ABSTRACT:

The study area (218 km2) is located on the northwestern part of the Central Valley of Costa Rica. The region is characterized by two well-defined seasons: dry (December to April) and wet (May to November). Daily precipitation samples (N=793) were collected from April 2021 to December 2022. Three passive rain collectors (Palmex Ltd., Croatia; Gröning et al., 2012) were installed at 938 m a.s.l (AC), 1,804 m a.s.l (AF), and 2,532 m a.s.l (VP). Precipitation and groundwater samples were stored below 6°C in 50 mL HDPE bottles with no headspace and plastic inserts to avoid evaporation. Samples were analyzed at the Stable Isotopes Research Group laboratory at the Universidad Nacional (Heredia, Costa Rica) using an IWA-45EP water analyzer (Los Gatos Research, Inc., California, USA). The precision was ±0.80‰ for d2H and ±0.08‰ for d18O (1sigma; 5 injections). Isotopic reference materials, USGS45 (d2H=−10.30‰, d18O=−2.24‰), USGS47 (d2H=−150.2‰, d18O=−19.80‰), and calibrated secondary standard PGW (d2H=−52.64 ‰, d18O=−8.20‰), were used for data normalization and quality control. 18O/16O and 2H/1H ratios are presented in delta notation relative to the VSMOW-SLAP scale and expressed as per mile (‰). Deuterium excess was calculated as d-excess= d2H - 8·d18O . Long-term d2H, d18O, and d-excess precisions are 0.29‰, 0.08‰, and 0.65‰, respectively.