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Chemical and isotopic signature of old groundwater and magmatic solutes in a Costa Rican rain forest: Evidence from carbon, helium, and chlorine

C, He, and Cl concentrations and isotopes in groundwater and surface water in a lowland Costa Rican rain forest are consistent with the mixing of two distinct groundwaters: (1) high‐solute bedrock groundwater representing interbasin groundwater flow (IGF) into the rain forest and (2) low‐solute local groundwater recharged in the lowlands. In bedrock groundwater, high δ13C (−4.89‰), low 14C (7.98 pM), high R/RA for He (6.88), and low 36Cl/Cl (17 × 10−15) suggest that elevated tracer concentrations are derived from magmatic outgassing and/or weathering of volcanic rock beneath nearby Volcan Barva. In local groundwater, the magmatic signature is absent, and data suggest atmospheric sources for He and Cl and a biogenic soil gas CO2 source for dissolved inorganic carbon. Dating of 14C suggests that the age of bedrock groundwater is 2400–4000 years (most likely at the lower end of the range). Local groundwater has 14C > 100 pM, indicating the presence of “bomb carbon” and thus ages less than ∼55 years. Overall, data are consistent with a conceptual hydrologic model originally proposed on the basis of water budget and major ion data: (1) large variation in solute concentrations can be explained by mixing of the two distinct groundwaters, (2) bedrock groundwater is much older than local groundwater, (3) elevated solute concentrations in bedrock groundwater are derived from volcanic fluids and/or rock, and (4) local groundwater has not interacted with volcanic rock. Tracers with different capabilities converge on the same hydrologic interpretation. Also, transport of magmatic CO2 into the lowland rain forest via IGF seems to be significant relative to other large ecosystem‐level carbon fluxes.