Nuclear Science and Technology, Vol. 2, No. 2 (2003), pp. 12 - 17
USE OF RADON-222 AS TRACER TO ESTIMATE
GROUNDWATER INFILTRATION VELOCITY
IN A RIVER BANK AREA
TRINH VAN GIAP
Institute for Nuclear Science and Technique
Vietnam Atomic Energy Commission
Abstract: Naturally occurring isotope Rn-222 has been used as a natural tracer to
determine the residence time of groundwater infiltrated from river into an aquifer in a
riverbank area. The applied method is based on the increasing radon concentration in
infiltrating water during it passes through the riverbank and reaches an equilibrium
value. Solid-state nuclear track detector technique is used to measure directly radon
concentration in water of a well. In order to confirm the relationship between radon
concentration and it’s residence time, a model was constructed in the laboratory.
Experiment carried out in Nam Dinh area showed that mean infiltrating velocity of
groundwater in the studied area as high as 5.1 m/day.
I. INTRODUCTION
Alluvial aquifers hydraulically connected to neighboring river are used as a major
water supply resource in many countries. Because of proximity to the river, the aquifers
always ensure high recharge and pumping rates, and this is the reason that these aquifers
are easily polluted. The contamination degree of groundwater depends on several
factors such as contamination status of surface water and the infiltration velocity of
groundwater. So determination of infiltration velocity is important for management and
exploitation of groundwater with ensured quality. Since artificial radioactive tracer
could not be used in these cases due to the high discharge rate and all wells are used to
produce fresh water. So radon-222 isotope already existing in groundwater seems to be
the most suitable tracer to estimate infiltration velocity of water from river into
neighboring aquifer in a river bank area.
Radon-222 is a product of the U-238 radioactive decay series and it’s the daughter
of α disintegration from Ra-226. Radon concentration in groundwater depends on Ra226 concentration in water and residence time of water in the aquifer. Rn-222 tracer was
used to study the groundwater, surface water interaction and the ability of pollutant
infiltration from surface water into groundwater [1,2].
USE OF RADON-222 AS TRACER TO ESTIMATE GROUNDWATER....
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The aim of study is to determine the relationship of radon concentration in
groundwater and those in surface water using track detector technique and to estimate
the infiltration velocity groundwater into an aquifer base on the residence time of
groundwater and distance between the surface water resource and the observed wells.
II. TECHNIQUE FOR MEASURING RADON IN WATER
Determination of radon concentration in groundwater by direct method demands
special water sampling devices as radon easy escapes from water during sampling and
transportation. Besides, it is more difficult in the cases of long distance from sampling
place to the laboratory because, radon concentration in water samples is measured by
electronic equipment with ZnS(Ag) detector or a liquid scintillation counter. In this
work, we used the indirect method to measure radon concentration using solid-state
nuclear track detectors. Detailed description of the radon-measuring technique is
presented in the previous paper [3].
The advantages of this technique are:
- Radon concentration is measured directly in groundwater, so it eliminates the
loss of radon during sampling and transportation.
- Individual measurement cannot be influenced by environmental parameters
such as: temperature, pressure, etc… because the measurements are carried out for
all wells at the same time.
III. INFILTRATION FLOW STUDIES
1. Method
The principle of the method is based on the fact that river water as a surface water
source contains low radon concentration because a turbulent current causes the radon
gas to escape. As infiltrating, river water with low radon concentration flows through
the aquifer; it continuously absorbs radon, which is emanated from soil of an aquifer
due to alpha decay of radium. So the increase of radon concentration in groundwater is
described by the following equation:
(
C t = C e 1 − e − λt
With:
)
(1)
Ct, radon concentration in groundwater at the time t
Ce, radon concentration in groundwater at equilibrium state
λ , Decay constant of radon ( λ Rn = 0.18 d-1)
Residence time of groundwater in an aquifer can be calculated using
t = (1/ λ )Ln[Ce/(Ce-Ct)]
(2)
TRINH VAN GIAP
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This requires knowing the radon concentration Ce, which can be determined using
a gauge at a distance which is sufficient far away from the river. If taking into account
radon concentration Co at the time to of the river water, and residence time tres= t - to,
equation (2) can be adopted to calculate the residence time of groundwater which
filtrates from the river:
tRes = (1/ λ )Ln[(Ce-Co)/(Ce-Ct)]
(3)
Equation (3) is used to calculate the residence time of groundwater in the cases
that radon concentration of surface water may not always be negligible. With half-life of
radon of 3.8 days, this method can be used to determine the residence time of
groundwater at the wells where the equilibrium state of radon concentration is not
reached. Fig.1 presents increase of radon concentration in groundwater as river water
flows into the aquifer.
