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A field study of tritium migration in groundwater
the Science of the Total Environment ELSEVIER
The Science of the Total Environment 173/174 (1995) 47-51
Afieldstudy of tritium migration in groundwater Zhang Li-Xing*, Zhang Ming-shun and Tu Gou-Rong Northwest Institute of Nuclear Technology, P.O. Box 69-17, Xian 710024, China
Abstract A field study of tritium migration from an underground nuclear explosion was carried out for more than 7 years. A series of satellite wells was drilled around the explosion cavity, which is within a nuclear test site water-supply aquifer. Samples from various wells were analysed. In this way, variations in the tritium concentration of water from different wells were determined, and the extent of tritium migration during the 7 years after the detonation was examined. The maximum tritium concentration reached in water from various wells is just 52-times higher than the maximum permissible concentration for drinking water and decreased afterwards. According to the results obtained, the flow rate of groundwater was inferred to be about 91 m per year and the maximum contamination distance of water supplies by the tritium were < 2 km from the explosion cavity. Keywords: Field study; Tritium migration; Groundwater; Radioactive nuclide
1. Introduction Monitoring of pollution of radioactive nuclides, such as ^Sr, 137Cs, U, Pu, 3 H, in groundwater is very important for the safety of a nuclear test site and its peripheral area. Tritium possesses the same chemical behaviour as hydrogen. It can displace hydrogen in water, form tritiated water (HTO) and migrate with groundwater. More than 99.9% of tritium which has resulted from underground nuclear explosion exists as HTO [1]. The migration rate of tritium is approximately the flow rate of groundwater. However the distribution coefficients of other radioactive nuclides
* Corresponding author.
between rock and groundwater are relatively high, and migration rates are slower than tritium. Therefore tritium can be used as a leader nuclide in the monitoring of pollution of radioactive nuclides in groundwater, and to predict the migration of radioactive nuclides and maximum possible area of pollution. The goals of this study were to examine the rates of radionuclide migration in groundwater, the potential for movement, both on and off the nuclear test site, of radioactivity from underground nuclear explosions and the possible contamination of water supplies. Besides, it might provide data applicable to the underground disposal of radioactive waste. A series of satellite wells was drilled around the explosion cavity, which is within a nuclear test
0048-9697/95/$09.50 © 1995 Elsevier Science BV. All rights reserved. SSDI 0048-9697(95)04734-1
48
L.-X. Zhang et al. / The Science of the Total Environment I73 /I 74 (I 995) 47-51
site water-supply aquifer. Samples from various wells were analysed. In this way, variations in the radionuclide content of water from different wells were determined, and the extent of radionuclide migration during the 7 years after the detonation was examined. In the present paper, the results for the field study of tritium migration are reported. Sufficient tritium was present in the explosion cavity to provide an easily measurable tracer for water from the cavity region. The maximum tritium concentration in water from various wells reached 52 times that of the maximum permissible concentration ( N 1 X 10’ Bq/l) for drinking water and decreased afterwards. According to the results obtained, the flow rate of groundwater was inferred to be about 91 m/year, and the maximum contamination distance of water supplies by the tritium was 5 2 km from the explosion cavity. 2. Experimental 2.1. Positions of satellite wells Eleven satellite wells were drilled around the explosion cavity, most of the wells were positioned down-stream of groundwater from the exTable 1 Parameters of the satellite well Well no.
6 I 8 9 11 12 13 14 15 16 17
Depthb
Position” azimuth angle 0
Cm)
distance
296 231 59.6 60.3 73 47 104 71 46 34 57.5
160 148 462 634 163 93 99 444 478 476 298
Cm) 30 30 250 250 251 122 131 100 100 100 2.50
%sing the explosion cavity as the reference point. bThe groundwater level of these wells is 8.8-20.17 m from the surface.
