4.1 Humic and Fulvic Acids

The first such indirect method for characterizing DOC was based on its humic and fulvic acid composition as has been discussed previously. Artinger and others (2000) provided an example of how quantifying humic and fulvic acids can be used to characterize groundwater DOC. This study collected 35 different samples of groundwater from four different aquifer systems in Germany. Because of the low DOC concentrations encountered, it was necessary to concentrate the DOC by adsorption onto XAD-8 resins as described by Thurman (1985). In samples exhibiting the low DOC typically observed in groundwater (0.1 to 2 mg/L; ~8.3 to ~170 µmol/L), humic acids comprised a relatively low percentage of the total DOC (< 10 percent). Fulvic acids, on the other hand, comprised up to 80 percent of total DOC in the low-DOC groundwaters. That is consistent with the greater solubility of fulvic acids. In contrast, in groundwater containing higher concentrations of DOC (10 to 100 mg/L; ~830 to ~8,300 µmol/L), humic acids comprised a much larger percentage of the total DOC (> 50 percent). Similar results have been found in other groundwater systems. In general, higher amounts of humic acids are associated with higher overall DOC concentrations.