Drilled Well

DRILLED WELL, the name for a borehole (see) that has the purpose of extracting artesian or groundwater from deep underground strata (horizons) for supplying populated places or for technical purposes. D. w. is executed by drilling methods, mostly by means of percussion drilling. The diameters of D. w. are determined depending on: 1) the depth of the D. w., which is determined by the occurrence of the water-bearing stratum; 2) the quantity of water that is planned to be extracted from the D. w.; 3) the system and size of water-lifting machines. The most frequently encountered diameters of D. w. are from 150 mm to 300 mm; D. w. of smaller diameters are suitable for extracting small quantities of water for individual use from shallow soil layers. D. w. with a diameter from 200 mm to 300 mm are suitable for supplying populated places and industrial enterprises. The largest diameter of D. w. encountered in our practice reaches up to 1 m. The depth of D. w. in the USSR rarely exceeds 250 m. The quantity of water extracted from one D. w. reaches 1,000 cubic m (80,000 buckets) per hour. The average quantity of water extracted from one D. w. of medium size (diameter 200-350 mm) can be taken as 50-150 cubic m per hour. The measurement of the capacity of D. w. is carried out by pumping or by the absorption of water by the D. w. In this case, the quantity of water pumped or absorbed by the D. w. and the corresponding lowering of the water level during pumping or raising during absorption are taken into account. The water level in the D. w., which is established during its operation, is called the dynamic level; it is measured from the initial level, called the static level. It is approximately considered that the lowering or raising of the water level in the D. w., within practical limits of use, is directly proportional to the quantity of pumped or absorbed water: the greater the quantity of water obtained per 1 liter of lowering, the more productive the D. w. Each water-bearing stratum (horizon) has a more or less constant elevation of the static water level; therefore, the static water level in the D. w. is deeper from its mouth, the higher the elevation of the mouth of the D. w. Wells located at elevations lower than the elevation of the static water level of the water-bearing stratum will have self-flowing water. When water is drawn for analysis, during trial pumping, elements of fresh water contamination (NH₃, HNO₂, Bact. coli) are usually discovered, which is usually brought into the D. w. with dirty tools, and during use during drilling with flushing, a mixture of water from the water-bearing stratum and water used for flushing is inevitably obtained, as part of it is always absorbed by the water-bearing stratum. The removal of foreign impurities from the water of the water-bearing stratum can be detected by a series of consecutive analyses. Undesirable water-bearing strata should be sealed with casing pipes. When constructing D. w., it is necessary to very carefully monitor the protection of casing pipes against upper water-bearing layers and contaminated water-bearing strata, as pipe joints cannot be considered hermetic. It is necessary to protect undesirable water horizons with two columns of pipes with filling of the space between pipes with thick cement or mastic. When using riveted pipes in this case, two rows of pipes and filling of the space between pipes are always necessary. When cutting inner pipes, careful sealing of the space between pipes at the cut end is necessary. The mouth of the D. w. should be carefully protected from the possibility of external contamination entering it. Water-bearing layers can be stable rocks-sandstones, limestones and other such rocks, or unstable-sands of various sizes, gravel and the like (see figure). Stable rock when receiving water is not reinforced in any way, while unstable ones have to be reinforced with filters, which are iron pipes with drilled holes for gravel, and for sands, the same pipe is tinned with copper mesh, sometimes tinned or nickel-plated. The density of the mesh is selected according to the fineness of the sand grains. Filters should, as much as possible, cover the entire water-bearing layer. If groundwater is being extracted, then the D. w. has to be located outside populated places, in order to eliminate the possibility of contamination of the water-bearing stratum by contaminated groundwater of the populated place. In this case, it is necessary to know all hydrogeological data to be sure that contaminated groundwater of the settlement cannot enter the water-bearing stratum exploited for water supply. It must be kept in mind that the water level and in general the regime of groundwater changes during operation. To protect groundwater from 1WI contamination, a sanitary protection zone (see) of the groundwater basin should be established. To determine the capacity and regime of the groundwater flow and the size of the sanitary protection zone, detailed topographic surveying, hydrogeological and chemical-bacteriological investigations must be carried out. D. w. extracting artesian water can be arranged within the boundaries of a populated place, since the artesian horizon is always protected by an impervious stratum that does not allow its contamination by groundwater, provided the D. w. is properly reinforced with pipes. Protection of D. w. from contamination should consist of the following: 1. Waters subject to isolation should not penetrate into the D. w. through the walls and joints of pipes and through the cut inner ends of pipes. 2. During drilling and cleaning, only drinking water should be allowed into the drilled well for flushing the bottom. 3. The construction of absorption wells to layers exploited for water supply should not be allowed. 4. The exposure of the water-bearing stratum by artificial structures within the populated place should not be allowed. 5. All D. w., both operating and conserved, must be well protected from the possibility of external contamination. 6. Old D. w. with rusted walls of casing pipes should be closed for use, tightly sealed with fat clay or poured with cement, or repaired in such a way as to eliminate the possibility of penetration of contaminated groundwater into the D. w. Inspection of old D. w. should be carried out in the presence of sanitary supervision. The service life of D. w. depends on the service life of the casing pipes. Casing pipes begin to rust primarily under the action of contaminated groundwater. The service life of casing pipes usually ranges from 10-25 years. The use of thick-walled pipes with filling of the space between pipes with cement or mastic extends the service life of the drilled well. The cost of drilling depends, mainly, on the depth of drilling and the quantity of pipes required. Pumps in D. wells can be placed either at ground level or in a more or less deep shaft, depending on the depth of the dynamic water level. The depths of shafts reach in some cases 10-12 m. If the dynamic water level in the well does not exceed 5-6 m from the axis of the pumps, then pumps of the ordinary type can be used; for small quantities of water, manual or horse-powered pumps are installed; for large quantities of water, centrifugal or piston pumps with mechanical engines are used. In cases where the dynamic water level of the D. w. is lower than 6 m from the axis of the pump and reaches 50 or more meters from the surface of the earth, it is necessary to use water-lifters that are lowered into the D. w. below the dynamic water level. Such water-lifters can be Mamut-type pneumatic water-lifters, vertical centrifugal pumps lowered into the D. w. and driven by engines installed at ground level. The area of application of D. w. for water supply of populated places is constantly expanding, and D. w. are gaining increasing sanitary significance. A number of cities, settlements, industrial enterprises arrange their water supply from D. w., obtaining artesian or groundwater from deep water-bearing strata. For supplying large settlements requiring large quantities of water, several D. w. are constructed. The water obtained from D. w., for the most part, is suitable for water supply without any preliminary treatment, therefore it is cheaper than river water, which requires filtration. The cities of Kharkov, Kiev, Vologda, Tver, Minsk and many others are supplied with water from D. w. Moscow, in addition to the city water supply, has about 400 D. wells, extracting up to 180,000 cubic m (14 million buckets) of water per day.