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TABLE 4 maintenance and operating costs over the long run, although VOLUME OF WATER PUMPED CONTINUOUSLY it should be recognized that there is a limit to what can be FROM SMALL CAPACITY WELLS achieved when compared to expenditure. Current design and construction technology is capable of producing wells with efficiencies of 80 to 90 percent. Pumping-plant or "wire-to- Pumping Rate Total Pumped in 24-Hours water" efficiency is currently at 65-70 percent. Gallons (litres) per minute Gallons Litres Sandia 0.25 ( ) 0 1 7360 20 Irrespective of size or composition, any loose 0.5 (22 72 720) 720 2 material entering a well is usually called "sand", and wells 5 that regularly produce significant quantities of loose 1 4 2900 5 00 0 material are termed "sanders". The continued influx of sand 2 8 900 11 0 to a well results in damage to pumps and leads eventually to 3 (11) 4,300 16 300 decreased capacity, and thus a reduction in well efficiency. 5 (19) 7,200 27,200 Further, enough sand may pass through the well to create 10 (38) 14,400 54 500 cavities in the aquifer around the intake section of the 50 (190) 72,000 273 000 well. As a result, such cavities can collapse and damage the well casing or screen. While most wells pump a minor amount of sand, excessive sanding is usually caused by poor well of the aquifer(s) in which the well is situated.. The design or inadequate development. discharge rate and drawdown established should be maintained Uncased ("Open-bottom"). Wells. Casing serves to for a specified time period. The ratio of the discharge rate hold up the walls of the borehole and provide a path for the to the drawdown is called the specific capacity of the well movement of the water. In formations with material that will for that time period. The units for specific capacity are not loosen and be carried away by the inflowing water, such gallons per minute per foot (litres per minute per metre) of as crystalline rock and other "hard rock" formations, the drawdown. Static water levels must be measured in advance of practice is to leave the intake sections Uncased. (Theoreti- the test and after the test during recovery. tally in such instances, well efficiency would be Detailed descriptions of procedures and methods 100 percent.) Unfortunately, in certain areas some drillers, believing the underlying material to be fully consolidated or used in conducting pump tests for large capacity wells and attempting for analyzing and interpreting the results are too lengthy to r save on costs, have drilled open-bottom wells p be included in this publication. Such information will be that later produced sand. Furthermore, as pumps lowered found in literature on ground water and on the design of following declining water levels, such wells developed water wells. sanding problems. This occurred in several areas in the Central Valley during the 1976-77 drought. In such instances, Well Efficiency the wells should have been completely cased to prevent caving and the intake section screened to prevent the entrance of Well efficiency is defined as the ratio of the sand. theoretical drawdown in the formation to the actual drawdown Inadequately Designed Intake Sections. Sanding is in the well. The difference between the two is caused by often the result of poor selection of screen size or perfora- frictional energy losses of the water as it moves from within tion dimensions and/or, where used, filter material (the the formation to the pump intake. Thus, well efficiency "gravel pack"). The well screen aperture (slot) openings or describes the effectiveness of a well in yielding water. the perforation size, together with the length of screen or Well efficiency should not be confused with pumping-plant (motor and pump) or "wire-to-water" efficiency used to perforated section, should be selected to provide sufficient measure um lant erformance open area to allow the desired quantity of water to enter pumping-plant p with minimal friction losses while keeping out 90 to 95 per- It should be obvious that well efficiency is cent of the natural aquifer material or filter material. related to the cost of pumping and the use of energy. If Artificial filter materials perform a similar efficiency improves, pumping costs and energy consumption function. In addition to allowing the water to enter the will drop. Thus, optimum well design is no less important well openings and preventing the entrance of fine-grained where a small capacity well is concerned than it is for one material, artificial filters are also used to increase the with a large capacity. Unfortunately, design and construction effective diameter of the well and increase the yield of practices that produce efficient wells are often sacrificed in certain wells by allowing numerous thin aquifers to produce order to save on the cost of constructing a well, particularly water. On the other hand they need not be used unless there in the case of small capacity wells. However, the increased are conditions that make their use desirable or necessary. cost of design and construction can be offset by decreased