Diffuse Groundwater Discharge Dominates Terrestrial Dissolved Inorganic
Carbon Export and CO2 Evasion From a Semiarid Headwater Stream
Abstract
Groundwater discharge to headwater streams and concomitant terrestrial
dissolved inorganic carbon (DIC) export play a significant role in
headwater stream CO2 evasion. However, previous studies rarely examined
diffuse groundwater discharge and its impact on headwater stream CO2
evasion, thereby lacking the understanding of the role of diffuse
groundwater discharge in terrestrial DIC export and stream CO2 evasion.
This study quantified diffuse groundwater discharge along a 43 km
semiarid headwater stream by combining hydraulic, isotopic (radon-222)
and chemical (electrical conductivity) approaches, and estimated the
reach-level CO2 budgets of the stream. Reach-scale water and mass
balance modeling yielded highly variable diffuse groundwater discharge
rates (n = 16, range: 1.08-7.80 m2/d, mean ± 1 sd: 4.57 ± 1.81 m2/d).
Groundwater was supersaturated with CO2 at all sites, with strongly
variable CO2 partial pressure (pCO2) and DIC concentrations at
1,223-27,349 μatm and 30-119 mg/L, respectively. Diffuse groundwater
discharge dominated terrestrial DIC export to the stream (12-111 g C m-2
d-1, normalized to water surface area). A portion of groundwater
dissolved CO2 transported to the stream was emitted to the atmosphere
with evasion rates varying at 0.62-3.18 g C m-2 d-1. However, most
dissolved CO2 was transformed into HCO3- through carbonate buffering
because of the regulation of carbonate equilibrium. Overall, the stream
CO2 evasion was driven by carbon transfer but limited by carbon supply.
This study provides a bottom-up perspective to understand terrestrial
DIC export and stream CO2 evasion in arid and semiarid areas.