Isotope study of monthly rainfall and its response in the Santos Formation phreatic aquifer, La Mesa de Los Santos, Santander (Colombia).

La Mesa de Los Santos is located in the Eastern Cordillera of Colombia and configures a plateau marked by a shortage of potable surface water. The isotopic study of the rain and its relation to Los Santos Formation phreatic aquifer was carried out. Los Santos Formation is divided into three members: Upper Member (well cemented quartz sandstones), Middle Member (siltstones, claystones and sandstones) and Lower Member (sandstones and conglomerates). Phreatic aquifer groundwater circulates in the Upper Member. On the plateau, 4 stations were installed to collect rainwater samples monthly for stable isotopes (n=46) from October 2020 to September 2021. The seasonal behavior of Oxygen-18 and Deuterium is similar in the four stations, and the Local Meteoric Water Line is δ²H=8.22×δ¹⁸O+13.8. The rain-weighted average values of Oxygen-18 and Deuterium (V-SMOW) in each station are: -8.99‰, -61.4‰ (height=1275masl, P=764mm); -8.78‰, -58.5‰ (height=1678masl, P=1869mm); -8.51‰, -54.5‰ (height=1684masl, P=1469mm); -8.13‰, -52.2‰ (height=1433masl, P=1030mm), respectively. The isotopic composition of the rainwater does not correlate with the elevation or the precipitation height. From the phreatic aquifer, 35 groundwater points (wells, borehole and springs) were monitored for stable isotopes (n=134) in the months of December 2020, February, April, June, and August 2021. Three behaviors were differentiated in the isotopic composition of Oxygen-18. Large temporal variability (differences greater than 1‰ in δ¹⁸O) apply to 10 groundwater points showing δ¹⁸O values between -9.64 and -5.19‰ (with two outliers at -1.1 and -3.8‰), in accordance with the monthly behavior of the rain, considering flows with rapid transit on a monthly scale. 21 groundwater points show a low temporal variability (differences less than 1‰ in δ¹⁸O), presenting a distribution of Oxygen-18 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 rain, although with a better mix of groundwater and a slower transit than the previous behavior. Finally, 4 groundwater points present signs of evaporation, showing values of Oxygen-18 between -6.2 and -3.1‰ and d-excess values between -6.4 and +4.3‰, which are related to surface water bodies that undergo evaporation before the infiltration process.