Heavy metal in suburban earthquake gardens

At my flat in Sydenham we have a respectful sized vegetable garden. OK – all that is growing there currently are weeds. But it has potential. The strawberry patch has been bountiful this season and produced fruits even till late autumn. We have two peach trees that provided us with big juicy peaches that we enjoyed throughout summer.

It is quite common for the New Zealand suburban home to come equipped with a vegetable garden and at least some sort of fruit tree. Growing produce has always been a part of my life growing up. However, research shows that the soil in your backyard may not be that safe.

The 2010 and 2011 earthquakes in Christchurch resulted in some 7000 homes being demolished, in what is called the ‘red zone’, leaving a 630 hectare space. All that remains are the roads, footpaths, trees and gardens of the people who once inhabited the area.

The area is not as overrun by weeds as you would think, because the city council has kept the lawns mown. Many insect and bird people, like myself, reckon they should’ve let the plants grow out and supportted the ecosystem that developed. But alas, Christchurch loves their immaculate gardens too much.

In the years following the the earthquakes and with the forming of the red zone it has been a hot topic of discussion as to who will own it and who will look after the area.  Some of the ideas of what to do with the red zone included making community gardens. But we may want to think twice before producing food on land that has been used for industrial and urban activities in the past.

A study done in Christchurch, New Zealand evaluated the levels of heavy metals in suburban garden soils, compared to soils in rural areas. Heavy metals (cadmium, lead, arsenic, copper, mercury, nickel and zinc) are often by-products of activities we humans do regularly. In excess amounts they are damaging to the environment and cause health issues.

These pollutants can get into the soil from humans carrying out many of our daily activities. Such things might be driving a car, constructing buildings, using fertilisers, using galvanised steel and the historic use of lead-based paints and fuels. Heavy metals (HM) soak into the soil and contaminate it. Urban soils often contain higher levels of heavy metals compared to soils in rural areas and older neighbourhoods were found to have higher HM levels.

The Christchurch residential Red Zone. Once this was all houses. Now it’s lawn with some old garden trees. CC: Jon Sullivan

The researchers, including Nick Dickinson, Nik Lehto and Johannes Welsch from Lincoln University, demonstrated in their article ‘Heavy metals in suburban gardens and the implications of land-use change following a major earthquake’ that often, HM levels in suburban Christchurch wereis above the residential land-use national standard (bit of a mouthful, I know). This essentially means the soil is often so contaminated that people should not be producing fruit and vegetables from it.

The authors recommended that instead of creating community gardens, trees and shrubs should be planted as they absorb heavy metals in the soil. Creating parklands and forests would reduce the amount of bare soil that would expose humans to heavy metals.

The red zone is now officially called the Ōtākaro Avon River Corridor and is being managed by Regenerate Christchurch. They have officially submitted a development plan which focuses on natural environment experiences and culture. The current plan that Regenerate Christchurch have submitted to the city council involves a 345 ha ‘green spine’ that runs 11 km along the banks of the Avon river, connecting the three reaches of the red zone. These areas are the Ōtākaro reach, the Horseshoe reach and the Eastern reaches.

The finished product, an estimated $830 million later, will boast ecological restoration sites, wetlands, cycle ways, nature paths and park areas. There will also be a focus on food and culture, ecotourism, water sports and other activities such as play grounds and sight seeing. There may also be a gondola and a small amount of housing. Based on the research done, this is a great way to utilise the land. It will also bring in revenue for the city.

Although it is true that the gardens in our Christchurch backyards contain heavy metals, most soils everywhere do as well. This research explained that yes there are heavy metals in our garden soils, but there is no need to tear up your vege patch or cut down your fejoa tree. In fact, planting more trees will help to reduce HM in the soil. Christchurch citizens are not experiencing ill effects from heavy metals in the soil more than other cities.

 Becky Clement is a postgraduate student in the Master of Science at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

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