Environmental Engineering Reference
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surficial hydrologic flowpaths that bypass mineral rich sorption surfaces present in
subsurface soil horizons (Inamdar et al. 2011 , 2012 ).
Past studies have generally focused on determining the bulk concentrations of
DOC and DON in wetland soils and watershed runoff (Hinton et al. 1997 ; Inamdar
and Mitchell 2006 ; Raymond and Saiers 2010 ). While these observations have been
important in highlighting the significant role of wetlands for DOM, bulk DOM
concentrations provide little information on the reactivity, bioavailability, molecu-
lar size, and mobility of DOM. To get an idea of these ecologically relevant
characteristics of DOM, we need to know the functional groups or the individual
constituents such as carbohydrates, proteins, carboxylic acids, lignins that make up
DOM. For example, labile fractions of DOM that are easily consumed by microbes
are found to be rich in carbohydrates and proteins (Benner 2003 ). Aromatic and
humic-rich fractions of DOM play a preferential role in the complexation and
transport of metals such as cadmium, arsenic, and mercury. Similarly, aromatic
compounds of DOM are predisposed to forming carcinogenic disinfection
by-products when water is chlorinated for drinking purposes (Nokes et al. 1999 ).
Hydrophobic DOM compounds are preferentially sorbed on Fe and Al oxides in
soils while hydrophilic DOM molecules remain in solution and move with runoff
waters (Jardine et al. 1989 ; Kaiser and Zech 1998 ; Ussiri and Johnson 2004 ). Thus,
to truly understand the fate and transport of DOM in watersheds and its implications
for terrestrial and aquatic ecosystems we need to move beyond bulk determinations
to characterizing the chemical constituents of DOM.
7.4.7.2 Characterizing Dissolved Organic Matter Using UV
and Fluorescence Spectroscopy
In the past, DOM composition or characterization of functional groups have usually
been performed using traditional chemical techniques that are labor-intensive,
time-consuming, involve high analytical costs, and require large sample volumes.
These challenges have precluded the routine characterization of DOM composition
for many ecosystem and watershed studies. The recent advances in ultra-violet
(UV) (Weishaar et al. 2003 ) and fluorescence technology (Coble et al. 1990 ;
McKnight et al. 2003 ), however, overcomes some of these challenges and promises
to be a useful tool for characterizing DOM chemistry, especially for studies that
generate a large number of DOM samples.
Ultra-violet and fluorescence techniques rely on the property of DOM that
different organic molecules absorb and reflect light at differing wavelengths.
Thus, investigation of the absorption and fluorescence spectra can provide critical
insights into the composition of DOM. It needs to be emphasized here that these
spectrofluorometric procedures do not provide information on the concentration or
chemical structure of the DOM functional groups but the proportion of fluorescence
contributed by specific DOM moieties can be determined through post-processing
of the spectra. Furthermore, only a small of fraction of the DOM pool responds to
UV and fluorescence measurements (McKnight et al. 2003 ). Despite these
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