1 Introduction
As illustrated in Figure 1, dissolved organic carbon (DOC) is the fraction of organic carbon present in water defined as that which can pass through a filter with a pore size 0.45 micrometers and expressed as a concentration (M/L3). The fraction remaining on the filter is called particulate organic carbon (POC) and is also expressed as a concentration (M/L3). Dissolved organic matter (DOM) is a closely related term often used interchangeably with DOC. While DOC refers specifically to the mass of carbon in the dissolved organic material, DOM refers to the total mass of the dissolved organic matter. DOM includes the mass of elements other than carbon that are present in organic material, such as nitrogen, oxygen and hydrogen. DOC is a component of DOM and there is typically about twice as much DOM as DOC (i.e., the concentration of DOM is typically twice the concentration of DOC).
Figure 1 – Organic matter in water is typically about 50 percent carbon and is generally referred to as composed of two fractions, that captured by a 0.45 filter (POC) and that passing the filter (DOC) (Mackay, 2022).
Concentrations of DOC are commonly analyzed in investigations of water quality for both surface water and groundwater systems. For surface-water systems, this reflects the fact that DOC-removal treatment is frequently required to make the water suitable for human consumption (Kornegay et al., 2000). That is not, however, typically the case for groundwater. One of the defining characteristics of groundwater used for human water supply is that it generally lacks the higher DOC concentrations requiring removal treatment. The question can reasonably be asked, therefore, what is the practical usefulness of DOC measurements in groundwater? The answer is threefold.
Organic carbon, in its dissolved (DOC), particulate (POC), and adsorbed (AOC) forms as illustrated in Figure 2, determines the kinds and direction of reduction/oxidation (redox) reactions that can occur in an aquifer system. Those redox processes, in turn, determine concentrations of redox-sensitive chemical species such as dissolved oxygen, nitrate, ferrous iron, manganous iron, sulfide, and methane, all of which affect the real and perceived chemical quality (defined as its usefulness to humans and/or ecosystems) of groundwater. DOC-driven redox processes also affect the fate and transport of human-derived chemical contaminants such as petroleum hydrocarbons and chlorinated solvents. The dissolved, particulate, and adsorbed compartments of organic carbon interact dynamically with each other in groundwater systems, thus the bioavailability of DOC may indicate the bioavailability of particulate and adsorbed organic carbon.
Figure 2 – Occurrence of AOC, POC and DOC in groundwater in a pore between grains or fracture walls within an aquifer. This is not a scale drawing, but meant to illustrate that typically there is a lot more organic carbon in AOC and POC than in DOC. AOC may not coat all particle surfaces nor coat them uniformly, as is illustrated here. Though not indicated, AOC may sometimes remobilize into the porewater, which increases DOC (Mackay, 2022).
Particulate organic carbon (POC) and adsorbed organic carbon (AOC) are always much more abundant than DOC in any given volume of aquifer material. In a study of ten shallow aquifers located throughout the continental United States (Chapelle et al., 2012b), measurements of total organic carbon (DOC + POC + AOC) were found to range from 50 to 10,000 milligrams of organic carbon per kilogram of aquifer material (mg/kg). To put those numbers in perspective, a typical clean white beach sand contains only about 50 mg/kg of total organic carbon. Given the density of quartz sand grains (1,631 kg/m3), and assuming 50 mg/kg of POC + AOC, one cubic m (1 m3) of sandy aquifer material contains 81,550 mg of organic carbon. Assuming a porosity of 0.3, that same volume of saturated aquifer material will contain 300 liters of water. If that water contains 5 mg/L of DOC (a relatively high value for groundwater), there is just 1,500 mg of organic carbon present as DOC. In other words, as illustrated in Figure 3, DOC in groundwater typically represents less than 1 percent of the total organic carbon present in any given volume of aquifer material.
Figure 3 – DOC typically makes up a very small percentage of total organic carbon in a given volume of aquifer material. This illustration is for a clean white beach sand described in the text (Mackay, 2022).
If DOC represents such a small portion of the total organic carbon, how can DOC be of relevance to evaluating groundwater? Studies of DOC in groundwater published over the last fifty years reveal facts relevant to that question:
- DOC concentration in groundwater systems that are not under the immediate influence of surface-water sources are ubiquitously low due to a combination of DOC adsorption onto aquifer solids and biodegradation processes, while DOC of surface waters is typically 100 to 1000 times higher; and,
- DOC adsorption onto aquifer solids is reversible (Figure 4), so AOC can be remobilized as DOC suggesting that the chemical and biological properties of DOC can reflect those of POC and AOC.
Thus measurement of DOC is relevant because:
- a high concentration of DOC in groundwater may reflect a significant surface water source indicating need for treatment if the water is used for drinking; and,
- OC concentrations influence the kinds and direction of redox reactions that determine concentrations of chemical species affecting the natural quality of groundwater as well as the fate and transport of human-derived chemical contaminants that enter groundwater.
Figure 4 – AOC, POC and DOC near a grain or rock surface in an aquifer. Thin double-headed arrows represent adsorption and desorption of DOC to POC, AOC or to mineral surface. Heavier arrow represents adsorption and desorption of POC to AOC as discussed in the text. Although not illustrated in this figure, biodegradation processes discussed in later sections can convert AOC, POC and/or DOC to metabolites (products that result from breaking down the organic carbon), including dissolved inorganic carbon (DIC) (Mackay, 2022).