Nitrate Legacy in a Tropical and Complex Fractured Volcanic Aquifer System

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, delta O-18(water), delta N-15(NO3), and delta O-18(NO3). 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 similar 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 mu mol/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. Plain Language Summary This study analyzed different nitrate sources in a complex tropical groundwater system and how they were affected by changes in land use over the last decades. In general, high-elevation areas near protected forests are represented by lower nitrate levels, while mid- and low-elevations areas exhibited greater nitrate and chloride concentrations. Our results suggest that groundwater nitrate reflects a legacy 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. The identified nitrate groundwater legacy in the Central Valley of Costa Rica, evidenced by unprecedented decadal nitrate isotope data, could serve as a valid example of the potential impact of abrupt urbanization growth across the wet tropics.