Uranium isotopes as a tracer of groundwater transport studies.

The activity concentrations of 234U and 238U in thermal groundwater, deep well water and river water samples from Central Poland were determined. Concentration of 234U and 238U in the examined waters varied from <0.013 (LLD) to 16.8 mBq/dm3 and from <0.013 (LLD) to 45.5 mBq/dm3 respectively. The highest uranium activity concentrations were measured in the thermal groundwater from Mszczonow and Cieplice, while the lowest were observed in thermal ground water from Uniejow and Poddebice. In thermal groundwater from Skierniewice, uranium activity concentrations were below lower limit of detection (0.013 mBq/dm3). The 234U/238U activity ratio varied from 0.37 (Cieplice) to 1.30 (Poddebice well water).

Introduction

Uranium chain radionuclides are usually used for a wide range of applications in the Earth Sciences. The 234U/238U activity ratio is used as a geochemical tool to investigate transport and flow relationships in major hydrological reservoirs, groundwater pattern and it is highly useful for interpreting timescales of weathering. In rocks older than a few million years, 234U/238U activity ratios should be in secular equilibrium [1]. Many studies on 238U and 234U in natural waters indicate that these isotopes occur in a disequilibrium state and that, with a few exceptions, water samples contain higher 234U activities than 238U [2-4]. The higher activity of 234U in waters is the result of the 234U atom displacement from the crystal lattice after 238U decay and recoil of the formed 234Th nuclide, which after two consecutive decays finally form the 234U nuclide. It allows 234U to be more mobile during the weathering of rocks by atmospheric water. The recoil 234U atoms are liable to be oxidized to the hexavalent stage and can be leached into the water phase more easily than its parent nuclide 238U. The oxidation of U(IV) to U(VI) is an important step in leaching into water, because compounds of U(VI) have a higher solubility in water [2, 3, 5, 6]. The uranium isotope ratio is also influenced by other factors: the age of the rocks, rock type and climate difference [1]. This fact has been successfully applied for geological tracing or the characterization of geochemical processes.

In the same way, the 235U/238U isotope ratio (natural value 7.25 × 10−3) has been used for the identification of anthropogenic pollution scenarios [7, 8]. 235U is a key isotope in nuclear fuel manufacture and reprocessing, and any deviation from the expected natural isotope ratio in environmental samples is an indication of contamination from nuclear activity. Large increases in the 236U/238U isotopic ratio represent have been used as a clear indicator of the presence of irradiated uranium [8, 9].

Average 238U and 234U activity concentrations in surface and ground water are in the range from 1 to 20 mBq/dm3 [10]. A major source of uranium is seawater, where its average concentration of 238U equal to 41.5 mBq/dm3 [11]. The highest uranium activity concentrations were detected in the drinking water in a private well in Finland −150 Bq/dm3 [12].

The uranium activity ratio can be measured in diverse matrices by radiometric, mass spectrometry and alpha spectrometry methods. Advances in the methods of uranium activity ratio determination have been recently overviewed [13-15].

Recently we have found significant differences in the some natural radionuclide activity concentrations in deep well water samples (including thermal groundwater) originating basically from the same Lower Cretaceous geological formation [16]. Since the salinity of these samples differs also substantially one possible explanation for this phenomenon is the possible infiltration of surface water to these underground reservoirs. Special attention has been devoted to a newly exploited source of the thermal groundwater from Poddebice close to the city of Lodz. In very close vicinity of this thermal groundwater source lying 2,000 m below ground level, there are two other important water sources: an underground water reservoir (70 m below surface) used as a source of municipal drinking water, and the Ner river. The Ner river flows mostly through the rural area, and the main source of uranium in its water should be the dissolved uranium from fertilizers.

Therefore, it seems to be interesting to compare the uranium activity ratio in the selected water samples in order to detect the possible vertical transport of the surface water to the deep underground thermal water reservoir.

Experimental

Study area

Samples of thermal groundwater, groundwater and surface water was collected from different sites from central Poland (Fig. 1).

Fig. 1

Location of sampling collection sites

In the case of Poddebice, the water samples were taken from three water sources:

The Ner river

Well municipal water (70 m below surface)

Thermal groundwater reservoir (2,000 m) as is shown in Fig. 2.

Fig. 2

Sites of samples collection in Poddebice

Ten litres of each water sample were collected in polyethylene bottles with screw caps. All samples were acidified to pH ≈ 2 directly after collection. As the water samples presented complete clarity, filtering of the samples was considered unnecessary.

For the Poddebice sites, several soil samples from different levels of the bore-hole were also taken for uranium radionuclides analysis.

Radiochemical method

Uranium isotopes were co-precipitated from water samples with hydrated manganese dioxide by a method described elsewhere [17-19].

