5.2 Biodegradation of DOC

DOC, produced by either allochthonous or autochthonous sources, is subject to biodegradation processes immediately upon entering soil or groundwater systems. These biodegradation processes support the extensive and diverse microbial populations found in these environments. In groundwater systems, these microbial populations are predominantly attached to mineral grain surfaces which may indicate that DOC adsorption to those same surfaces facilitates ongoing biodegradation processes. If that is the case, then DOC removal from groundwater by sorption and biodegradation processes are effectively linked.

5.2.1 Kinetics of DOC Adsorption and Biodegradation in Groundwater Systems

Groundwater systems have a high capacity for removing DOC from solution. A striking example comes from the Floridan aquifer of southern Georgia, USA (McConnell and Hacke, 1993). The aquifer is being recharged via sinkholes with high-DOC water from the blackwater Withlacoochee River. This river water has a median total organic carbon (TOC) concentration of 1,200 µmol/L (16 mg/L) and median color of 110 potassium-cobalt units (PCU). Within two hundred meters of the recharge zone, however, TOC concentrations in groundwater decline to less than 300 µmol/L (3.9 mg/L) and the color decreases proportionally. By the time the “plume” of river-derived water has been transported 10 kilometers downgradient in the aquifer, DOC concentrations have dropped below 80 µmol/L (1 mg/L) and the color disappears completely (< 1 PCU). This shows that the Floridan aquifer has a substantial capacity to attenuate DOC concentrations, a capacity that reflects both adsorption and biodegradation processes. This implies that the kinetics of DOC removal are relatively rapid.

In some cases, it is possible to quantify the removal kinetics of DOC in groundwater systems due to the combined processes of sorption and biodegradation. An example of how DOC removal kinetics can be quantified was given by Chapelle and others (2016). The aquifer in that study is a crystalline piedmont aquifer in South Carolina, USA that is recharged by water percolating through leave litter on a forest floor. Long-term monitoring showed that DOC concentrations in groundwater increased following rainfall events and decreased in between rainfall events (Figure 18). Those DOC concentration decreases, in turn, provided a way to quantify the DOC removal kinetics.

Graph showing concentrations of DOC in groundwater from a single well over time

Figure 18  Concentrations of DOC in groundwater from a single well over time in the Piedmont of South Carolina, USA. DOC concentrations increase following precipitation events and then subsequently decline due to sorption and biodegradation processes. Reproduced from Chapelle and others (2016), with permission.

The removal of DOC in between recharge events can be modeled with an equation in which the DOC removal rate constant (k) combines the contributions of both biodegradation and sorption as shown on Equation 3.

x(t) = C1 + C2 ekt (3)


x(t) = DOC concentrations as a function of time, typically in micromoles per liter (M/L3)
C1, C2 = constants of integration (M/L3)
k = first-order removal rate constant (T1)
t = time since recharge event (T)

When t = 0, x = C1 + C2, which is the initial concentration of DOC in the wellbore following a recharge event. As t becomes larger following the recharge event, the second term of Equation 3 approaches zero and x(t) → C1. Therefore, C1 represents DOC that is recalcitrant to biodegradation and sorption and remains in solution after the reactive DOC fraction (C2) has been removed. The estimates of DOC removal in this study ranged from 0.093 to 0.21 micromoles per liter per day (µmol/L/d) and the DOC removal rates for time periods A, B, and C (Figure 18) ranged from 0.21 to 1.1 percent per day. Assuming a DOC removal rate of one percent per day, then virtually all non-recalcitrant DOC (C2) will be removed from groundwater in less than two years. DOC removal rates of that magnitude, in turn, can fully explain the observed DOC removal in the Floridan aquifer described by McConnell and Hacke (1993). Assuming that similar DOC removal rates are observed in other groundwater systems, these kinetic values provide an explanation for the fact that DOC concentrations in groundwater are typically so low (Leenheer et al., 1974).


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