Proceedings of the 11th International Conference on Environmental
Remediation and Radioactive Waste Management
ICEM2007
September 2-6, 2007, Oud Sint-Jan Hospital Conference Center, Bruges, Belgium

ICEM07-7202

INNOVATIVE IN-SITU DETERMINATION OF UNSATURATED HYDRAULIC PROPERTIES IN
DEEP LOESS SEDIMENTS IN NORTH-WEST BULGARIA
Dirk Mallants/Belgian Nuclear Research Centre(SCK•CEN), B-2400 Mol, Belgium

Doncho Karastanev/Geological Institute, Bulgarian
Academy of Sciences, Bulgaria

Dimitar Antonov/Geological Institute, Bulgarian
Academy of Sciences, Bulgaria

Janez Perko/ Belgian Nuclear Research Centre(SCK•CEN), B-2400 Mol, Belgium

values, although the test sites were several meters apart.
Apparently the size of the affected volume of soil was large
enough to reduce the effect of spatial variability and to produce
average field-scale hydraulic parameters that are relevant forlarge-scale predictions of flow patterns and radionuclide
migration pathways.

ABSTRACTIn the framework of selecting a suitable site for final
disposal of low- and intermediate level short-lived radioactive
waste (LILW-SL) in Bulgaria, site characterization is ongoingat the Marichin Valog site, North-West Bulgaria. The site ischaracterized by a complex sequence of loess, clayey gravel,and clay layers, of which the first 30-40 m are unsaturated.
Proper knowledge about unsaturated water flow andconcomittant radionuclide transport is key input to safetyassessment calculations. Constant-head infiltrometer tests werecarried out at several meters below ground surface to determine
the unsaturated hydraulic properties of silty loess, clayey loess,and clayey gravel layers. Individual infiltrometers were
equipped with 0.5-m-long filter sections; the shallowest filter
was from 2 to 2.5 m depth, whereas the deepest was from 9.5 to
10 m depth. Infiltration tests provided data on cumulative
infiltration and progression of the wetting front in the initiallyunsaturated sediments surrounding the infiltrometer. A
cylindrical time-domain reflectometry TRIME probe was used
to measure water content variations with time during
progression of the wetting front. Access tubes for the TRIME
probe were installed at 0.3 to 0.5 m from the infiltrometertubes. By means of an inverse optimization routine
implemented in the finite element code HYDRUS-2D, fieldscale soil hydraulic parameters were derived for all layers.
Results show a great consistency in the optimized parameter

INTRODUCTION
At present, the only nuclear power plant in operation in
Bulgaria is located in Kozloduy. At Kozloduy, six reactor units
exist, with four in operation, while two were shut down in
December 2002 (WWER 440/230 units 1 and 2) as a result ofan agreement between the Bulgarian Government and theEuropean Union. Two further units are planned to be shut downin the beginning of 2007 (units 3 and 4). Units 3 and 4 are also
440 MW WWER reactors, while units 5 and 6 are 1000 MW
WWER reactors. For the Kozloduy plant, the total volume of
LILW-SL for surface disposal is estimated at ~60 000 m3.
The Bulgarian State Enterprise Radioactive Waste
Management (SERAW) is currently investigating the suitabilityof three sites for disposal of LILW-SL. Two sites are located in
the vicinity of the Kozloduy NPP, notably Brestova Padina andMarichin Valog. The third site is located in Belene, along the
Danube river. Marichin valog is located at about 2.5 km to thewest-southwest direction from the Kozloduy NPP, while theother sites are located at a greater distance.

