6.2 The Age of Aquifer Sediments and DOC Bioavailability

The model of organic carbon compartment interactions shown in Figure 8 of Section 2.2 also suggests that the bioavailability of DOC should reflect the age of POC and AOC sources to an aquifer and not just groundwater age. One would expect, therefore, that DOC present in groundwater presently receiving recharge through active modern soil zones will be more bioavailable than groundwater of the same age receiving recharge from much older sediments. Two shallow water table aquifers exhibiting these differences are located at Naval Submarine Base Kings Bay (hereafter referred to as KB) in the coastal plain of Georgia and the Naval Air Warfare Center in New Jersey.

The KB site is a sandy aquifer of marginal marine origin receiving modern atmospheric recharge that passes through an organic-rich backwater swamp deposit of late Miocene age (5 to 6 million years BP) that is two to three meters thick. The NAWC site, in contrast, is a bedrock aquifer of Triassic age that receives modern atmospheric recharge passing through an organic-rich modern soil zone that is also two to three meters thick. Groundwater at both sites is anoxic (dissolved oxygen < 0.05 mg/L) due to these POC and AOC sources. That simply indicates that the amount of bioavailable carbon exceeds the flux of oxygen being delivered to each aquifer. It does not, however, indicate any differences in DOC bioavailability between the two sites.

The bioavailability of DOC in groundwater from these two aquifers was compared using biochemical indicators similar to those discussed in Section 6.1 but including the ratio of DOC to total nitrogen (DOC:TN ratio). The DOC:TN ratio has been widely studied in the context of leaf litter biodegradation in forested soils because leaf litter decomposition drives nutrient cycling in forested ecosystems. Surprisingly, it has been consistently found that microbial respiration rates, as measured by CO2 production, increase with increasing C:N ratios (Ohtonen, 1994; Gödde et al., 1996; Michel and Matzner 2002; review by Spohn, 2015). That result is counterintuitive, as it might be expected that increasing amounts nitrogen would have a “fertilizer” affect, thus increasing microbial respiration rates. Gödde and others (1996) hypothesized that less bioavailable DOC would be expected to contain a higher proportion of lignin (a component of plant cells). The low C:N ratios characteristic of lignin would then require soil microorganisms to cycle more C in order to obtain their N requirements, thus increasing CO2 production. Whatever the underlying mechanism(s) are, however, the much higher DOC:TN ratios in groundwater from the NAWC site than the KB site (Figure 19, Figure 11) indicates greater DOC bioavailability. Furthermore, because DOC and nitrogen species are routinely measured in studies of groundwater chemistry, it may be that the DOC:TN molar ratio can be a useful indicator for comparing DOC bioavailability between different aquifer systems.

Graph showing DOC:TN molar ratios in groundwater

Figure 19  DOC:TN molar ratios in groundwater suggesting greater DOC bioavailability at NAWC relative to the KB site.

Consistent with the apparent differences in DOC bioavailability inferred from biochemical indicators discussed above, a strong correlation between DOC and dissolved inorganic carbon (DIC), a product of DOC mineralization, was observed in groundwater from the NAWC site (Table 2). In contrast, no significant DOC-DIC correlation was observed at the KB site. Because DIC is produced directly by oxidation of bioavailable DOC, the presence or absence of such a correlation is a potential indicator of differences in DOC bioavailability between different aquifers. The DOC/DIC correlation for both the NAWC and KB sites, as expressed by the coefficient of determination (r2), is shown on Table 2 along with the other biochemical indicators.

Importantly, the DOC:TN ratio and the correlation of DIC with DOC (Table 2) are bioavailable indicators based solely on water-chemistry measurements routinely made in groundwater quality studies and do not require specialized laboratory facilities. As such, they may be the most accessible DOC bioavailability indicators available to field hydrogeologists.

Table 2  Comparison of bioavailability indicators between the Kings Bay and NAWC sites.

Bioavailability indicator Mean Value
Kings Bay
Mean Value
Consistent?1 Statistical significance2
THNS yield 0.55 1.14 yes 0.003
Mole percent of THNS 18.15 31.50 yes 0.001
THAA yield 0.42 1.32 yes 0.004
Mole percent glycine of THAA 38.13 29.37 yes 0.019
C/TN ratio 0.81 23.19 yes 0.003
DOC-DIC correlation (r2) 0.72 0.18 yes 0.003

1 Consistent with NAWC DOC more bioavailable than KB DOC?
2 Statistical significance of difference (p value).


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