A plot comparing
238 U concentrations along thc Gironde salinity gradicnt with the theoretical dilution line of seawater by river water inputs clearly shows that, at the time of sampling, dissolved uranium in
the Gironde estuary behaved non-conservatively, with removal taking
placc at salinities in the range 0-16‰ (tig. 2). This contrasts with previous observations by Martin et al (1978), which found uranium in
the Gironde estuary to behave conservatively. It is important to note,
however, that while our sampling took place during a period of relativcly low river discharge, sampling by Martin d ai took place during
the winter months, when discharges are signtficantly greater. Indeed,
other studies have suggcsted that the non-conservative behaviour of
uranium along estuaries is a seasonal feature related to the river discharge regime prevailing at the time of sampling (Maeda & Windom,
1982; Sarin & Church, 1994).
As in other estuaries which exhibit non-conservative behaviour, the
mechanism responsible for the removal of uranium at low salinities is
likely to be thc salt-induced coagulation of riverine colloidal material
(Boyle efal, 1977; Swarzenski et al., 1995; Sanudo-Wilhelmy et al., 1996). In
the Gironde estuary, coagulation has been shown to take place at salinities
between 0.1 and l%o (Cjibbs et at, 1989). Uranium concentrations,
however, only gradually approach the ideal dilution line in the midsalinity region, suggesting that flocculation may extend up to 16 per
thousand. Ihe elevated suspended matter ctjncentrations in this (intermediate salinity) turbidity /one may account for the removal of uranium at these relatively high salinities (Swarzenski etai, 1995).
Figure 2 -
238 U salinity profile in the Gironde estuaiy (non-conservative behaviour).
Above 16 %o, dissolved uranium concentrations are essentially indicative
< >f conservative mixing. Such laehaviour extends r< > < >pen ocean salinities and
has been previously obsemed in other estuarine systems (Sarin & Church,
1994; Swar/enski etai, 1995). /V likely explanatton for this obsetvation lies
300
238 U concentrations along thc Gironde salinity gradicnt with the theoretical dilution line of seawater by river water inputs clearly shows that, at the time of sampling, dissolved uranium in
the Gironde estuary behaved non-conservatively, with removal taking
placc at salinities in the range 0-16‰ (tig. 2). This contrasts with previous observations by Martin et al (1978), which found uranium in
the Gironde estuary to behave conservatively. It is important to note,
however, that while our sampling took place during a period of relativcly low river discharge, sampling by Martin d ai took place during
the winter months, when discharges are signtficantly greater. Indeed,
other studies have suggcsted that the non-conservative behaviour of
uranium along estuaries is a seasonal feature related to the river discharge regime prevailing at the time of sampling (Maeda & Windom,
1982; Sarin & Church, 1994).
As in other estuaries which exhibit non-conservative behaviour, the
mechanism responsible for the removal of uranium at low salinities is
likely to be thc salt-induced coagulation of riverine colloidal material
(Boyle efal, 1977; Swarzenski et al., 1995; Sanudo-Wilhelmy et al., 1996). In
the Gironde estuary, coagulation has been shown to take place at salinities
between 0.1 and l%o (Cjibbs et at, 1989). Uranium concentrations,
however, only gradually approach the ideal dilution line in the midsalinity region, suggesting that flocculation may extend up to 16 per
thousand. Ihe elevated suspended matter ctjncentrations in this (intermediate salinity) turbidity /one may account for the removal of uranium at these relatively high salinities (Swarzenski etai, 1995).
Figure 2 -
238 U salinity profile in the Gironde estuaiy (non-conservative behaviour).
Above 16 %o, dissolved uranium concentrations are essentially indicative
< >f conservative mixing. Such laehaviour extends r< > < >pen ocean salinities and
has been previously obsemed in other estuarine systems (Sarin & Church,
1994; Swar/enski etai, 1995). /V likely explanatton for this obsetvation lies
300