Theme: 01. Remediation Technology Innovations

Topic Area: 01m. In Situ Technologies: Lessons Learned

Formatted Title:

Sustainable Approach for Persistent Arsenic-Contaminated Aquifers with Groundwater Circulation Wells: A Pilot Plant Study

Background/Objectives:

Background/Objectives. Persistent arsenic (As) pollution sources from anthropogenic activities pose a serious threat to groundwater quality. This work aims to illustrate the application of hydraulic manipulation to enhance As mobilization and removal from a heavily contaminated aquifer at a historically polluted industrial site. Groundwater circulation well (GCW) technology was tested at a pilot scale to verify the possibility to significantly increase and accelerate the mobilization and removal of As in the source area. The objective of this pilot test was to demonstrate the effectiveness of the GCW system in mobilizing significant quantities of arsenic in the site-specific hydrogeological context in which it is installed.

Approach/Activities:

Approach/Activities. The pilot plant was installed in February 2023 and consists of a 24 m IEG-GCW® groundwater circulation well with three screened sections piped to a treatment plant. The IEG-GCW® extracts and re-injects groundwater at different depths of a vertical circulation well. By pumping out and reinjecting in different screen sections of the well, the resulting vertical hydraulic gradients create recirculation cells and affect and mobilize trapped contaminants that cannot be influenced by traditional pumping systems. The plant removes arsenic by oxidizing it from arsenite to arsenate with sodium hypochlorite and then coagulating and co-precipitating it with ferric chloride. The ferric arsenate flakes thus generated are then removed by filtration on Macrolite® ceramic media. The filtered water is returned to the aquifer with the IEG-GCW® while the arsenic is concentrated in the filter backwash water and disposed of. In order to effectively monitor the development of the GCW circulation cells, two multilevel sampling wells (MLSW) with three screened section each were installed within the radius of influence of the GCW. During the seven months of operation, the well worked on an average recirculation flow rate of 2 m³/h. In the first fifty days the GCW operated by pumping from the middle and lower horizons and re-injecting into the superficial one, to maximize the extracted mass of arsenic, which is mainly present at the bottom of the aquifer. Then the system switched to pumping from the lower horizon and re-injection distributed over the superficial and middle horizons.

Results/Lessons Learned:

Results/Lessons Learned. In the first two months of operation, a remarkable ability to mobilize As in the aquifer was demonstrated, increasing the plant influent concentrations from 2-3 mg/L to 7-8 mg/L. The development of the recirculation cell was also observed due to the variation of the chloride parameter in MLSW wells. Considering the proximity to the sea, as the depth increases the aquifer becomes gradually more saline. Initially the chloride concentration was 60 mg/L in the surface horizon, 0.5 g/L in the intermediate and 10 g/L in the deep. Within 90 days, the chloride concentrations all rose to the value of 10 g/L and remained fixed at that value during the next four months of operation. The homogenization of chloride concentrations over 6 meters from the well along a 24-meter vertical over three months clearly indicates the effect of recirculation. Regarding the mass balance, the single well can mobilize more than 100 kg per year of arsenic by recirculating 99.3 percent of the emitted water (0.7 percent used for backwashing). The results above highlight how the IEG-GCW® system implemented in this context is an extremely valuable tool for greatly accelerating the remediation process. The results acquired during this pilot test will be used for the design of the full-scale well field that will cover the entire area of the secondary source of contamination.

Felice Tatangelo

Affiliation

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(Group 1, Poster Board #245) Sustainable Approach for Persistent Arsenic-Contaminated Aquifers with Groundwater Circulation Wells: A Pilot Plant Study