Hydrated manganese dioxide was dissolved in 50 mL of 3% H2O2 in 1 M HCl solution and evaporated to dryness. The residue was dissolved in 50 mL of 9 M HCl and was passed through the anion exchangeable resin Dowex 1X8 (50–100 mesh, Cl− form). Each column was first conditioned by 50 mL 9 M HCl. Under this condition, uranium is adsorbed by the resin, whereas thorium and other elements are passed with the solution and discarded [20]. Then the column was washed with 20 mL of 9 M HCl to eliminate residual thorium traces. Cations of U+4, U+6 and Fe+3 were retained on the column. Iron ion was eluted with 25 mL of 8 M HNO3. The remaining uranium was eluted in two portion: with 25 mL of 8 M HNO3 and with 50 mL deionized water, and the total collected solution was evaporated. After evaporation the solid residue containing uranium was dissolved in 0.75 M (NH4)2SO4 solution and then it was transferred to an electrolytic cell. Uranium isotopes (234U and 238U) are electrodeposited on a stainless steel disc at a current density of 1.2 A/cm2 for 1.5 h. Fifteen minutes before ending the electrolysis, 0.5 mL NH4OH solution was added to the electrolyte to complete the uranium deposition on the disk. The disk with uranium was washed with deionized water and acetone.

The activity concentrations of uranium isotopes were determined using an α spectrometry system with PIPS detector (Canberra Packard). Lower limit of detection, for 300,000 s counting time, were equal to 0.013 mBq/dm3 for each of the uranium isotopes.

The activity concentrations of 238U via its daughter 234Th in the soil samples were determined by gamma spectrometry with HPGe detector using its principal γ-line 63.3 keV.

Quality assurance of the elaborated method

The accuracy of the developed method for uranium determination was evaluated by checking the activity concentration of 234U and 238U in two Standard Reference Materials: IAEA Soil 327, IAEA Phosphogypsum 434 and IAEA water samples distributed for intercomparison measurements—CU-2010-03. The average uranium recovery was almost quantitatively (varied from 92 to 100%). The results for intercomparison measurements—CU-2010-03 are presented in Table 1.

Table 1

The results of 234U and 238U activity in International Atomic Energy Agency (IAEA) samples

Sample	234U		238U
	IAEA value (Bq/kg)	Measured value (Bq/kg)	IAEA value (Bq/kg)	Measured value (Bq/kg)
1	0.04 ± 0.00	<LLD	0.04 ± 0.00	<LLD
2	1.30 ± 0.03	1.29 ± 0.02	0.70 ± 0.02	0.69 ± 0.02
3	0.47 ± 0.01	0.47 ± 0.02	0.31 ± 0.01	0.29 ± 0.02

Results and discussion

Activity concentration of uranium isotopes in Poddebice region

The activity concentration of 234U and 238U in the water samples taken from three sources around the geothermal bore-hole of Poddebice in different months of 2011 are presented in Table 2.

Table 2

Activity concentration of 234U and 238U in thermal groundwater, well water and river water from Poddebice (mBq/dm3)

Months	Geothermal reservoir				River water-NER				Well water Poddębice
	234U		238U		234U		238U		234U		238U
January	na		na		8.54	±0.19	7.96	±0.18	4.12	±0.13	4.17	±0.13
February	0.151	±0.025	0.159	±0.026	6.95	±0.17	6.36	±0.16	1.29	±0.07	1.23	±0.08
March	0.110	±0.021	0.119	±0.022	5.16	±0.15	5.05	±0.14	0.79	±0.06	0.83	±0.06
April	0.102	±0.020	0.106	±0.021	4.83	±0.16	4.31	±0.15	1.19	±0.07	0.92	±0.06
May	0.168	±0.026	0.176	±0.027	5.03	±0.14	4.66	±0.14	0.97	±0.07	0.82	±0.07
June	0.087	±0.020	0.092	±0.021	4.78	±0.15	3.98	±0.14	1.73	±0.12	1.92	±0.13
July	0.037	±0.012	0.041	±0.013	3.09	±0.12	2.83	±0.11	1.65	±0.08	1.62	±0.08
August	0.049	±0.014	0.053	±0.014	4.35	±0.14	3.76	±0.13	0.45	±0.04	0.43	±0.04
September	0.064	±0.014	0.071	±0.015	3.23	±0.11	2.84	±0.10	0.62	±0.05	0.70	±0.05
October	0.123	±0.019	0.132	±0.020	1.43	±0.07	1.15	±0.06	0.85	±0.05	0.82	±0.05

na Not analyzed

The lowest activity concentration of 234U and 238U were found in the thermal groundwater samples, and varied from 0.087 to 0.168 mBq/dm3 and from 0.092 to 0.176 mBq/dm3 for 234U and 238U respectively. As expected, the highest contents of uranium isotopes were found in the river water, from the Ner, as a result of additional anthropogenic uranium input from agricultural application of phosphoric fertilizer [21]. In this water activity, concentrations varied from 1.43 to 8.54 mBq/dm3 and from 1.15 to 7.96 mBq/dm3 for 234U and 238U respectively.

