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DIRECCIÓN DE INVESTIGACIÓN Y DESARROLLO

RADON DETERMINACIÓN IN' GROUND WATER

INFORME TÉCNICO IA-91-26 AGOSTO, 1991. RADON DETERMINATION IN GROUND WATER

N. Segovia A- y S. Bulbulian Gerencia de Investigación Aplicada Dirección de Investigación y Desarrollo Instituto Nacional de Investigaciones Nucleares..

INFORME TÉCNICO IA-9.1-26

AGOSTO, 1991 RADON DETERMINATION IN GROUND WATER

N. Segovia and S. Bulbulian Instituto Nacional de Investigaciones Nucleares Ap. Post. 18-1027, México D.F.,

INTRODUCTION The natural radioactivity of ground water varies markedly from site to site. Elevated levels of 2 2Rn and other natural radionuclides in ground water seems to favor granite areas and areas with abundant pegmatite. 222Rn has been used as a tracer in a number of studies of the aquatic environment . Among the early measurements of radon, those of Ramsey in 1915 have shown that radon concentrations in shallow springs increase with increasing discharge. More recent investigations in Japan discussed the 222Rn content in shallow unconfined ground water(3). The 222Rn source is the decay of ^^a within the solid matrix of the aquifer and exhaled from 226Ra iissolved in the ground water. 6Ra, with a half life of 1600 years, supplies a continuous source of 222Rn to the fluid in the pores of the matrix. Being chemically inert, radon incorporation in ground water systems is dependent apon molecular diffusion. Ground water flow is then the iominant mechanism by which radon is transported; the Limiting factor in the transport length being its ialf-life. In areas where surface waters infiltrate to aquifer, radon may be used as a tracer for qualitative Investigations. Studies on natural radioactivity in ground water íere started in Mexico in San Luis Potosi state followed Dy samplings from deep wgells and springs in the states of léxico and Michoacan ' ' . The samples were analyzed for solubilized and 22^Ra-supported 22Rn. Some of them íere also studied for U/ U activity ratio. In this paper we discuss the activities obtained and their relationship with the geologic characteristics of the studied zones.

En Homenaje al Dr. A. Mondragon. Coloquio Internacional sobre Tópicos de Física y Química. UNAM., México, D.F., 15-16 de Agosto, 1991. SITES DESCRIPTION 1.- San Luis Potosi State The water samples were collected from 4 wells in the Villa de Reyes valley and 3 wells in the San Luis Potosi . valley, both located in the State of San Luis Potosi, Mexico, approximatelly 400 km NE of Mexico City. The Villa de Reyes valley is formed by cemented sandstones, conglomerates and volcanic tuffs. A non confined aquifer is found formed by elastics and rhyolites. Ground water movement is non-horizontal; downward movement occurs in the center of the valley where rainfall and water from excess irrigation recharges the aquifer system. On the other hand, the San Luis Potosi valley has a shallow aquifer and a deep one separated by a clay and clayey sand body located in the middle of the valley . 2.- Mexico State The sampling includes 46 wells drilled up to 200 m from the City of , and 6 springs located at the SW part of the state. The aquifers of the studied zone in the are found mainly in basaltic rocks, particularly around the City of Toluca where ignous rocks are found produced by ancient eruptions from volcano 3.- Michoacan State Water samples from 2 springs belonging to the geothermal field of Los Azufres in the state of Michoacan have also being studied. The geothermal field of Los Azufres belongs to the Solfataras region located at the Sierra de San Andrés in the Central Part of the Mexican Neovolcanic belt. The rocks found in the area are basalts, rhyolites, dacites and andesites. Hydrothermal springs are. found in this under exploitation geothermal field 6 . The sampling locations are indicated in Figure 1.

Fig. 1.- Sampling Locations. 1.- San Luis Potosi State 2.- Mexico State 3.- Michoacan State EXPERIMENTAL

SAMPLING 222Rn in natural water easily escapes from water samples, therefore, the sampling has to be done with great care. The water must run steadily in the one liter washed and decontaminated container and without any air bubbles, and perfect sealing has to be achieved after sampling in order to avoid degassing. Finally the sample must be transported to the laboratory to be analysed for 2Rn content within 24 h from the time of sampling.

RADON DETERMINATION IN THE WATER SAMPLES Measurement of radon and short lived radon daughters activity in water samples was performed with a Packard No. 4530 scintillation detection system. The toluene extraction method reported by Noguchi and Wakita 7 was modified in order to measure very low 2Rn content in the water samples. Aliquots of 100 ml of sample and 10 ml of toluene were transferred to a separatory funnel and vigorously shaken for 2 minutes for radon separation. When the mixture resettled in two phases, the organic phase was transferred into a counting vial to which 10 ml of INSTAGEL scintillation solution was added. Samples were counted immediately after radon separation and again upon reaching radioactive equilibrium ( about 4 hrs) with his daughters. Corrections for decay of radon and decay and growth of the daughter products in the samples were necessary. Calibration of the counter was performed with a 226Ra-standard solution from NBS. Systematic countings of selected samples were made during several days in order to ascertain that the decay of the extracted nuclide in toluene corresponded to the 222Rn half-life. RADON SUPORTED RADIUM DETERMINATION We determined 226Ra support of 222Rn in some samples from the same water aliquots from where original Rn present in the samples was extracted. Those aliquots were sealed and left for one month in order to reach equilibrium between 226Ra present in water and 222Rn. Once the time elapsed, 222Rn extraction was performed from the samples in the same way as specified in the previous paragraph. MEASUREMENT OF 23*U/238U ACTIVITY RATIO Uranium was concentrated from twenty-liter water samples by co-precipitation in basic media. The precipitate was dissolved in 0.1N HCl solution. Uranium was then purified by the extraction-chromatographic method using di-2-ethylhexylphosphoric acid. Thin foils of U compound were prepared for use in a-spectrometry. Alpha-counting was performed utilizing a surface barrier detector coupled to a 1024 channel analyzer. The time of measurement of the activity of the sources ranged from several hours to two days, depending on the intensity of the a-sources.

