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1. Overview of Iron ochre
Iron ochre can cause clogging of the subsurface (tile) drainage system. The clogging can occur in the pipe perforations, valleys of corrugations, or inside of the pipe. Clogging from iron ochre can lead to under-performance or failure of the subsurface drainage system. This bulletin describes the iron ochre issue and its mitigation methods.
2. Iron ochre occurrence
When oxygen in groundwater is depleted after prolonged ponding or saturation, bacteria convert the insoluble ferric iron (Fe3+) to soluble ferrous iron (Fe2+) in the presence of organic matter. When the soluble iron enters the pipe where there is oxygen, bacteria oxidize the soluble iron back to insoluble iron and create ochre. Iron ochre shows up as an orange-brown slimy filamentous deposit in and around the drain (Figure 1). Iron ochre may not occur in the entire drainage system. It can occur in only parts of the drainage system while other areas of the system are without any problem.
3. Iron ochre potential
The highest potential of iron ochre is in fine sand, silty sand, organic soils, organic pans, and mineral soils with mixed organic matter. Waterlogged depressional areas with high organic matter are also prone to iron ochre formation. Soils with the least potential for iron ochre are silty clays, clay loam, and clay soils.
4. Iron ochre identification
To identify the potential for iron ochre, measure soluble ferrous iron (Fe2+) concentration in the shallow groundwater. With an excavator or auger, dig a hole about 3 to 5 ft deep and take a sample of the groundwater. It is better if the water table is less than 3-ft deep. Use an iron indicator test strip to measure ferrous iron. See Table 1 for the risk of iron ochre based on ferrous iron concentration. Analyzing a soil sample for total iron is useless because it includes iron forms that are insoluble and may not cause iron ochre problems. Iron alone does not cause iron ochre; the presence of bacteria generates ochre. Another simple indicator of iron ochre potential is observing orange-brown slime in the drainage ditch.
Table 1. Iron ochre potential based on an iron indicator test strip.
|
Iron ochre potential |
Ferrous iron (Fe2+) concentration in test strips (mg/L) |
|
Very high |
>25 |
|
High |
10-25 |
|
Moderate |
5-10 |
|
Low |
1-5 |
|
Negligible |
<1 |
5. Temporary or permanent issue?
5.1. Temporary issue
When the source of iron is local and there are no extensive quantities of iron in the soil, iron ochre can be a temporary issue. In this case, use mole drains to leach the soluble iron from the soil. For more information about mole drains, see Ghane (2022b). The iron leaching usually peaks over one to three years after installation, followed by a gradual decline of up to about eight years. After the iron leaching has declined, install subsurface drainage.
5.2. Permanent issue
If the iron is moving with groundwater from another source upstream or there are extensive quantities of iron in the soil, ochre can be a permanent issue. Fields having a permanent iron ochre problem should not be subsurface drained without a plan for continuous maintenance and special design considerations.
6. Iron ochre mitigation methods
6.1. Increase soil aeration
Soil aeration prevents the iron from becoming soluble in groundwater. If iron remains insoluble, it cannot move toward the drain. Anything that improves soil aeration minimizes prolonged saturated or ponding conditions. If a plow-pan or compacted layer is preventing soil aeration, break it up with subsoiling or moling to improve water movement to the drains, thereby improving soil aeration. To prevent the recurrence of plow-pan or compacted layers, improve soil structure with reduced tillage, cover crops, manure or compost, and diverse rotations to enhance water movement in the soil profile. Those practices minimize waterlogging conditions, thereby improving soil aeration.
6.2. Use pipes with large circular perforations
Use pipes with large circular perforations (0.2 to 0.5 inches) to extend the time before they get clogged (Bryant and Shaw, 1988). Pipes with slotted perforations present the greatest risk of clogging and limit the effectiveness of jet flushing (Stuyt et al., 2005). This is because ochre bacteria bridge the narrower perforation width more quickly.
