Radium isotopes for groundwater age and sustainability in the highland of Antananarivo, Madagascar

Abstract Nowadays, the access to drinkable water becomes more and more difficult for many regions around the world and the highland of Antananarivo, Madagascar, does not escape this problem. The groundwater resources of Antananarivo region could be a solution to consider for water needs, especially during drought. Knowing the groundwater age permits to estimate the aquifer potentiality, to predict the sustainability if the water source is health safe, and then to help the decision maker for the best management of the groundwater resources. Radium isotopes in the groundwater of this area have been measured to estimate the residence time of the water in these shallow aquifers. The analyses were carried out by using high resolution gamma spectrometry on MnO 2 -impregnated fibers as radium collectors from groundwater. 226 Ra activity ranges from 29.2 ± 10.2 to 241.7 ± 26.9 mBq/L -1 and 228 Ra activity ranges from 22.5 ± 4.9 to 684.9 ± 13.5 mBqL -1 whereas 228 Ra/ 226 Ra activity ratio ranges between 0.20 ± 0.05 and 4.86 ± 0.88. Three variations of a same basic model depending on the variation of 228 Ra/ 226 Ra activity ratio in function of alpha-recoil, dissolution, adsorption and time were used to estimate the groundwater residence time. The most reliable ages of the highlands’ groundwater, calculated in this way, range from 0.2 to 10 years. According to these results, the groundwater of the E-NE sector of the area have the highest average residence time of 6.5 years and a total potential volume of available groundwater estimated to about 152· 10 6  m 3 . These features, coupled with radium content below the WHO guidance levels for drinking water, show that the groundwater of this sector constitute a reliable resource to supply the region of its needs for human as well as for agricultural purposes.