H090-0004
Determining Sources and Residence Times of Groundwater and Carbon in a Semi-Arid Basin

Thursday, 10 December 2020
Poster
Jennifer Souza, Idaho State University, Geosciences, Pocatello, ID, United States, Kathleen A Lohse, Idaho State University, Biological Sciences and Geosciences, Idaho Falls, ID, United States, Melissa E Schlegel, Idaho State University, Pocatello, ID, United States, Sara R Warix, Idaho State University, Geosciences, Pocatello, United States, Erin M Murray, Boise State University, Department of Geosciences, Boise, ID, United States, Sarah Godsey, Idaho State University, Department of Geosciences, Idaho Falls, ID, United States, Zane Cram, Hydrologist, Agricultural Research Services, Boise, ID, United States and Mark S Seyfried, USDA-ARS, Northwest Watershed Research Center, Boise, ID, United States
Abstract:
Groundwater is a globally important reservoir for freshwater and carbon. Despite its importance, the residence time and sources of this water and carbon are often poorly constrained, especially in dryland regions. We collected samples from ten groundwater wells along a hypothesized east-west flowpath and six springs in the headwaters of the Reynolds Creek Experimental Watershed, a semi-arid basin located in southwestern Idaho. We analyzed samples for a combination of age (14C-DIC, tritium) and environmental tracers (DIC/DOC, 13C-DIC/DOC, stable water isotopes) to track and understand the movement and evolution of water and carbon through groundwater aquifers. We hypothesized that carbon would increase in concentration and age from the headwaters to the lower basin and along deeper flowpaths. Dissolved inorganic carbon (DIC) concentrations were generally lower in the headwaters than the basin wells and 13C-DIC became more fractionated moving west along flowpaths, -14.8‰ to -10.94‰. Age tracers were positively correlated with the depth of the wells indicating that deeper flowpaths tend to have longer residence times; tritium ages varied from 13 years at a spring site to 87 years at the deepest well. Carbon-14 dating showed a similar trend with 1212 years BP and 7343 years BP for the spring and basin well respectively. Additionally, stable water isotopes showed well samples were 36.6% evaporated compared to the Local Meteoric Water Line in Boise, whereas springs were closer to present-day precipitation. Collectively, our results indicate that water and carbon evolve from the headwaters along intermediate aged flowpaths and emerge as springs under open-system conditions, whereas water and carbon derived from basin wells evolve along regional flowpaths under more closed-system conditions. The well waters have longer residence times and water-rock interactions that lead to calcite saturation, resulting in higher concentrations of carbon and higher pH levels.
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