Pass the silt: Your questions on liquefaction answered

This information has been prepared by the Department of Soil and Physical Sciences,  Lincoln University,  to answer frequently-asked questions about the tonnes of silt that appeared around Christchurch after the February 22 earthquake.

Many people have had sand and water in their gardens as a result of the liquefaction effects of the earthquake.  Often the sand will not appear to be contaminated – there is no discolouration other than grey, and no nasty smells.

  • Why is the sand grey?
    1. Most of the sediment extruded by the liquefaction process ranged from fine sand to silt with very little clay present. Some deposits were more sandy than others. The sediment is typically grey in colour having resided in a waterlogged, anoxic (oxygen-free) environment.
    2. If the sand had been mixed with sewage, it will have a very strong odour; if so, appropriate health precautions need to be taken.
  • I want to get back to some gardening. Can I still eat the vegetables in the ground like carrots and leeks?
    Unless the garden was sited over an old landfill the material coming up will not have any dangerous contamination other than possibly sewage.
  • Do I need to cook the vegetables first or can I eat them raw?
    1. Potatoes/ parsnips/leeks cleaned of soil and boiled will be completely safe.
    2. Carrots if washed thoroughly in clean water and peeled would also be a very low risk, but boiling would remove any risk.
  • What about tomatoes, herbs and salad plants?
    Herbs, tomatoes and leafy salad plants that have no soil on them are low risk, but should be washed in clean water.
  • Can I dig the sand into my garden or do I have to replace it?
    1. The ejected liquefaction material is of low nutrient status (according to conventional soil tests) but benign. The only risk is if it has high salt level. In river-deposited sand this is very unlikely. In areas close to the Avon-Heathcote Estuary a risk of salinity exists. A simple electrical conductivity test could be carried out.
    2. If salinity is not an issue, the material can be incorporated into the existing soil. A depth of 4-5 cm maximum of ejected sand should be incorporated into the top 20 cm of soil.  More could be incorporated if it was dug into a greater depth.
    3. It is not a good idea to leave the material on the soil surface as more than 1 – 2 cm thick (half an inch) may limit gas exchange into the soil and therefore affect plants, especially if they are covered by this material. In situations where the layer of sand is thicker than 1 – 2 cm it should be removed or dug into the soil.  If the sand is left on the soil surface it can lead to elevated surface temperatures which may have a localised effect on low-growing plants and grass.
    4. The material may well increase readily-available water depending on its particle size characteristics and may also improve soil structure and drainage if dug into clay rich soils.
    5. Over time, this sand would make a good garden soil with additions of compost.  But as a sand it lacks structure (i.e. single grains of fine sand and silt) and will tend to pack into a tight solid with little pore space.
  • My property was flooded and has a lot of sand from liquefaction.  Does the timber used in raised gardens need to be replaced?
    The timber in the raised gardens does not need to be replaced on account of it having been saturated by flood waters. Human pathogens in the flood water will not remain viable long in the wood or the soil.

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