3.2 Autochthonous Sources of DOC

The model of allochthonous sources of DOC shown in Figure 8 predicts that both the concentration and bioavailability of DOC being delivered to an aquifer should decline as groundwater moves along the hydraulic gradient. Logically, therefore, if the sole sources of DOC to groundwater are allochthonous, then at some point concentrations of DOC should decline below measurable levels (~0.1 mg/L; ~8 µmol/L). That, however, is not what is observed. Measurable DOC concentrations are observed in groundwaters that are thousands or even millions of years old (time since recharge) which have moved hundreds of kilometers along aquifer flow paths (Leenheer et al., 1974). The reason for this observed behavior is that, in addition to allochthonous sources of DOC, there are many potential sources of autochthonous DOC in groundwater systems as well. In addition to delivering a steady supply of DOC to groundwater, autochthonous organic carbon in subsurface environments is also the source of petroleum and natural gas (Espitalie et al., 1980). Whereas the origin of petroleum and natural gas has been subject to active study for many years, the origin of autochthonous DOC in groundwater system used for water supply has only recently been investigated.

Initially, the motivation for investigations into autochthonous sources of DOC was the puzzling observation that, while concentrations of DOC in deep confined aquifers did not seem to change significantly along aquifer flow paths, concentrations of dissolved inorganic carbon (DIC) increased (McMahon and Chapelle, 1991b). Because the groundwater in that study was saturated with respect to calcite, it seemed unlikely that the increase in DIC could come exclusively from calcite dissolution. An alternative explanation was that there was a source of organic carbon that, when oxidized, could provide the observed DIC. While little organic carbon was present in sandy aquifer material in that system, there was abundant organic carbon in the clayey beds that confined the aquifers. Cores were recovered from both the aquifer and confining beds and their pore water was analyzed for the presence of organic acids. The results showed that while concentrations of formate and acetate were low in the pore water of sandy aquifer sediments, concentrations were much higher in the pore water of clayey confining beds. Furthermore, diffusion of the organic acids from the clayey beds to the sandy aquifers, as illustrated in Figure 9, could transport the DOC to the aquifer, which upon oxidation could provide a source of dissolved inorganic carbon (McMahon and Chapelle, 1991a). A later study (Chapelle and Bradley, 1996) demonstrated that the presence of organic acids in clayey sediments was associated with active microbial acetogenesis (production of acetate).

Figure showing release of old DOC by diffusion from low permeability geologic units

Figure 9  Low permeability geologic units with almost no active flow (e.g., clay beds) can release old DOC by diffusion (brown dots in inset or shading) to the active flow in the permeable zones (e.g., sands), so the groundwater flowing around the clay beds may contain DOC from the clay beds that may oxidize to DIC along the flow path in the permeable zone. Only a few large clay beds are shown for clarity (Mackay, 2022).

The production and diffusion of DOC from confining beds to aquifers has been observed in other deep confined aquifer systems (Lawrence et al., 2000, Hendry and Wassenaar, 2005). Figure 10 shows concentrations of formate, acetate, and propionate in pore water derived from deep sediments (810 to 890 m) of the Atlantic Coastal Plain underlying Hilton Head Island, South Carolina (Chapelle and Bradley, 1996). Note the millimolar concentrations of organic acids associated with pore water from the clayey sediments and their near absence in pore water from the sandy sediments. These sediments are of Upper Cretaceous age (~80 MY) demonstrating that organic carbon capable of supporting microbial organic acid production can persist for tens of millions of years in low-permeability clayey sediments.

Graphs showing data from Hilton Head Core Hole

Figure 10  Data from the Hilton Head core hole showing the relationship between concentrations of dissolved formate, acetate, and propionate in pore water of the permeable zones and confining bed within the Black Creek aquifer system (reprinted from Chapelle and Bradley, 1996, with permission).


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