In-situ immobilization of arsenic in the subsurface on an anthropogenic contaminated site - In-Situ Remediation of Arsenic-Contaminated Sites - page 148

to 4.1 m 3 h − 1 (operational maximum of the pilot plant with high DO-saturation) with unchanged
discharge. Therefore, the amount of process water was increased and thus the reactive domain
in the aquifer. Van Halem et al . (2010) stated that the efficiency of the As-immobilization is
improved by this procedure.

7.4 FIELD EXPERIMENT RESULTS

7.4.1 Arsenic concentration
The analysis results from the infiltration experiment are depicted in the following diagrams. The
sampling and analysis methods have been described in Stummeyer et al . (1996) and Holländer
et al . (2008). Circles and triangles mark the mean value of all 16 measurements during one cycle.
Triangles indicate results from well 1, circles from well 2. The error bars mark the minimum and
maximum value within a cycle. The black bars at the bottom show times when the plant was not
working properly or shut down. The dosage of the FeCl 2 solution is shown as a bar diagram in
the background. Changes in pump rate or dosage are marked by vertical dashed lines.
Figure 7.5 shows the t-As concentration throughout the entire experiment. Generally, the t-As
concentration decreases over the runtime although no significant reduction has been observed
at a dosage of 50 mL/cycle. The mean concentration in well 1 is lower than in well 2 in the
beginning of the study. This is most likely due to the position of the well screens. They were
built in different depths and thus most likely different environments. The increase of the dosage
to 100 mL/cycle resulted in a significant decrease in the t-As concentration. The concentration
decreased to 1 mg L − 1 t-As at the FeCl 2 -dosage of 150 mL/cycle. After a phase of limited plant
functionality, the t-As concentration is temporarily increased but decreased again when the plant
was switched on again. No significant trend in the t-As concentration was noticed at 200 mL/cycle.

Figure 7.5. Mean values of t-As-concentration per cycle in well 1 (triangles) and well 2 (circles); error bars
mark minimum and maximum of values in a cycle, gray bars denote the FeCl 2 -dosage.