202
2.5 Interactions with Freshwater Sediments and Uptake
by Freshwater Biota
Freshwater sediments often serve as a sink for uranium, with uranium partitioning
between sediments and water affected by pH, hardness, alkalinity (as bicarbonate),
redox potential, dissolved organic matter concentration, phosphorus concentration,
presence of complexing agents and sediment particle size and mineral composition
(Allard et al. 1979, 1982; Brunskill and Wilkinson 1987; Herczeg et al. 1988;
Premuzic et al. 1995; Markich 2002; USGS 2008; Goulet et al. 2012; ATSDR 2013).
Reported distribution coefficients range from 9 to 15,000 L kg
−1
(IAEA 2018).
The factors that affect the partitioning of uranium between freshwater sediments
and water also affect the bioavailability of the element. An important consideration
is competition with calcium and magnesium for uptake. In addition, at high pH the
complexation of uranium with carbonates may reduce its bioavailability. Uptake by
biota may occur both by ingestion of food and sediment and by passive diffusion
through the gills (Langston and Spence 1995; Rand et al. 1995; Köster 2004;
Marsden and Rainbow 2004; Goulet et al. 2012). However, the assimilation efficiency from the gastrointestinal tract is likely to be low (Simon and Garnier-Laplace
2005), and it seems likely that uranium is primarily taken up via the gills (Goulet
et al. 2012). Typical transfer factors for freshwater biota are 100 to 1000 for algae
and plankton, and for freshwater invertebrates, but are about 1 to 30 for freshwater
fish (IAEA 2009, 2010, 2014).
2.6 Interactions with Marine Sediments and Uptake by Marine
Biota
Uranium can enter estuarine, coastal and marine systems either directly in groundwater or in freshwater discharges. Rather limited data are available on K d values for
uranium in marine sediments. The IAEA (2004) provide reference values based on
exchange with carbonates in the sediments. These values are expressed as the ratio
of Bq kg
−1
sediment to Bq kg
−1
seawater, i.e. as dimensionless values. However, it
is more usual to give values expressed as Bq kg
−1
sediment to Bq per m
3
of seawater,
i.e. with units of m
3
kg
−1
. This convention is adopted here. The reference values are
0.5 m
3
kg
−1
for deep ocean sediments (with a reported range from 0.1 to 5 m
3
kg
−1
)
and 1.0 m
3
kg
−1
for ocean margin sediments (with reported values of 1.0 and 5 to
10 m
3
kg
−1
).
The distribution of dissolved uranium in sea water is very similar across marine
systems, and a concentration of 3.3 μg L
−1
of
238
U is a reasonable average in sea
water from coastal environments as well as the deep sea.
As in freshwater systems, absorption of uranium by organisms occurs to a small
extent, and there are no identified cases of uranium buildup in food chains.
Concentration ratio values have been reviewed and compiled by the IAEA (2004).
M. C. Thorne
2.5 Interactions with Freshwater Sediments and Uptake
by Freshwater Biota
Freshwater sediments often serve as a sink for uranium, with uranium partitioning
between sediments and water affected by pH, hardness, alkalinity (as bicarbonate),
redox potential, dissolved organic matter concentration, phosphorus concentration,
presence of complexing agents and sediment particle size and mineral composition
(Allard et al. 1979, 1982; Brunskill and Wilkinson 1987; Herczeg et al. 1988;
Premuzic et al. 1995; Markich 2002; USGS 2008; Goulet et al. 2012; ATSDR 2013).
Reported distribution coefficients range from 9 to 15,000 L kg
−1
(IAEA 2018).
The factors that affect the partitioning of uranium between freshwater sediments
and water also affect the bioavailability of the element. An important consideration
is competition with calcium and magnesium for uptake. In addition, at high pH the
complexation of uranium with carbonates may reduce its bioavailability. Uptake by
biota may occur both by ingestion of food and sediment and by passive diffusion
through the gills (Langston and Spence 1995; Rand et al. 1995; Köster 2004;
Marsden and Rainbow 2004; Goulet et al. 2012). However, the assimilation efficiency from the gastrointestinal tract is likely to be low (Simon and Garnier-Laplace
2005), and it seems likely that uranium is primarily taken up via the gills (Goulet
et al. 2012). Typical transfer factors for freshwater biota are 100 to 1000 for algae
and plankton, and for freshwater invertebrates, but are about 1 to 30 for freshwater
fish (IAEA 2009, 2010, 2014).
2.6 Interactions with Marine Sediments and Uptake by Marine
Biota
Uranium can enter estuarine, coastal and marine systems either directly in groundwater or in freshwater discharges. Rather limited data are available on K d values for
uranium in marine sediments. The IAEA (2004) provide reference values based on
exchange with carbonates in the sediments. These values are expressed as the ratio
of Bq kg
−1
sediment to Bq kg
−1
seawater, i.e. as dimensionless values. However, it
is more usual to give values expressed as Bq kg
−1
sediment to Bq per m
3
of seawater,
i.e. with units of m
3
kg
−1
. This convention is adopted here. The reference values are
0.5 m
3
kg
−1
for deep ocean sediments (with a reported range from 0.1 to 5 m
3
kg
−1
)
and 1.0 m
3
kg
−1
for ocean margin sediments (with reported values of 1.0 and 5 to
10 m
3
kg
−1
).
The distribution of dissolved uranium in sea water is very similar across marine
systems, and a concentration of 3.3 μg L
−1
of
238
U is a reasonable average in sea
water from coastal environments as well as the deep sea.
As in freshwater systems, absorption of uranium by organisms occurs to a small
extent, and there are no identified cases of uranium buildup in food chains.
Concentration ratio values have been reviewed and compiled by the IAEA (2004).
M. C. Thorne