6.1 The Age of Groundwater, Aquifer Sediments and DOC Bioavailability

The model of organic carbon compartment interactions shown in Figure 8 suggests that DOC bioavailability will reflect both groundwater age (time since recharge) and the age of POC and AOC with which groundwater interacts. This prediction can be examined by comparing DOC bioavailability between different aquifers exhibiting substantially different groundwater and sediment POC ages. Two aquifers that meet these criteria are a shallow water table aquifer in the coastal plain of South Carolina (hereafter referred to as SC) and the Central Valley aquifer system of California (hereafter referred to as CA).

Groundwater from SC receives recharge directly through modern agricultural soils with relatively high amounts of organic carbon (1 to 5 weight percent; 10,000 to 50,000 mg/kg), the water table varies from 1 to 3 m below land surface, and the groundwater ranges in age (time since recharge) from one to five years (Puckett and Hughes, 2005). The SC aquifer sediments are Holocene (~10,000 BP) in age. In contrast, groundwater from the CA site is much deeper (100 to 300 m), contains lower amounts of POC (~0.1-0.5 weight percent; ~1,000 to 5,000 mg/kg) in sediments of Pliocene to Pleistocene age (2 to 5 million years), and the groundwater age ranges from several hundred to several thousand years (Landon and Belitz, 2008).

The bioavailability of DOC from these two aquifer systems was compared using eight different indicator parameters (Chapelle et al., 2012b). These eight indicators are:

1. concentrations of total hydrolyzable neutral sugars (THNS) of DOC;
2. mole percent glucose (a sugar) of THNS;
3. concentrations of total hydrolyzable amino acids (THAA) of DOC;
4. mole percent glycine (an amino acid) of THAA;
5. initial bacterial counts;
6. bacterial growth over time during incubation;
7. specific ultraviolet absorbance at wavelength 254 nM (SUVA₂₅₄); and,
8. bioassays of carbon dioxide production/consumption over time during incubation.

Concentrations of total hydrolyzable neutral sugars (THNS) present in DOC have been shown to be proportional to DOC bioavailability (Volk et al., 1997; Weiss and Simon, 1999; Routh et al., 2001; Benner, 2003) and may be an indicator of bioavailability (Indicator 1). It has also been observed that the mole percent glucose of THNS increases between young surface ocean waters and ancient deep ocean waters (Benner, 2003). A higher mole percent glucose, therefore, may indicate lower DOC bioavailability (Indicator 2). Concentrations of total hydrolyzable amino acids (THAA) present in DOC are positively correlated with bioavailability (Dauwe et al., 1999, Benner, 2003), so that higher THAA of DOC may indicate higher bioavailability (Indicator 3). The amino acid glycine has been observed to become enriched in DOC as biodegradation proceeds (Dauwe et al., 1999), so that a higher mole percent glycine of THAA may be associated with lower bioavailability (Indicator 4). Numbers of bacterial cells present in water (Indicator 5) have been used as a qualitative indicator of available carbon and nutrients in groundwater (Marxsen, 1988). Similarly, bacterial growth rates during incubation (Indicator 6) have commonly been used as an indicator of DOC bioavailability in both groundwater (Hirsch and Rades-Rohkohl, 1988) and surface-water systems (Kroer, 1993). Naturally occurring DOC is a complex mixture of aromatic and aliphatic organic compounds (Aiken, 1989). It has been observed that as DOC is subjected to biodegradation, the aliphatic portion tends to be preferentially utilized relative to the aromatic portion (Sun et al., 1997). This, in turn, increases the aromaticity of the remaining DOC which reflects decreased bioavailability. It has been shown (Weishaar et al., 2003) that the aromaticity of DOC is proportional to the specific ultraviolet absorbance at wavelength 254 nanometers (SUVA₂₅₄). Thus, higher values of SUVA₂₅₄ imply higher DOC aromaticity and thus lower bioavailability (Indicator 7). Finally, microbial metabolism of DOC can result in either the production or consumption of carbon dioxide. Heterotrophic bacteria growing or maintaining biomass can result in the net release of carbon dioxide during incubation (McDowell et al., 2006). Alternatively, heterotrophic bacteria also have the capability to fix carbon dioxide in order to build biomass (Šantrůčková et al., 2005) which can lead to a net consumption of carbon dioxide. Both carbon dioxide production (McDowell et al., 2006) and consumption (Roslev et al., 2004) during incubation have been used as indicators for the bioavailability of DOC in surface-water systems. Thus, it is the net change of carbon dioxide during incubation that is a potential indicator of DOC bioavailability (Indicator 8).

Table 1 shows the observed differences between the populations for each indicator parameter (greater or less than) and whether those differences are consistent with the hypothesis that SC DOC is more bioavailable than CA DOC. Also shown in Table 1 is the statistical significance of the differences between the CA and SC sample populations. All of the indicators are consistent with the supposition that the SC samples were more bioavailable than the CA samples, although the statistical significance of mole percent glucose of THNS, mole percent glycine of THAA, and final cell count is not conclusive.

Table 1 – Comparison of bioavailability indicators measured in groundwater produced from wells in California and South Carolina.

¹ Differences consistent with SC DOC being more bioavailable than CA DOC?
² Statistical significance of the difference between populations (p value).