Fig. 4: Contrasting dissolved organic matter (DOM) processing pathways in groundwater and non-groundwater (river, lake and ocean) environments.

a DOM in shallow groundwater is removed following exposure of hetero-atom containing DOM and DOM with high nominal oxidation state of carbon (NOSC) to biodegradation in suboxic conditions, and the adsorption of aromatic and high NOSC formulae. Accumulation of low O/C, non-Island of Stability (IOS) carboxylic-rich alicyclic molecules (CRAM) occurs in this region. In deep groundwater where conditions are strongly anoxic, respiration rates are low and aerobically biolabile aliphatic and hetero-atom containing DOM are accumulated from microbial biomass in conjunction with a preferential removal of high O/C DOM. Hetero-atom containing DOM may be further accumulated through the hydrogenation of unsaturated formulae and inorganic sulfurization of lipids¹¹⁵. Aromatic DOM also accumulates from microbial biomass with possible contribution from DOM desorbed under high pH¹¹⁶. These processes result in an overall decrease in O/C and an increase in H/C with DOC age. Dark blue dashed arrows denote flow direction. Groundwater DOM processing contrasts with processing of DOM in rivers, lakes and oceans (b) where primary production and subsequent photodegradation of aromatic formulae results in either low or high O/C formulae, and high H/C formulae such as aliphatics which are then rapidly biodegraded. Primary producers may also release biolabile hetero-atom containing DOM⁶⁸. Any build up of DOM in an anoxic pocket of seawater which is biolabile to aerobic microbes would be circulated and biodegraded upon return to oxic conditions. Exposure to sunlight and the primarily oxic conditions of marine environments result in an overall decrease in DOM H/C, and increase in O/C and IOS formulae with DOC age. Processing pathways for both environments are shown in van Krevelen Diagrams in upper right corners of each panel.