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Whereas the typical domestic well is located as a matter of convenience without any detailed hydrogeologic exploration and definition of most favorable drilling site, municipal and other large yielding wells are most often constructed at sites selected on the basis of technical investigation. Methods of investigation for sand and gravel wells are different from those used for fractured bedrock wells because of the major differences in geologic nature of the two types of aquifer.
Sand and Gravel Aquifers
Available surficial geologic and overburden aquifer maps and reports are first consulted to determine what is known about sand and gravel aquifers present in a given area. This work is followed by interpretation of land forms on aerial photographs, and by on-site inspection of geologic features, particularly those evident in sand and gravel pits. Areas that appear favorable for well construction may then be further analyzed by geophysical techniques (seismic refraction profiling and electrical resistivity sounding) to determine depth to water table, saturated thickness, and general stratigraphy. Sites that are shallow to bedrock, for example, are easily eliminated by geophysical evaluation. The ideal test drilling site has a significant thickness of saturated and very permeable sand and gravel, good potential for recharge from precipitation and possibly by inducement from a hydraulically coupled stream or lake, and isolation from potential sources of contamination.
Once favorable test drilling sites are selected, 2 1/2 inch diameter wells are installed by driving and washing casing into the aquifer to depths that are typically 30 to 150 feet. Occasionally, very bouldery deposits require test drilling by rotary or percussion machines. Each test well is pumped for a few minutes or more to determine the potential yield. If a suitable well site is found, an observation well usually 2-feet distant from the previous test well is installed and a 4-hour pump test is completed. Water samples are collected for chemical quality analysis. Where results are favorable, an 8-inch diameter test well is constructed at the same site and test pumped for a minimum of five days.
Finally, if all the test drilling and test pumping results are favorable, a permanent production well is installed with a length and diameter of screen that provides the most suitable well in the given aquifer. This permanent well is also fully test pumped and analyzed before it is put into use.
Fractured Bedrock Wells
Exploration for high yielding fractured bedrock wells includes initial consultation of available bedrock and surficial geology maps and reports, but requires more extensive use of remotely sensed data than is usually employed to locate a sand and gravel well site. Water-bearing fracture zones that are not readily apparent to a ground observer are mapped in detail using aerial photographs, satellite imagery, side-scanning radar, and various geophysical techniques. Field study of bedrock outcrops and regional landforms is also done in the final selection of favorable test drilling sites. The ideal site is underlain by highly fractured bedrock that stores and transmits ground water readily, is covered by thick and permeable materials that provide recharge to the fractures, and is sufficiently isolated from potential sources of contamination.
Initial test drilling is completed using 6-inch diameter wells. Air-percussion ("down-the-hole-hammer") methods are typically used to reach depths of between 200 and 700 feet. Each test well is evaluated by air-lifting water out of the hole using the same compressed air system that is used in the drilling process. If the results are favorable, and a yield of more than 100 gpm is desired, an 8-inch diameter test well is drilled at the most favorable site. This well is equipped with a submersible pump and is tested for several days. Drawdown measurements are taken in the previously completed 6-inch wells, and water samples are withdrawn for analysis of chemical quality.
Where yields from the bedrock aquifer in the range of 300 to 600 gpm are needed, a 10 or 12-inch diameter permanent production well is constructed in the same favorable area. This well accommodates the required large submersible pump and provides greater hydraulic efficiency for more reliable and economical long-term operation. A full scale pumping test is also completed for the production well.
Last updated on March 25, 2009
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