Standing column wells consist of a borehole that is cased until competent bedrock is reached. The New York State Department of Mineral Resources DMR requires that the well casing be driven 75 feet into competent bedrock. Upon indication of solid and unfrac-tured rock the DMR has agreed upon a reduction of that depth to as low as 35 feet. The remaining depth of the well is then self-supporting through bedrock. A central pipe is dropped to form a core through which the water is pumped up, and an annulus into which the water is returned. The length of the central pipe at the bottom is perforated to form a diffuser. Water is drawn into the diffuser and up the central riser pipe. The well pump is usually located at some depth below the water table in-line with the central riser
pipe. The standing column well combines the supply and diffusion wells into one, and is not dependent upon the presence or flow of ground water, although fractures in the bedrock that allow flow across the well can enhance performance, and reduce the required depth.
In practice, standing column wells are a trade-off between the open well systems and the closed loop earth coupling discussed below. The standing column well is advantageous during the design phase because the performance can be predicted without an extensive hydrogeological study. The savings in design fees and time are attractive. Given that standing column wells are predictable in projected performance, the use of this type of geothermal heat exchanger in most of Manhattan, the Bronx, northern Queens and western Staten Island, where bedrock is close to the surface is recommended. The standing column well can be designed and expected to perform to specification without the need for a test well. A heating or cooling capacity of 420 to 480 MBH (thousands of Btu/hr, equivalent to 35 - 40 tons of cooling) can be reliably expected from a 1,500 foot deep standing column well. Ideal spacing between SCW is at 50-75 feet. Closer spacing will affect the performance of the well field and can be projected with available design software (typically GLPRO, see below). Geothermal re-injection well water is considered a Class V water use and is ^^
regarded by the EPA as a 'beneficial' use. Permitting or notice ^Cj may be required dependent upon average daily water flow rates. SCW's are serviced by qualified well contractors with minimal training.
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