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At Marichin Valog two sites were equipped with
infiltrometer devices for the determination of unsaturated
hydraulic properties ([1], [2]). The sites are at an elevation of
96 m (no. 1) and 88 m (no. 2) above mean sea level. The
second site was subdivided into two subsites, i.e. site 2 and 3,
with one infiltrometer device being installed in each site. The
two sites were chosen in such a way that the first site would
have a maximum thickness of all stratigraphic layers, whereas
in the second site the top layers would have a significantly
reduced thickness, or were even absent, due to erosion. As aresult, the depth to the presumed foundation layer for repository
construction (Pliocene clay) would be about 3 m for the secondsite versus ~16 m for the first site. Since the primary objectiveof this study was to hydraulically characterize the last few
meters of the layer above the foundation layer, a much shorter
infiltrometer device was necessary in the second site comparedto the first. The primary layer to be hydraulically characterized
by in-situ tests was classified as clayey gravel, which waschosen because 1) the contrast in hydraulic conductivity withthe underlying Pliocene clay (i.e. ~10-7 – 10-6 m/s versus 10-10 –10-9 m/s) presumably is large enough to generate an importantlateral flow component (note the inclined interface between
Pliocene clay and clayey gravel) which could dominate thelong-term flow pattern in the loess hill with much horizontal
longer flow paths compared to flow paths associated withvertical drainage, (2) the presence of gravel makes coresampling and subsequent laboratory determination on coresamples problematic.
The objective of the in-situ test at the first site was tohydraulically characterize the silty (2% sand (2-0.1 µm), 82 %loess (0.1-0.005 mm), and 16 % clay (< 0.005 mm)) and clayey
loess layers (10% sand, 70% loess, and 20 % clay). Based on
the particle size distribution curve, 50% of the particles waslarger than 1 cm. For this purpose an infiltrometer was installedwith filter sections at depths of 5.5 and 9.5 m (see further). The
objective of the in-situ test at the second site was tohydraulically characterize the last few meters of the gravelly
clay layer (66% sand, 30% loam, and 4% clay), for which amuch shorter infiltrometer device was sufficient compared to
the first site.

At present the Marichin Valog site is the most extensively
characterized site; this work is mainly done by the Geological
Institute of the Bulgarian Academy of Sciences. Because of theexistence of relatively thick unsaturated loess/clay formations
at the potential repository site, detailed characterization of theselayers is needed in view of a defensible site evaluation. This
paper documents the initial steps of the characterization of thehydraulic properties of the unsaturated loess/clay formations.
The results reported are the outcome of collective efforts of theGeological Institute of the Bulgarian Academy of Sciences andthe Belgian Nuclear Research Centre (SCK•CEN), in theframework of a bi-lateral cooperation between both countries.
SITE DESCRIPTION
Topography and stratigraphy
The Marichin valog site is located in an undulatinglandscape (Fig. 1) developed on Pliocene clay covered with
Quaternary sediments. The latter consists of two layers: silty
loess and clayey loess with a variable thickness between 5 and14 m, and a clayey gravel layer of about 2 m thickness whosebase is the interface with the Pliocene clay (Fig. 1). The
repository is planned to be founded on the top of the Pliocene
clay layer (at an elevation of ~85 m above mean sea level),
after the removal of the Quaternary sediments. The inclinationof the interface between Pliocene clay and clayey gravel is innorth-west – south-east direction. The presumed outlet of thecatchment is the Ogosta river, which is at a distance of 8-9 km.
A thin low-yield aquifer is formed in a 1.5-2 m thick sandysilty intercalation within the Pliocene clay layer. A seepage
zone exists at a distance of about 300-350 m northeast of thesite, where the aquifer is intersected by the nearby valley (Fig.
1). The top of the site (borehole BH 34) is at approximately 100m above mean sea level, whereas the lowest point in the valley
is at 64 m, and the groundwater table is at 63 m above mean sea
level.

Site 1Site 2 & 3

Figure 1 Cross-sectional profile of Marichin Valog sitewith approximate location of test sites indicated.
Infiltrometer tests were carried out at Site 1, 2 & 3.

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Copyright © 2007 by ASME

Observed
moisturemeasurements

profilesfrom

The moisture content was calculated by the TRIME device;
we required a custom made calibration because thicker accesstubes were used in our experiments compared to thecommercially available tubes from IMKO. The TRIME probe
has the advantage that one single probe can be used todetermine nearly continuous moisture profiles within different
access tubes.
Several tubes have been installed at the site, mainly for the
purpose of calibration of the probe, for determining time seriesof moisture profiles, and for carrying out in-situ infiltration
tests in undisturbed soil at depths up to 10 m and more. Usuallyapplications with the TRIME apparatus are limited to depths ofa few meters ([5]), while in our application the measurement
depth is up to 10 m.
TRIME access tubes installed as a component of theinfiltrometer set-up have also been used to obtain time series ofnatural moisture profiles. Details of the access tubes and themeasurement depths are shown in Fig. 2. The standard
commercial length of access tubes is 3 m. For our purpose, thetotal length of access tubes was extended until 10 m, using 2m-long sections with SCK•CEN-made provisions at both ends
to allow two neighboring tubes to be smoothly attached andglued together. For the TRIME probe to be useful at thosedepths, a special extension cable with a maximum length of 21m was used together with the TRIME IPH module. Until
present, moisture profiles have been measured at four different
times (see further). A second set of moisture profiles wasdetermined on the basis of undisturbed core samples collectedfrom two boreholes. Careful analysis of these core samples also
served for determining a refined stratigraphic description.

