Mito City, Ibaraki Prefecture, Hexavalent Chromium Contamination Remediation Measures - December 2021

Mito City, Ibaraki Prefecture, Hexavalent Chromium Contamination Remediation Measures - December 2021

2021, a survey conducted at the site of a former industrial waste treatment facility in Mito City, Ibaraki Prefecture, revealed that the concentration of hexavalent chromium in the groundwater was 10 times higher (0.5 mg/L) than the standard value. This contamination was attributed to improper waste disposal since the 1980s, and there were strong concerns about the health risk to residents.

The city and prefecture secured a total budget of 1.5 billion yen and began a large-scale cleanup project in December 2021. The project included the following detailed measures

1. **Soil excavation**.
- The excavation of contaminated soil covered an area of approximately 1,000 square meters and a depth of 10 meters.
- Approximately 5200 tons of contaminated soil was removed and detoxified at a nearby treatment facility.

2.**Chemical injection**.
- In order to reduce hexavalent chromium in the groundwater to trivalent chromium, iron sulfate (FeSO₄) solution was injected into the ground through 10 injection wells.
- A total of 20 tons of iron sulfate was used, and injection operations were conducted over a period of approximately three months.

3.**Monitoring system installed**.
- Fifteen monitoring wells were installed in the remediation area to monitor the progress of the remediation work.
- Sensors were installed to measure the concentration of hexavalent chromium in the groundwater in real time, and daily data was collected.

**Accomplishments and Current Status**.
In the final report in 2023, it was confirmed that the concentration of hexavalent chromium in the groundwater reached below the standard value (less than 0.05 mg/L). Additional sources of contamination have been identified through post-excavation soil investigation, and further remediation plans are underway. An additional 200 million yen will be budgeted for stabilization of the entire remediation area.

This project is considered an important model case in the groundwater contamination problem faced by Mito as well as other municipalities. In addition, future measures to prevent recontamination require strict monitoring and tighter regulation of illegal waste disposal.