plosion cavity. The positions and the parameters of various wells are listed in Table 1. 2.2. Sampling In the first month after the detonation, the cells were sampled once every ten days. After that they were sampled once each month. 2.3. Treatment of samples Water samples were filtered to remove all solids if present. The samples were distilled under reduced pressure. A 2-ml volume of the distilled water sample was pipetted into the bottle for liquid scintillation measurement. Ten ml of scintillation solution was added, the bottle capped and shaken thoroughly. The tritium content of the samples was measured with an SG30 liquid scintillation counter (Intertechnique Inc., France). For water samples without contamination of tritium, in which the contents of tritium were background level, sodium hydroxide solution (NaOH concentration 1%) was added to prepare 100 ml electrolyte after filtering and distillation. The electrolytic solution obtained was electrolyzed and tritium was enriched. The enriched water sample was distilled under reduced pressure, then prepared into a scintillation solution and measured with the SL-30 liquid scintillation counter. 2.4. Preparation of scintillation solution [2] 100 g naphthalene, 6 g PPO (2,5diphenyloxazole), and 0.3 g POPOP Cl,6bisU-phenyloxazol-2-yl) benzene) were dissolved in dioxane and diluted to 1 litre. 3. Results and discussion 3.1. Source of tritium in underground nuclear explosions [3,41 Tritium is mainly produced from nuclear fusion in a nuclear explosion. The yield of tritium produced from nuclear fission is very low (about 10e2%). If a fission device of 1 kT TNT equivalent is fired, only 0.1 mg tritium is generated. The yield of tritium produced from fusion reactions is
L.-X. Zhang et al. / The Science of the Total Environment 173 / 174 (1995) 47-51
obviously closely related to the specific design of the device and to the environment in which it is detonated, 1 kT TNT equivalent of fusion reaction can generate OS-5 g tritium. In underground nuclear explosions the leakage neutrons react with boron and lithium in peripheral rock. Neutrons, which possess more than 1.5 MeV of energy, can induce a “B(n,2 LY)T reaction, therefore tritium is produced. 6Li may undergo a 6Li(n,a)T reaction; this reaction does not have a neutron energy threshold. About 2 X 1O23 neutrons/kT escape from both an unshielded fission explosion and a fusion explosion. About l-3% of the neutrons escaping into the rock will produce tritium. The amount of tritium that is formed will depend on the contents of 6Li, “B and other absorbers present in the rock and on the temperature of the rock at the time of neutron capture. By the incorporation of shielding internal and/or external around the explosive, the potential for neutron escape from the explosive, and hence the tritium formed from “B and 6Li in the rock, will be significantly reduced. Therefore the amount of tritium formed from “B and 6Li is small in comparison with that produced from the fusion process. 3.2. Variation of tritium concentration in groundwater after the detonation The production of tritium in nature is mainly from the reaction between neutrons in cosmic rays and nitrogen in the atmosphere, 14N(n,T)12C. The amount of tritium resulting from natural cosmic processes is about 400-800 g per year. A steady-state tritium inventory is about 10 kg. The tritium naturally formed will be converted into HTO by actinism and isotopic exchange, then brought to surface water by rain and finally into groundwater or the sea. The concentration of tritium in groundwater varies with geological and hydrological conditions. The concentration of tritium naturally formed in our test site is shown in Table 2. The range of variation is 3.63-13.5 Bq/l. The analytical results obtained after the detonation indicated that the concentrations of tritium in groundwater were significantly enhanced. The highest tritium concentration (5.70 x lo6
49
Table 2 Concentration of tritium naturally formed in the groundwater Well no.
Concentration of tritium (Bq/l)
6
5.59 4.51 3.63 5.85 13.5
7 9
11 13
Bq/l) was reached in the groundwater of No. 12 satellite well, which was located downstream and 93 m from the explosion cavity. The highest concentrations reached in the groundwater of various wells decreased, as shown in Fig. 1, when the distances between wells and the explosion center increased. Besides radioactive decay there are two principal processes contributing to the change of concentration of the radioactive species. They are sorption and desorption of the radioactivity on surrounding mineral matter and hydraulic or geometric dispersion. In the case of tritium the former process is negligible. Thus the decrease of tritium concentration in groundwater was mainly caused by radioactive decay and dispersion processes. The analytical results from the samples from wells 6 and 7, which were located upstream, indicated that the tritium could be brought into upstream groundwater after the detonation. This was probably caused by leakage of radioactive gas and reverse-flow of groundwater, which took place
C&q/4
::: &
015
105
*9
I .
