Temporal dynamics of groundwater-dissolved inorganic carbon beneath a drought-affected braided stream: Platte River case study

Authors

Audrey R. Boerner, University of Nebraska-LincolnFollow
John B. Gates, The Climate Corporation

Date of this Version

2015

Citation

Boerner, A. R., and J. B. Gates (2015), Temporal dynamics of groundwaterdissolved inorganic carbon beneath a drought-affected braided stream: Platte River case study, J. Geophys. Res. Biogeosci., 120, 924–937, doi:10.1002/2015JG002950.

Comments

Copyright © 2015 American Geophysical Union. All Rights Reserved. Used by permission.

Abstract

Impacts of environmental changes on groundwater carbon cycling are poorly understood despite their potentially high relevance to terrestrial carbon budgets. This study focuses on streambed groundwater chemistry during a period of drought-induced river drying and consequent disconnection between surface water and groundwater. Shallow groundwater underlying vegetated and bare portions of a braided streambed in the Platte River (Nebraska, USA) was monitored during drought conditions in summer 2012. Water temperature and dissolved inorganic carbon (dominated by HCO₃^-) in streambed groundwater were correlated over a 3 month period coinciding with a decline in river discharge from 35 to 0 m³ s^-1. Physical, chemical, and isotopic parameters were monitored to investigate mechanisms affecting the HCO₃^- trend. Equilibrium thermodynamic modeling suggests that an increase of pCO₂ near the water table, coupled with carbonate mineral weathering, can explain the trend. Stronger temporal trends in Ca²⁺ and Mg²⁺ compared to Cl^- are consistent with carbonate mineral reequilibria rather than evaporative concentration as the primary mechanism of the increased HCO₃^-. Stable isotope trends are not apparent, providing further evidence of thermodynamic controls rather than evaporation from the water table. A combination of increased temperature and O₂ in the dewatered portion of the streambed is the most likely driver of increased pCO₂ near the water table. Results of this study highlight potential linkages between surface environmental changes and groundwater chemistry and underscore the need for high-resolution chemical monitoring of alluvial groundwater in order to identify environmental change impacts.