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The upper catchment of Ikopa has great potential for surface or sub-surface water resources. The Ikopa River plays an important role in the water supply of the plain of Antananarivo. Knowing that surface water is abundant but exposed to the high risk of pollution, and groundwater remains less exploited because of the insufficiency of the study concerning the recharge and the sources of their mineralization. The objective of this research is to use the isotope method to characterize groundwater to determine their origin and recharge process. The results of chemical analysis show three different facies type of groundwater which dominated by the sodium, nitrate and chloride ions. The isotopic compositions of the waters sampled vary respectively from -7.25‰ to 5-.09‰ for δ18O and from -46.9‰ to -24.7‰ for  δ2H. water form the lakes are more enriched compared to those rivers and groundwater. The diagram δ18O- δ2H shows that groundwater is recharged from local rainwater but undergoes low evaporation before infiltration. However, the results obtained from the parameters measured in situ show that a lateral recharge of groundwater from the streams feeds the neighboring aquifers. Lakes are fed by run-off during the rainy season.

Identification of groundwater origin using environmental isotopes (18O and 2H) in the upper catchment of Ikopa, district Antananarivo