4.3 Elemental Composition of DOC

Another method for characterizing DOC in groundwater (Artinger et al., 2000; Murphy et al., 1989) is by direct chemical analysis. Reported analyses of groundwater show that DOC is typically composed of carbon (50 to 58 percent), oxygen (33 to 42 percent), hydrogen (4.4 to 6 percent), sulfur (1.6 to 4.5 percent), and nitrogen (0.5 to 2 percent). The usefulness of elemental composition in understanding the sources and diagenetic history of DOC, however, is not always clear. Artinger and others (2000) compared differences in elemental composition between a young aerobic groundwater sampled from a quaternary-age aquifer of glacial origin (Munich) and an older anaerobic groundwater sampled from a Triassic-age sandstone aquifer (Franconian) in Germany. The DOC concentrations in Munich groundwater were somewhat higher (0.6 to 0.9 mg/L; 50 to 75 µmol/L) than Franconian groundwater (0.1 to 0.5 mg/L; ~8 to ~40 µmol/L). There were not, however, dramatic differences in elemental composition. The carbon content of DOC was slightly higher in Franconian DOC (56.3 ± 2.1 percent) relative to Munich DOC (52.2 percent ± 0.6 percent). Similarly, the hydrogen content of Franconian DOC (6.1 ± 0.5 percent) was slightly higher than Munich DOC (4.4 ± 0.2 percent), but it is unlikely those differences are statistically significant. The oxygen content of both Franconian and Munich DOC was virtually identical (~37 percent). Because of the differences in groundwater ages and aquifer types, those systems might have been expected to exhibit more differences in elemental composition. That, however, was not observed.

Murphy and others (1989), in a study of the Milk River aquifer system in Canada, showed a general decrease in DOC concentrations along the regional hydrologic gradient (from ~6 to 1.5 mg/L; i.e., 500 to 125 µmol/L). There was not a systematic change in DOC carbon content (~56 percent), or hydrogen content (~6.5 percent) between upgradient and downgradient groundwater. There was, however, a slight decrease in oxygen content between upgradient (~35 percent) and downgradient (~31 percent) DOC samples. In any case, the utility of using bulk elemental composition to characterize groundwater DOC remains an open question.

More recently and using more sophisticated analytical procedures for elemental analyses of DOC have shown further promise (McDonough et al., 2020). Using a combination of liquid chromatography-organic carbon detection (LC-ODC) and negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) the Connected Waters Initiative Research Centre in Sydney, Australia has shown how the elemental character of DOC in a shallow coastal aquifer varies temporally and spatially depending on atmospheric precipitation events. Those variations, which have only recently come to light, are very likely to be observed in other groundwater systems.


Dissolved Organic Carbon in Groundwater Systems Copyright © 2022 by Francis H. Chapelle. All Rights Reserved.