RESULTS AND DISCUSSION Results of 222Rn, U and 234U/238U activity ratio as obtained from water samples of seven wells in San Luis Potosi and Villa de Reyes valleys are shown in Table 1. The water temperatures at sampling time are also indicated in this table.

Table 1.- Results obtained in San Luis Potosi State

234 238 Well Radon U u/ u iPemperatur (Bq/1)

VR 397 5.26 0.34 1.03 31 416 8.7 0.72 1.09 28.6 398 3.2 0.32 1.68 27.8 354 1.21 0.48 1.07 41.6 From these results we can see that radon solubility in water decreases with increasing temperature. No correlation was found between radon and uranium concentration levels in water. This lack of correlation has often been reported in aquifers since radon migration into the pore fluids is higher than uranium. The values of the activity ratios obtained, very close to equilibrium, together with the low uranim content seem to confirm a rapid water transit from the recharge zone to the sampling site. Radon content for the samples from Villa de Reyes are higher than those of San Luis Potosi by an average factor of 2.8. This suggests radon enrichment from a flow of more ancient subsurface water into the aquifer of Villa de Reyes. The distribution of 222Rn concentration as found in 46 wells from the City of Toluca is shown in Figure 2. It can be observed that 58% of the sampled wells have less than 2.4 Bq/1; the highest value obtained being 11.3 Bq/1. Bq/L 4

Dnttl WELL NUMBER Fig 2.- Radon Concentration at the City of Toluca Sampled Wells Those wells with the higher Rn concentration were found in regions where local rock composition is mainly andesites and rhyolites. 2Rn supported by Ra dissolved in the water samples was below the detecting threshold of the measuring equipment (0.009 Bq/1 as reported by Olguin et al. ), showing that radon input into the fluids of the aquifer is much higher than that of Ra. Radon content in the water from eight springs in the states of Mexico and Michoacan is shown in Table 2. The spring water samples were taken in the same month (November) as those of the wells of the City of Toluca. Table 2.- Radon Concentrations in Spring Water Samples from Mexico and Michoacan States

Spring Radon (Bq/1) Temperature(°C) Mexico State Las Tazas 0.491 17 1.518 33 2.3 62 32 Nevado de Toluca 0.375 16 Ixtapan del Oro I 1.037 31 Ixtapan del Oro II 0.117 25 Michoacan State San Alejo 1.335 41 Los Azufres 0.25 82 Our results which give extremely low radium and radon contents for ground water that circulate largely through volcanic rocks, can be explained if we consider that the samples were taken in november, at the end of seasonal rains, when the aquifers are recharged, and therefore become highly diluted.

ACKNOWLEDGEMENTS The authors are grateful to the technitians of the Chemistry and Nuclear Tracks Departments, ININ. We acknowledge finantial support from CONACyT, Mexico.

REFERENCES,

1.- Hoehn E. rand Von Gunten H.R. Radon in Groundvater: A Tool to Asses Infiltration from Surface Waters to Aquifers. Water Resour. Res. 25 (8), 1795-1803 (1989). 2.- Ramsey R.R. The Variation of the Emanation Content of Certain Springs. Philos. Mag. 6 (30), 815-818 (1915). 3.- Fukui M. 222Rn Concentrations and Variations in Unconfined Groundwater. J. Hydrol. 79, 83-94 (1985). 4.- Olguin M.T., Segovia N., Carrillo J. , Ordonez E., Iturbe J.L., Bulbulian S. Rn Content and 23iU/238U Activity Ratio in Groundvaters. J. Radioanal. Nucl. Chem. Art. 141, 1, 17-23 (1990).

5.- Olguin M.T., Segovia N., Tamez E., Alcantara M., Bulbulian S. Radon Concentration Levels in Ground Water from the City of Toluca, Mexico. Sci. Tot. Environ, (in rpress, 1991).

6.- Servün N., Segovia N., Armienta M.A., Aguayo A., Ceniceros N., Juarez F. Estudio Geoquímico y Emanométrico de Aguas de Manantiales. Informe Técnico IA-90-25, ININ, México (1990).

7.- Noguchi M. and Wakita H. A Method for Continuous Measurement of Radon in Groundvater for Earthquake Prediction.J. Geophys. Res. 82, 8,1353-1357, (1977).