Tactic CR-7: Flooding
The use of flooding with clean water depends on the nature of the spilled substance:
- Crude Oil and Diesel: Flooding raises or maintains the water level on the tundra surface, reducing the contact of oil with vegetation and making the use of skimmers (Tactic CR-6) or sorbents (Tactic CR-1) more effective (Fig. 41). In dry tundra, flooding also fills pore spaces in the root mat or soil with water, reducing the amount of oil that can infiltrate. Repeated flooding, followed by removal of the floating oil, can greatly increase recovery of hydrocarbons.
- Water-Soluble Substances (salts, methanol, glycol): Flooding reduces toxicity by diluting the contaminants. The diluted contaminants can then be recovered by pumping (Tactic CR-6). Flooding and pumping can be repeated as needed.
Flooding and flushing (Tactic CR-8) are similar approaches. The potential for erosion is the primary factor to assess when choosing which of these two tactics to use. Use flooding when the potential for erosion is moderate or higher, and use flushing when the potential for erosion is low.
Most sites should be divided into several cells that are small enough to manage efficiently (Fig. 42). Water pressure and flow rate should be kept low to minimize erosion; using a Manta Ray skimmer in reverse to diffuse the input of water works well (Fig. 43). Move the input hose periodically to prevent erosion. Water may be pumped from a nearby tundra pond or creek, or transported to the site using trucks with clean tanks. Do not use seawater or produced water to flood tundra vegetation. Land barriers (Tactic CR-5) are needed to contain water on site (Fig. 44), especially during snowmelt (Fig. 45). In summer, flood with cold or warm water. Hotter water will be needed during winter to allow recovery before the water freezes. 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. In winter, water can be hauled to the site in heated or insulated tanks.
Surfactants reduce adhesion of crude oil and fuels to vegetation by increasing the ability of water to mix with hydrocarbons. Flooding with surfactants is appropriate for final cleanup of hydrocarbon spills after most of the spilled product has been removed (Fig. 44). Surfactants can be mixed with water in tanks, or added to the stream of water flowing out of the input hoses. 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.
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
- Maintaining a constant water level is important to prevent exposure of previously unaffected vegetation on higher areas (e.g., polygon rims) to floating or dissolved contaminants, as well as preventing repeated contact of oil with vegetation within the flooded area.
- Create a current in flood water or set-up petroleum collection downwind to remove floating hydrocarbons immediately.
- Surfactants decrease the ability of sorbent pads, booms, and skimmers to recover hydrocarbons, and are generally used during the final flooding, after most of the spilled product has been recovered.
- Surfactants can create enough suds to make operations difficult; add soap carefully.
- 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 flooded area, 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.
- Ensure water is free of hydrocarbons and salts before using it to flood tundra.
- Assess concentrations of contaminants in floodwater periodically using field screening techniques.
- Flood as few times as possible, to minimize physical damage to vegetation.
- Flooding is feasible during winter, but precautions for worker safety are necessary. Flooding may not be practical at extremely cold temperatures.
- Protect tundra being flooded by walking on plywood boardwalks, sandbags, rig mats, etc.
- Flooding may also be used to irrigate (Tactic TR-4) a site during the growing season.
- This tactic has been adapted from Tactic R-4 in the Alaska Clean Seas Technical Manual (http://www.alaskacleanseas.org/techmanual.htm).
Equipment, Materials, and Personnel
- Water truck or upright tank (1 operator) - to provide water source.
- Pumps and suction and discharge hose (1 to 2 operators each) – to pump water to and from site.
- Land barriers (Tactic CR-5) (number of people needed is site-dependent) – to contain water on site and to provide collection point.
- Clean water (not seawater or produced water).
- Plywood, sandbags or rig mats – to prevent trampling.