Groundwater recharge in highly fractured volcanic aquifers in the humid tropics remains poorly understood. In this region, rapid demographic growth and unregulated land use change are resulting in extensive surface water pollution and a large dependency on groundwater extraction. Here we present a multi‐tracer approach including δ18Oδ2H, 3H/3He dating, and noble gases (NG) within the most prominent multi‐aquifer system of central Costa Rica, with the objective to assess dominant groundwater recharge mechanisms and age distributions. We sampled wells and large springs across an elevation gradient from 868 to 2421 m asl. Our results indicate relatively young apparent ages ranging from 0.0 ± 3.2 up to 43.5 ± 7.6 years within the unconfined aquifer system. Helium isotopes (R/Ra up to 5.4) indicate a dominant signal from the upper mantle and preclude 3H/3He dating in 50% of the samples. Potential recharge elevations (based on NG and δ18O) ranged from ~1350 to 2670 m asl. NG‐derived recharge temperatures ranged from 11.0°C to 19.4°C. Recharge estimates varied from 129 ± 78 to 1605 ± 196 mm/yr with a mean value of 642 ± 117 mm/yr, representing 20.1 ± 4.0% of the total mean annual rainfall as effective recharge. The shallow unconfined aquifer is characterized by young and rapidly infiltrating water, whereas the deeper aquifer units have relatively older water (>60 years). These results are intended to guide the delineation and mapping of critical recharge areas in mountain headwaters to enhance water security and sustainability in the most important headwater dependent systems of Costa Rica.
Numerous socio-economic activities depend on the seasonal rainfall and groundwater recharge cycle across the Central American Isthmus. Population growth and unregulated land use changes resulted in extensive surface water pollution and a large dependency on groundwater resources. This work combines stable isotope variations in rainfall, surface water, and groundwater of Costa Rica, Nicaragua, El Salvador, and Honduras to develop a regionalized rainfall isoscape, isotopic lapse rates, spatial-temporal isotopic variations, and air mass back trajectories determining potential mean recharge elevations, moisture circulation patterns, and surface watergroundwater interactions. Intra-seasonal rainfall modes resulted in two isotopically depleted incursions (W-shaped isotopic pattern) during the wet season and two enriched pulses during the mid-summer drought and the months of the strongest trade winds. Notable isotopic sub-cloud fractionation and near-surface secondary evaporation were identified as common denominators within the Central American Dry Corridor. Groundwater and surface water isotope ratios depicted the strong
Nitrate legacy is affecting groundwater sources across the tropics. This study describes isotopic and ionic spatial trends across a tropical, fractured, 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 potential source contributions to the nitrate legacy in groundwater. Land use change was evaluated using satellite imagery from 1979 to 2019. The lower nitrate concentrations (<1 mg/L NO3−N) 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− and moderate NO3−/Cl− ratios, indicating the potential influence of soil nitrogen (SN) inputs. Wells and low‐elevation springs exhibited greater NO3−/Cl− ratios and Cl− concentrations above 100 μmol/L. Bayesian calculations suggest a mixture of sewage (domestic septic tanks), SN (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.
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