2.2 DOC Fate Summarized in a Regional Chromatography Model
Based on these observed interactions, Aiken (1989) suggested that groundwater systems behave much like chromatographic columns which separate the components of DOC according to their atomic weights, their hydrophobicity, and their affinity for mineral grain surfaces. In Aiken’s own words:
“Application of chromatographic theory to subsurface transport can aid in understanding and quantifying the chemical processes in subsurface systems. Chromatography is essentially the transport of a chemical in a mobile fluid phase through a column packed with a stationary phase. A chemical introduced at the beginning of this column moves at a rate proportional to the average velocity of the fluid and inversely proportional to the strength and nature of sorptive interactions with the stationary phase. These interactions include ionic and complexation interactions, hydrogen bonding, van der Waal’s interactions, and equilibrium partitioning. In a ground-water system, the geologic matrix, representing the stationary phase, generally consists of sand grains coated with organic matter. The mobile phase is the water, which can contain significant quantities of dissolved organic and inorganic chemicals. Organic compounds move through the system as a result of the flow rate of water and the strength and nature of interactions with the stationary phase. The nature and distribution of organic matter in the system is determined, to a large extent, by the interactions between the various phases in the environment (emphasis added).”
Shen and others (2015) came to similar conclusions in a study of DOC being delivered from forest floor leaf litter to a fractured-rock aquifer in the piedmont of South Carolina. In that paper, Shen and others (2015) proposed what they termed a “regional chromatography model” to illustrate the DOC dynamics that they observed (Figure 8).
Figure 8 – Regional Chromatography Model – precipitation and surface water leaches dissolved organic matter (DOM) from vegetation and plant litter and percolates through the soil column to the saturated zone. The concentration, composition, and bioavailability of DOC are altered during transport through the soil column by various physicochemical and biological processes, including sorption, desorption, biodegradation, and biosynthesis. Hydrophobic molecules are preferentially partitioned onto soil minerals and have a longer retention time in soils than hydrophilic molecules. The hydrophobicity and retention time of colloids and dissolved molecules in soils are controlled by their size, polarity, charge, and bioavailability. Bioavailable DOC is subjected to microbial decomposition, resulting in a reduction in size and molecular weight. Novel molecules are synthesized by soil microbes, and some of these metabolites enter the DOC reservoir in groundwater. Reprinted with permission from Shen and others (2015).