6.3. Use gravel or knitted-sock envelope
Soils with low clay and organic matter may need an envelope because soil particles do not stick together (for example loamy sand, sandy loam, loam, silt loam), thereby causing sediment-clogged pipes. In a sandy soil with drain sedimentation and ochre problems, it is best to use a graded gravel envelope to minimize ochre clogging. To determine if sedimentation is a problem for your drain pipes, see Ghane (2022a). If using a gravel envelope is unfeasible, use a muck pipe wrapped with a knitted-sock envelope instead of a sand-slot pipe. Synthetic thin envelopes perform better than sand-slot pipes with iron ochre. Among all synthetic envelopes, a knitted-sock envelope performs better with iron ochre.
6.4. Use steeper lateral pipe grade
When designing the drainage system, use steeper lateral pipe grades (within the allowable limit) to promote self-cleaning and low-pressure cleaning (Section 7.1). With a steeper lateral grade, water moves faster and has more force to flush the ochre.
6.5. Run shorter laterals into the ditch
Run lateral pipes into the ditch in the ochre-prone area instead of connecting them to a main pipe. In this case, if one lateral pipe clogs, the entire drainage system does not clog. High-pressure cleaning from the outlet is also easier if lateral pipes flow into the ditch (Section 7.2). Also, install shorter lateral pipes for easier jet flushing from the outlet.
6.6. Use woodchips
Backfilling around the pipes with tannin- containing woodchips (such as oak, spruce, pine, cypress, conifer bark) has been shown to adsorb the soluble iron (Fe2+) and remove it from water. Also, these tannin-containing woodchips appear to be toxic to ochre-producing bacteria, so they reduce ochre clogging.
Besides backfilling woodchips around the pipes, you can also fill mesh bags with woodchips and place the bags in access facilities (control structures or manholes) along the main pipe. Water in the drainage system is forced to go through the bags for the removal of soluble iron. The woodchip bags will need to be replaced regularly because woodchips adsorb the iron and turn blue over time. This method has been successful in Scotland.
6.7. Eliminate the oxygen supply
To reduce ochre formation, eliminate the oxygen supply by keeping the outlet submerged and install deep drain pipes. Keep water in the pipes throughout the year by maintaining a raised weir in a control structure at the system outlet, thereby minimizing the oxygen supply to the pipes. This practice is known as controlled drainage. Keeping the pipes submerged is easier when deep drain pipes are installed from 4- to 5-ft depth. Installing deep drain pipes can reduce iron ochre because pipes are as far away as possible from the aerated zone, so water at the drain depth has less oxygen to supply to the bacteria. In the case of a sump pump, the float switch needs to be set higher so that the main and lateral drains are always underwater.
7. Iron ochre removal methods
7.1. Use low-pressure cleaning
Low-pressure cleaning is useful when the ochre is fresh. To perform low-pressure cleaning, follow these steps. First, use a manhole or a control structure at the system outlet to prevent the drainage system from flowing temporarily. Second, pump water into the drainage system either from the system outlet or from an access facility (manhole or vent) on the uphill end of the pipe. Third, after the pipes are full of water, open the outlet to release water quickly and flush the ochre. This approach is useful with steeper lateral grades, so water moves faster to flush the ochre.
7.2. Use high-pressure cleaning
High-pressure cleaning or jet flushing can remove iron ochre, if it is done early enough, that is typically during the first year after installation. Afterward, clean the pipes regularly rather than waiting for the drains to clog. In the absence of regular jet flushing, ochre will harden and become difficult to remove.
High-pressure cleaning can be done from the outlet to loosen the ochre, so it flows out with water. This approach is useful when shorter lateral pipes run into the ditch. If connecting laterals to a main pipe, install access facilities (manhole or vent) on the uphill end of the laterals and use wye connections in the ochre-prone area for better jetting. If access facilities are not available, dig up lateral-main connections for jetting. Contact a local sewer, plumbing, or septic tank company for jet flushing (Figure 2).
8. Summary and recommendations
Iron ochre can result in under-performance or failure of the drainage system. An Iron ochre problem occurs mostly in sandy and muck soils. There is no simple solution to fully address an iron ochre problem. First, identify the risk of iron ochre. An iron indicator test strip is a simple method for identifying the potential for iron ochre. If the risk of ochre is high, perform mitigation methods along with continuous maintenance and special design considerations. Some of the mitigation and removal methods have limitations and may not fully address the ochre issue. Therefore, use a suite of methods to reduce iron ochre clogging.
