4.2 Hydrophilic, Hydrophobic, and Neutral Fractions of DOC

A more advanced procedure, derived from the humic/fulvic dichotomy, is termed “dissolved organic carbon fractionation analysis” (Leenheer and Huffman, 1979). In this method, DOC is first concentrated on XAD-8 resins and separated into hydrophobic and hydrophilic fractions by selective desorption using acidic and basic eluents. Those two fractions are then each further separated into acids, bases, and neutral fractions, giving a total of six fractions. This method of analysis was used to characterize the composition of soil water and groundwater in a forested watershed of the Adirondack Mountains in New York (Cronan and Aiken, 1985). Their results showed, for the first time, that groundwater DOC was dominated by hydrophilic and hydrophobic acids, with a lesser contribution from bases and neutral organic compounds. This fractionation technology has continued to evolve and is still used to characterize DOC in groundwater and surface-water systems (Chow et al., 2004; Ratpukdi et al., 2009).

Modifications of this fractionation methodology has been used to characterize DOC in a variety of aquifer systems. Hendry and Wassenaar (2005) described a study of DOC in confining-bed porewaters of a deep aquifer system in western Canada. They focused their study on the high-molecular weight (HMW) and low molecular weight (LMW) fractions of DOC and their respective carbon-13 and carbon-14 compositions. The results showed a systematic decrease in DOC concentrations and the HMW and LMW fractions with depth (42 m) in pore waters of the confining bed material. They also showed a systematic decrease in the δ13C and 14C activity with depth, suggesting the observed DOC was derived from soil organic carbon of Holocene (~10,000 years BP – before present) soils, and older (15,000 years BP) pore water.


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