Geoscience Reference
In-Depth Information
3.4.2 Groundwater NOM Fluorescence Characteristics
EEM analysis of groundwater natural organic matter fluorescence demonstrates that
groundwaters have natural organic matter fluorescence characteristics very similar to those
of freshwater and marine samples (
Figure 3.3
). In terms of specific peaks, the following
general observations can be made:
1.
Peak C and peak A
C
fluorescence is the dominant fluorescence signature, typical of
humic-like material. This material is presumed to be derived predominantly from
the overlying soil, although systematic investigations of groundwaters have not been
undertaken, and it is probable that some material is derived from organic matter pro-
duction and degradation within the aquifer.
2.
Although systematic data are sparse, peak C and peak A
C
fluorescence is likely to
be blue-shifted compared to the overlying soil water fluorescence (Baker and Genty,
1999
), and definitely blue-shifted when compared with surface water samples in the
same region (e.g., see Conmy
2008
). The relatively hydrophobic properties of NOM
would lead to the adsorption of truly dissolved material to the bedrock, leaving only
the relatively hydrophilic fraction to be transported, which could explain some or all of
this blue-shifted emission, as well as the processing of organic matter by groundwater
microbial communities. Conmy (
2008
) compared surface, ground, and marine water
samples, and demonstrated that peak C fluorescence emission wavelength is interme-
diate to marine and surface samples.
3.
Peak C fluorescence intensity correlates with dissolved organic carbon concentration
and decreases with depth in the aquifer. The latter reflects the combination of physical
losses in the aquifer (e.g., adsorption) as well as potential biological degradation. A
decrease in both peak C and peak T intensity with depth is well observed in Lapworth
et al. (
2009
). The peak C fluorescence intensity-DOC relationship leads to the poten-
tial for the use of fluorescence as a surrogate measure of DOC, as first observed by
Smart et al. (
1976
). However, the gradient and strength of this relationship varies from
site to site (Mudarra et al., submitted), depending on the overall content of fluorescent
material.
4.
Peak T and peak B fluorescence peaks are rarely observed in groundwater organic
matter fluorescence EEMs, unless a contaminant source is present (see
Section 3.4.3
).
Peak M fluorescence has yet to be definitively observed in groundwater samples, but if
derived from a microbial source it might also be expected to be observed in microbially
contaminated groundwaters.
NOM fluorescence properties have therefore been mostly utilized as a natural tracer of
surface water-groundwater connectivity, either from the soil directly to the groundwa-
ter (e.g., Baker et al.,
1999
), or through river-groundwater exchanges in the hyphoreic
zone (Lapworth et al.,
2009
). In the future, NOM fluorescence is likely to be used to
understand better the chemical and biological processing of organic matter, or the char-
acter and function of groundwater organic matter, following research already undertaken
