09 January 2017

Are we helping or harming when we 'clean' oiled mangroves?

It is heartbreaking to see oil land on mangrove trees. The trees breathe air through pores on their roots and trunk and oil can seal these pores making it hard for the trees to breathe. Oil is also toxic. Fortunately, "damage to Chek Jawa's ecosystem in Pulau Ubin following last week's oil spill has, so far, been minimal, said the National Parks Board (NParks)." reported the Straits Times on 7 Jan (Sat).
Oil on mangrove trees at Sungei Puaka
which was not boomed until 4 days after the oil spill.
Photo by Rachel Quek.
Do we do more harm than good when we try to 'clean' the oil from mangrove trees? Here's what the US NOAA (National Oceanic and Atmospheric Administration) says.

The following text are extracts taken from Oil Spills in Mangroves: Planning and Response Considerations by the US NOAA (National Oceanic and Atmospheric Administration).

Hat tip to Ivan Kwan for the link. Photos of workers and volunteers cleaning mangrove trees at Chek Jawa on 7 Jan (Fri) by Mohammad Juhari.

Cleanup Options for Oiled Mangroves

If mangrove shorelines are oiled, extreme caution must be exercised in selecting cleanup activities. Potential benefits of oil removal must be weighed against the risks of potential additional harmful impacts from the cleanup technique.

Natural Recovery

There are several circumstances under which it is appropriate to do nothing. The foremost of these situations is when cleanup would cause more harm than benefit to mangroves or other associated habitats, or when shorelines are inaccessible.

When no cleanup is conducted, oil will slowly degrade and be removed naturally, assisted by natural and storm-generated flushing. (See Era spill case study, Chapter 5).

Cleanup also is not recommended for small accumulations of oil, regardless of product type. Impacts caused by light accumulations generally do not warrant the tradeoffs associated with cleanup activity.

Even for major spills, there may be cases for which it is best to rely on natural recovery, depending on the nature of the oiling and the characteristics of the mangrove forest affected.

Generally, cleanup should not be conducted in interior areas of mangrove forests because of the risk of damaging mangrove roots and seedlings, trampling oil into the sediment where it will degrade much more slowly, and spreading oil into previously unoiled areas.

Exceptions may be made if access is possible from upland areas or if vegetation is sparse enough to permit access without injury to pneumatophores and prop roots.

If cleanup is attempted in interior mangroves, experienced personnel must constantly oversee cleanup crews to prevent further injury.

In any case, attempts should be made to control the movement and spread of any mobile oil within the mangroves to prevent contamination of adjacent areas.
Photo of clean up at Chek Jawa mangroves by Mohammad Juhari.

Manual Oil Removal

Manual removal, using hand tools and manual labor, is often conducted to remove bulk oiling by heavier oils, such as crude oil or intermediate fuel oils, stranded in mangroves.

Manual removal can help prevent other areas from becoming contaminated as the oil moves around, and helps limit longterm sediment contamination.
Photo of clean up at Chek Jawa mangroves by Mohammad Juhari.

Consideration should be given, however, to the trade-off between these benefits of manual removal and the damage to the mangroves that often accompanies manual cleanup.

It is nearly impossible to reach the tangle of prop roots and pneumatophores of most mangroves without causing physical damage.
Photo of clean up at Chek Jawa mangroves by Mohammad Juhari.

Trampling of oil deeper into the sediment from foot traffic can be another harmful consequence of manual cleanup.
Photo of clean up at Chek Jawa mangroves by Mohammad Juhari.

Garrity and Levings (1996) observed that black mangrove pneumatophores along paths used by cleanup workers were significantly more likely to be killed than those in areas accessed by one or a few workers.

Where pneumatophores had been dense at the time of the spill, paths often were bare substrate by 15 months post-spill as broken pneumatophores died and rotted away. (See Bahía las Minas case study.)

If manual removal is conducted in mangroves, and particularly in interior areas, consideration should be given to ways to minimize foot traffic and other impacts.

Conducting activities from boats, when possible, is advisable. Close supervision of cleanup crews is essential.
Photo of clean up at Chek Jawa mangroves by Mohammad Juhari.

Passive Collection with Sorbents

Even when natural recovery is the selected option, sorbents are often deployed to recover any oil released from the area. Sorbents are composed of materials that either adsorb oil on the surface or absorb oil into the pores of the material. There are many types: natural organic substance (e.g., peat, wood, cotton, straw, shredded sugarcane process residue called “bagasse”), synthetic organic substance (e.g., polypropylene, polyurethane), inorganic mineral substance (e.g., clay, vermiculite, diatomite), or a mixture of the three. The material may also be treated with oleophilic (oil-loving) or hydrophobic (water-hating) compounds to improve performance. They come in various forms: round sausage “boom,” snare, sweeps, pads, rolls, loose particulates, pillows, and socks.

Sorbents have been used to wipe heavy oil coating from mangrove surfaces. Before using sorbents in this way, consideration should be given to associated physical damage. This activity is best conducted under close supervision and only in areas where substrate is firm enough to support foot traffic and prevent mixing of oil into the sediments.
Photo of clean up at Chek Jawa mangroves by Mohammad Juhari.

Removal of Oiled Wrack (seaweeds) and Debris

Heavily oiled wrack and debris should be removed if it can be done without significantly damaging prop roots, pneumatophores, and seedlings or trampling oil into the sediment.

However, oiled wrack should not be removed until the threat of oiling has passed, since wrack and leaf litter can act as a sort of natural barrier sorbent and actually protect the trees from direct oil contact.
Photo of clean up at Chek Jawa mangroves by Mohammad Juhari.
Unoiled and lightly oiled wrack and leaf litter should not be removed because they provide habitat and contribute to the ecosystem.

Inappropriate Response Techniques for Mangroves

Under no circumstances should live mangrove vegetation be cut or burned. Both techniques will destroy trees and mangrove habitat.

Mangrove trees are slow-growing and take decades to reach a mature stage. The loss of a large number of trees may compromise the forest structure, making it unlikely to recover naturally.

Other cleanup techniques used at some oil spills but inappropriate in mangroves include mechanical oil removal, high-pressure or hot-water flushing, steam-cleaning, slurry sand blasting, trenching, and sediment reworking, tilling, or removal.

All these methods would severely damage or destroy mangrove forests and associated organisms and habitats. All of these techniques may also cause or contribute to severe erosion.


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