Tracing chemical and biological O2 consumption in the subsurface by 18O isotope enrichment factors

Dissolved O 2 (DO) levels in the shallow subsurface, controlled by both chemical and biological processes, play an important role in contaminant transformation and attenuation. However, methods for tracing chemical and biological DO consumption are not available. Here we show that oxygen isotope enrichment factors ( ε ) can be used to trace the mechanisms of chemical and biological O 2 consumption in the shallow subsurface. In batch solution experiments, the ε values for O 2 consumption by dissolved Fe(II), dissolved organic carbon (DOC), and their mixtures at different C/Fe ratios (0.5∼20) were measured to range from -1.74 ‰ to -3.32 ‰. In the oxygenation of reduced clayed sediment suspension, the ε values were relatively small (-6.46 ‰) in the initial 15 h and increased to a large level (-18.14 ‰) afterward; while for the sand sediment suspension, the ε values were always small (ranging from -4.56 ‰ to -4.65 ‰) throughout the duration time of 24 h. Chemical probe and sterilized control experiments proved that the initial stage characterized with small ε values (<-10 ‰) was dominated by chemical O 2 consumption and the subsequent stage with large ε values (>-10 ‰) was dominated by biological O 2 consumption. As an effective indicator for distinguishing subsurface chemical and biological oxygen consumption pathways, ε will provide a new parameter for exploring carbon and nitrogen cycling and pollution transformation.