The Christchurch Residential Red Zone

The Canterbury Earthquakes and the Red Zone

Starting on the 4th of September 2010, the Canterbury Region experienced a series of earthquakes with a magnitude 7.1 tremor. While this event caused infrastructure damage, the magnitude 6.3 aftershock on 22 February 2011 proved catastrophic. Striking shallowly and close to the Christchurch Central Business District (CBD), it generated violent ground shaking and resulted in 185 fatalities.

The earthquakes caused unprecedented land damage, particularly in the eastern suburbs and the Port Hills. Extensive liquefaction (where soil behaves like liquid) and lateral spreading rendered large areas of land unstable.

In June 2011, the Government announced a zoning system to guide the recovery. The "Red Zone" (RZ) designated land where remediation was uneconomic and rebuilding infeasible. The government implemented a buyout scheme for property owners in these areas. This process led to the demolition and removal of more than 8,000 residential properties across approximately 630 hectares. Entire neighbourhoods ceased to exist as residential areas.

Soil Contamination

Soil analysis in the Red Zone identified elevated concentrations of heavy metals, including lead (Pb), arsenic (As), and coal tar, a source of polycyclic aromatic hydrocarbons (PAHs) that present a threat to human health and ecosystem function. Some 20% of Red Zone soil samples exceed New Zealand safety guidelines for Pb.

Why Not Conventional Remediation?

Remediation using established techniques such as excavation or capping is infeasible for the Red Zone:

Excavation ("Dig and Dump")

Excavating the top 500 mm of soil across the 630-hectare Red Zone yields over 3 million cubic metres of waste. This volume exceeds local landfill capacity and requires extensive transport logistics, generating high carbon emissions.

Capping

Alternative remediation via capping (importing 0.5 m to 1.0 m of clean soil) creates hydrological risk. Raising ground levels eliminates the flood retention capacity of the Otakaro Avon River Corridor, removing a primary defence against future city flooding.

Phytomanagement: A Nature-Based Solution

Phytomanagement is an alternative remediation strategy that uses plants and soil conditioners to simultaneously mitigate environmental risks and generate economic or ecological value. Unlike excavation or capping, phytomanagement is far less costly and minimises land disturbance.

For the Christchurch Red Zone, phytomanagement aligns with future land-use goals by establishing native vegetation that enhances biodiversity, creates green corridors, and potentially offers economic returns through products like essential oils or honey, while managing soil toxicity.

The Field Trial

A field trial site has been established within the Residential Red Zone. This experiment evaluates the performance of specific vegetation and soil conditioners in managing heavy metal contamination, particularly reducing contaminant leaching into the groundwater or entering the food chain. The vegetation cover physically stabilises the surface, mitigating wind erosion and the subsequent inhalation of metal-laden dust.

Soil Amendments from Municipal Waste

The trial uses soil amendments derived from Christchurch's municipal waste stream:

• Compost — Produced from the city's kerbside green waste collection, it introduces high levels of organic matter containing humic and fulvic acids, which immobilise contaminants. Compost boosts nutrients and soil structure, and supports beneficial microbes.
• Biochar — A carbon-rich product created through the pyrolysis of organic waste. Its porous structure provides a vast surface area for adsorption. The addition of biochar increases soil pH, causing metals to precipitate out of the soil solution and become immobile. Biochar also improves water retention and provides long-term carbon storage.
• Fishery Waste — Provides nitrogen, phosphorus, and potassium (NPK) and enhances plant growth.

Plant Species

• Phormium tenax (Harakeke) — Native New Zealand flax used for metal stabilisation.
• Solanum laciniatum (Poroporo) — A native plant trialled for phytoremediation.
• Beta vulgaris (Fodder Beet) — Evaluated for metal accumulation capacity.

Anthropic Soil

The soil profile from the field trial site reveals an anthropogenic soil, containing anthropogenic objects such as brick, concrete, glass, and timber fragments throughout the profile. These artefacts are remnants of the demolished residential properties. The soil is low in soil organic carbon, reflecting its degraded condition. The dark surface layer is biochar, applied as a soil amendment to immobilise heavy metals and improve soil structure.

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