TRIME

In the framework of the characterization of the unsaturated
sediments at the Marichin Valog site, a cylindrical TRIME T344 moisture apparatus ([3]; [4]) was used. The TRIME
apparatus is based on the Time Domain Reflectometer
technique (TDR) ([5]), and should be used together with special
polycarbonate access tubes. The TRIME device generates ahigh-frequency pulse (between 600 MHz and 1.2 GHz) which
propagates along the wave guides generating anelectromagnetic field around the probe. At the end of the wave
guides, the pulse is reflected back to its source. The resulting
transit time (10 ps to 2 ns) and dielectric constant are dependent
on the moisture content of the medium.

T1 (03-02-2006)
T1 (15-05-2006)
T1 (29-06-2006)
T1 (22-11-2006)
0Depth below surface (m)

2

4

6

8

10
10

20

30

Volumetric water content (cm3 cm-3, %)40

Figure 3 Observed water content based on TRIME
readings.

Figure 2 Observation borehole T-1 for water content
measurements using TRIME probe, and infiltrometersF-1a and F-1b. Elevations in m above sea level.

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Copyright © 2007 by ASME

infiltrometer tube with the recording depth 1 cm below thelevel of the overflow. When the water level dropped 1 cm
below the overflow level, a signal was given and the pumping
rate was increased until the original water level was restored.
Such situation did almost not occur during the measurements,
since the pumping rate was put slightly larger than theinfiltration capacity, with excess water being evacuated throughthe overflow device, then collected and finally recycled. When
steady-state flow conditions were reached, water application
was stopped and the infiltration test was finished.

TRIME-based natural water content measurements at
different observation times are shown in Fig. 3. For borehole T1 drying of the profile as time progresses from February 2006
till November 2006 is clearly seen, especially up to depths of 3
m. Deeper in the profile water content variations with time aresmaller. For the clayey gravel, average water content increased
from 26.6 % at 7.5 m was till 36.3 % at 10 m (Fig. 3).
INFILTROMETER SET-UP

1

At site 1 the deepest infiltrometers were installed at thedepths of 5.5 m and 9.5 m (Fig. 2). The first one is located in
the silty loess layer, hence is used to determine the hydraulicproperties characteristic of the silty loess. The second one islocated in the clayey loess, and can be used to determine thehydraulic properties of that layer. Owing to the relatively great
depth of the filter sections, much higher constant head values
will be applied for infiltrometer 1 compared to the head values
used for the less deep filters in site 2.
For the installation of the infiltrometers at site 1, a 150mm-OD borehole was drilled up to a depth of 6 and 10 m,
respectively. The infiltrometer was at 0.5-m distance from the
TRIME access tubes (Fig. 2). The infiltrometer consisted of a
75-mm-OD PVC (72-mm-ID) tube with a screened section of
0.5 m at its bottom end (screen started at 7 cm from bottom).
Vertically oriented sleeves were made in the PVC to arrange
the filter section. After installation of the PVC tube, gravel was
poured in the hole until the first meter of the borehole wasfilled. Then loess from the borehole was poured on the gravel
until a layer of 0.7 m loess was obtained. Then bentonite
powder was poured in the hole up to a thickness of 0.5 m. Next
a 0.5-m-thick concrete layer was made to seal off the filter
section. The remaining space until the soil surface was filledwith loess from the site. Installation of infiltrometers at theother site was done in the same way.
Each infiltrometer was used as a so-called constant head
infiltrometer. Water was injected into the PVC cylinder bymeans of a flexible tube, to which a heavy metallic weight was
attached to keep the end of the flexible tube at a fixed positionat the bottom of the cylinder. The flexible tube should be
attached to a pump which provides a constant influx of water
for the entire duration of the test. The PVC cylinder extended
approximately 2 m above the soil surface, where an overflow
was provided. In this way a constant water level could bemaintained throughout the entire infiltration test.
CONSTANTHEADINFILTROMETER
SUMMARY OF DATA AND ANALYSIS