100
300
I
500
,
D,m
Fig. 1. Variation in the highest tritium concentrations in the groundwater of various wells with distance from the explosion cavity.
50
L.-X. Zhung et al. / The Science of the Total Environment 173/l 74 (1995) 47-51
after the detonation. The tritium concentration in the upstream groundwater reached a maximum of 6.9 X lo5 Bq/l and decreased to less than the maximum permissible concentration for drinking water afterwards. 3.3. Deducing the flow direction of groundwater According to the variation of tritium concentrations in the groundwater of various satellite wells, the flow direction of groundwater could be deduced. The analytical results showed the following.
(9 Wells 6 and 7 were located upstream from
(iii)
the detonation cavity. The tritium concentration of the groundwater in well No. 7 reached its peak value and reduced rapidly. Wells 12 and 13 were located downstream and about 90 m from the detonation cavity. The tritium concentration of the groundwater in well 12 first reached its peak value and then held a higher value for a relatively long time. Wells 14, 8, 15 and 16 were located downstream and about 450 m from the detonation cavity. Among these, the tritium concentration of the groundwater in well 15 first reached its peak value and then held a higher value for a relatively long time.
From the above phenomena we deduced that the flow direction of groundwater in this area was well 7 --) well 12 + well 15, which was N47”E as shown in Fig. 2. This deduction coincided with the result of a geohydrological survey. 3.4. Estimation of the velocity of groundwater flow The rate of groundwater movement is governed by the permeability of the aquifer and the hydraulic gradient. The natural rate of movement of the groundwater is generally not greater than 1.5 m per day and not less than 1.5 m per year. According to the analytical results of tritium concentration in the groundwater of wells 12 and 15, the difference in time at which the peak value of tritium concentration appeared was obtained.
Fig. 2. Tritium contamination range in the test site 6 years after the detonation; 0 the explosion cavity; 0 satellite well.
From the time difference and the distance between the two wells the average velocity of tritium migration with groundwater was estimated to be about 91.2 m/year. The average flow rate of the species may be calculated from the equation [5]: K = &/Cl+
rK,)
where Vj V, r
= = =
Kd =
the average velocity of the ionic species, the average velocity of the groundwater, the ratio of the weight of rock to volume of water per unit volume of aquifer material, the distribution coefficient of the given ionic species between rock and groundwater.
The quantity (1 + &) is sometimes referred to as the retardation factor. For tritium Kd = 0 and (1 + &J = 1. Thus the average velocity of the groundwater approximates to that of tritium migration, that is 91.2 m/year. 3.5. Evaluating the range of groundwater contamination by tritium Six years after the test, the range of groundwater contamination by tritium is as shown in Fig. 2. Because of the migration of tritium, the tritium concentration in the groundwater of the test site
51
L.-X. Zhung et al. / The Science of the Total Environment 173 / 174 (1995) 47-51
increased above the maximum permissible concentration for drinking water. The range of groundwater contamination was 400 m wide and 700 m long in the downstream direction. From the above results, the time required for tritium migration of 1 km is about 11 years. During the migration process the tritium concentration in the groundwater decreases due to radioactive decay of tritium (TI,2 = 12.33 years ) and dispersion processes. According to the results of monitoring the tritium migration from well 12 to well 15, the factor of reduction was about 5 after a migration of 437 m. This means that the maximum distance of tritium migration is about 1760 m and the time required for the migration is about 19.3 years before the tritium concentration in the groundwater decreases to below the maxi-
mum permissible ter.
concentration
for drinking
wa-
References DC. Hoffman, W.R. Daniels, K. Wolfsberg, J.L. Thomp son, R.S. Rundberg, S.L. Fraser and KS. Daniels, A review of a field study of radionuclide migration from an underground nuclear explosion at the Nevada test site, IAEA-CN-43/469,1983. PI J.F. Camron, Radioactive Dating and Method of Counting, IAEA, Vienna, 1967, pp. 543-568. [31 J.A. Miskel, Production of tritium by nuclear weapons, UCRL73270,1971, [41 J.B. Green, Jr. and R.M. Lessler, Reduction of tritium from underground nuclear explosives, UCRL-73258,197l. 151 H.B. Levy, On evaluating the hazards of groundwater contamination by radioactivity from an underground nuclear explosion, UCRL51278, 1972.
