Characterization of Natural Organic Matter in Alluvial Aquifer Sediments: Approaches and Implications for Reactivity

Abstract:

Sediment-associated natural organic matter (NOM) is an extremely complex assemblage of organic molecules with a wide range of sizes, functional groups, and structures, which is intricately associated with mineral particles. The chemical nature of NOM may control its’ reactivity towards metals, minerals, enzymes, and bacteria. Organic carbon concentrations in subsurface sediments are typically much lower than in surface soils, posing a distinct challenge for characterization. In this study, we investigated NOM associated with shallow alluvial aquifer sediments in a floodplain of the Colorado River. Total organic carbon (TOC) contents in these subsurface sediments are typically around 0.1%, but can range from 0.03% up to approximately 1.5%. Even at the typical TOC values of 0.1%, the mass of sediment-associated OC is approximately 5000 times higher than the mass of dissolved OC, representing a large pool of carbon that may potentially be mobilized or degraded under changing environmental conditions. Sediment-associated OC is much older than both the depositional age of the alluvial sediments and dissolved OC in the groundwater, indicating that the vast majority of NOM was sequestered by the sediment long before it was deposited in the floodplain.

We have characterized the sediment-bound NOM from two locations within the floodplain with differing physical and geochemical properties. One location has relatively low organic carbon (<0.2%) and is considered suboxic [dissolved oxygen is low or absent, but no dissolved Fe(II) observed], while the other is a naturally reducing zone with higher organic carbon (0.2-1.5%) and Fe(II)-reducing conditions. An extraction scheme was developed using a combination of sequential extraction [water and sodium pyrophosphate (pH 10)] and purification in order to isolate different fractions of sediment-associated NOM. Analysis of these different NOM fractions was then carried out by FTIR and ESI-FTICR-MS to allow for comparison of NOM structure and composition both across sites and across fractions for a single location. Using this combination of analytical techniques we can probe the variation in NOM chemical composition and mineral association across different biogeochemical regimes and assess the potential reactivity of various NOM pools.