Cumulative infiltration (m3)

0.8

0.6

Marichin Valog site
Infiltrometer F1-a (2006)

0.4

Best fit: F1-a
Infiltrometer F1-b (2006)
Best fit: F1-b
Infiltrometer 2 (2006)

0.2

Best fit: 2
Infiltrometer 3 (2006)
Best fit: 3
Infiltrometer 2 (2005)
Infiltrometer 3 (2005)

0
0

0.1

Time (days)

0.2

0.3

Figure 4 Measured cumulative infiltration for
infiltrometer F-1, F-2 and f-3. Best fit simulations
using HYDRUS-2D are also included.

Cumulative infiltration for all infiltrometer tests is shownin Fig. 4. In total four tests were carried out in 2006: two tests
in the shallow clayey gravel layer (infiltrometer F-2 and F-3),
and two tests in deeper loess sediments to evaluate themethodology for deep sediments (F-1a and F-1b). For theformer infiltrometer, initial tests carried out in 2005 wererepeated in 2006 to evaluate the reproducibility of such tests.
Different cumulative infiltration was observed for different
infiltrometers (Fig. 4). The faster increase in cumulative
infiltration for infiltrometer F-1a and F-1b is due in part to thehigher imposed heads: approximately 12 m for F-1a and 7 m
for F-1b, versus approximately 4.5-5 m for infiltrometer F-2
and F-3. In less than 0.05 days (1.2 hours) a total amount ofnearly 0.8 m3 has infiltrated. Approximately twice as muchtime is needed to infiltrate the same amount for infiltrometer F2. For infiltrometer 3 a total amount of 0.5 m3 had infiltrated
within 0.1 day. For all infiltrometers a steady-state infiltration
rate was obtained within the duration of the experiment.

TESTS:

Constant head infiltrometer tests were carried out with
infiltrometer F-1a, F-1b, F-2 and F-3. The distance from thedifferent filter sections to the soil surface and the extension was
different. As a result, the constant head at the top of the filter
section was, respectively 11.4, 7.3, 3.9, and 4.4 m for
infiltrometer F-1a, F-1b, F-2, and F-3. Constant head was
obtained by supplying water through a flexible tube by meansof an adjustable pump. A water level recorder was put in the4

Copyright © 2007 by ASME

Cumulative infiltration for the 2005 and 2006 tests aredifferent, with the smallest difference being observed for
infiltrometer F-2 and largest for infiltrometer F-3. For theformer the same slope is observed indicating a very similar
infiltration rate (6.55 m3/d in 2006 versus 6.9 m3/d in 2005),
while diverging slopes are seen for the latter resulting inslightly different infiltration rates (2.46 m3/d in 2006 versus 3
m3/d in 2005). Taking account of the different initial watercontent conditions (wetter conditions in 2006), the repeatibilityof the test is certainly acceptable and the suitability of the test istherefore confirmed.
NUMERICAL MODELING
INFILTROMETER TESTSOFCONSTANT

Water flow through the unsaturated sediments was
described by means of the van Genuchten water retention
characteristic, or water retention curve, θ(h), defined as ([7]):θ s -θ r
 θr+
h<0

(1 + ( α | h | )n )m
θ (h) = 
(1)
 θ
h≥0
s
where θ is water content (cm3/cm3), h is matric head (m), θr
and θs are residual and saturated water content (cm3/cm3),
respectively, and α (1/m), n, and m are constants which define
the shape of the curve, with m = 1-1/n (with the requirement n> 1 if Eq. 1 is used in combination with K(h)).
In addition to the parameterisation of θ(h), description ofwater flow through unsaturated soil also requires theparameterisation of the unsaturated hydraulic conductivity
K(h). Most models for K(h) assume that the water-filled porespace consists of a set of capillaries with the distribution ofpore radii being determined by the soil water characteristic.
One very attractive unsaturated hydraulic conductivity functionis that derived by van Genuchten ([1]), and is based on thestatistical pore size distribution of ([8]). The van GenuchtenMualem K(h) model is defined as follows:

HEAD

Inverse estimation of hydraulic properties using
borehole infiltration data
The purpose of the in-situ borehole infiltration test is thedetermination of soil hydraulic functions at a measurement
scale that is commensurate with the scale of modelling. The
procedure that will be used is outlined below. The parameter
estimation by means of numerical inversion is demonstratedusing a conceptual model that was specifically developed for
the Mariching Valog site ([1],[2]). Initial values for thehydraulic functions that had to be provided for the numericalinversion were estimated on the basis of neural network
predictions provided in HYDRUS-2D ([6]). In the predictions
we used particle size distribution data as input parameters.
Soil hydraulic properties may be determined by means of
inverse optimization techniques such as provided in HYDRUS2D. Numerical inversion with HYDRUS-2D allows
determination of van Genuchten ([7]) soil hydraulic properties(see further) when sufficient and appropriate soil hydraulic data
is measured, e.g. during constant head infiltration in a borehole
when cumulative infiltration and history of water content in theundisturbed sediment at several locations are measured.
For our applications, inverse optimization comprises
computer simulation of the expected soil water redistribution
history while adjusting the soil hydraulic parameters until thebest possible agreement is obtained between measured andcalculated cumulative infiltration and soil moisture profile. The
simulation starts with "guess" or "trial" values of the soil
hydraulic properties; these values may be estimated usingneural network predictions based on particle size data, or byusing some other prior information, such as the optimized
values from laboratory testing ([1]). In the optimization
hydraulic properties are gradually changed until the calculated
cumulative infiltration and soil moisture profile agrees wellwith the measurements.
Application of the inverse optimization required
developing an axisymmetric model in HYDRUS-2D for each of
the four infiltrometers. The vertical dimension of the model
was limited to the soil layers that would be immediately
influenced by the infiltrating water. The lateral dimension of
the model was sufficiently large such that lateral boundary
conditions would not influence the simulation results.

 K s K r (h) h < 0(2)K(h) = 
h≥0
 Ks
where
τ
1/m m 2K r = S e [1 - (1 - S e ) ] (3)and Ks and Kr are saturated and relative hydraulic conductivity(m/s), respectively, τ is an empirical constant assumed equal to
0.5 ([8]), and Se is saturation degree, defined as:Se =θ −θ r(4)
θ s −θ r

where θ is soil water content (cm3/cm3), θr and θs are theresidual and saturated water contents (cm3/cm3), respectively.
As a first approximation and on intuitive ground, θs = η (total
porosity) and θr = 0. In reality, however, the saturated watercontent θs of field soils is generally 5 to 10% smaller than the
total porosity η because of entrapped air and the presence oflarge pores which drain too rapidly to become saturated ([9]).
The residual water content θr is likely to be larger than zero,
because of the presence of adsorbed water. Furthermore, theresidual water content is an extrapolated parameter, and may
therefore not necessarily represent the smallest possible watercontent in a soil. Most often θr is treated as a fitting parameter
without physical significance ([9]).
The advantage of the van Genuchten-Mualem K(h) modelis that nearly all its parameters, except Ks, can be determinedfrom the soil water characteristic. In other words, a complete
description of θ(h) and K(h) may be obtained with fourparameters, i.e. θr, θs, α, n, and one additional parameter, Ks.

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Copyright © 2007 by ASME

In the optimization of the θ(h) and K(h) relationships, theθr and θs parameters were fixed to decrees the degrees of
freedom and to reduce problems of non-uniqueness in the
optimization. Parameter θs was put equal to total porosity while
θr was fixed at 0.05.
The automatic parameter optimization routine provided in
HYDRUS-2D was invoked to further optimize the parameters
α and Ks. HYDRUS adopts the minimization of the sum of thesquares (SSQ) of the residuals:

observation is in agreement with the general soil physicalconcept that coarser materials have larger α-values ([10]). For
this material the saturated water content, θs, was also fitted,
since no estimates of total porosity were available. The fittingof θs was done only after α and Ks were fitted, and the latter
two parameters were kept fixed at their optimal value while
fitting θs. Fitting of all three parameters together resulted in
non-uniqueness in the solution. A graphical comparison
between observed and calculated cumulative infiltration is
given in Fig. 4.