ill
The Science of the Total Environment 173/174 (1995) 47-51
Afieldstudy of tritium migration in groundwater Zhang Li-Xing*, Zhang Ming-shun and Tu Gou-Rong Northwest Institute of Nuclear Technology, P.O. Box 69-17, Xian 710024, China
Abstract A field study of tritium migration from an underground nuclear explosion was carried out for more than 7 years. A series of satellite wells was drilled around the explosion cavity, which is within a nuclear test site water-supply aquifer. Samples from various wells were analysed. In this way, variations in the tritium concentration of water from different wells were determined, and the extent of tritium migration during the 7 years after the detonation was examined. The maximum tritium concentration reached in water from various wells is just 52-times higher than the maximum permissible concentration for drinking water and decreased afterwards. According to the results obtained, the flow rate of groundwater was inferred to be about 91 m per year and the maximum contamination distance of water supplies by the tritium were < 2 km from the explosion cavity. Keywords: Field study; Tritium migration; Groundwater; Radioactive nuclide
1. Introduction Monitoring of pollution of radioactive nuclides, such as ^Sr, 137Cs, U, Pu, 3 H, in groundwater is very important for the safety of a nuclear test site and its peripheral area. Tritium possesses the same chemical behaviour as hydrogen. It can displace hydrogen in water, form tritiated water (HTO) and migrate with groundwater. More than 99.9% of tritium which has resulted from underground nuclear explosion exists as HTO [1]. The migration rate of tritium is approximately the flow rate of groundwater. However the distribution coefficients of other radioactive nuclides
* Corresponding author.
between rock and groundwater are relatively high, and migration rates are slower than tritium. Therefore tritium can be used as a leader nuclide in the monitoring of pollution of radioactive nuclides in groundwater, and to predict the migration of radioactive nuclides and maximum possible area of pollution. The goals of this study were to examine the rates of radionuclide migration in groundwater, the potential for movement, both on and off the nuclear test site, of radioactivity from underground nuclear explosions and the possible contamination of water supplies. Besides, it might provide data applicable to the underground disposal of radioactive waste. A series of satellite wells was drilled around the explosion cavity, which is within a nuclear test
0048-9697/95/$09.50 © 1995 Elsevier Science BV. All rights reserved. SSDI 0048-9697(95)04734-1
48
L.-X. Zhang et al. / The Science of the Total Environment I73 /I 74 (I 995) 47-51
site water-supply aquifer. Samples from various wells were analysed. In this way, variations in the radionuclide content of water from different wells were determined, and the extent of radionuclide migration during the 7 years after the detonation was examined. In the present paper, the results for the field study of tritium migration are reported. Sufficient tritium was present in the explosion cavity to provide an easily measurable tracer for water from the cavity region. The maximum tritium concentration in water from various wells reached 52 times that of the maximum permissible concentration ( N 1 X 10’ Bq/l) for drinking water and decreased afterwards. According to the results obtained, the flow rate of groundwater was inferred to be about 91 m/year, and the maximum contamination distance of water supplies by the tritium was 5 2 km from the explosion cavity. 2. Experimental 2.1. Positions of satellite wells Eleven satellite wells were drilled around the explosion cavity, most of the wells were positioned down-stream of groundwater from the exTable 1 Parameters of the satellite well Well no.
6 I 8 9 11 12 13 14 15 16 17
Depthb
Position” azimuth angle 0
Cm)
distance
296 231 59.6 60.3 73 47 104 71 46 34 57.5
160 148 462 634 163 93 99 444 478 476 298
Cm) 30 30 250 250 251 122 131 100 100 100 2.50
%sing the explosion cavity as the reference point. bThe groundwater level of these wells is 8.8-20.17 m from the surface.
