declined linearly from thesoil surface to zero at the soil-liner interface at a depth of 50.
Changing the root distribution to only the upper 30 cm also did
not materially affect the comparison between Ks and net infiltration, except for very small increases in the extremely low
rates (those less than about 1 cm/year in Fig. 5).
Once the results in Fig. 5 were established, we calculated theflow regime through the waste, the lower concrete liner and theunderlying vadose zone. Figure 6 shows the simulated pressure
head and water content distributions during the assumed 600year life span of the repository. As initial condition for thepressure head we used the steady-state profile calculated withan upper boundary flux approximately equal to the saturated

206

Estimated infiltration rates into the repository were used
next for the 137Cs transport calculations. Figure 7 shows calculated 137Cs concentrations of fluid leaving the waste and thelower concrete liner into the underlying vadose zone. Noticethat the concentrations are already very much lower than thosein the waste layer, mostly because of the very low fluid flow
rates and the relatively high Kd value (463 cm3/g) for 137Cs in
the waste. Figure 8 further shows the Cesium distributions atthree locations below the lower concrete liner, i.e., at locations
N1 (12 cm), N2 (23 cm) and N3 (34 cm) below the liner. Notice that concentrations considerable decreased in time becauseof slow movement and radioactive decay. 137Cs concentrations
eventually approach zero because of the decay.
For completeness we also present, in Fig. 9 on a semi-log
scale, the entire 137Cs concentration profiles in the waste andunderlying vadose zone. Notice again the slowly moving concentration front due to the high Kd value of the soil below therepository (Table 2), with the retardation factor being about
1530, as well as rapidly decreasing concentrations due to radioactive decay. Radionuclides hence remained restricted nearly
entirely to the upper 50 cm of the vadose zone. These results
suggest that very little if any radioactive material will reach thewater table during the life span of the repository.
All of the calculations thus far assumed applicability of thestandard Richards equation for flow in variably-saturated media, the equilibrium advection-dispersion equation for Cesiumtransport, and the presence of slowly degrading concrete liners.
These relatively ideal conditions may not necessarily be met
in practice. For this reason we also considered two scenariosthat would alter the above standard approach: More rapidly
degrading concrete liners, and preferential flow through thewaste layer and underlying vadose zone.

Reassessment of the Goiânia radioactive waste repository in Brazil using HYDRUS-1D

0

0

-200

-200

-400

-400

-600

-600

-800

-800

-1000
-400

-300

-200

-100

0

-1000
0.20

0.25

Pressure Head, h [cm]

0.30

0.35

0.40

0.45

Theta [-]

Fig. 6. Calculated pressure head and water content distributions in the repository (below the upper concrete liner) and underlying vadosezone. Distributions are given at every 100 years from left (t = 0, representing the initial conditions) to right (t = 600 years). The distributions after 500 and 600 years on the right were essentially identical.
0.06

0

0.05

-200

0.04

-400

0.03

-600

0.02

-800

0.01
0.00
0

100

200

300

400

500

600

-1000
10E-9

10E-7

Time [years]

10E-5

10E-3

10E-1

10E+1

Conc [Bq/cm3]

Fig. 7. Calculated 137Cs concentrations just below the lower concrete liner at the bottom of the waste layer.
0.0006

Fig. 9. Calculated 137Cs concentration within the waste, the lower
concrete liner and underlying vadose zone. Distributions are given
every 100 years from right (t = 0, representing the initial condition)
to left (t = 600 years).

N1

0.0005N20.0004N3

0-200

0.0003-400

0.0002-600

0.00010.0000

-800

0

100

200

300

400

500

600

Time [years]
137

Fig. 8. Calculated Cs concentrations in the vadose zone 12, 23
and 34 cm below the lower concrete liner of the Goiânia repository.

For the first scenario we assumed that the concrete liners
completely failed after installation, possible due to the development of cracks and/or dissolution or other alterations of theupper liner due to repeated wetting and drying cycles andperhaps the effect of plant roots. We used for these calculations
a net infiltration rate of 65 cm/year (0.187 cm/day), which we
obtained for the case when the saturated hydraulic conductivity(Ks) of the upper liner would not limit the long-term downward
infiltration rate as derived from the atmospheric boundary con-

-1000
10E-9

10E-7

10E-5

10E-3

10E-1

10E+1

Conc [Bq/cm3]

Fig. 10. Calculated 137Cs concentration at three locations in thevadose zone below the lower concrete liner. Distributions are given
every 100 years from right (t = 0, representing the initial condition)
to left (t = 600 years).

ditions (Fig. 3). Figure 10 shows the resulting 137Cs distribution
in the profile below the upper concrete liner. The calculations
indicate that the concentration front now moves slightly deeper
into the vadose zone, but still not to depths and at concentrations that should be a concern.

207

Elizabeth M. Pontedeiro, Paulo F. Heilbron, Jesus Perez-Guerrero, Jian Su, Martinus Th. van Genuchten

0

0

-200

-200

-400

-400

-600

-600

-800

-800

-1000
10E-9

10E-7

10E-5

10E-3

10E-1

10E+1

-1000
10E-9

10E-7

10E-5

10E-3

10E-1

10E+1

Conc [Bq/cm3]

Conc [Bq/cm3]

Fig. 11. Mobile concentration distributions in the repository and underlying vadose zone obtained with the dual porosity model using theparameters listed in Table 3 for Case A (left plot) and Case B (right plot). Distributions are given every 100 years from right (t = 0, representing the initial condition) to left (t = 600 years).
Table 3. Dual-porosity model parameter values used for the simulation in Figure 10.