2

SSQ = ∑ (q p ,i − qo ,i ) (5)N

Table 2 Parameter values from inverse optimization using
HYDRUS-2D (infiltrometer F-2, clayey gravel). Parametern was fixed at 2, while θr = 0.05 and θs = fitted. Values insquare brackets from 2005 campaign ([1]).

i =1where N is the number of calibration points (here only the
cumulative fluxes are used), qp,i is the ith predicted values, andqo,i is the ith observed value. HYDRUS uses the MarquardtLevenberg optimization algorithm to minimize the objectivefunction (Eq. 5), to come up with parameters that produce theminimum difference between the observed and predictedvalues. In the optimization the parameter n was excluded frombeing optimized because initial calculations where n was
included showed high correlation with parameters α and Ks,
and a high standard error coefficient for n indicative of nonuniqueness of the solution. The parameter n was therefore kept
constant and equal to 2 [2].

Parameter

Best fit
S.E.
value
coefficient
3 [0,53]
1.167
α (1/m)Ks (m/day)
0,091250,000739[0,041]θs
0,413#
0,00011SSQ
0,00251 (0,000605)#
R20,99935 (0,99940)##
when θs fitted separately

Lower
95%
0,7014
0,089

Upper
95%
5,298
0,0927

0,411

0,416

Results from inverse optimization
CONCLUSIONS

Results from parameter optimization for infiltrometer F1-aare summarized in Table 1. Since the filter section of F1-a is incontact with clayey loess material, parameters for this layer
were determined first. Best fit values for α and Ks with a low
standard error (SE) coefficient were obtained for the clayey
loess. After five iterations the SSQ did not further decrease(true for all infiltrometers). A comparison between observedand calculated cumulative infiltration is given in Fig. 4.

The Marichin Valog site located in the vicinity of
Kozloduy NPP is considered by the State EnterpriseRadioactive Waste (SE-RAW) a potential site for final disposalof low-and intermediate level short-lived radioactive waste. To
prepare for an in-depth evaluation of the suitability of the
Marichin Valog site, site characterization was carried out in2005 and 2006 by the Geological Institute of the Bulgarian
Academy of Sciences and SCK•CEN emphasizing on
determination of the unsaturated flow characteristics of theunsaturated geological materials. Over the past two years, thehydraulic characteristics of the variably saturated Quaternary
sediments (a sequence of silty and clayey loess, red clay andclayey gravel) at the Marichin Valog experimental site havebeen determined by means of laboratory and field tests. The
present document reports results from the constant head
infiltrometer field tests carried out in 2005 and 2006.
The objective of in-situ field tests was to evaluate thefeasibility of the infiltrometer set-up for several unsaturated
layers with varying depths below soil surface. For this purpose
four infiltrometer tests were carried out. The first two to
confirm earlier results for infiltrometers installed up to thedepth of 3 m in very heterogeneous clay layer mixed withgravel or carbonate concretions. The second two to test thetechnique in clayey and silty loess at depths up to 10 m.
Testing the repeatability of the infiltrometer test bycomparing results from 2005 and 2006 was partly successful.
Estimated saturated hydraulic conductivity values for one andthe same infiltrometer were in good agreement. The 2006
shape-curve parameters α were not in good agreement with theTable 1 Parameter values from inverse optimization using
HYDRUS-2D (infiltrometer F1-b, silty loess). Parameter n
was fixed at 2.
Parameterα (1/m)Ks (m/day)
SSQ
R2Best fitvalue
0,0586
0,04490,002810,99939

S.E.
coefficient
0,00849
0,00068

Lower
95%
0,04177
0,0436Upper
95%
0,0754
0,0463

Optimized parameter values for infiltrometer F1-b are
given in Table 1, illustrating again a very good fit, withacceptable SE coefficients. Other parameters (n, θr, and θs) were
fixed during the optimization, with θs = 0.44, the independently
derived total porosity ([1]). Further comparison between
observed and fitted cumulative infiltration is given in Fig. 4.
Results for infiltrometer 2 whose filter section was placedin the clayey gravel are summarized in Table 2. A much higher
α value was obtained compared to infiltrometer F1-a. This