plosion cavity. The positions and the parameters of various wells are listed in Table 1. 2.2. Sampling In the first month after the detonation, the cells were sampled once every ten days. After that they were sampled once each month. 2.3. Treatment of samples Water samples were filtered to remove all solids if present. The samples were distilled under reduced pressure. A 2-ml volume of the distilled water sample was pipetted into the bottle for liquid scintillation measurement. Ten ml of scintillation solution was added, the bottle capped and shaken thoroughly. The tritium content of the samples was measured with an SG30 liquid scintillation counter (Intertechnique Inc., France). For water samples without contamination of tritium, in which the contents of tritium were background level, sodium hydroxide solution (NaOH concentration 1%) was added to prepare 100 ml electrolyte after filtering and distillation. The electrolytic solution obtained was electrolyzed and tritium was enriched. The enriched water sample was distilled under reduced pressure, then prepared into a scintillation solution and measured with the SL-30 liquid scintillation counter. 2.4. Preparation of scintillation solution [2] 100 g naphthalene, 6 g PPO (2,5diphenyloxazole), and 0.3 g POPOP Cl,6bisU-phenyloxazol-2-yl) benzene) were dissolved in dioxane and diluted to 1 litre. 3. Results and discussion 3.1. Source of tritium in underground nuclear explosions [3,41 Tritium is mainly produced from nuclear fusion in a nuclear explosion. The yield of tritium produced from nuclear fission is very low (about 10e2%). If a fission device of 1 kT TNT equivalent is fired, only 0.1 mg tritium is generated. The yield of tritium produced from fusion reactions is
L.-X. Zhang et al. / The Science of the Total Environment 173 / 174 (1995) 47-51
obviously closely related to the specific design of the device and to the environment in which it is detonated, 1 kT TNT equivalent of fusion reaction can generate OS-5 g tritium. In underground nuclear explosions the leakage neutrons react with boron and lithium in peripheral rock. Neutrons, which possess more than 1.5 MeV of energy, can induce a “B(n,2 LY)T reaction, therefore tritium is produced. 6Li may undergo a 6Li(n,a)T reaction; this reaction does not have a neutron energy threshold. About 2 X 1O23 neutrons/kT escape from both an unshielded fission explosion and a fusion explosion. About l-3% of the neutrons escaping into the rock will produce tritium. The amount of tritium that is formed will depend on the contents of 6Li, “B and other absorbers present in the rock and on the temperature of the rock at the time of neutron capture. By the incorporation of shielding internal and/or external around the explosive, the potential for neutron escape from the explosive, and hence the tritium formed from “B and 6Li in the rock, will be significantly reduced. Therefore the amount of tritium formed from “B and 6Li is small in comparison with that produced from the fusion process. 3.2. Variation of tritium concentration in groundwater after the detonation The production of tritium in nature is mainly from the reaction between neutrons in cosmic rays and nitrogen in the atmosphere, 14N(n,T)12C. The amount of tritium resulting from natural cosmic processes is about 400-800 g per year. A steady-state tritium inventory is about 10 kg. The tritium naturally formed will be converted into HTO by actinism and isotopic exchange, then brought to surface water by rain and finally into groundwater or the sea. The concentration of tritium in groundwater varies with geological and hydrological conditions. The concentration of tritium naturally formed in our test site is shown in Table 2. The range of variation is 3.63-13.5 Bq/l. The analytical results obtained after the detonation indicated that the concentrations of tritium in groundwater were significantly enhanced. The highest tritium concentration (5.70 x lo6
49
Table 2 Concentration of tritium naturally formed in the groundwater Well no.
Concentration of tritium (Bq/l)
6
5.59 4.51 3.63 5.85 13.5
7 9
11 13
Bq/l) was reached in the groundwater of No. 12 satellite well, which was located downstream and 93 m from the explosion cavity. The highest concentrations reached in the groundwater of various wells decreased, as shown in Fig. 1, when the distances between wells and the explosion center increased. Besides radioactive decay there are two principal processes contributing to the change of concentration of the radioactive species. They are sorption and desorption of the radioactivity on surrounding mineral matter and hydraulic or geometric dispersion. In the case of tritium the former process is negligible. Thus the decrease of tritium concentration in groundwater was mainly caused by radioactive decay and dispersion processes. The analytical results from the samples from wells 6 and 7, which were located upstream, indicated that the tritium could be brought into upstream groundwater after the detonation. This was probably caused by leakage of radioactive gas and reverse-flow of groundwater, which took place
C&q/4
::: &
015
105
*9
I .
100
300
I
500
,
D,m
Fig. 1. Variation in the highest tritium concentrations in the groundwater of various wells with distance from the explosion cavity.
