Thursday, August 09, 2012 1:06 PM

For all farmers in Waikato, Bay of Plenty, Rotorua-Taupo, Taranaki, Ruapehu, Tararua, Wanganui, Gisborne-Wairoa, Manawatu-Rangitikei, Hawke’s Bay and Wairarapa

Initial analysis of ash confirms no current human-health or agricultural threat beyond the immediate vicinity of the volcano

Initial analysis of ash produced from the 6 August 2012 eruption from Mt. Tongariro has shown moderate levels of soluble Fluorine (F).

The measurements were made by Massey University’s Volcanic Risk Solutions and the Fertiliser and Lime Research Centre, together with University of Canterbury researchers on ash samples collected near the volcano beside State Highways 46 and 1, along with one sample collected from Gisborne.

Concentrations between 20 and 70 mgF/kg of ash were measured, which is in a similar range to the widespread volcanic ash produced during the 1995 and 1996 eruptions of Ruapehu. Due to the restricted distribution and very thin ash fall, this currently poses no current human-health or agricultural threat beyond the immediate vicinity of the volcano.

In addition, heavy rainfall since the eruption has removed much of the ash and associated contaminants.
If future, larger eruptions of Mt. Tongariro produce ash with similar concentrations of Fluorine, a significant agricultural hazard can be anticipated:

The impact of ashfall on pastoral grazing systems is the covering of pasture, meaning it is ingested by grazing sheep, cattle and deer. Further, livestock drinking water in open troughs may be contaminated. Additionally, rural dwellers with roof-catchment drinking water sources should be vigilant to avoid ash runoff into water tanks. During and immediately following ashfall, the intake pipe to water tanks should be disconnected until ash has washed off the roof with rain. 

In grazed pastoral systems, following ash cover, some livestock will be put-off grazing due to high levels of acidic and abrasive ash, while others will continue to graze. If supplementary feed is unavailable, this may lead to starvation of stock, especially pregnant or lambing/calving stock facing high energy demands at this time of year. If significant ash is ingested along with pasture, livestock are also at risk of the disease fluorosis.

Experience from the 1995/1996 eruptions has shown that ash coverings of >2 mm, low-grazed pastures, and low rainfall following ash deposition are critical factors increasing hazard. Deaths of stock normally begin 4-10 days after ashfall if no supplementary feed is available. Heavy or persistent rainfall quickly disperses such levels of ash and also rapidly leaches the F; reducing the hazard considerably. 

In general Deer are likely to be the most susceptible to Fluorosis, followed by cattle, with sheep being the most resistant. Fluoride is adsorbed rapidly by grazing animals from ingested ash or contaminated water. In moderate levels of excess, it does not pass into milk. 

Latest mitigation advice to farmers in the event of future ashfall is as follows:

If ashfall exceeds 2 mm or coats >50 percent of pasture/feed crops, either move stock to less affected areas of the farm or supply supplementary feed

In these situations refill stock drinking troughs from bore or river supplies.

If ash has not washed off pastures after 2-3 days, raise the quantity of supplementary feed and monitor stock condition closely

In general, to reduce impacts from ashfall, maintain pasture length by regular rotation rather than close cropping. Longer pastures are less likely to be completely covered.

For rural residents with roof-fed tank water supplies:

Temporarily remove downpipe connections to water tanks during and following ashfall to protect stored water

If possible, wait until rainfall clears ash from roofs before reconnecting intake

Water affected by ash will appear turbid (cloudy) and may have a bitter taste. If this is the case, avoid drinking this water and seek alternative supplies.

For more information:

Professor Shane Cronin of Volcanic Risk Solutions (021 228 9882 or 06 356 9099 x7207)

Professor Mike Hedley of the Fertiliser and Lime Research Centre (06 356 9099 x7419)

Dr Thomas Wilson, Natural Hazards Research Centre, Geological Sciences, University of Canterbury, 021 434 596