The slight seasonal decrease of uranium concentration in the Ner river, connected with periods of rain, can be also observed (Fig. 3). Similar levels of 238U activity concentration and its seasonal fluctuation were observed for other Polish main rivers [21, unpublished data].

Fig. 3

Activity concentration of 234U and 238U in the Ner river

234U/238U isotopic ratio in Poddebice region

As was mentioned, the interesting information concerning mutual transfer between underground and surface water can be obtained from a comparison of 234U/238U activity ratio in these water samples. The results of the seasonal fluctuations of 234U/238U in water samples from Poddebice are shown in Fig. 4.

Fig. 4

Uranium isotopic ratio (234U/238U) in thermal groundwater, deep well water and river water from Poddebice

In thermal groundwater from Poddebice, the activity ratio of 234U/238U was the lowest with an average value 0.935 ± 0.021, and practically does not change seasonally.

In deep well water exploited from 70 m below ground level, the uranium isotope activity ratio changed from 0.9 in June to 1.30 in April.

As was mentioned, most groundwater samples have 234U/238U activity ratio above 1. An excess of 234U is explained by α-recoil of 234Th into solution and by preferential leaching of 234U from soil [22, 23]. However this phenomenon plays a role for water reservoirs with adjacent rock with a higher concentration of uranium, for example granitic minerals. It should be mentioned that α-recoil mechanism is efficient when dissolution rates are low and the residence times of adjacent water in contact with the rock are very long. Such conditions do not exist in the case of Poddebice thermal groundwater, which has a temperature of about 60 °C, and a sandy soil environment with very low uranium content. The profile of the uranium activity concentration in the Poddebice bore-hole is shown in Fig. 5.

Fig. 5

Uranium-238 activity concentration profile for the Poddebice bore-hole

As is evident from Fig. 5, uranium surface soil activity is equal to 17 Bq/kg, whereas for the deeper sandy geological formation, which surround the thermal groundwater reservoir, this activity concentration decreases to very low values of ≈11 Bq/kg.

In this case, the dissolution rate of both uranium isotopes is higher and the input of 234U recoil directly to the solution is negligible. However, because of very low uranium concentration in the surrounding soil, leachable uranium radionuclides in the time scale of these processes (the dozen thousands of years) are exhausted and there is no renewal of material (uranium) exposed to water weathering. Then, such water would be expected to evolve to have values of 234U/238U ratio below unity even up to 0.3 as presented in models by Anderson et al. [24] or Rihs et al. [25].

This data clearly show lack of any water transport from the surface water to thermal groundwater reservoir. However, such transfer from the Ner river to the underground municipal water is possible.

Uranium activity concentration in thermal groundwater samples from Central Poland

Lower values of 234U/238U activity ratio were also observed for the geothermal waters in other places in Central Poland. The results are presented in Table 3.

Table 3

Activity concentration and activity ratio of uranium isotopes in the water samples from central Poland

		Activity (mBq/dm3)		234U/238U
		234U	238U
Geothermal reservoir	Mszczonow	13.32 ± 0.26	15.98 ± 0.29	0.83
	Skierniewice	<0.013	<0.013	–
	Uniejow	0.39 ± 0.04	0.54 ± 0.04	0.75
	Poddebicea	0.12 ± 0.03	0.13 ± 0.03	0.94

aAverage from ten month in 2011 year

As shown in Table 3, the activity concentration of 234U and 238U in water samples varied in a wide range, from <0.013 to 16.84 mBq/dm3 and from <0.013 to 45.47 mBq/dm3, respectively. The highest uranium activity concentrations were measured in thermal groundwater from Mszczonow and Cieplice, whereas the lowest uranium activities were found in thermal ground water from Uniejow and Poddebice.

The values of 234U/238U activity ratio below 1 confirm basically the same hydro geochemical conditions for all these thermal groundwater reservoirs in Central Poland. Similarly, lower values of uranium activity ratio for most geothermal waters are reported by Osmond and Cowart [4].

Conclusions

234U and 238U activity concentrations were determined in the water samples (river water, deep well water and thermal groundwater) collected from Central Poland. Uranium activity concentrations in the analyzed waters varied in a wide range. On the basis of the measured values, the uranium activity ratio (234U/238U) was calculated. In deep well water and river water, uranium activity ratio (234U/238U) was higher than 1 (from 1.06 to 1.20), while in thermal groundwater it was lower than 1 (from 0.75 to 0.95). A comparison of the average uranium isotopic ratio in the well water from Poddebice (1.06) and river water from the Ner (1.09) show possible infiltration of surface water to this reservoir.

The lower 234U/238U ratios in the geothermal water are caused by the fact that the majority of both uranium isotopes come from the uranium present in low concentrations in the adjacent rocks and its relatively quick dissolving at higher temperatures leads to its exhausting from the surface layer of rocks contacting with water. Therefore, the input of the recoil 234U atoms is negligible. The remarkably lower 234U/238U data for thermal groundwater in the Poddebice region in comparison with data for surface and well water in this region, clearly show lack of any water transport from the surface water to this reservoir.