50
L.-X. Zhung et al. / The Science of the Total Environment 173/l 74 (1995) 47-51
after the detonation. The tritium concentration in the upstream groundwater reached a maximum of 6.9 X lo5 Bq/l and decreased to less than the maximum permissible concentration for drinking water afterwards. 3.3. Deducing the flow direction of groundwater According to the variation of tritium concentrations in the groundwater of various satellite wells, the flow direction of groundwater could be deduced. The analytical results showed the following.
(9 Wells 6 and 7 were located upstream from
(iii)
the detonation cavity. The tritium concentration of the groundwater in well No. 7 reached its peak value and reduced rapidly. Wells 12 and 13 were located downstream and about 90 m from the detonation cavity. The tritium concentration of the groundwater in well 12 first reached its peak value and then held a higher value for a relatively long time. Wells 14, 8, 15 and 16 were located downstream and about 450 m from the detonation cavity. Among these, the tritium concentration of the groundwater in well 15 first reached its peak value and then held a higher value for a relatively long time.
From the above phenomena we deduced that the flow direction of groundwater in this area was well 7 --) well 12 + well 15, which was N47”E as shown in Fig. 2. This deduction coincided with the result of a geohydrological survey. 3.4. Estimation of the velocity of groundwater flow The rate of groundwater movement is governed by the permeability of the aquifer and the hydraulic gradient. The natural rate of movement of the groundwater is generally not greater than 1.5 m per day and not less than 1.5 m per year. According to the analytical results of tritium concentration in the groundwater of wells 12 and 15, the difference in time at which the peak value of tritium concentration appeared was obtained.
Fig. 2. Tritium contamination range in the test site 6 years after the detonation; 0 the explosion cavity; 0 satellite well.
From the time difference and the distance between the two wells the average velocity of tritium migration with groundwater was estimated to be about 91.2 m/year. The average flow rate of the species may be calculated from the equation [5]: K = &/Cl+
rK,)
where Vj V, r
= = =
Kd =
the average velocity of the ionic species, the average velocity of the groundwater, the ratio of the weight of rock to volume of water per unit volume of aquifer material, the distribution coefficient of the given ionic species between rock and groundwater.
The quantity (1 + &) is sometimes referred to as the retardation factor. For tritium Kd = 0 and (1 + &J = 1. Thus the average velocity of the groundwater approximates to that of tritium migration, that is 91.2 m/year. 3.5. Evaluating the range of groundwater contamination by tritium Six years after the test, the range of groundwater contamination by tritium is as shown in Fig. 2. Because of the migration of tritium, the tritium concentration in the groundwater of the test site
51
L.-X. Zhung et al. / The Science of the Total Environment 173 / 174 (1995) 47-51
increased above the maximum permissible concentration for drinking water. The range of groundwater contamination was 400 m wide and 700 m long in the downstream direction. From the above results, the time required for tritium migration of 1 km is about 11 years. During the migration process the tritium concentration in the groundwater decreases due to radioactive decay of tritium (TI,2 = 12.33 years ) and dispersion processes. According to the results of monitoring the tritium migration from well 12 to well 15, the factor of reduction was about 5 after a migration of 437 m. This means that the maximum distance of tritium migration is about 1760 m and the time required for the migration is about 19.3 years before the tritium concentration in the groundwater decreases to below the maxi-
mum permissible ter.
concentration
for drinking
wa-
References DC. Hoffman, W.R. Daniels, K. Wolfsberg, J.L. Thomp son, R.S. Rundberg, S.L. Fraser and KS. Daniels, A review of a field study of radionuclide migration from an underground nuclear explosion at the Nevada test site, IAEA-CN-43/469,1983. PI J.F. Camron, Radioactive Dating and Method of Counting, IAEA, Vienna, 1967, pp. 543-568. [31 J.A. Miskel, Production of tritium by nuclear weapons, UCRL73270,1971, [41 J.B. Green, Jr. and R.M. Lessler, Reduction of tritium from underground nuclear explosives, UCRL-73258,197l. 151 H.B. Levy, On evaluating the hazards of groundwater contamination by radioactivity from an underground nuclear explosion, UCRL51278, 1972.
ill