Disentangling dissolved oxygen sources in shallow riparian groundwater by stable isotope analysis

This study focused on the Selke River in the Harz Mountains (Germany) with steep DO gradients between the stream water and the shallow, adjacent groundwater and associated stable isotope shifts. δ13CDIC values decreased from -13 􏰀 to -18 permille versus the Vienna Pee Dee Belemnite standard (VPDB) from May to November 2016 and indicated the dominant influence of microbial respiration on the observed DO gradients. With such respiration dominance, we have expected a simultaneous enrichment of δ18ODO to values higher than the one of atmospheric O2 (+23.9 􏰀permille versus Vienna Standard Mean Ocean Water standard - VSMOW). However, our measurements revealed anomalously low δ18ODO values between +22 􏰀 and +18 􏰀permille versus VSMOW for the same time period. These δ18ODO values were lower than those found in the river. Latter were close to equilibrium with the atmo- sphere (24.9 􏰀permille versus VSMOW). The observed δ18ODO ratios in the shallow groundwater can be explained with DO from the river that is subject to fractionation by microbial respiration with a typical fractionation factor (αr) of 0.995. In addition, mass balances revealed that this oxygen pool receives contributions of up to 25 % by diffused oxygen from the vadose zone. Consequently, isotope shifts by respiration and admixture with surface water are masked by diffusion effects that result in a decoupling of carbon and oxygen isotope systematics in the near river subsurface environment. They also demonstrate that DO in shallow groundwaters has additional sources than admixture from surface waters alone.