Recent testing of sediment and silt deposited in and around Wairoa by Cyclone Gabrielle has revealed most flood sediment is not hazardous.

However, residents are still urged to use good hygiene practices when dealing with silt as there is a risk of potential for pathogen infection.

Wairoa District Council staff collected silt samples on February 24, 10 days after the cyclone, to gather information on contamination risks and to enable assessment and decisions on public safety advice and on silt disposal.

Sediment was sampled directly from undisturbed areas of flood deposits at three sites to represent three different types of urban environments that had been inundated: a petrol station (Mobil Wairoa), some front yards in a residential area (Ruataniwha Road), and the Destination playground at Alexandra Park.

All samples collected were found to be safe except for two samples which indicated the potential for pathogen infection. The concern for pathogen risks and infections after flooding is typical and expected and has been part of Council’s guidance issued during the clean-up phase. A pathogen is defined as an organism that can cause disease and comprise viruses and bacteria.

The testing has confirmed that the silt does not need to be contained, go to the landfill or be treated as hazardous waste. However, the presence of pathogens, such as bacteria and viruses needed to be managed. While immediately after flooding the levels will be high, they will naturally reduce over time.

Wairoa District Council Chief Executive Kitea Tipuna said while there is some pathogen risk, it does not extend to all areas affected by silt. The silt tested from Alexandra Park was found to be safe for children to play without risks of infection or any long-term health issues.

The silt can be used as ‘ordinary soil’, but people need to be careful to ensure that infection risks of fresh silt deposits are managed by using good hygiene, such as wearing gloves, avoiding breathing in dust, and washing hands and exposed skin frequently. Freshly, soiled clothing should be washed regularly, but no extra precautions such as disinfectants are needed for the washing process to be effective; ordinary detergent will do the job well enough. The longer the soil has been exposed to the air and sunshine, the lesser the risk and the lesser the need for hygiene precautions.

Further testing might be needed to show the risk has diminished.

Council advises that fresh silt is not safe for incorporating into vegetable gardens immediately, with the need to wait until a period that allows pathogen levels to decrease. However, it can be stockpiled for a few months, and in spring it could be incorporated into vegetable gardens for summer harvest crops.

Photo credit: NZ Herald

9 May 2023

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