Geochemical Fingerprinting - Application of Geochemical Tracers to Fluvial Sediment - page 36

significantly between the seven subcatchments. The observed variations in source
area inputs to the river not only reflected the proportion of the basins covered by the
sediment source, but also a host of other factors that controlled the erosion of the
source sediments. Collins et al. ( 2010a ) argued, for instance, that sediment inputs
from bank erosion were probably influenced by such factors as “channel morphology
and density, river bank dimensions, and riparian land use pressures”. Regardless of the
controls, the data clearly indicated that sediment mitigation strategies will need to be
tailored to individual subcatchments, rather than using a one-method fits all approach.
Evrard et al. ( 2013 ) utilized geochemical fingerprinting (and an alternative, diffuse
reflectance infrared Fourier Transform spectroscopy method) to assess the predomi-
nant source of sediments to tropical rivers in centralMexico underlain by different soil
types. In this case, the goal was to determine whether the sediments were primarily
produced by gully erosion or sheet erosion. As they were attempting to decipher
sediments eroded from surface and subsurface sites, they relied on fallout radionu-
clides ( 137 Cs and 210 Pb ex ) and biogenic elements (C, N) as fingerprints. These para-
meters are known to differ significantly between surface and subsurface sources (as
will be discussed in the next chapter). The analyzed river sediments were obtained in
2009 from the channel bed, and were assumed to represent average contributions to
the river during the rainy season. Evrard et al. ( 2013 ) found that within the Huertitas
subcatchment dominated byAcrisols themajority of the sediment (between 88 and 98
%) was derived fromgullies. The amount of sediment derived fromcroplands by sheet
erosion decreased during the rainy season, possibly as a result of increased vegetation
cover that helped stabilize the soil surface. In contrast, the majority of the sediment
(50-85%) within the Andisol dominated La Cortina catchment was derived from the
surface of croplands.Within the Potrerillos catchment, characterized by bothAcrisols
and Andisols, contributions of sediment from gullies and croplands were highly
variable between storms. Gullies, for example, generated between 5 and 86% of the
sediment, while the sheet erosion of rangelands generated between 14 and 95% of
the sediment. However, when combined with other data, it appears that fine-sediment
delivered to the Cointzio reservoir was primarily derived from gullies developed in
Acrisols, even where Acrisols covered small areas of the basin (

0.5%). Thus, they
suggested that sediment mitigation efforts should focus on stabilizing gully networks.
While the three analyses described above focused on contemporary sediments,
other studies have combined their investigation of contemporary sediment sources
with an analysis of the changes in sediment provenance through time. These studies
typically focus on cores taken of semi-continuously accumulated sediments on flood-
plains (e.g., overbank deposits) or within reservoirs, lakes, or wetlands that receive
sediments from the upstream drainage network. Such fingerprinting studies can be
extremely useful from a management perspective in that the sediment cores can be
dated, allowing changes in sediment provenance to be linked to (1) an analysis of
sediment accumulation rates and the changes in those rates through time, (2) tem-
poral changes in the concentration of various contaminants including trace metals
and nutrients within the system, and (3) alterations in land-use/land-cover and other
human activities within the catchment. Thus, a link can be made between sediment-
contaminant source, sediment influx rates to the river, and anthropogenic activities.

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