Chris Herries1, Jon Turner2
1 Regional Manager, Horticentre Hawke’s Bay
2 John Turner Consultancy Ltd
Chris.Herries@horticentre.co.nz
Straight away once the water disappeared there was concerns being voiced about the possible toxic effects of the silt, including what possible toxins could be contained in the silt, and what the long term affects the silt may have on the soil. Soil tests were taken straight away following the flooding from four of the major flooded area’s being Esk Valley, Dartmoor Valley, Twyford and Pakowhai. Analysis included looking at heavy metals and agrichemical contamination. Soil pH and nutrient values were also measured. The impact of these results on the parent soil material was assessed.
The ‘bush telegraph’ started immediately following the cyclone, with growers expressing various concerns about the silt as assistance was being provided to affected properties. The response effort drew on prior experience with flooding, including lessons from Cyclone Bola in 1988, which had caused severe flooding in the Gisborne region between Paratai and Manutuke.
Common concerns voiced by growers included:
- The silt was toxic and would poison the ground, necessitating its removal
- Septic tanks had overflowed, contaminating the silt
- Spray sheds had been washed away, potentially contaminating the silt
- The silt had sealed off the original topsoil, and tree roots would die from lack of oxygen
- The silt was full of heavy metals and carcinogens
Without data to confirm or refute these concerns, rapid sampling was initiated. This presented considerable logistical challenges, as the work commenced just five days after the cyclone when roads were still being cleared and laboratory access was limited. Opening roads, particularly from Hastings through to Esk Valley, was essential before sampling could be conducted.
Despite initial fears about toxic contamination, comprehensive testing revealed that the deposited silt, whilst challenging to manage, did not contain dangerous levels of heavy metals or agrichemical contaminants. All detected heavy metal levels fell within normal ranges for New Zealand soils, and no agrichemical contamination was detected despite testing for 487 different chemicals.
The silt’s characteristics varied considerably depending on location, with sandy deposits near river sources that drained quickly, transitioning to fine silty clay material in areas such as Pakowhai where drainage was slower and anaerobic conditions developed. The nutritional profile of the silt reflected its composition as predominantly subsoil material, with high calcium content, elevated pH, and low phosphorus levels.
The industry’s response, including the rapid testing programme and the organised collection and disposal of displaced agrichemicals, demonstrated effective collaboration and commitment to safety. The successful recovery of affected properties to commercial production, achieved through intensive manual labour by growers, represents a significant achievement in the face of substantial challenges. The experience has provided valuable data and insights that will inform future responses to similar events.
Appreciation is extended to Jon Turner for his inputs and expertise drawn from previous flood response work in different parts of the country. Recognition is also given to all growers for their extraordinary efforts in recovery and remediation, and to Hill Laboratories for expediting sample analysis during the critical initial response period.