In-situ immobilization of arsenic in the subsurface on an anthropogenic contaminated site - In-Situ Remediation of Arsenic-Contaminated Sites - page 149

This might be due to problems occurred with the Fe dosage during this phase so that potentially a
lower amount of Fe was added. Increasing the dosage to 300, 375 and 500 mL decreased the t-As
concentration below 0.4 mg L − 1 .
Noticeably, the mean concentration in well 2 started out higher than the one in well 1 and
remained higher for about 200 days of the experiment. After 200 days almost no difference could
be noticed between the As concentrations in the two wells. Finally, the concentration in well 2 was
lower than in well 1 after day 330. This was estimated earlier since well 2 received the downstream
water from well 1 which has already decreased As-concentration.
Changing the amount of discharged water to the pump-and-treat facility from 0.5 to 1.0 m 3 h − 1
on day 334 leads to an increase in t-As concentration up to 0.8 mg L − 1 . An increase in concen-
tration was expected since the amount of injected Fe was constant while the flow of ambient
(contaminated) groundwater into the subterranean reactor was larger due to the higher discharge
which increased by 0.5 m 3 h − 1 . The deviation of the minimum and maximum values from the
mean value of a cycle was increased as well ( Fig. 7.5 ). An increase in dosage to 750 mL and
1000 mL decreased the mean concentration to 0.4 mg L − 1 again.
The immobilization phase was followed by a 45 days remobilization phase. After switching the
Fe dosage off, the t-As concentration increased by 0.5 mg L − 1 which is an increase of more than
100%. Nevertheless, the concentration were below 1 mg L − 1 and thus much lower than the initial
concentration of about 1.6-1.7 mg L − 1 . Hydraulic observations showed an effective flow velocity
of 1md − 1 . Therefore, the groundwater flows about 45 m within the 45 days remobilization
period. The distance between the two wells is 30 m so that the subterranean reactor was still
able to immobilize As because the resulting concentrations were still much lower than the initial
concentration which is still flowing into the system from upstream. Hence the precipitated and
adsorbed As had not been remobilized.
The cumulative graph of the immobilized t-As is presented in Figure 7.6 . The data were
calculated by using the mean As-concentration of each dosage period. The immobilized mass of
t-As can be calculated by taking the amount of discharged water to the pump-and-treat facility

Figure 7.6. Cumulative graph of immobilized t-As.