Effects of diffuse groundwater discharge, internal metabolism and carbonate buffering on headwater stream CO₂ evasion

Groundwater discharge to headwater streams and concomitant terrestrially derived CO₂ input play a key role in stream CO₂ evasion. However, previous studies rarely examined the transport and transformation of terrestrially derived CO₂ in headwater streams, thereby limiting our understanding of stream CO₂ evasion. Here, we firstly used a numerical flow model to quantify groundwater discharge rates along a 43 km headwater stream (semiarid northwest China) by combining stream flow rate, radon-222 isotopic data, and then estimated internal metabolism through a reach-scale dissolved O₂ mass balance and carbonate buffering through carbon budget for the stream. Modeling results indicate that groundwater discharge rates were highly variable (2.17–7.03 m² d⁻¹, mean ± 1 standard deviation: 4.48 ± 1.25 m² d⁻¹). Groundwater was supersaturated with CO₂ at all sites (5.45 ± 3.87 mg C L⁻¹). Our results also show that diffuse groundwater discharge was a major control on terrestrial carbon input to the stream. Terrestrial CO₂ input (3.92 ± 2.03 g C m⁻² d⁻¹) contributed more CO₂ to the stream than internal metabolism (1.74 ± 2.33 g C m⁻² d⁻¹). However, most terrestrially derived CO₂ was transformed into HCO₃⁻ through carbonate buffering after entering the stream, inhibiting stream CO₂ evasion. This carbonate buffering process depends on the hydrochemistry and proportion of mixing waters. Overall, our study provides a bottom-up and holistic perspective to understand the transport and transformation of terrestrially derived CO₂ in headwater streams.