4.5 UV Absorbance

Much of the brownish color associated with some natural groundwater and surface water is due to the presence of DOC. Some DOC is capable of absorbing ultraviolet (UV) and visible (V) light thereby conferring the brownish color. This color-producing DOC is referred to as chromophoric or cDOC.

The absorbance of radiation by compounds such as cDOC depends on the compound’s electronic structure. In the case of the near UV (γ = 200-380 nm), conjugated organic molecules (those with delocalized electrons such as are present in benzene) have the greatest UV absorbances. Because humic substances present in DOC are characterized by chains of alternating single and double-bonded aromatic carbon atoms, they have delocalized electrons and are able to absorb UV radiation. The absorbance of UV radiation by naturally occurring DOC is typically proportional to its concentration. For that reason, absorbance at 254 nm is often used in the water-treatment industry as a surrogate parameter for DOC concentrations. However, because the aromatic content of DOC can vary significantly between different groundwater systems, that approach is not typically useful in groundwater studies.

Variation of UV absorbance between hydrologic systems is illustrated by Figure 15 and Figure 16. Figure 15 shows the locations of eight different aquifer systems located throughout the United States (Chapelle et al., 2016). Samples from the South Carolina sites showed DOC concentrations that ranged from < 0.1 to 5.6 mg/L (< 8 to 430 µmol/L) and absorption coefficient values at 254 nm, aγ254, ranging from zero to 420 m-1. The calculation of aγ254 from measured UV absorbance is given by Equation 1.

\displaystyle a_{\gamma 254}=2.303\;A_{\gamma }/r (1)


aγ = absorption coefficient (L-1)
Aγ = measured UV absorbance at 254 nm (dimensionless)
r = Path length (L)

Use of absorption coefficients reflects the fact that the low absorbances typical of most groundwaters are measured using a 10 cm pathlength cuvette whereas higher absorbance samples are measured using a 1 cm pathlength cuvette. The South Carolina samples were used to delineate how DOC concentrations varied relative to aγ254 and that “DOC/aγ254 path way”. That path way was compared to DOC concentrations and aγ254 values from seven other aquifer systems in the United States. The results of this comparison are shown in Figure 16.

Map showing locations of the Piedmont and Coastal Plain sites in South Carolina

Figure 15  Locations of the South Carolina (SC) Piedmont and SC Coastal Plain sites and locations of the eight Principal aquifers of the United States. Reprinted from Chapelle and others (2016), with permission.

Graph showing DOC concentrations and UV absorbance coefficients for eight aquifer systems

Figure 16  DOC concentrations and UV absorbance coefficients for eight aquifer systems of the United States plotted against: a) the DOC/aγ254 evolution pathway exhibited by the SC piedmont and coastal plain aquifers for the 1-450 µM (µmol/L) concentration range; and, b) for the 1-100 µM concentration range. Reproduced from Chapelle and others (2016), with permission.

Figure 16 illustrates several important characteristics of UV absorbance in groundwater. First, the South Carolina samples were relatively young groundwaters with residence times of ten years or less. In contrast, most of the groundwaters from the other seven aquifers exhibited residence times of greater than 50 years and the absorbance coefficients plot well below the South Carolina DOC/aγ254 pathway. That is consistent with the expectation that biodegradation and sorption processes systematically remove cDOC from groundwater systems. Figure 16 also illustrates another feature of UV absorbance that must be kept in mind when it is applied to groundwater. While most of the samples from the seven aquifers plot below the South Carolina DOC/aγ254 pathway, a cluster of samples from the California Central Valley and the Edwards/Trinity aquifer in Texas plot on or above the South Carolina curve. Those samples are characterized by nitrate concentrations that exceed 10 mg/L, and nitrate, like cDOC, absorbs UV radiation. Dissolved ferrous iron also absorbs UV radiation (Weishaar et al., 2003). For those and other reasons (aromatic content of DOC), UV absorbance is often not a useful surrogate for DOC concentrations in groundwater systems.

One important use of UV absorption measurements in groundwater studies is that they provide an indication of the aromatic composition of cDOC. Weishaar and others (2003) have shown, using a combination of UV absorption measurements and solid state 13C-NMR measurements, that the aromaticity of DOC is directly proportional to its specific ultraviolet absorbance (SUVA), as defined by Equation 2.

SUVA254 = A254 / [DOC] (2)


SUVA254 = specific ultraviolet absorbance at 254 nm (dimensionless)
A254 = absorbance at 254 nm in units of inverse meters (L-1)
DOC = DOC concentration in units of milligrams per liter (mg/L)

Because the bioavailability of DOC decreases as its aromatic composition increases, SUVA254 may provide an indication of DOC bioavailability.


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