Introduction - Application of Geochemical Tracers to Fluvial Sediment - page 2

It was not until the middle of the 1900s, however, that the potential for tracer studies
to provide meaningful data on sediment dynamics began to be appreciated (Walling
et al. 2013 ). Early studies were primarily aimed at understanding particle entrainment
thresholds and transport distances of large bed material clasts within short reaches
of the channel and were based on what Black et al. ( 2007 ) calls 'particle tracking'.
Essentially, particle tracking refers to (1) the practice of tagging individual clasts
in some fashion so their movement can be documented, especially during storm
events, or (2) the addition of exotic constituents to a mixture of sediment so that
the movement of sediment of similar characteristics can be monitored. These studies
initially relied on rather unsophisticated methods (e.g., painting of a particle surface),
but have evolved so that particle tracking now includes such sophisticated technolo-
gies as inserting magnets or radio-transmitters into individual clasts of varying size,
or incorporating Rare Earth Elements, magnetic constituents (e.g., magnetite), and
other materials in the sediment to monitor their incipient motion and transport dis-
tances in near real-time (Parsons et al. 1993 ; Zhang et al. 2003 ; Kimoto et al. 2006 ;
Mentler et al. 2009 ; Guzmán et al. 2010 ;Huetal. 2011 ; Spencer et al. 2011 ). These
techniques can also be used to assess such things as transport step lengths and rest
periods for variously sized particles, and have been applied to other problems such
as soil erosion rates and redistribution patterns on hillslopes.
The 1980s and 1990s saw an expansion of tracer research to address a num-
ber of additional aspects of the sediment system, including the origin and transport
mechanisms of particles found in both consolidated (sedimentary) and unconsoli-
dated deposits. Walling et al. ( 2013 ) point out that these studies differed from earlier
particle tracking methods in three important ways. First, particle tracking as origi-
nally conducted required the addition of a tracer material which was costly to use
over large areas; thus, the addition of a tracer was (and continues to be) restricted
to short reaches of river channel or small soil plots. To circumvent this problem,
investigators began to utilize natural characteristics of the sediment (e.g., its miner-
alogy, grain size, color, chemical composition, and magnetic properties) as a tracer,
or utilize some pre-existing constituent within the sediment. With respect to the
latter, tracers often consisted of anthropogenic constituents (e.g., 137 Cs from surfi-
cial nuclear bomb tests or trace metals from mining operations). Second, the use
of natural and pre-existing tracers allowed the area of study to be greatly expanded
from short river reaches or small soil plots to the landscape scale. From this larger
scale perspective, the sediment system can be envisioned as an integrated sediment
generation and dispersal network in which sediments are produced in upland areas
and ultimately deposited downstream in a basin that acts as a long-term repository
(Fig. 1.1 ). These zones of sediment production and deposition are connected by a
drainage network that intermittently moves sediment, primarily during flood events,
from source to sink (Schumm 1977 ; Weltje 2012 ). Tracers, at this scale, can be
used to address aspects of the entire, and highly complex, sediment dispersal system
over a variety of temporal scales. Third, fingerprinting and tracing methods began
to focus upon the fine-grained sediment fraction, rather than the coarse-grained bed
load (Walling et al. 2013 ). Interest in fine-sediments resulted from the fact that the
excessive generation and transport of particulates

< ∼

2mm in size pose a direct threat