Local Meteoric Water Line of the Central Zone of Boyacá, Colombia
Jaison MALO, Claudia ALFARO
[Servicio Geológico Colombiano, Colombia]
This paper presents information of stable isotopes composition of meteoric water obtained in the period July 2014 – November 2016, for Boyacá Central North Zone, which includes Paipa geothermal area. The work based on the analysis of monthly samples collected in a network shaped by 41 stations: 28 rainwater totalizers, 12 sampling points in rivers at different elevations and, 1 dam. Measurements of monthly precipitation and in situ physicochemical parameters (pH and electrical conductivity) were performed. The mean precipitation levels are relatively low; in general they range between 200 y 350 mm with a maximum value around 900 mm, in the stations located in the eastern side of the area. The magnitude of pH in rain water oscillates between 4.07 to 8.9 and the electrical conductivities, between 0.7 to 356μS/cm. From the results of stable isotopes analyses, a local meteoric isotopic line was determined (dD [‰] as function of d18O [‰]) to the Boyacá Central North Zone. The resulting values are 10.37 ‰ for deuterium excess and 7.86 for the slope. From the rain distribution pattern, a particular behavior of the stations located to the west of Tibasosa – Toledo Anticline and south of Chicamocha River was established. On the basis of the selection of information for those stations, another function was determined, considered as the local meteoric line for Paipa geothermal area. In this case, the resulting deuterium excess was 7.15‰ and the slope, 7.7. The magnitude of these variables show a significant variation with respect to the global meteoric line water (D=8*18O+10), which could be related to a high evaporation process. The variation of the isotope composition of D and 18O with the elevation, relation used for the estimation of the elevation of groundwater recharge, resulted in a linear function with wide scattering, when all the data were considered (typical error between 6 to 8 for D and, 0.8 to 0.9 to 18O). However, the same relation for selected stations on the basis of the rain distribution pattern, for Paipa geothermal system, shows a much lower and acceptable scattering (standard error of 1.9 to 2.2 for D and 0.2 to 0.4, to 18O). Based on the assumption of that isotope composition of hot springs water do not have a significant isotopic enrichment and it is the result of mixing between saline sodium sulfate water (isotopically enriched) and precipitation water, a recharge between 2900 and 300 m.a.s.l. for the hot fluid of the geothermal system, was estimated. Although the results of this work, summarized mainly in the local meteoric isotopic line of the studied zone and in the variation of isotope composition as function of elevation, can be applied to the interpretation of isotope analyses measurements for groundwater points, in future studies, the impact of El Niño phenomenon during the sampling collection period, should be taken into account. The isotope analytical work on which this study is based, was carried out in the Stable Isotopes Analysis of the Colombian Geological Survey (SGC for its acronym in Spanish). Results of a comparison exercise of the local laboratory with the IAEA (International Atomic Energy Agency) laboratory in Vienna, are included. They show no significant variations between the results.
|        Topic: Geochemistry||Paper Number: 14088|