{"id":62,"date":"2022-12-12T05:35:54","date_gmt":"2022-12-12T05:35:54","guid":{"rendered":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/chapter\/uv-absorbance\/"},"modified":"2022-12-25T07:19:05","modified_gmt":"2022-12-25T07:19:05","slug":"uv-absorbance","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/chapter\/uv-absorbance\/","title":{"raw":"4.5 UV Absorbance","rendered":"4.5 UV Absorbance"},"content":{"raw":"
\r\n

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.<\/p>\r\n

The absorbance of radiation by compounds such as cDOC depends on the compound\u2019s electronic structure. In the case of the near UV (\u03b3<\/em>\u00a0=\u00a0200-380\u00a0nm), 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\u00a0nm 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.<\/p>\r\n

Variation of UV absorbance between hydrologic systems is illustrated by Figure\u00a015 and Figure\u00a016. Figure\u00a015 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 <\u00a00.1 to 5.6\u00a0mg\/L (<\u00a08 to 430\u00a0<\/em>\u00b5<\/em>mol\/L) and absorption coefficient values at 254\u00a0nm, a<\/em>\u03b3<\/em>254<\/sub>, ranging from zero to 420\u00a0m-1<\/sup>. The calculation of a<\/em>\u03b3<\/em>254<\/sub> from measured UV absorbance is given by Equation\u00a01.<\/p>\r\n\r\n\r\n\r\n\r\n
<\/td>\r\n[latex]\\displaystyle a_{\\gamma 254}=2.303\\;A_{\\gamma }\/r[\/latex]<\/td>\r\n(1)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

where:<\/p>\r\n\r\n\r\n\r\n\r\n\r\n\r\n
a<\/em>\u03b3<\/em><\/sub><\/td>\r\n=<\/td>\r\nabsorption coefficient (L-1<\/sup>)<\/td>\r\n<\/tr>\r\n
A<\/em>\u03b3<\/em><\/sub><\/td>\r\n=<\/td>\r\nmeasured UV absorbance at 254\u00a0nm (dimensionless)<\/td>\r\n<\/tr>\r\n
r<\/em><\/td>\r\n=<\/td>\r\nPath length (L)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

Use of absorption coefficients reflects the fact that the low absorbances typical of most groundwaters are measured using a 10\u00a0cm pathlength cuvette whereas higher absorbance samples are measured using a 1\u00a0cm pathlength cuvette. The South Carolina samples were used to delineate how DOC concentrations varied relative to a<\/em>\u03b3<\/em>254<\/sub> and that \u201cDOC\/a<\/em>\u03b3<\/em>254<\/sub> path way\u201d. That path way was compared to DOC concentrations and a<\/em>\u03b3<\/em>254<\/sub> values from seven other aquifer systems in the United States. The results of this comparison are shown in Figure\u00a016.<\/p>\r\n

\"Map<\/p>\r\n

Figure\u00a0<\/strong>15<\/strong>\u00a0-<\/strong>\u00a0Locations 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.<\/p>\r\n

\"Graph<\/p>\r\n

Figure\u00a0<\/strong>16<\/strong>\u00a0-<\/strong>\u00a0DOC concentrations and UV absorbance coefficients for eight aquifer systems of the United States plotted against: a) the DOC\/a<\/em>\u03b3<\/em>254<\/sub> evolution pathway exhibited by the SC piedmont and coastal plain aquifers for the 1-450\u00a0\u00b5<\/em>M (\u00b5<\/em>mol\/L) concentration range; and, b) for the 1-100\u00a0\u00b5<\/em>M concentration range. Reproduced from Chapelle and others (2016), with permission.<\/p>\r\n

Figure\u00a016 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\/a<\/em>\u03b3<\/em>254<\/sub> pathway. That is consistent with the expectation that biodegradation and sorption processes systematically remove cDOC from groundwater systems. Figure\u00a016 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\/a<\/em>\u03b3<\/em>254<\/sub> 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\u00a0mg\/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.<\/p>\r\n

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 13<\/sup>C-NMR measurements, that the aromaticity of DOC is directly proportional to its specific ultraviolet absorbance (SUVA<\/em>), as defined by Equation\u00a02.<\/p>\r\n\r\n\r\n\r\n\r\n
<\/td>\r\nSUVA<\/em>254<\/sub> = A<\/em>254<\/sub> \/ [DOC<\/em>]<\/td>\r\n(2)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

where:<\/p>\r\n\r\n\r\n\r\n\r\n\r\n\r\n
SUVA<\/em>254<\/sub><\/td>\r\n=<\/td>\r\nspecific ultraviolet absorbance at 254 nm (dimensionless)<\/td>\r\n<\/tr>\r\n
A<\/em>254<\/sub><\/td>\r\n=<\/td>\r\nabsorbance at 254\u00a0nm in units of inverse meters (L-1<\/sup>)<\/td>\r\n<\/tr>\r\n
DOC<\/em><\/td>\r\n=<\/td>\r\nDOC concentration in units of milligrams per liter (mg\/L)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

Because the bioavailability of DOC decreases as its aromatic composition increases, SUVA<\/em>254<\/sub> may provide an indication of DOC bioavailability.<\/p>\r\n\r\n<\/div>","rendered":"

