Groundwater-surface water interactions are important in controlling lake water residence time, biogeochemistry, and water availability for downstream communities in tropical volcanic catchments. To better understand these complex seasonal interactions, a multi-tracer approach including water and inorganic carbon stable isotopes (δ2H, δ18O, δ13CDIC), hydrochemistry, and 222Rn was applied in Lake Hule, northern Costa Rica. Seasonal isotope mass balance calculations using lake, stream, precipitation, and groundwater isotope compositions were supplemented with local hydrometeorological information. Evaporation to inflow ratios (E/I) revealed a small variability between the dry (December-April) and wet seasons (May-November), with relatively low evaporation losses, 2.9±1.0 % and 3.2±1.8 %, respectively. Bayesian end-member analysis indicated that annual inputs from groundwater, precipitation, and runoff represented 61.3±8.1%, 24.4±8.4, and 14.3±5.9% of total inflow, respectively. Temporal variations of δ13CDIC also confirmed the key role carbonate buffering plays in this lake and indicated greater CO2 degassing from groundwater sources in the wet season. This first tracer-aided assessment in a volcanic lake maar of northern Costa Rica provides evidence of previously unknown groundwater-surface water interactions and poses a promising tool for estimating seasonal variability of groundwater discharge into natural lakes across the volcanic front of Central America.

Nitrate contamination is affecting groundwater across the tropics. This study describes isotopic and ionic spatial trends across a tropical and volcanic multi-aquifer system in central Costa Rica in relation to land use change over four decades. Springs and wells (from 800 to 2,400 m asl) were sampled for NO3- and Cl- concentrations, δ18Owater, δ15NNO3, and δ18ONO3. A Bayesian isotope mixing model was used to estimate source contributions to the nitrate legacy in groundwater. Land use change was evaluated using satellite imagery from 1979 and 2019. The lower nitrate concentrations (< 1 mg/L) were reported in headwater springs near protected forested areas, while greater concentrations (up to ~63 mg/L) were reported in wells (mid- and low-elevation sites in the unconfined unit) and low-elevation springs. High-elevation springs were characterized by low Cl- concentrations and moderate NO3-/Cl- ratios, indicating the potential influence of soil nitrogen inputs. Wells and low-elevation springs exhibited greater NO3-/Cl- ratios and Cl- concentrations above 100 mg/L. A decreasing trend in NO3-/Cl- ratios coupled with greater Cl- values was also detected. Bayesian calculations suggest a mixture of sewage (domestic septic tanks), soil nitrogen (forested recharge areas), and chemical fertilizers (coffee plantations), as a direct result of abrupt land use change in the last 40 years. Our results confirm the incipient trend in increasing groundwater nitrogen and highlight the urgent need for a multi-municipal plan to transition from domestic septic tanks to regional sewage treatment and sustainable agricultural practices to prevent future groundwater quality degradation effectively.