Layer

Depth below upperconcrete liner (m)Waste layer
Lower concrete liner
Vadose zone

0–4.38
4.38–4.58
4.58–9.58

θim
(–)
0.20
0.20
0.20

For the second scenario we superimposed on the first scenario the effects of possible preferential flow in both the waste
layer and vadose zone. This since the waste layer was very
heterogeneous by containing separate drums with the 137Cs
contaminated material, which likely caused infiltrating water tobypass much of the radioactive waste. Preferential transport
was simulated using a mobile-immobile type dual-porosity
model of the form (van Genuchten and Wagenet, 1989)∂βθ RCm=∂t∂C  ∂qcm∂ 
θ m Dm m  −− α s (cm − cim ) − βθ Rμ Cm∂x ∂z  ∂z∂(1 − β )θ RCim
= α s (cm − cim ) − (1 − β )θ RμCim
∂t

(8)

(9)

in whichβ=θ m + f ρb K d
θ + ρb Kd(10)

where the subscripts m and im refer to the mobile and immobile
regions of the soil (or concrete) such that θm + θim = θ, f is thefraction of 137Cs sorption sites located in the mobile region, andαs is a first-order mass transfer coefficient governing the rate ofsolute exchange between the mobile and immobile liquid regions.
Figure 11 gives two plots based on dual-porosity simulations
using the parameters listed in Table 3. For θim we selected areasonably large (and hence conservative) value that always
remained below the minimum water contents in the three layers. The value of f similarly is a fairly conservative value sug-

208

Case A
f
(–)
0.20
0.50
0.50

αs
(year–1)
0.01
0.1
0.1

θim
(–)
0.20
0.20
0.20

Case B
f
(–)
0.20
0.20
0.20

αs
(year–1)
0.1
1.0
1.0

gesting that 80% of the sorption sites are not in direct contact
with the moving fluid. This again appears very reasonable forthe heterogeneous waste material where water likely will bypass the containers filled with contaminated material. Finally,
two values for the mass transfer coefficient αs were used tocover relatively fast and slow solute exchange between themobile and immobile liquid domains (Cases A and B in Fig.
11), based in part by an inventory by Maraqa (2001) of previously published values. The lower value of αs for the waste
layer (as compared to the vadose zone) was again motivated by
the likely imperfect contact between the waste and presumed
preferential fluid paths through the repository.
The results in Fig. 11 indicate that preferential flow in certain situations will lead to faster movement of the concentration
front through the vadose one, and hence earlier breakthrough at
the water table. However, the concentrations, as obtained especially for Case B in Fig. 11, are extremely low and well below
the detection limit for 137Cs. Also, interestingly, is the effect ofpreferential flow in the waste layer as predicated by the selectedvalues of f and αs, leading to considerable bypassing of thewaste. Preferential flow in that layer causes less rapid transportof 137Cs out of the contaminated waste, and hence less pollutionof underlying layers. These results are consistent with an earlier
study (Pontedeiro et al., 2010) showing that preferential flow
through a mining waste disposal site will lead to less rapid
pollution of the subsurface by considerably lowering the concentrations leaving the disposal site. This is in contrast to situations where contaminated water originates at the soil surface,for example through some industrial accident or as part ofagricultural practices such as the surface application of pesticides, in which case preferential flow will lead to more rapid
downward transport and contamination of the vadose zone andunderlying groundwater (e.g., Jarvis et al. 2016; Mallants et al.,2011).

Reassessment of the Goiânia radioactive waste repository in Brazil using HYDRUS-1D

CONCLUSIONS
Decontamination of areas affected by the Goiânia radiation
accident produced about 3,500 m3 of solid radioactive wastes,
which were disposed of in two near-surface low-level repositories built in concrete in 1995. In this paper we presented ananalysis over a time period of 600 years of the repository containing the higher 137Cs levels. Using HYDRUS-1D, we firstobtained estimates of water infiltrating through the soil cover
on top of the repository into and through the waste and its concrete liners and the underlying vadose zone towards groundwater. For the calculations we used atmospheric boundary conditions accounting for local precipitation and evapotranspiration
rates, including root water uptake by the grass cover. Special
concern was possible degradation of the concreate liners on top
and below the waste material. We used HYDRUS-1D next tosimulate long-term water fluxes and 137Cs transport from therepository towards groundwater, thus providing an assessmentof potential consequences to the environment and long-term
exposure to the public. Results indicate that very little if any
radioactive material will reach the water table during thelifespan of the repository.
Complete failure of the concrete liners from the very beginning also would not lead to any significant 137Cs movement togroundwater below the repository. We note that recent tests (in2017) revealed no noticeable degradation of the concrete liners,
thus indicating that the adopted approach with concrete degradation was a very conservative scenario that may well haveoverestimated actual flow rates through the repository. Additional simulations assuming preferential flow caused fastertransport to the groundwater table, but at concentrations far
below the detection limit. In all, the current study confirms
previous assessments by the Brazilian Nuclear Energy Commission (Heibron et al., 1996, 2002) that it is very unlikely tofind in the future any unsafe concentrations of 137Cs in theaquifer for populations living near the site.
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Note: Colour version of Figures can be found in the web version of this article.

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Received 9 May 2017
Accepted 31 August 2017