Depositional and paleohydrogeological controls on the distribution of organic matter and other reactive reductants in aquifer sediments

The reactivity of sedimentary reductants is the main control for the natural attenuation of common groundwater contaminants, such as nitrate or chlorinated hydrocarbons. Middle Miocene to Late Pleistocene (0.01–20 My old) aquifer sands of marine, fluvial, fluvio-glacial and aeolian origin were characterized to determine the control of sediment depositional environment, sediment age and paleohydrology on the distribution and reactivity of sedimentary reductants. In addition to the variability in the molecular composition of sedimentary organic matter (SOM), the reduction reactivity of these sediments was determined. Oxygen consumption and carbon dioxide production during sediment incubations indicated that SOM, pyrite and ferrous iron-bearing carbonates were the main reductants. Bulk δ13Corg-values (∼25‰) indicated that terrestrial higher land plants were the main precursor of SOM, regardless of sediment origin. However, pyrolysis-GC/MS analysis showed that SOM in the marine Tertiary sands contained lignin with preserved side-chains, while in the Pleistocene fluvial and fluvio-aeolian sediments, highly degraded lignin and recalcitrant macromolecular aliphatic structures dominated SOM, indicative of aerobic degradation. The higher dynamics of these terrestrial depositional environments as compared with their marine counterpart, likely allows for increased oxygen exposure times and thus more intense aerobic degradation of SOM. The aquifer sediment that fills a large erosional valley created during the Saale ice-age, likely consists of fluvio-glacially reworked marine Tertiary sediments, as supported by carbonate isotopic evidence. The more degraded status and decreased reactivity of SOM in this sediment than in its precursor, is likely due to the increased oxygen exposure during dynamic fluvio-glacial reworking. Despite the highly degraded nature of SOM and the absence of pyrite in the Pleistocene fluvial and fluvio-aeolian sediments, oxygen consumption rates were high during the incubation of sediments from the shallowest part of the aquifer. A reactive ferroan carbonate phase was inferred as the main source of oxidant demand in shallow fluvial and aeolian sands. Depleted oxygen and carbon isotopes indicated that this phase was groundwater derived. Aerobic degradation during sediment deposition appears to control the molecular composition and reactivity of SOM in aquifer sediments and to affect the potential of subsequent diagenetic pyrite formation. In addition, (paleo)hydrological conditions may result in the accumulation of a ferrous iron-bearing carbonate phase precipitated during the exfiltration of Fe(II)-containing anoxic groundwater.