While the standards presented here (see Chapter II following) are designed to protect the continued utility of the State's groundwater resources, they are only incidentally related to the effective use of these resources. Events of the past decade have emphasized the need for conservation of water and energy. Furthermore, consumers (in this case, well owners) have become more aware of problems resulting from inefficient operation (as reflected in increased energy consumption) and inadequate maintenance.
Accordingly, this section was prepared to provide well owners and drillers with guidelines for measuring performance that will lead to the design and construction of more efficient wells as well as those requiring less maintenance.
Testing for Capacity
Every well owner is interested in how much water the well will produce and how dependable the production will be with time. To make that determination a capacity test or performance test must be made. Usually this involves installing a pump and operating it at the expected production rate over a certain length of time. There is considerable variation in actual practice on how such tests are performed depending on the dimensions of the well, including expected capacity and intended use as well as geologic conditions at the site. Obviously, for a small capacity well, i.e., one that produces under 50 gallons per minute, the test would not be as elaborate as it would be for a high capacity well, but is no less important.
The amount of water needed is determined by the intended use of the water. For example, on the average, each person in a household uses 100 gallons of water a day. To the daily household use must be added seasonal uses such a lawn and garden irrigation, swimming pools, etc. Table 4 lists the volume of water supplied from a small capacity well, assuming continuous pumping for 24 hours. Thus, a well supplying one to three gallons per minute is a reasonable amount for a single family dwelling. Additional amounts, such as for watering livestock or irrigating small acreages of crops, must be added to these values. Table 4 also indicates that a family of four could subsist on the water supplied by a well pumping constantly at the rate of only one-quarter gallon per minute. Unfortunately, at this rate there is little margin for error.
Small Capacity Wells. Performance tests for small capacity wells are relatively simple. A widely used test for small capacity wells is a pump test which lasts for four hours or until an apparently stable pumping level has been achieved at a rate equal to that expected for the permanent pump. However, in the hilly and mountainous "hard rock" areas of the State there are no defined aquifers and supplies are related to fracture patterns, the nature and extent of the soil mantle, faults, changes in stratigraphy, etc. In such areas the production potential of a well cannot be accurately assessed. Further, wells in these areas often exhibit a satisfactory initial production, which then declines due to poor recharge characteristics of the surrounding material. In such situations a longer than usual test, upwards of 12 to 24 hours (and longer) duration, may be desirable.
Bailing or air-blow tests give an approximate indication of production. They do not provide information of the accuracy needed to determine well capacity or to design an efficient pump system. (Air lift testing differs from air-blow testing. It involves pumping with air, not blowing the water out of the well as is the case with the air-blow test.)
The ability of the water level in a small capacity well to recover should be observed. If the water level fails to return to nearly its original level after 24-hours, the reliability of the producing zone is open to question.
Large Capacity Wells. Where large capacity wells are concerned, capacity tests are more elaborate and extensive. Such wells are usually located in defined, productive groundwater basins, where considerable information on existing conditions is normally available to aid in the evaluation of their performance. All should be pump tested; bailer tests are of little value. The test pump should be capable of pumping 125 percent of the desired yield of the well. Pumping should be continued at a uniform rate until the "cone of depression" reflects any boundary condition that could affect the performance of the well. This could be as short as six hours and as long as several days, depending on aquifer characteristics and knowledge of the aquifer(s) in which the well is situated. The discharge rate and drawdown established should be maintained for specified time period. The ratio of the discharge rate to the drawdown is called the specific capacity of the well for that time period. The units for specific capacity are gallons per minute per foot of drawdown. Static water levels must be measured in advance of the test and after the test during recovery.
Detailed descriptions of procedures and methods used in conducting pump tests for large capacity wells and for analyzing and interpreting the results are too lengthy to be included in this publication. Such information will be found in literature on groundwater and on the design of water wells.
Well efficiency is defined as the ratio of the theoretical drawdown in the formation of the actual drawdown in the well. The difference between the two is caused by frictional energy losses of the water as it moves from within the formation to the pump intake. Thus, well efficiency describes the effectiveness of a well in yielding water. Well efficiency should not be confused with pumping-plant (motor and pump) or "wire-to-water" efficiency used to measure pumping-plant performance.
