Geoscience Reference
In-Depth Information
The residence times of contaminants in flood-
plains can be very long, ranging from 10
1
to
10
3
years. This was illustrated by Helgen &
Moore (1996), who estimated that it would
take several thousand years to completely
remobilize metal-bearing tailings deposited on
the floodplain of the Clark Fork River flood-
plain, Montana, USA. The length of time the
contaminants remained stored in the floodplain
environment generally depends on the rates of
post-depositional physical, biological and chem-
ical remobilization (see below), and on the
geomorphology of the basin or reach. In stable
reaches (e.g. floodplains far from the active
channel, valley reaches with low stream power)
or reaches where aggradation rates are high,
contaminants can be stored for extremely long
periods ( James 1989; Miller 1997), whereas in
more active reaches (e.g. near-channel flood-
plains, valley reaches with high stream power),
the storage period may be short ( James 1989).
Lewin & Macklin (1987) showed that the
deposition and storage patterns of sediment-
associated metals can be related directly to flood-
plain geomorphological processes and channel
sedimentation styles. In laterally mobile braided
and meandering river environments, aggradation
of contaminated sediments occurs in floodplains
and bars in reworked valley floors (Wolfenden
& Lewin 1977; Lewin et al. 1983; Macklin &
Lewin 1989). The patterns of sediment metal
concentrations vary considerably across flood-
plains in these environments, according to the
ages and quantities of contaminated sediments
deposited (Lewin et al. 1983; Macklin & Lewin
1989).
Floodplain morphology often affects the
nature of contaminated sediment deposition.
In reaches or rivers that experience little lateral
movement, contaminated sediments are added
to floodplains by overbank deposition as a
thin veneer over the whole floodplain surface
(Lewin & Macklin 1987; Bradley & Cox 1990;
Chesnokov et al. 2000) (see Case Study 3.3).
River-bed incision at the end of the eighteenth
century in the lower South Tyne valley, England,
restricted metal-contaminated overbank sediment
deposition to a relatively narrow trench adjacent
to the contemporary river (Macklin 1988). In
the River Aire, England, the construction of flood
embankment structures at Beal confined and
accelerated sedimentation in the near-channel
Case Study 3.3 Styles of deposition of sediment-borne radionuclide contaminants in the River
Techa, Urals, Russia
The River Techa in the Chelyabinsk region of the Urals in Russia is typical of river systems affected
by releases from nuclear facilities. Liquid radioactive wastes with a total activity of 10
17
Bq, of
which 12.2% was the radiogenic isotope caesium-137 (
137
Cs), were released to the River Techa
from the Mayak plutonium facility between 1949 and 1952. These waste releases coincided
with large floods, resulting in widespread contamination of the Techa floodplain by
137
Cs. Water
reservoirs and by-pass channels were constructed between 1951 and 1961 to prevent further
radionuclide release to the river system, but the contaminated floodplain is currently exploited
for fishing, bathing, pasture and hay collection by residents of the village of Muslomovo.
Subsequent study showed that much of the liquid waste had precipitated or sorbed onto
sediment grains, with the total deposition of
137
Cs within a surveyed area of 2.5 km
2
estimated
at 6.6 TBq. Two major styles of deposition of
137
Cs-contaminated sediment occurred. In many
areas, deposition of highly contaminated sediment was restricted to a narrow zone near the
river (Case Fig. 3.3a). This zone is confined by steep river banks (gradient of 0.0015) and has
a relatively narrow floodplain (up to 75 m); maximum
137
Cs deposition above 7.5 MBq m
−2
is
localized in areas of bank height up to 1 m above normal water level (Case Fig. 3.3b). Further