Reactivity of Organic Matter and other Reductants in Aquifer Sediments

Abstract

The molecular composition and the carbon isotope signature of sedimentary organic matter (SOM) and indicate that SOM is predominantly derived from higher land plants in sediments of both terrestrial as marine origins. The reactivity of SOM in the aquifer sediments studied is determined by the extent of molecular SOM degradation. Although all SOM studied has been degraded extensively from its biological precursor, the presence of relatively preserved lignin-derived components represents the least degraded status of SOM. Interestingly, the preservation of SOM is not inversely related to the age of the aquifer sediments. Instead, SOM present in sediments of a marine origin was more preserved and more reactive than SOM in their fluvial and fluvio-glacial counterparts. The more frequent exposure to atmospheric oxygen of sediments in the highly dynamic terrestrial depositional environments probably explains this difference. As a result, SOM in terrestrial sediments suffers from more intense, aerobic degradation. Similarly, fluvio-glacial erosion result in the re-exposure of sediments to oxygen. Besides SOM, pyrite and ferrous iron bearing carbonates were the most reactive sedimentary reductants found. The importance of these other reductants also strongly depend on sediment history (depositional environment, diagenetic processes and (palaeo)hydrological conditions) as well as the relative preservation of SOM. Locally, the exfiltration of anoxic groundwater is likely responsible for the precipitation an important reactive ferrous iron bearing carbonate phase. Acid conditions inhibited the microbial degradation of SOM while, under circum-neutral pH conditions, the reactivity of pyrite decreased due to the formation of ironhydroxide coatings on its surface. In contrast with oxygen reduction, microbial adaptation to nitrate exposure controlled the rates and pathway of nitrate reduction. In addition, results indicate that SOM oxidation was favored over pyrite oxidation during nitrate reduction under carbonate buffered conditions.