In-situ immobilization of arsenic in the subsurface on an anthropogenic contaminated site - In-Situ Remediation of Arsenic-Contaminated Sites - page 146

Figure 7.4. Work flow for the pilot plant operation.

The above described steps comprise one cycle ( Fig. 7.4 ) . The cycles are completely computer-
ized as well as the internal control and measurements within the pilot plant. For the second cycle,
all steps are repeated with the difference that water is pumped from well 2 and infiltrated into
well 1. Thus a reactive domain around both wells is established due to the forced precipitation of
Fe. A part of the pumped water is always discharged to the external remediation facility during the
pilot plant operation. This part reached up to 20% (1 m 3 h − 1 ) of the withdrawn discharge during
the pilot experiment. Thus, a higher amount of water is withdrawn than infiltrated. Therefore,
water from outside of the subterranean reactive domain will enter the domain, and the As of
this water is removed by adsorption. The pump-, infiltration- and discharge rates for the steps
throughout the experiment can be seen in Table 7.2. As the main result, the As-concentration
of the water which is pumped to the external remediation plant is lower than in the ambient
groundwater.
A field study was carried out over a period of 705 days for the infiltration of Fe. In the beginning
50 mL FeCl 2 solution containing 5.9 g Fe has been dosed per cycle (11.8 mg Fe L − 1 infiltrated
water). The dosage was increased stepwise up to 1000 mL (118 g Fe) per cycle (236 mg Fe L − 1 ).
The amount was increased when there was no or little Fe measured in the water pumped from
the wells. The low dosage in the beginning was chosen to achieve the first aim of the pilot study:
Immobilization of the i-As as shown in the batch experiments targeting on a rate of 10 g Fe to
1 g i-As. The second reason for choosing the low dosage was to avoid clogging of the pores near
the wells or even at the well screens. An indicator is dissolved Fe in the pumped water. If Fe
were measured in the pumped water, not all of the infiltrated Fe was precipitated in the aquifer.
The second indicator was the immobilization of org-As. Therefore, the Fe dosage was increased
throughout the study.
The amount of dosed Fe is shown in Table 7.2. Additionally the pump and infiltration rates are
given. A typical schedule for one day is depicted in ( Fig. 7.4 ).
The steps have always been carried out in the sequences (1), (2), (3) and (4). Every entry in
Table 7.2 marks a change in the schedule. If a step is not listed in the table, the parameters for the
step remained unchanged. In the beginning, all steps had a pump rate of 3.0 m 3 h − 1 and discharge
water of 0.5 m 3 h − 1 . The infiltration rate was 1.0 m 3 h − 1 in step (1) and 2.5 m 3 h − 1 in all other
steps. The durations of the steps were 0.5 h, 5 h and 2 h for steps (2), (3) and (4). Step (1) had