Carbon, Chloride, and Oxygen Isotopes as Tracers of Interbasin Groundwater Flow at La Selva Biological Station, Costa Rica

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Title: Carbon, Chloride, and Oxygen Isotopes as Tracers of Interbasin Groundwater Flow at La Selva Biological Station, Costa Rica
Author: Webb, Mathew Douglas
Advisors: David Genereux, Committee Chair
Neal Blair, Committee Member
John Fountain, Committee Member
Abstract: Groundwater and surface water samples were taken at 14 locations at a lowland rainforest site (La Selva Biological Station) in Costa Rica for the analysis of DIC, DOC, 14C, 13C, 36Cl, 18O, and other geochemical parameters. The data are consistent with the mixing of two endmember groundwaters. One is a local water having low Cl concentrations (<0.07 mM), low DIC (<3.0 mM), high 14C (>100 pmc), δ13C between -22‰ and -26‰, and highly variable 36Cl⁄Cl ratios. This chemistry is consistent with locally recharged shallow groundwaters having short residence times in which the DIC originates from plant root respiration and atmospheric deposition is the only source of Cl. The other endmember is bedrock groundwater, representing interbasin groundwater flow (IGF) into La Selva and having relatively high Cl concentration (>0.9 mM), high DIC (about 14 mM), low 14C (<8 pmc), high δ13C (-3‰ to -5‰), and a low and more consistent 36Cl⁄Cl ratio. This chemistry is consistent with the expectations for bedrock groundwater recharged on the flanks of Volcan Barva to the north of La Selva, with a majority of the DIC and Cl derived from magmatic degassing and dissolution of the volcanic rocks that make up the aquifer. A 14C age of 750 — 4650 years before present was estimated for the bedrock groundwater endmember using NETPATH geochemical mass-balance modeling software, suggesting an average linear velocity of 3-20 m⁄yr for this groundwater; the actual age is probably closer to the upper limit, and velocity closer to the lower limit. The results of this study are consistent with prior work using major ion, 18O, and physical hydrologic data, suggesting that the conclusions about IGF and groundwater mixing at this site are correct. Also, new DIC data for bedrock groundwater and previous hydrologic data on bedrock groundwater inputs to the Arboleda watershed at La Selva suggest that IGF of bedrock groundwater is responsible for a large inorganic carbon flux into lowland watersheds (about 740 grams of carbon per m2 of watershed each year for the Arboleda).
Date: 2007-04-04
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences

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