Tactic CR-8: Flushing
Flushing with clean water is used to mobilize oil from ice (Fig. 46), vegetation, and the tundra surface. If necessary, gently agitate and compress the tundra surface with a rubber squeegee (Tactic CR-2) while directing water flow with the discharge hose. Agitation is most useful in wet tundra where the organic mat is relatively thick and resistant to erosion.
Flushing typically adds and removes water continuously. Keep water pressure and flow rate low enough to minimize erosion. Flush toward a collection area, such as a natural depression or a trench (Tactic CR-9) lined with plastic sheeting, where the oil can be recovered with direct suction (Tactic CR-6) or sorbents (CR-1). A land barrier (Tactic CR-5) is typically needed to contain fluids.
Flushing and flooding (Tactic CR-7) are similar approaches. The potential for erosion is the primary factor to assess when choosing which of these two tactics to use. Use flushing when the potential for erosion is low; use flooding when the potential for erosion is moderate or higher.
Water may be obtained from a nearby tundra pond or creek, or transported to the site in trucks with cleaned tanks. Do not use seawater or produced water to flush tundra vegetation. Flushing water must be contained using land barriers (Tactic CR-5). In summer, flush with cold or warm water. Hotter water will be needed during winter to allow recovery before the water freezes. In winter, water can be hauled to the site in heated or insulated tanks. Snow melters can generate very hot water (up to 180°F), and may be the best choice during winter at remote sites with no road access, if the volume of water produced is sufficient.
Surfactants reduce adhesion of crude oil and fuels to vegetation by increasing the ability of water to mix with hydrocarbons. Flushing with surfactants is appropriate for final cleanup of hydrocarbon spills after most of the spilled product has been removed (Fig. 47). Dawn™ detergent is the recommended surfactant because it is not toxic to soil microbes at concentrations used during flushing (Jorgenson and Cater 1992a); it is commonly used for cleaning oiled wildlife because of its effectiveness and low toxicity (Hemenway 1990); and it is readily available. Apply Dawn™ at a 0.1% (by volume) concentration. Surfactants also decrease the ability of sorbent pads, booms, and skimmers to recover hydrocarbons, and should only be used after these methods are no longer needed.
Surfactants can be mixed with water in tanks, or added to the stream of water flowing out of the input hoses. Most sites should be divided into several cells that are small enough to manage efficiently (Fig. 48).
Avoid thawing of frozen soil to the extent possible, to minimize infiltration of contaminants into the rooting zone, and exposure of dormant vegetation to freeze-thaw cycles.
Considerations and Limitations
- Flush as few times as possible, to minimize physical damage to vegetation.
- Move the input hose periodically to minimize erosion.
- Surfactants are not effective for removing substances that mix with water (e.g., salts, glycol).
- Insulated water tanks lose heat at the rate of approximately 10°F every 12 hours.
- Ensure that land barriers (Tactic CR-5) are strong enough to contain water in the area being flushed, and that the seal with the tundra surface will not leak.
- If ice berms are used as the land barrier, hot water may cause the berm to fail.
- Skimmers and sorbents will not be effective after surfactants have been applied to the site.
- Protect tundra being flushed by walking on plywood boardwalks, sandbags, rig mats, etc.
- Ensure water is free of hydrocarbons and salts before using it to flush tundra.
- Flushing is feasible during winter, but precautions for worker safety are necessary. Flushing may not be practical at extremely cold temperatures.
- This tactic has been adapted from Tactic R-4 in the Alaska Clean Seas Technical Manual (http://www.alaskacleanseas.org/tech-manual).
Equipment, Materials, and Personnel
- Water truck (1 operator), tank, tundra pond or stream – to provide water source.
- Clean water (not seawater or produced water)
- Surfactant (Dawn™ detergent) – to enhance recovery of spill residue.
- Trash pump (1 to 2 operators each) – to pump water to and from site.
- Suction hose (1 operator) – to take up water from water source.
- Discharge hose (3- to 6-inch) with adjustable valve (1 operator) – to discharge water on site.
- Mop, squeegee (1 operator) – to agitate and gently compress tundra mat to release spill residue.
- Land barriers (Tactic CR-3) (number of people needed is site-dependent) – to contain water on site and to provide collection point.
- Plywood, sandbags or rig mats – to prevent trampling.