rebound effect was probably caused by diffusion and/or desorption of Cr(VI) from
the very low-permeable environment of weathered granite to more permeable layers.
The dithionite infiltrated into the saturated zone gradually decomposed into
sulfate, whose concentration in the groundwater increased to approximately
1000–5000 mg/L. After the infiltration was completed, there was a downward
trend in the concentrations of sulfates in most of the wells. A more significant
decrease was revealed for the concentrations of potassium and sodium as a result
of sorption processes; in the case of bicarbonates, it was a result of the neutralization
of hydrogen ions and weathering of carbonates such as calcite and rhodochrosite.
Due to Cr(VI) rebounding in the above-mentioned wells, there was an additional
phase of infiltration of the reducing solution. The reducing agent was the same as for
the main phase, i.e., sodium dithionite (14 g/L) with K 2 CO 3 (56 g/L) and KHCO 3
(2.8 g/L) buffers. In total, 270 m
3 of the solution and 94 m
3 of pure water were
infiltrated into 12 wells. The concentration of Cr(VI) in the groundwater in the
source area was significantly below 0.1 mg/L two and four months after the
completion of this infiltration phase. Post-remedial monitoring was conducted at
monthly intervals over the following 2 years in ten selected wells in the source area.
During the reporting period, the target limit was only sporadically exceeded in a
maximum of two wells.
3.3 Reduction of Hexavalent Chromium by Metabisulfite
The Permon Křivoklát site with proven massive contamination of groundwater and
soil by Cr(VI) was selected to test the efficiency of the sodium metabisulfite method.
This contamination comes from an inadequately sealed sewerage system in the
chrome production plant. The total amount of the released contaminant was not
recorded and the investigation work prior to the remediation found Cr(VI) at the
concentration of 60 mg/L in the groundwater (well HV-22, Fig. 3.4).
Geological and hydrogeological conditions of the site are complicated. The area
has two different hydrogeological rock types—upper fluvial sand-gravel sediments
(maximum thickness of 8 m—porous permeability) and lower silty shale (fissure
permeability) with local spilites and tuffs. Rock of a Proterozoic age is relatively
impermeable or semipermeable underlying rock with fluvial sediments. The transmissivity coefficient of both rock types ranges in the order of 10
À5 m
2 /s and the
hydraulic conductivity is in the order of 10
À4
–10
À6 m/s, which corresponds to rather
weakly permeable to lightly permeable rock.
The groundwater level in these rocks is unconfined to slightly confined and has
been documented (mainly in periods of intense precipitation) at about 7 m below
ground level (base of the sandy gravel). The data from the monitoring equipment/
system showed significant levels of oscillation, which probably caused the high
concentration of dissolved oxygen (on the saturation level).
Before the application of metabisulfite, laboratory tests were conducted using
three different reducing agents:
60
J. Němeček et al.
the very low-permeable environment of weathered granite to more permeable layers.
The dithionite infiltrated into the saturated zone gradually decomposed into
sulfate, whose concentration in the groundwater increased to approximately
1000–5000 mg/L. After the infiltration was completed, there was a downward
trend in the concentrations of sulfates in most of the wells. A more significant
decrease was revealed for the concentrations of potassium and sodium as a result
of sorption processes; in the case of bicarbonates, it was a result of the neutralization
of hydrogen ions and weathering of carbonates such as calcite and rhodochrosite.
Due to Cr(VI) rebounding in the above-mentioned wells, there was an additional
phase of infiltration of the reducing solution. The reducing agent was the same as for
the main phase, i.e., sodium dithionite (14 g/L) with K 2 CO 3 (56 g/L) and KHCO 3
(2.8 g/L) buffers. In total, 270 m
3 of the solution and 94 m
3 of pure water were
infiltrated into 12 wells. The concentration of Cr(VI) in the groundwater in the
source area was significantly below 0.1 mg/L two and four months after the
completion of this infiltration phase. Post-remedial monitoring was conducted at
monthly intervals over the following 2 years in ten selected wells in the source area.
During the reporting period, the target limit was only sporadically exceeded in a
maximum of two wells.
3.3 Reduction of Hexavalent Chromium by Metabisulfite
The Permon Křivoklát site with proven massive contamination of groundwater and
soil by Cr(VI) was selected to test the efficiency of the sodium metabisulfite method.
This contamination comes from an inadequately sealed sewerage system in the
chrome production plant. The total amount of the released contaminant was not
recorded and the investigation work prior to the remediation found Cr(VI) at the
concentration of 60 mg/L in the groundwater (well HV-22, Fig. 3.4).
Geological and hydrogeological conditions of the site are complicated. The area
has two different hydrogeological rock types—upper fluvial sand-gravel sediments
(maximum thickness of 8 m—porous permeability) and lower silty shale (fissure
permeability) with local spilites and tuffs. Rock of a Proterozoic age is relatively
impermeable or semipermeable underlying rock with fluvial sediments. The transmissivity coefficient of both rock types ranges in the order of 10
À5 m
2 /s and the
hydraulic conductivity is in the order of 10
À4
–10
À6 m/s, which corresponds to rather
weakly permeable to lightly permeable rock.
The groundwater level in these rocks is unconfined to slightly confined and has
been documented (mainly in periods of intense precipitation) at about 7 m below
ground level (base of the sandy gravel). The data from the monitoring equipment/
system showed significant levels of oscillation, which probably caused the high
concentration of dissolved oxygen (on the saturation level).
Before the application of metabisulfite, laboratory tests were conducted using
three different reducing agents:
60
J. Němeček et al.