Sediment and Soil Remediation — Southern China

Project snapshot

Location: Southern China

Client/Delivery: iEnvi Principal Environmental Scientist Michael Nicholls, in partnership with Arcadis

Start date: 2012 (programme value reported at US$2.1 billion)

Scope: Series of 11 creek sediment remediation projects plus four major industrial‑zone soil and groundwater remediation projects.

Background and drivers

The programme followed community concern about earlier, inadequate sediment works (2008) that reportedly resulted in elevated cadmium in fish and impacts to local residents and potable water supplies. The initial work targeted approximately 60,000 m3 of sediment impacted by cadmium (Cd), lead (Pb), mercury (Hg), arsenic (As) and hydrocarbons associated with a former gasworks, from a contaminated 4.3 km creek reach that fed a major river. Several creek sections were noted as highly acidic and exothermic.

Remediation approach — plain English

The remediation combined careful delineation of contamination with an in‑the‑dry excavation and treatment workflow designed to be cost‑effective and to use locally available materials and treatment facilities:

• Site delineation and risk assessment to confirm volumes and contaminant distribution.
• Cofferdams were installed to isolate creek sections and enable dewatering.
• Extracted water was pumped to nearby industrial wastewater treatment plants for processing.
• Sediment excavation with on‑site separation of large debris during works.
• Transport of excavated sediment to a treatment area where gravity dewatering was applied; recovered water returned for treatment.
• Mechanical solidification/stabilisation (S/S) of dewatered sediment using locally selected blend ratios — typically around 10% Portland cement, 10% steel slag, 10% local clay and 10% fly ash (ratios varied by sediment section following treatability testing).
• Placement of stabilised material as engineered fill for a constructed dry‑dock shipyard, subject to ongoing leachability monitoring and management for acid‑rain infiltration.
• Hydrological modelling informed the design of residual sediment capping matrices where full removal was not practicable — typically sands, gravels and cobbles to provide physical stability and reduce exposure risk.

Practical issues and mitigations

• Laboratory and contractor capacity constraints were managed by simplifying field test programmes where appropriate and using international cooperation to transfer technical know‑how.
• Access limitations were addressed with phased work areas and logistics planning to maintain continuity of treatment and disposal operations.
• Using nearby industrial wastewater treatment facilities and locally available binder materials reduced haulage costs and supported a more economical treatment solution.

Outcomes and ongoing management

The project delivered mass removal and stabilisation of heavily contaminated creek sediments and provided a beneficial reuse pathway for treated material. Key ongoing activities included leachability monitoring of placed fill, groundwater and surface water monitoring downstream, and adaptive management where acid‑drainage risk required additional precautionary measures.

Commercial and development benefits

• Reduced long‑term liability by treating and stabilising contaminated sediments rather than uncontrolled in‑river disposal.
• Cost savings from using local materials and nearby treatment plants, and from beneficial reuse of stabilised material as engineered fill.
• Improved stakeholder confidence where visible remediation and monitoring addressed community health concerns.
• Support for redevelopment outcomes (e.g. dry‑dock construction) by converting waste into engineered fill that meets specification.

Practical takeaways for developers and asset owners

1. Start with robust delineation and treatability testing — it drives the remediation approach, cost and reuse opportunities.
2. Plan for water management and off‑site treatment early; nearby wastewater treatment can materially reduce programme cost and complexity.
3. Consider S/S and beneficial reuse where leachability and engineering performance can be demonstrated — this can reduce disposal costs and create value for redevelopment.
4. Account for local laboratory and contractor capability in project schedules and consider international technical partners where gaps exist.
5. Establish a clear monitoring and complaints‑handling plan to manage community and regulator expectations through to completion.

Next steps

If you are assessing remediation strategy, site risk or redevelopment options for contaminated sediment and soils, iEnvi can help with technical due diligence, treatability testing, remediation design and monitoring plans. Call 13000 43684 or visit /contact/ to discuss your site.