Figure 9-10.—Protection of spring from surface contamination.

Figure 9-10.—Protection of spring from surface contamination. enter the spring during periods of flood. It is not necessary to ventilate spring structures; therefore, all openings should be avoided, except for an inspection manhole fitted with a tight, locked cover. When ground and surface water supplies are inadequate or cannot be used, ground water sup- plies are developed by constructing wells. Wells are classified into five types, according to their method of construction. These are dug, bored, driven, jetted, and drilled wells. Each type of well has its particular advantages, which may be ease of construction, type of equipment required, storage capacity, ease of penetration into certain types of formations, or ease of safeguarding against pollution. In the event of chemical, biological, and radiological operations, it is important to note that ground water would probably remain essen- tially uncontaminated by airborne or surface dissemination, in contrast to surface water, which could become severely contaminated. This does not mean that ground water is always pure and safe to drink. It can be naturally contaminated or could, in some cases, become contaminated with CBR agents. Well water should be thoroughly tested before use. The production of ground water involves the method of recovery of water stored in the zone of saturation below the waterline or water table. The ground water table does not always remain at the same elevation, as it is controlled by rain- fall, tides, the pumping rate from wells, and so forth. A dug well is a large diameter well, seldom less than 3 feet in diameter, excavated with hand tools, and lined with brick, stone, steel, wood cribbing, or tile. That portion of the lining through the water-bearing formation is porous. This shallow type of well is usually dug from 20 to 40 feet deep, depending upon favorable loca- tion for water. Because of the large opening and perimeter to be protected against the incursion of surface drainage, dug wells are easily polluted by surface wash. Bored wells are constructed in soft water- bearing formations that will not cave in while the hole is being bored. They are usually bored with hand or powered earth augers to a depth ranging from 25 to 60 feet without caving in. Jetted wells are suitable in soft, uncon- solidated, alluvial deposits. The well consists of an inner tube which is a drilling or jetting tube and an outer tube which is the well-casing. A 9-11