200
T. Schondorf et al.
A remarkable fact is that the concentrations of dissolved mercury initially
increase downgradient of the larger source area (kyanization building) before
they decrease after some 100 m distance, instead of continuously decreasing
right from start of the plume. This can be explained by the time function of
various release scenarios such as the termination of quasi-operationally related
releases after shutdown of the plant and by former massive releases by accidents,
war impacts, etc., as well as by releases still occurring today by leaching of the Hg
depot in the groundwater fluctuation zone dependent on fluctuating groundwater
levels. With regard to Hg transport behaviour in the groundwater, therefore,
retardation factors between 400 to 2000 can be estimated, which characterize the
decreased mobility due to sorption to the aquifer material with respect to water
flow velocity. However, no constant retardation factor either in time nor in space
can be assumed in the case of Hg due to alternating Hg species and
concentrations, hydrochemical conditions, mineral surfaces and inner surfaces
as well as influences of organic substances.
9
Mercury in Groundwater: Speciation
9.1
Operationally Defined Hg Fractions
9.1.1
Results and Discussion
Table 3 summarizes the results of various investigation campaigns. It has to be
pointed out that the significantly contaminated groundwater monitoring wells
contain mainly easily reducible inorganic mercury (HgIIa), an important fraction
of which is present as Hgo. At first sight, the occurrence of dissolved metallic Hgo
in groundwater at concentrations of partly over 200 ~lg/l seems to be improbable
because of limiting solubility (cf. Table 3). Richter-Politz et al. (1995), however,
have shown that the water solubilitiy of Hgo increases from the already known
60 ~tg/l (25°C) in oxygen-free water to the mg/l range in the presence of airl
oxygen, as in this case. A balance of these fractions could not be achieved in
some of the investigation campaigns due to difficulties during determination of
Hgo and particulate Hg. These difficulties, which are still to be investigated, may
result from sorption at container walls, from volatile organic matter absorbing in
the UV range, from Hg phase transition and species transformation (cf. below) as
well as from cross-contamination and evaporation of not stabilized samples. The
fraction of the solved organic complex Hg forms gave reliable results, as
demonstrated by similar results of various laboratories at various sampling dates.
The HgIIb fraction predominates mostly in groundwater monitoring wells with
low total concentrations in the range of several ~tg/l. In the course of a flow path,
this fraction has an increasing tendency with decreasing total Hg. This may
T. Schondorf et al.
A remarkable fact is that the concentrations of dissolved mercury initially
increase downgradient of the larger source area (kyanization building) before
they decrease after some 100 m distance, instead of continuously decreasing
right from start of the plume. This can be explained by the time function of
various release scenarios such as the termination of quasi-operationally related
releases after shutdown of the plant and by former massive releases by accidents,
war impacts, etc., as well as by releases still occurring today by leaching of the Hg
depot in the groundwater fluctuation zone dependent on fluctuating groundwater
levels. With regard to Hg transport behaviour in the groundwater, therefore,
retardation factors between 400 to 2000 can be estimated, which characterize the
decreased mobility due to sorption to the aquifer material with respect to water
flow velocity. However, no constant retardation factor either in time nor in space
can be assumed in the case of Hg due to alternating Hg species and
concentrations, hydrochemical conditions, mineral surfaces and inner surfaces
as well as influences of organic substances.
9
Mercury in Groundwater: Speciation
9.1
Operationally Defined Hg Fractions
9.1.1
Results and Discussion
Table 3 summarizes the results of various investigation campaigns. It has to be
pointed out that the significantly contaminated groundwater monitoring wells
contain mainly easily reducible inorganic mercury (HgIIa), an important fraction
of which is present as Hgo. At first sight, the occurrence of dissolved metallic Hgo
in groundwater at concentrations of partly over 200 ~lg/l seems to be improbable
because of limiting solubility (cf. Table 3). Richter-Politz et al. (1995), however,
have shown that the water solubilitiy of Hgo increases from the already known
60 ~tg/l (25°C) in oxygen-free water to the mg/l range in the presence of airl
oxygen, as in this case. A balance of these fractions could not be achieved in
some of the investigation campaigns due to difficulties during determination of
Hgo and particulate Hg. These difficulties, which are still to be investigated, may
result from sorption at container walls, from volatile organic matter absorbing in
the UV range, from Hg phase transition and species transformation (cf. below) as
well as from cross-contamination and evaporation of not stabilized samples. The
fraction of the solved organic complex Hg forms gave reliable results, as
demonstrated by similar results of various laboratories at various sampling dates.
The HgIIb fraction predominates mostly in groundwater monitoring wells with
low total concentrations in the range of several ~tg/l. In the course of a flow path,
this fraction has an increasing tendency with decreasing total Hg. This may