REACTIVITY OF REDUCTANTS IN SEDIMENTARY AQUIFERS: DEPOSITIONAL AND PALEOHYDROLOGICAL CONTROLS

The reactivity of sedimentary reductants mainly controls the natural attenuation of common groundwater contaminants, such as nitrate or chlorinated hydrocarbons.

Middle Miocene to Late Pleistocene aquifer sands with marine, fluvial, fluvio-glacial and aeolian origins were studied to determine the influence of depositional environment and diagenesis on the presence, distribution and reactivity of sedimentary reductants.

Oxygen consumption and carbon dioxide production during sediment incubations indicated that sedimentary organic matter (SOM), pyrite and ferrous iron bearing carbonates were the main reductants. Bulk d¹³C_org–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-glacial sediments, highly degraded lignin and recalcitrant macromolecular aliphatic structures dominated SOM, indicative of aerobic degradation. Co-oxidation of SOM and pyrite occurred during the incubations of the Tertiary marine sands, while pyrite oxidation dominated during the incubation of the fluvio-glacial sands. Therefore, the smaller relative importance of SOM oxidation likely reflects its more degraded nature of SOM as the higher dynamics of fluvio-glacial as compared with marine sediment deposition likely allows for longer and more intense aerobic degradation of SOM.

Despite the highly degraded nature of SOM and the absence of pyrite, oxygen consumption rates are elevated during the incubation of sediments from the shallow part of the aquifer. A reactive ferroan carbonate phase was identified 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 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 that precipitated during the exfiltration of Fe(II)-containing anoxic groundwater.