Hexavalent Chromium Groundwater and Soil Remediation – Steel Processing

Project overview

iEnvi Principal Michael Nicholls led a soil and groundwater assessment and remediation program for hexavalent chromium (Cr6+) at a steel processing factory with more than 50 years of operations. Contaminants migrated through corroded drains and drip trays into the clay and weathered basalt vadose zone and into a fractured basalt aquifer approximately 450 m downgradient of the site.

Site context and scale of impact

• Primary infrastructure failures included corroded drainage pits, pipelines, stormwater drains and drip trays that allowed chromic acid to escape the process area.
• Estimated impacted media: ~35,000 m3 of hydrocarbon and hexavalent chromium–impacted soil and ~1,260,000 m3 of impacted groundwater extending to dozens of private properties to the south of the site.
• Fractured basalt aquifer conditions influenced contaminant transport and remediation delivery options.

Community and regulatory liaison

A community liaison plan was developed and implemented to manage local stakeholder concerns and support regulatory engagement during source control and remediation activities.

Remediation approach

Remediation comprised two complementary in situ strategies focused on (a) controlling and treating primary soil source zones and (b) treating downgradient groundwater in the fractured basalt aquifer.

Soil source treatment

Pilot soil treatments included injection of a polylactate polymer and a benign organosulfur compound formulated to provide a long‑term controlled release of reducing substrates. The intention was to stimulate reductive transformation of Cr6+ in the vadose and near‑saturated zones and to promote precipitation of trivalent chromium species.

Groundwater treatment

Groundwater pilot approaches included a zero‑valent iron (ZVI) plus carbon mixture with a sulfur component to support biotic reactions, and injections of emulsified vegetable oil (soybean oil) combined with an oxidant (sodium persulfate) where appropriate. The emulsified oil product was prepared as negatively charged micro‑droplets to maintain dispersion and adhesion to sediments, providing a carbon source for anaerobic bacteria for an estimated 3–5 year substrate release period.

Pilot performance and outcomes

• Reported pilot trial results indicated reductions in Cr6+ groundwater concentrations from up to 96 mg/L to below 0.002 mg/L in treated zones.
• Hydroxide solids (Cr(OH)3 and related phases) were reported to form during treatment and demonstrated apparent stability under the measured site redox conditions.
• While technically effective in the pilot, the cost of the ZVI + carbon approach was considered prohibitive at full scale for the impacted plume extent.

Commercial, compliance and development implications

• Liability and transaction risk: documented offsite groundwater impacts to private properties create material transaction and insurance risk for owners and prospective purchasers. Early assessment and a clear remediation pathway reduce transaction friction.
• Regulatory engagement: in situ approaches require careful design, consenting and performance monitoring to demonstrate long‑term stability and protect sensitive receptors.
• Cost vs benefit: pilot success does not automatically translate to feasible full‑scale solutions where plume volumes are large. A staged approach — source control, targeted hot‑spot treatment, monitored natural attenuation where appropriate, and remediations aligned to receptor risk — is typically most cost‑effective.
• Remediation delivery in fractured rock: fractured basalt complicates amendment distribution and monitoring; delivery techniques and monitoring networks must be tailored for preferential flow paths.

Practical takeaways

1. Prioritise source control to stop ongoing mass loading before large‑scale aquifer treatments.
2. Use pilot trials to test both geochemical efficacy and realistic delivery in fractured media; measure both Cr6+ and total Cr plus relevant geochemical parameters.
3. Balance remediation effectiveness with lifecycle cost — consider hybrid programs (targeted in situ treatment + monitored attenuation + institutional controls) where plume size makes blanket treatment uneconomic.
4. Maintain transparent community and regulator engagement to manage expectations and approvals.

Contact iEnvi

For pragmatic advice on contamination at industrial sites — including site assessments, pilot testing and remediation planning — contact iEnvi on 13000 43684 or via our contact page.