It should be obvious that well efficiency is related to the cost of pumping and the use of energy. If efficiency improves, pumping costs and energy consumption will drop. Thus, optimum well design is no less important where a small capacity well is concerned than it is for one with a large capacity. Unfortunately, design and construction practices that produce efficient wells are often sacrificed in order to save on the cost of constructing a well, particularly in the case of small capacity wells. However, the increased cost of design and construction can be offset by decreased maintenance and operating costs over the long run, although it should be recognized that there is a limit to what can be 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-water" efficiency is currently at 65-70 percent.
Irrespective of size or composition, any loose material entering a well is usually called "sand", and wells that regularly produce significant quantities of loose material are termed "sanders". The continued influx of sand to a well results in damage to pumps and leads eventually to decreased capacity, and thus a reduction in well efficiency. Further, enough sand may pass through the well to create cavities in the aquifer around the intake section of the 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 design or inadequate development.
Uncased ("Open-bottom") Wells. Casing serves to hold up the walls of the borehole and provide a path for the movement of the water. In formations with material that will not loosen and be carried away by the inflowing water, such as crystalline rock and other "hard rock" formations, the practice is to leave the intake sections uncased. (Theoretically in such instances, well efficiency would be 100 percent.) Unfortunately, in certain areas some drillers believing the underlying material to be fully consolidated or attempting to save on costs, have drilled open-bottom wells that later produced sand. Furthermore, as pumps lowered following declining water levels, such wells developed sanding problems. This occurred in several areas in the Central Valley during the 1976-77 drought. In such instances, the wells should have been completely cased to prevent caving and the intake section screened to prevent the entrance of sand.
Inadequately Designed Intake Sections. Sanding is often the result of poor selection of screen size or perforation dimensions and/or, where used, filter material (the "gravel pack"). The well screen aperture (slot) openings or the perforated section, should be selected to provide sufficient open area to allow the desired quantity of water to enter with minimal friction losses while keeping out 90 to 95 percent of the natural aquifer material or filter material.
Artificial filter materials perform a similar function. In addition to allowing the water to enter the well openings and preventing the entrance of fine-grained material, artificial filters are also used to increase the effective diameter of the well and increase the yield of certain wells by allowing numerous thin aquifers to produce water. On the other hand they need not be used unless there are conditions that make their use desirable or necessary. Artificial filters are desirable when the aquifer has a "uniformity coefficient"Note 3 of less than 2.5 (some authorities recommend a value of less than 3), or in poorly consolidated rock, i.e., rock that tends to cave when pumping occurs.
Detailed information on the design of intake sections, including the selection of well screen aperture openings and artificial filter materials, will be found in most publications dealing with groundwater and water wells, a number of which are listed in Appendix E.
Incomplete Development. Well construction causes compaction of unconsolidated material about the walls of the drilled hole and drilling fluid also invades these walls, forming a mud cake. In consolidated rocks, cuttings, fine particles and mud can be forced into joints and fractures. Thus, the borehole walls become clogged, reducing the potential yield and causing a drawdown to be increased. Proper well development breaks down the compacted walls (or opens fractures) and draws the material into the well where it can be removed. Obviously, the more thorough the development the better the well will perform. Adequacy of development is largely a matter of experience and judgment. The success of development can be measured by the amount of sand produced during interrupted pumping and the final specific capacity of the well.
Testing for Sand. The sand content should be tested after development and performance (pump) testing. Sand production should be measured by a centrifugal sand samplerNote 4 or other acceptable means. Following development (i.e., stabilization of the formation and/or gravel pack) and pump testing, the sand content should not exceed a concentration of 5 ppm (parts per million) by weight 15 minutes after the start of pumping.
Sand production exceeding this limit indicates that the well may not be completely developed or may not have been properly designed. In that event, redevelopment may be appropriate or as an alternative, a sand separator installed. In existing wells should this value be exceeded significantly, rehabilitation (redevelopment) or repair is probably needed. Again, as an alternative, a sand separator may need to be installed.
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