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Mesa de Los Santos is an elevated plateau located in the Eastern Cordillera of Colombia (Andean region) in the Department of Santander (altimetric variation 300 to 1800 m a.s.l.). The region is marked by a shortage of potable surface water. The isotopic study of the rain and its relation to Los Santos Formation phreatic and fractured aquifer was carried out. Four rain stations were installed on top of the plateau (1275 to 1684 m a.s.l.) to collect monthly rainwater samples for stable isotopes. The rainwater stations recorded 1469 to 764 mm from North to South, and the seasonal patterns of stable isotopes were similar in all stations. The preliminary Local Meteoric Water Line is δ2 H = 8.22×δ18 O + 13.9, slightly above the GMWL with an intercept of + 13.9 ‰, possibly indicating continental vapor recycling. The rain-weighted annual isotope means showed more depleted values in the southmost rain station (δ18 O = -8.99 ‰, 1275 m a.s.l.). From the phreatic and fractured aquifer, 35 groundwater points were monitored bimonthly for stable isotopes (n=134) and three trends were differentiated in the isotopic content value of δ18 O. Large temporal variability characterized 10 groundwater points (amplitudes greater than 1 ‰ in δ18 O for each point) consistently with the seasonal behaviour of the rain, indicating flows with rapid transit on a monthly scale. 21 groundwater points showed a low temporal variability with year-round differences under 1 ‰ and presenting a spatial distribution of δ18 O with enriched values towards the north (between -6.71 ‰ and -6.00 ‰) and depleted values towards the south (-8.97 ‰ to -8.14 ‰), which tends to be also consistent with the rainwater isotope distribution observed. The stability of year-round groundwater isotopic values is a sign of efficient mixing of groundwater and a slower transit. Finally, 4 groundwater points presented signs of evaporation, showing d -excess values between -6.4 ‰ and + 4.3 ‰, in connection with surface water bodies that undergo evaporation before infiltration. [ABSTRACT FROM AUTHOR]