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Maine's Highest Yielding Well
The State of Maine is blessed with abundant ground water resources. If you happen to live in a rural setting, your water supply is likely to be a dug, driven-point, drilled gravel, or most commonly, drilled bedrock well. If your home is served by a municipal or public water supply, there is also a very good chance that your water may come from a large-capacity drilled gravel-packed or naturally developed well installed in sand and gravel. Although there are some very high yielding drilled bedrock wells in Maine, they are not as common or as high yielding as those installed in sand and gravel deposits. July's Site of the Month focuses on what likely is Maine's highest yielding gravel well or well field. The well, which is owned and operated by a major water district, occurs along a large river in the mid-coast area of Maine.
This water district well and many high capacity gravel wells like it are sited along large rivers and streams to increase the potential recharge. When a gravel well adjacent to a river is pumped, the cone of depression or the surface expression of this cone known as the area of influence typically would extend out to the river (Figure 1). In some cases depending upon site conditions, this cone of depression may even extend to the opposite side of the river from the well (Figure 2). When a well is pumped and recharged in this way, the condition is known as induced infiltration or induced recharge. In some settings, low permeability sediments such as clay may isolate or partially isolate the well from the river. Therefore, the hydraulic connection between a well and the river is largely dependent upon the permeability, thickness, and continuity of the underlying sediments.
Figure 3 is a generalized site map showing the well locations relative to the river as well as two associated geologic cross-sections. As is evident from the cross-sections, the area is underlain by significant thicknesses of clay, sand, and gravel. Based upon this subsurface information, the aquifer is only partially confined by the clay in the sense that there are areas where shallow and deep coarse materials are continuous. These "windows" allow water to be filtered though the sand and gravel from the river bed to the well as a seemingly infinite supply of recharge. This connection to the river is clearly seen when the downstream river hydrograph is compared to the hydrographs of the production well and a monitoring well immediately adjacent to the river (and west of the production well) (Figure 4).
The original well at the site was drilled in 1956 using cable tool drive and wash methods. The completed well was drilled to a total depth of 138 feet (depth of bedrock surface) with 12-inch diameter casing and 25 feet of screen with variable slot openings sized according to the grain size of the material (Figure 5). Yield at the time was determined to be 2910 gallons per minute (gpm). Subsequent pumping tests have revealed that this yield is very conservative and that theoretical potential yields based upon constant rate pumping test results for the well are likely to be in the range of 5900 to 8800 gpm. Since the original well was approaching 50 years in age and there were some concerns about the screen's long-term integrity, it was decided to replace this well. In December 2004, a 24-inch diameter naturally developed gravel well was installed immediately adjacent to the original 12-inch well with a total depth of 140 feet, a screen length of 29.5 feet, and a total theoretical yield based upon a 2500 gpm constant rate pumping test of 11,000 gpm (Figure 6). This well would not likely be operated at this rate because of the very high approach and entrance velocities. Larger diameter wells would likely need to be installed in this well field to realize this tremendous yield.
The following photographs (Figures 7-16) show the new well being drilled and developed as well as views of the site in general.
The Maine Geological Survey wishes to thank the owners of the well and the Maine Department of Human Services Center for Disease Control and Prevention for their kind assistance and cooperation in preparing this information.
U.S. Geological Survey Water Science for Schools, Ground Water: Wells
Text and photos by D. Locke
Originally published on the web as the July 2008 Site of the Month.
Last updated on August 11, 2008
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