acids, they produce deadly toxins that sometimes leak into the groundwater or into the human food chain. Some of these toxins remain reactive and hazardous for up to 100 years before they become nontoxic. When any of these materials are spilled onto the ground, it must be considered a major contaminating spill with dangerous aquifer polluting potential. Some of the larger CESE used by NCF contain enormous amounts of petrochemicals. A 25-ton hydraulic crane contains over 214 gallons of potential pollutants, and a twin-engine scraper contains over 350 gallons. Spillage or dumping of these amounts of contaminants can be disastrous!

Because of the varied conditions that affect migration and recovery of a spill, recovery systems must be tailored for each site. Some of these systems are covered in NAVFAC DM 5.14, Groundwater Pollution Control.

Small spills that encounter a shallow groundwater table or that are contained by a natural barrier, such as rock, stone, or impermeable clay, prevent vertical migration and can be recovered by using an interceptor trench, as shown in figure 7-1. This interceptor trench system is relatively simple and can be built by using materials and equipment normally available on a construction site. The trench must bisect the entire width of the spill to contain it; therefore, the interceptor system is useful only on spills that can be contained quickly. The trench depth usually is limited to 6 to 8 feet, because beyond that depth the ground becomes unstable. An impermeable barrier, such as rubber sheeting, should be installed on the downgrade side of the trench. This barrier prevents migration of the accumulated spill product and still allows water to pass beneath the barrier. Since most petroleum products float on water, the spill can be pumped out into a separator, as shown in figure 7-2. The separated spill can then be disposed of off-site at an authorized waste-handling facility.

Another method of clearing a spill area is to completely remove all contaminated soil from the site to a facility or landfill that is designed to receive such material. This is called stripping. Stripping must be done carefully so that the underlying and adjacent soil is not contaminated by the removal process. Once all of the contaminants are removed from the site, the excavation may then be backfilled with dry, clean soil.

The best method, of course, is to be sure that spills do not occur on jobsites.

AIR POLLUTION

As a first class petty officer, you should be aware of work conditions that cause air pollution and of the efforts required to minimize or correct the problem.

When incomplete combustion occurs in base boilers, space heaters, and stoves, the unburned hydrocarbons and various other fuel components combine chemically to form by-products. Many of these by-products are harmful to people and the environment.

The by-products that have the most adverse effect on the air are carbon monoxide, particulate matter, sulfur oxides, unburned hydrccarbons, nitrogen oxides, and lead. The most effective means of controlling air pollution from incomplete fuel combustion is to properly and frequently maintain the equipment. In this way, the equipment is operating at an optimal fuel and oxygen mixture. Another means of lessening air pollution, not always under your control, is the use of only the best grade of fuel. High-grade fuel contains low particulate matter, low water and sulfur content, and few contaminants.

Figure 7-1. - Interceptor trench with skimmer pump.