Distribution and Reactivity of O 2 -Reducing Components in Sediments from a Layered Aquifer NIELS HARTOG,* ,† JASPER GRIFFIOEN, ‡ AND CORNELIS H. VAN DER WEIJDEN † Departm ent of Geochem istry, Faculty of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, Utrecht, The Netherlands The redox status of subsurface aqueous systems is controlled by the reactivity of solid redox-sensitive species and by the inflow of such species dissolved in groundwater. The reactivity toward molecular oxygen (O 2 ) of solid reductants present in three particle size fractions of sediments from a pristine aquifer was characterized during 54 days. The stoichiometric relationships between carbon dioxide (CO 2 ) production and O 2 consumption was used in combination with sulfate production to discriminate between the contributions of sedimentary organic matter (0-87%), pyrite (6-100%), and siderite (0-43%) as the dominant reductants. The observed simultaneous oxidation of these reductants indicates that they are reactive on the same time scales. The measured reduction capacity (8-84 μmol O 2 /g) ranged from 8 to 42% of the total reduction capacity present as pyrite and organic carbon in the total sediment fraction (<2 mm). Fine fractions (<63 μm) were 10-250 times more reactive than their corresponding total fractions. Oxygen consumption rates decreased continuously during carbonate buffered conditions, due to a decreasing reactivity of reductants. Acidification accelerated pyrite oxidation but impeded SOM respiration. Our findings indicate that the geological history of aquifer sediments affects the amounts of organic matter, pyrite and siderite present, while environmental conditions, such as pH and microbial activity, are important in controlling the reactivity of these reductants. These controls should be considered when assessing the natural reduction activity of aquifer sediments in either natural or polluted systems. Introduction Thenaturalpotentialofaquifersedimentstoreduceoxidants is ofgeneralinterest in groundwater chemistry.For instance, due to excessive fertilization and manuringextensive leaching of nitrate from agricultural fields occurs (1-4), and the fate ofthis nitrate is controlled bythe reactivityofthe reductants present in the subsurface (5-11). Degradation of organic contaminants is also controlled by the redox status of the contaminated groundwater (12-15). The anaerobic degra- dation of benzene is of prime interest (16, 17), as is the reductive dechlorination of chlorinated hydrocarbons by reactive reductants (18, 19). The injection of oxidants such as oxygen, nitrate, or sulfate mayenhance the breakdown of monoaromatics (16, 17, 20), but an important drawback for stimulated in-situ bioremediation in contaminated aquifers isthe competition ofnaturalreductantsfor injected oxidants (21, 22). Understanding the reactivity of reductants present in aquifer sediments thus deserves attention. Common reduc- tants in aquifer sediments are sedimentary organic matter (SOM) and pyrite (FeS2), but ferrous iron in silicates, siderite (FeCO3), and vivianite as well as exchangeable ferrous iron are potentiallyreactive reductants too (23).Pyrite and siderite are commonlyfound in close association with organic matter due to redox processes occurring during or after deposition (24).Therefore,a relationship between the reduction capacity and the diagenetic history of sediment can be expected. Furthermore, fine-grained sediments are generally richer in organic material and associated reduced mineral phases (8, 25), and higher total reduction capacities for aquifer sedi- ments with a larger fine fraction have been suggested (26). Recently,Christensen et al.discussed studieson the reduction capacity of aquifer sediments (27). The TRC of sediments can be calculated if all relevant reduced components are recognized, and their quantification is sufficiently accurate. However, this approach yields a maximum potential, since it does not account for the reactivities ofthese components. In this study, we focus on the reduction reactivity of pristineaquifersedimentsbymeasuringtheO2 consumption during incubations. Together with the overall change in aqueous composition,we use the stoichiometrybetween the O2 consumption and CO2 production to identifythe ongoing oxidation reactions. Our objectives were (1) to determine the relative contribution of the identified reductants to the reduction activity,(2)to assess the difference in the reduction capacity of different grain size fractions, and (3) to evaluate the impact ofgeologicalstratification on the reduction activity within a layered single aquifer unit that consists of three geological units. Materials and Methods Sample Collection and Processing. Six core samples were taken from a borehole in a sandy aquifer at the drinking water production site “De Steeg” near Langerak, The Netherlands.This aquifer was selected since it contains three distinct geological formations, covering a range from coarse to fine sands(Figure 1).Furthermore,thislocation isproposed as a site for recharge through riverbank infiltration, which *Corresponding author phone: (+31) 30 253 4991; fax: (+31) 30 253 5302; e-mail: nhartog@geo.uu.nl. † Utrecht University. ‡ TNO Netherlands Institute of Applied Geosciences. FIGURE1. Geological description of the sediments and geochemical characteristics of the total fractions (0-2 mm) used. Depth is referenced in meters below surface level. A log scale was used for the TOC (%) to show also the data for the fine fraction (<63 μm). Environ. Sci. Technol. 2002, 36, 2338-2344 2338 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 11, 2002 10.1021/es015681s CCC: $22.00  2002 American Chemical Society Published on Web 05/03/2002