Iron ochre results from a chemical reaction or a biological process, and both of these causes may occur either individually or simultaneously:
Chemical reaction: When the soil contains iron, it migrates along with the water towards the drainage system. When it comes in contact with the atmosphere, it forms a sludge of iron hydroxide.
Biological process: When iron bacterium is present within the water table, this bacterium, following oxidation of the iron upon coming into contact with the atmosphere, produces a gel-like mass.
When the biological process adds up to the chemical reaction, the oxidation effect is considerably increased.
Both cases result in the formation of a viscous deposit on the corrugated wall of drain pipes. This deposit is what we call iron ochre.
What conditions give rise to iron ochre?
This substance is more likely to develop in a more aerated soil, such as the sandy soils containing silt, and will less likely develop in clay soils.
The chemical or biological reaction is also quicker:
where soil tends to be neutral (pH of approximately 7);
when the wall of the drain is corrugated;
when the openings of the drain are punched instead of cut out, which creates
attachment zones for the deposits;
when the drain is wrapped in a filtering membrane.
Under the action of iron ochre, even crushed gravel loses its filtering properties. The soils rich in iron and in iron bacteria will produce ferrous deposits indefinitely.
What consequences has the presence of iron ochre?
Iron ochre causes several issues with buildings:
It causes ocher-colored slimy sludge deposits to accumulate in the rain water catch basin and in ditches.
It creates reddish deposits on the concrete slab of basements. These deposits give a sulfur-like odour.
It obstructs agricultural tile drains: since runoff water is no longer channelled at a distance from the foundations, it infiltrates the basement, at the junction between foundation walls and slab.
It obstructs the backflow valves.
Can one eliminate iron ochre or prevent this bacterial growth?
Iron is naturally present in the soil. Thus, it is very difficult to eliminate altogether. Even if one would replace the soil surrounding a building, iron originating from the neighbouring soils would still migrate towards the drain of the property through natural water runoff.
To prevent bacterial growth, one would need to eliminate from the soil two natural elements - water and air - which is impossible.
It is therefore not feasible to prevent the chemical reaction giving rise to iron ochre.
How can we prevent iron ochre deposits, and how can we remedy it when present?
It is possible to prevent iron ochre deposits, either before or after the construction.
Before the construction
One must keep vigilant when tell-tale signs appear such as the presence of reddish waters, after a rainfall, in ditches or on the ground surface.
During the construction of a property, it is recommended to enquire, with the neighbours and the municipality, about any history of iron ochre formations in the area.
A professional evaluation of the current conditions should also be made before proceeding with the construction. This evaluation should determine:
The type of soil;
The pH of the soil;
The presence of iron or iron bacteria in the soil;
The annual variation of the level of the water table.
The results of this evaluation will enable the contractor to adapt his building strategy in order to avoid the effects of iron ochre formations.
Such adaptations may be as follows:
Location of the basement concrete slab above the water table level;
Selection of the weeping tile (diameter, size and locations of openings, material);
Location of the geomembrane;
Type of granular backfill.
If the location of the construction site remains at risk, it is recommended to fit up two vertical cleaning conduits to enable the cleaning-up of the drain as needed.
After the construction
Keep vigilant in the presence of tell-tales signs such as a high humidity level in the basement, a recent flood event, or the presence of reddish waters in ditches or on ground surfaces after a rainfall.
The possible remedial actions with existing buildings in order to alleviate the symptoms of iron ochre deposits will vary with the locations. Such actions may require:
A change to the grade;
The sealing of the junction between concrete walls and slab;
The installation of vertical cleaning conduits;
The draining system under the slab;
The installation of a waterproofing coating;
The installation of a watertight membrane;
The uplifting of the basement slab;
Condemning the basement.
Example of remedial action from the outside
Example of remedial action from the inside
What are your rights of recourse?
The Régie du bâtiment du Québec (RBQ) recommends to the purchasers of a new house as well as to home builders to verify with the municipality if the location of the lot is subject to the formation of iron ochre deposits. If such is the case, some prevention measures will be called for.
The RBQ wishes to stress that one cannot prevent the process leading to iron ochre formation. We recommend to the owners confronted with this problem to refer to a professional or a contractor holding an appropriate licence from the RBQ, who will help them avoid or at least minimize the adverse effects of such environmental condition.