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Copyright © 2007 by ASME

2005 values. Possibly the sampling of a different part of the soilmaterial surrounding the infiltrometer could have caused thisdifference, in combination with large spatial variability.
Results from the two deeper infiltration tests in clayey
loess and silty loess were satisfactory: steady-state flow was
obtained within a reasonable short period (i.e., less than 0.1day), and no technical difficulties were observed. Cumulative
infiltration curves could be fitted well with similar unsaturated
parameters, although the much lower content of large-sizedparticles such as gravel and carbonate concretions resulted inmuch lower α-values more typical of loess or loam material.
Difference in saturated hydraulic conductivity between
different materials was relatively small, i.e. less than a factor offive between minimal and maximum value.
One of the most difficult to characterize layers was theclayey gravel and carbonate rich layer. Owing to the large-sized
concretions and gravels, laboratory determination of hydraulicproperties is nearly impossible, leaving only fielddetermination as a viable option. The use of borehole
infiltration data in the inverse optimization routine of thecomputer code HYDRUS-2D is a practical and reliable
methodology to obtain such field-scale hydraulic properties.
The innovative features of our approach comprise use of
constant-head infiltrometers at relatively great depth below
surface whereby in combination advancing water fronts aremonitored by means of cylindrical TRIME TDR probes andvolumes of water infiltrated. All data may be integrated in theparameter optimization routine of HYDRUS-2D for inverse
optimization of hydraulic parameters.

[4] Stacheder, M., Blume, P., Köhler K., and Ruf, R. (2002).
Profiling soil moisture with a time domain reflectometry tube
probe. Proceedings of 17th WCSS, 14-21 August 2002,
Thailand, Paper no. 2068.
[5] International Atomic Energy Agency (IAEA) (2000).
Comparison of soil water measurement using the neutron
scattering, time domain reflectometry and capacitance methods.
IAEA-TECDOC-1137.
[6] Šimůnek J., Šejna, M., and van Genuchten M. Th.(1999).
The HYDRUS-2D software package for simulating water flow
and solute transport in two-dimensional variably saturated
media. Version 2.0, U.S. Salinity Laboratory, Riverside,
California.
[7] van Genuchten, M.Th. (1980). A closed-form equation forpredicting the hydraulic conductivity of unsaturated soils. Soil
Sci. Soc. Am. J, 44:892-898.
[8] Mualem, Y. (1976). A new model for predicting thehydraulic conductivity of unsaturated porous media. Water
Resour. Res., 12:513-522.
[9] Van Genuchten, M. Th., Leij, F.J., and Yates, S.R. (1991).
The RETC code for quantifying the hydraulic functions of
unsaturated soils. EPA/600/2-91/065. U.S. Salinity Laboratory,
USDA, Riverside, CA.

ACKNOWLEDGMENTS[10] Carsel, R.F., and Parish, R.S., 1988. Developing joint
probability distributions of soil water retention characteristics.
Water Resour. Res., 24(5): 755-769.

This study was financially supported by the bilateral
cooperation programme “Royal Decree 20 June 2005 and 2
June 2006 on Belgian support to improve the safety of nuclear
installations in East and Central Europe and in the CIS (year2005 and 2006, project No. 8)”, (contract CO-90-99-1220-02
between SCK•CEN and Federale Overheidsdienst Economie,
K.M.O., Middenstand, en Energie).
REFERENCES[1 ] Mallants, D., Karastanev, D., and Antonov, D. (2005). The
2005 site characterization campaign at the Mariching Valog site
(Kozloduy), Bulgaria. Assistance to selection and site
evaluation for low- and intermediate level waste disposal in
Bulgaria. SCK•CEN report R-4285, SCK•CEN, Mol, Belgium.
[2] Mallants, D., Karastanev, D., and Antonov, D. (2006). The
2006 site characterization campaign at the Mariching Valog site
(Kozloduy), Bulgaria. Assistance to selection and site
evaluation for low- and intermediate level waste disposal in
Bulgaria. SCK•CEN report R-4419, SCK•CEN, Mol, Belgium.
[3] IMKO GMBH (1996). Comments on the TRIME-tube
access probe, http://www.alive.de/imko/trime-tube-com.htm,
Abrolat, T, Köhler, K. Germany.

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