Water Well Journal

March 2015

Water Well Journal

Issue link: http://read.dmtmag.com/i/465664

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Page 36 of 73

WWJ March 2015 33 This article is the fifth in a six-part series that will detail each of the 11 sections of the ANSI/NGWA Water Well Construction Standard. T he purpose of this article is to highlight key aspects of the stan- dard in respect to well perform- ance testing and the data to be recorded in the well completion report. Testing for Performance Sometimes confused with an aquifer performance test, which is used to eval- uate aquifer characteristic parameters, a well performance test is conducted to collect data to determine peak capacity, long-term capacity, or well efficiency of a fully developed water well. Well efficiency is the ratio of the the- oretical drawdown in the formation in respect to the actual drawdown in the well. A well cannot be 100% efficient due to frictional energy losses as water is drawn from the aquifer to the pump intake. Current design and construction technology is capable of producing wells with efficiencies of approximately 80% to 90%. Generally speaking, the efficiency of a well is an estimate of how capable the well is in yielding water. A well with a higher efficiency results in lower costs of pump operation and lower energy costs in operating the well. It should be noted during the rehabilitation of old water wells the overall pumping costs and energy consumption will drop if well efficiency improves. The difference between a good well contractor and a great one is the per- sonal interest and commitment made by the contractor in the newly constructed well to not only produce water, but to master their craft so that each well con- structed is reliable and efficiency will be sustained over long periods of time. Dependability starts with the efficiency of the water well. The only way to determine a newly constructed well's efficiency is to con- duct performance testing. This usually involves the installation of a pump and operating the well at the expected or estimated production rate over a certain length of time. Well performance testing should fol- low the development of the newly con- structed water well. In theory, during the well development phase of construction the performance of the well should im- prove as effects of the drilling operation are addressed. During the well perform- ance test, the two principal measure- ments to be collected are withdrawal (pumping) rates and hydraulic head changes over time. Withdrawal (pumping rate) is a criti- cal measurement used in determining well efficiencies. It is good practice to have at least two points of measure- ments of withdrawal from the well. A measurement at the wellhead in con- junction with an inline measurement can allow for a better understanding of the flow exiting the well. Assuming both flowmeters are cali- brated, the two measurements should be comparable once the pump has been dialed in. The other reason for recom- mending using two meters is each meter is a backup for the other. Failure of one meter during the test is less detrimental to the efficiency evaluation when two meters are used—as opposed to a test with one meter that fails, which results in stoppage of the test. In the age of electronics, it is also wise to consider traditional flowmeters in conjunction with electronic meters. Electronic meters can collect a signifi- cant amount of data at sampling fre- quencies far exceeding any human. However, manual measurements in conjunction with the electronic meas- urements allow for quality assurance checks. (Were the meters calibrated correctly? Are the meters operating correctly? Are the electronic data comparable to the manual data?) They also allow for the analysis of a less intensive data set. During the well performance test, the pump should be operated without interruption and the withdrawals from the well maintained within plus or minus 5% of the desig- nated rate for the duration of the testing period. The change in the hydraulic head from the static water level is the re- sponse to the withdrawal (stimulus). The use of the term "static water level" is defined in the ANSI/NGWA Water Well Construction Standard as less than 1 foot of hydraulic head difference between two consecutive water level measurements taken a minimum of 60 minutes apart. The standard adds all measurements of hydraulic head shall be taken at the same base datum and measured to the nearest 0.1 foot. To assure this occurs, it is a good practice to place a notch into the surface casing of the water well to establish a set measurement point. Much like the flow measurements, it is good practice that hydraulic head measurements taken electronically be supplemented with manual measure- ments. In theory, hydraulic head meas- urements should be taken in the manner specified by the data requirements of the well performance test, and at time inter- vals that reflect the logarithmic nature of a drawdown curve. The ANSI/NGWA Water Well Con- struction Standard provides a reference guide for sampling time intervals. For most situations, the intensity of the data collection is best accomplished with the use of pressure transducers supple- mented with manual measurements as a quality assurance measure. Prior to recovery testing at the well, a final round of measurement should be conducted and pressure transducers downloaded and reprogrammed for a set start time for measurements of the loga- rithmic nature of the recovery of the well. Recovery readings of water level Go to the NGWA Bookstore to purchase your copy of the ANSI/NGWA-01-14 Water Well Construction Standard. It encompasses municipal, residential, agricultural, monitoring, and industrial water production wells. Topics covered include: well site selection; casing and casing installation; well screens, filter pack, and formation stabilizer; grouting; plumbness and alignment; well development; testing for performance; data recording; disinfection with chlorine; water sampling and analysis; and permanent well and test-hole decommissioning. Go to the bookstore at www.NGWA.org. Twitter @WaterWellJournl STANDARD continues on page 34

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