Fluorescence Indices and Their Interpretation - Aquatic Organic Matter Fluorescence - page 320

reduced quinone-like moieties (RI = 0.51) are transported from the hyporheic zone to the
main stream channel, where they are rapidly oxidized (RI = 0.39). The study highlighted
the usefulness in biogeochemical studies of fitting EEMs to an existing PARAFAC model
constructed based on EEMs from a range of DOM sources and redox conditions, such as
the Cory and McKnight ( 2005 ) model, in this case necessary to calculate RI. A discussion
of the potential advantages and disadvantages to building a new PARAFAC model versus
fitting EEMs to an existing PARAFAC model is provided by Fellman et al. ( 2009 ) and
PARAFAC modeling is considered in detail in Chapter 10 (this volume).
The RI has been shown to be useful in understanding DOM- metal interactions across
redox gradients in groundwater systems. Mladenov et al. ( 2010 ) calculated the RI for sur-
face and groundwater samples collected in Bangladesh in the context of investigating the
role of DOM source and redox reactivity in controlling dissolved iron and arsenic mobil-
ization. In surface waters, the quinone-like moieties were oxidized (RI = 0.34-0.46) and
had low dissolved iron (<0.1 mg L -1 ) and arsenic (~5 μg L -1 ) concentrations. In contrast,
shallow groundwater samples had more reduced quinone-like moieties (RI = 0.47-0.48)
and higher dissolved iron (6-10 mg L -1 ) and arsenic (>200 μg L -1 ) concentrations. Based
on these results and results from a sediment incubation experiment, a pathway for microbi-
ally mediated electron shuttling via humic DOM resulting in mobilization of dissolved iron
and arsenic in shallow groundwater was hypothesized ( Figure 9.9 ) and applied to a similar
system in the Okavango Delta (Mladenov et al., 2007 , 2008 ). The RI has also been used to
characterize DOM redox state in oxidizing environments such as in the water column in
alpine lakes (Miller et al., 2009 ; Mladenov et al., 2009 ), and in chlorination during drinking
water treatment (Beggs et al., 2009 ).
9.3 Applications of Fluorescence Indices
Table 9.1 and Figure 9.1 demonstrate that many of the fluorescence indices may be related,
especially those that identify variations in the location of maximum intensity of the humic
peak (the Kalbitz et al. [ 1999 ] and the McKnight et al. [ 2001 ] FI) or those which are ratios
of microbiologically derived organic matter to soil-derived organic matter (BIX and the
peak T/peak C ratio). Here we highlight several studies and show examples of how fluores-
cence indices have been used to further ecosystem understanding.
9.3.1 Using Fluorescence Indices to Identify Environmental
Controls on Soil Organic Matter
As was discussed earlier in this chapter, much of the initial work using fluorescence and
fluorescence indices to study and characterize organic matter was developed to under-
stand changes in soil organic matter due to land use or environmental changes. Kalbitz
et al. ( 1999 ) developed the HIX SYN from the observation that aqueous extracts of topsoil
from arable and intensively used sites demonstrated a higher C/N and smaller degree of