Abstract

The molecular composition and the carbon isotope signature of sedimentary organic matter (SOM) indicate that SOM is predominantly derived from higher land plants in sediments of both terrestrial and marine origins. While SOM content is related to available mineral surface area, 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 younger 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 results in the re-exposure of sediments to oxygen. Besides SOM, pyrite and siderite 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 of 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.

⁽¹⁾ N Hartog, J Griffioen, and PF van Bergen, Depositional and Paleohydrogeological Controls on the Distribution of Organic Matter and Other Reactive Reductants in Aquifer Sediments. Chemical Geology, 2005. 216 (1-2): p. 113-131.

⁽²⁾ N Hartog, PF van Bergen, JW de Leeuw, and J Griffioen, Reactivity of organic matter in aquifer sediments: geological and geochemical controls. Geochimica et Cosmochimica Acta, 2004. 68 (6): p. 1281-1292.

⁽³⁾ N Hartog, Reactivity of Sedimentary Organic Matter and other Reductants in Aquifer Sediments, in Geologica Ultraiectina. 2003, Department of Earth Sciences, Utrecht University: Utrecht. p. 176.

⁽⁴⁾ N Hartog, J Griffioen, and CH Van der Weijden, Distribution and reactivity of O2-reducing components in sediments from a layered aquifer. Environmental Science and Technology, 2002. 36 (11): p. 2338–2344.

Association of SOM with Mineral Surfaces

Total organic carbon versus specific surface area of the mineral phase. Data for the aquifer sands is presented in my PhD Thesis. Data for marine clay is taken from a study on black shale (Kennedy et al., 2002). Clay aquitard data are taken from a study on four different aquitards (Allen-King et al., 1995). All specific surface areas (SSA) were determined by sorption of ethyl-glycol monoethyl (Churchman et al., 1991). For more information on this topic, see the Introduction of my PhD Thesis.

 

Eenvoudig Verhaal
Nitraat is een van de meest verontrustende oxidanten in Nederlands grondwater en bedreigt verscheidene drinkwaterwinningen. De sedimenten waardoor het grondwater stroomt bevatten bestandsdelen die in staat zijn oxidanten te verwijderen. Van die bestandsdelen is organisch materiaal er een van. Dit organisch materiaal is afkomstig van plantenmateriaal dat met de sedimenten is afgezet. Plantenmateriaal is een mengsel van vele complexe moleculen en de factoren die bepalen hoe reactief organisch materiaal in aquifers is, is grotendeels onbekend. In dit onderzoek is het organisch materiaal in verscheidene Nederlandse types aquifer sediment onderzocht op zijn reactiviteit en moleculaire samenstelling. Het bleek dat de mate van organisch materiaal preservatie direct gekoppeld was aan de snelheid waarmee het afbrak. Verder bleek dat organisch materiaal in de jongste sedimenten niet beter gepreserveerd was dan in sedimenten van vele miljoenen jaren oud. In plaats daarvan bleek dat de preservatie van organisch materiaal vooral gerelateerd was aan het afzettingsmilieu. De dynamische condities tijdens de afzetting van sediment in rivieren en gedurende glaciale periodes, zorgen er waarschijnlijk voor dat organisch materiaal langer in contact is met atmosferisch zuurstof in dus in een vroeg stadium reeds intensief wordt afgebroken. Daarentegen zorg snelle begraving van sedimenten in zee voor een relatieve preservatie van organisch materiaal.