Characterization of DOC in riparian wetland pore water and its interference in stable water isotope analysis of δ²H and δ¹⁸O

Wetlands play an important role in carbon retention, which is greatly dependent on hydrological conditions. Therefore, the interest in wetland hydrology has increased over recent decades. In wetland hydrology, stable water isotopes (δ²H, δ¹⁸O) have found increasing use, as laser-based instruments have become readily available and allow for faster analysis and lower costs. However, the use of laser-based determinations of stable hydrogen and oxygen isotopes in wetland pore waters might be challenging because of dissolved organic carbon (DOC), especially as wetland pore waters often contain elevated DOC concentrations. To study the potential interference of DOC on laser-based stable water isotopes determination, we determined concentration, fluorescence and absorbance characteristics of DOC in pore waters collected from six restored and six near-natural riparian wetlands. The water samples’ δ²H and δ¹⁸O values were determined by two laser absorption spectrometers (LAS) and, for reference, in parallel by dual-inlet isotope-ratio mass spectrometry (DI-IRMS). The two LAS methods showed significant deviations from the DI-IRMS-determinations. Variations in the specific UV absorbance index (SUVA₂₅₄, the quotient of the absorbance at 254 nm and the DOC concentration), which indicates the aromaticity of the DOC, partially accounted for interference for δ²H measurements. Elevated SUVA₂₅₄ index values were especially linked to restored wetlands. Studies that investigate pore waters with high DOC aromaticity but where the DOC is not characterized, and which require a high accuracy of the δ²H-values for interpretation of the hydrological system, may be severely affected by DOC interference.