River
Non-equilibrium
area
Equilibrium
area
Rn
Aquifer
Fig.1 Radon in groundwater is due to emanation of alluvial deposit as river water
infiltrates into an aquifer.
2. Study on a model in laboratory
In order to control the relationship between radon concentration and residence
time of groundwater in an aquifer, a model in laboratory was simulated. Detailed
structure of the model is presented in fig.2.The tube 1 with L=300cm, D=20cm was
packaged with alluvial taken from the study area. By adjusting position of the valves 4,
one can change water flow rate through the model. Thus, the residence time of water in
the model could be change. Radon emanated from the alluvial soil in the tube 1 into
water is measured by solid-state nuclear track detector technique [3]. At equilibrium
state, radon concentration in water of the model was determined as high as Ce =
1576 ± 190 (Bq/m3) and radon concentration at various residence time of water are
USE OF RADON-222 AS TRACER TO ESTIMATE GROUNDWATER....
15
presented in table 1. Fig. 3 plots the relationship of radon concentration and residence
time in water as it flows through the model.
Accuracy of radon measurement in water is about 10-12 %, so results of radon
concentration measured in water of the model completely corresponds to the theoretical
results calculated from the equation 3.
Fig 2. Model to study the relationship between radon concentration
in water and residence time of water in a aquifer
1.Plastic tube with L=300 cm, D=20 cm; 2. Filter
3. Alluvial aquifer; 4. Valves; 5. Track detectors; 6. Base
Table 1.
Residence time of water
0.84
Days
1.75
2.56
3.25
9.1
20
22
-
222
428
600
701
1264
1534
1547
CRn(Th.)
Bq/m3
-
CRn(exp.)
202
Bq/m3
437
564
722
1215
1515
1565
-
3. Study area
Field works were carried out for several wells at a rural area of Nam Dinh
province, where a Holocene aquifer hydraulically connected directly to the river No10.
The aquifer consists of alluvial sediment, with a thickness up to 25 m deep. The position
of wells in the studied area is presented in the fig.4. The measurements of radon
concentration in groundwater for the wells were carried out at the same time in the rainy
TRINH VAN GIAP
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season using the solid-state nuclear track detectors. The results of radon concentration
in water from corresponding wells are presented in table 2. It shows that, within the
accuracy of the method, the radon concentration in water from wells number 2; 3; 4; 5;
6 are the same and reach the equilibrium value of 24,944 ± 1147 Bq/m3. The radon
concentration in river water is 2100 ± 45 Bq/m3. Using equation (3), the residence time
of groundwater for the well number 1 was determined using equation (3): tres ≈ 7.8 days.
Thus, the mean value of the infiltration flow velocity as high as 5.1 m/day is calculated.
1800
Concentrated of Rn in
water, Bq/m3
1500
1200
900
600
300
0
0
5
10
15
20
25
Time, Day
Small Canal
5
1
4
Dyke
River No.10
Fig.3 The relationship between radon concentration and residence time of water
V. Road
2
3
6
Fig.4 Position of water production wells in the studied area
USE OF RADON-222 AS TRACER TO ESTIMATE GROUNDWATER....
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Table 2.
Wells
1
2
3
4
5
6
Distances to river
40
M
130
211
330
419
501
-
Rn concentration in water
18,887
Bq/m3
25,551
26,400
23,466
25,166
24,178
-
IV. CONCLUSIONS
The results of measurements on the model in the laboratory as well as in the study
area indicate that the environmental radon isotope really is a natural tracer to determine
infiltration flow velocity of groundwater in riverbank area.
The solid-state nuclear track detector technique has more advantages than other
techniques for measuring radon in water because many measurements can be carried out
at the same time, so the effects of the environmental parameters to the measured results
can be excluded.
The existence of homogeneous radon emanation along the flow path in an aquifer
is expected to be investigated by this technique.
V. ACKNOWLEDGEMENTS
This work is one of the tasks of the research project for years 1998-1999, and was
funded by the Institute for Nuclear Science and Techniques, VAEC
REFERENCES
1. Clotilde Bertin. Env. Sci. Technol,.1994 ,v. 28, p.794-798.
2. K.K. Ellins et al., J.Hydrology , 1990, v. 115, p.319-341.
3. T.V.Giap et al., Proceedings of the third National Conference on Nuclear Physics and
Techniques, Da Lat, 22-24 March, 1999, p.337-339