Journal ofAfricanEarth Sciences, Vol. 15, No. 3/4, pp. 399.403, 1992. 0899-5367./92 $5.00+0.00 Printed in Great Britm © 1993 Pergamon Press Year-long variability of Rn-222 in a groundwater system in Nigeria I. P. FARAI and A. O. SANNI Department of Physics, Universityof Ibadan, Ibadan, Nigeria (First received 1lth October, 1989; revised form received 26th September, 1991) Abstract-Radon 222 was measured continuously in a groundwater system in Nigeria for one year by y-spectroscopy. The mean Rn concentration for a particular month is 15,1 Bq/1 with a standard deviation of 1.0 Bq/1while the mean for the whole year is 14.8Bq/1with a standard deviationof 1.4 Bq/l. All the values for the year lie essentially within +~ of the mean. Rainfall appears to be the only major factor causing variations. INTRODUCTION Rn-222, the only gaseous member of the U-238 series, is an isotope of radon, element 86 in the periodic table. Like other members of the rare gas group, it is an inert gas. The other two common isotopes of the element are Rn-220 (T~/2 = 55.6s) and Rn-219 (T~/2 = 3.96s) which respectively, belong to the Th-232 and U-235 series. Because of their short half-lives, they are not usually observed in groundwater samples. Rn-222 on the other hand has a half life of 3.83 d, which is long enough for it to migrate through long distances in rocks, soils and water solutions within the earth. It is quite ubiquitous. When confined with its imme- diate parent, Ra-226 (T~/2 = 1622 y) it attains a secular equilibrium with Ra according to the equa- tion Rn=Ra (1 - e "~t) where ~ is the decay constant of Rn (3 x 10-6s-q, t is the time and Ra and Rn are the activities of radium and radon respectively. This equilibrium is attained within 20 days and conversely, no signi- ficant Rn exists after about 20 days (-5 T]/2) of separation from Ra. It is therefore a useful index in U-exploration. In the absence of man-made radionuclides, radioactivity in groundwater is determined mainly by the amount and distribution of the natural radionuclides of U, Th and K in the materials which constitute the aquifer. Rn-222 very easily ema- nates from the solid matrix into groundwater through several processes (Tanner, 1980) at the low concentration of U and Ra usually encounter- ed in natural rocks. Rn/Ra activity ratio can be as high as I0 e in groundwater (Hess et aL, 1985), presenting an extreme separation and disequili- brium between parent and daughter in the same medium. Data on radon are very sparse in this part of the world in spite of its radiological, geological and seismic importance. This work for the first time in Nigeria presents the profile of Rn level variability in a typical groundwater system over a period of one year. SAMPLING The use of groundwater through boreholes has greatly increased in Nigeria in recent times. One of such boreholes was selected for study in this work. It is about 20 m from the measuring laboratory situated at the Physics Department, University of Ibadan, and this greatly reduced logistic problems. It is about 80 m deep with its aquifer in a fairly large local fracture in the basement com- plex rocks of SW Nigeria. It has a high yield and its rate of use is fairly constant and stable. Geo- chemical studies of the rocks by Oshin and Rahaman, 1986 indicate that the Schists and quartzites have low U-content (0.3-1.1 pm). The Rn level is however measurable with the high efficiency y-spectroscopic system assembled for this work. Sampling of the well was done at least three days a week on a continuous basis for one whole year (Nov. 1987 - Oct. 1988). Sampling on a daffy basis was achieved completely in the month of March 1988. There were no significant changes in the conditions of the well or its environment through- out the period. MEASUREMENT Gamma-ray detection method was employed in the measurements. The y-rays with energies 0.61 and 1.12 MeV from Bi-214, short-lived progeny of Rn have been used, as Rn is itself not a strong y- emitter. Each sample was assayed after 4 hours of 399