Decontamination

Overview

Flooding can create associated hazards, such as downed electrical lines or the presence of dead animals and faecal matter in the water and then left on the surface of the land after the water recedes contaminating its surroundings.

Where flooding has occurred near farmland, there is an increased risk of phosphates, salt deposits (efflorescence), nitrates and sulphates being present. All of these products can cause damage to the building and surrounding land.

Black-water contamination (sewage) will almost certainly result in absorbent materials such as carpets and upholstery being removed, as decontamination would not be cost effective unless the item was of particularly high value or of historic significance. The disaster recovery contractor will make an inventory of items removed and those that are considered beyond economic repair. This will go to the loss adjuster for disposal approval.

Before decontamination and cleaning can take place, there needs to be a selective removal of items or fittings that are considered to be beyond economic repair. In the case of insured buildings disposal of items will need to be approved by the loss adjusters.

Mud, dirt and debris

Mud, dirt and flood debris should be removed from all surfaces with fresh water as soon as possible after the floodwater has receded. Do not wait until the deposits have dried; it is safer and easier to remove the mud while it is still wet. Plastic shovels will cause less damage than metal ones.

Silt and mud will accumulate not only on the floor and furnishings but also in interior wall and floor cavities. Open electrical outlets, ducts and chases and rinse them thoroughly. Let these areas dry completely before closing them up again. Clean out any services ducts as well as drains, gullies and inspection chambers.

Salts (efflorescence)

Walls

As they recover from flood saturation, masonry walls can be damaged by inherent soluble salts. When masonry is saturated, the salts dissolve but when the water evaporates, the salts are carried nearer to the surface, where they crystallise and can appear as a powdery white residue called 'efflorescence'. Although unsightly, this is not usually harmful and can be brushed, vacuumed or washed away.

Problems arise if the salts are trapped behind a relatively impermeable coating, such as a water-repellent sealant, cement renders, or an oil-based or acrylic paint. The salts crystallise within the substrate and expand, pushing off the surface of the brick or stone in processes known as 'spalling' or 'exfoliation'.

Masonry flooded by sea water should be rinsed down several times with clean salt-free water as soon as possible after the flood has subsided and then allowed to dry out slowly so as to minimise the effects of chloride-salt damage to soft masonry.

Paving

Where stone, terracotta or tile paving laid on to bare earth appears to be suffering from salt migration following flooding, rake out the mortar joints and leave them open. These can be filled with clean dry sand to increase the surface area for drying and thus provide sites for crystallisation to take place. Later, re-point the joints with sacrificial, permeable, lime-based mortars to help wick the salts away from the historic paving and into the mortar. Periodically, the mortar may become saturated with salts and have to be replaced. However, this will be far less damaging and more cost effective than having to deal with damaged paving.

Do not seal the paving with wax, oil or polyurethane varnish as sealants will encourage salt sub-florescence and the breakdown, powdering or exfoliation of the paving slabs themselves, as well as the wicking of moisture into any surrounding permeable walls.