\n

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.<\/p>\n

The absorbance of radiation by compounds such as cDOC depends on the compound\u2019s electronic structure. In the case of the near UV (\u03b3<\/em>\u00a0=\u00a0200-380\u00a0nm), 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\u00a0nm 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.<\/p>\n

Variation of UV absorbance between hydrologic systems is illustrated by Figure\u00a015 and Figure\u00a016. Figure\u00a015 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 <\u00a00.1 to 5.6\u00a0mg\/L (<\u00a08 to 430\u00a0<\/em>\u00b5<\/em>mol\/L) and absorption coefficient values at 254\u00a0nm, a<\/em>\u03b3<\/em>254<\/sub>, ranging from zero to 420\u00a0m-1<\/sup>. The calculation of a<\/em>\u03b3<\/em>254<\/sub> from measured UV absorbance is given by Equation\u00a01.<\/p>\n\n\n\n
<\/td>\n\"\displaystyle a_{\gamma 254}=2.303\;A_{\gamma }/r\"<\/td>\n(1)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

where:<\/p>\n\n\n\n\n\n
a<\/em>\u03b3<\/em><\/sub><\/td>\n=<\/td>\nabsorption coefficient (L-1<\/sup>)<\/td>\n<\/tr>\n
A<\/em>\u03b3<\/em><\/sub><\/td>\n=<\/td>\nmeasured UV absorbance at 254\u00a0nm (dimensionless)<\/td>\n<\/tr>\n
r<\/em><\/td>\n=<\/td>\nPath length (L)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Use of absorption coefficients reflects the fact that the low absorbances typical of most groundwaters are measured using a 10\u00a0cm pathlength cuvette whereas higher absorbance samples are measured using a 1\u00a0cm pathlength cuvette. The South Carolina samples were used to delineate how DOC concentrations varied relative to a<\/em>\u03b3<\/em>254<\/sub> and that \u201cDOC\/a<\/em>\u03b3<\/em>254<\/sub> path way\u201d. That path way was compared to DOC concentrations and a<\/em>\u03b3<\/em>254<\/sub> values from seven other aquifer systems in the United States. The results of this comparison are shown in Figure\u00a016.<\/p>\n

\"Map<\/p>\n

Figure\u00a0<\/strong>15<\/strong>\u00a0–<\/strong>\u00a0Locations 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.<\/p>\n

\"Graph<\/p>\n

Figure\u00a0<\/strong>16<\/strong>\u00a0–<\/strong>\u00a0DOC concentrations and UV absorbance coefficients for eight aquifer systems of the United States plotted against: a) the DOC\/a<\/em>\u03b3<\/em>254<\/sub> evolution pathway exhibited by the SC piedmont and coastal plain aquifers for the 1-450\u00a0\u00b5<\/em>M (\u00b5<\/em>mol\/L) concentration range; and, b) for the 1-100\u00a0\u00b5<\/em>M concentration range. Reproduced from Chapelle and others (2016), with permission.<\/p>\n

Figure\u00a016 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\/a<\/em>\u03b3<\/em>254<\/sub> pathway. That is consistent with the expectation that biodegradation and sorption processes systematically remove cDOC from groundwater systems. Figure\u00a016 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\/a<\/em>\u03b3<\/em>254<\/sub> 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\u00a0mg\/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.<\/p>\n

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 13<\/sup>C-NMR measurements, that the aromaticity of DOC is directly proportional to its specific ultraviolet absorbance (SUVA<\/em>), as defined by Equation\u00a02.<\/p>\n\n\n\n
<\/td>\nSUVA<\/em>254<\/sub> = A<\/em>254<\/sub> \/ [DOC<\/em>]<\/td>\n(2)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

where:<\/p>\n\n\n\n\n\n
SUVA<\/em>254<\/sub><\/td>\n=<\/td>\nspecific ultraviolet absorbance at 254 nm (dimensionless)<\/td>\n<\/tr>\n
A<\/em>254<\/sub><\/td>\n=<\/td>\nabsorbance at 254\u00a0nm in units of inverse meters (L-1<\/sup>)<\/td>\n<\/tr>\n
DOC<\/em><\/td>\n=<\/td>\nDOC concentration in units of milligrams per liter (mg\/L)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Because the bioavailability of DOC decreases as its aromatic composition increases, SUVA<\/em>254<\/sub> may provide an indication of DOC bioavailability.<\/p>\n<\/div>\n","protected":false},"author":1,"menu_order":10,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-62","chapter","type-chapter","status-publish","hentry"],"part":120,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/pressbooks\/v2\/chapters\/62","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/wp\/v2\/users\/1"}],"version-history":[{"count":10,"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/pressbooks\/v2\/chapters\/62\/revisions"}],"predecessor-version":[{"id":320,"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/pressbooks\/v2\/chapters\/62\/revisions\/320"}],"part":[{"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/pressbooks\/v2\/parts\/120"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/pressbooks\/v2\/chapters\/62\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/wp\/v2\/media?parent=62"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/pressbooks\/v2\/chapter-type?post=62"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/wp\/v2\/contributor?post=62"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/books.gw-project.org\/dissolved-organic-carbon-in-groundwater-systems\/wp-json\/wp\/v2\/license?post=62"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}