Water Well Journal

February 2015

Water Well Journal

Issue link: http://read.dmtmag.com/i/451364

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Page 16 of 75

A s both a hydrogeologist and a certified geoexchange designer, I am often approached by potential customers wondering if their existing water well might also serve for operation of a geothermal heating and cooling system. This is not always an easy question to answer and requires considerable understanding of the local aquifer, well construc- tion, pumping system, and a means for discharge or recharge of the spent water. Perhaps you or a number of geothermal contractors you work with may benefit from this survey template and installa- tion suggestions that are the result of more than 40 years of experience in working with geothermal open and closed loop systems. Locating the well log A survey of state and local water resource records to locate a copy of the original well log will help reveal if the well is a likely candidate for use with a geothermal system. I live in in Ohio where the Ohio Department of Natural Resources maintains an extensive database of well log and county map information. Most states and provinces in Canada also maintain well log databases available for public use. Well construction Wells drilled into a sand and gravel formation showing no well screen listed on the log are likely drilled with only the open end of the casing exposed to the sand and gravel. This limited area as an intake is easily plugged with minerals and may be okay for domestic use, but can be a problem for long- term withdrawal as would be needed for a geothermal system. It is best to avoid using this type of well as a source to supply a geothermal system. If a sand and gravel well is listed with a well screen, its capacity is likely substantially higher than a simple open- ended well casing and may prove adequate for use with a geothermal system, but only when tested for capacity and water drawdown. The ideal well will have a static water level within 80 feet of the surface and a change in pumping water level of no more than 20 feet with the pump in operation. In the case of measuring drawdown in a water well, less is better. This suggestion is to prevent the use of wells where the water level is deep and pumping costs consume operation savings or where sustained pumping is more likely to risk depletion of a deep aquifer having limited recharge. I once talked to a customer who was using a 5 hp pump to lift water from 280 feet to operate his geothermal system. The well pump was operating at twice the kilowatt load of the ge- othermal compressor. Consequently, his energy bills actually increased more than those of his old oil furnace. This was clearly an installation that needed a conversion to a closed loop if he was to realize any savings. Wells drilled into bedrock are best if the casing extends below the static water level, and when the pump operates, the drawdown in water level stays within the cased depth of the well and does not expose the rock surfaces of the open well below the casing to air. Air infiltrating areas of the aquifer providing water to the well will cause natural iron in the water to oxidize and form a red clay-like filter cake that can plug wells and deposit itself inside pump impellers and the piping system. For this reason, deeper wells with a shallow casing set where bedrock is close to the surface and where surface water and oxygen can enter the well are not good candidates for use with geothermal systems. Ideal wells for use with a geothermal system are set with well screens into prolific sand and gravel deposits or drilled deep and cased into a fractured or porous high-yield rock formation. Ohio and many other states and municipalities have water regulations protecting the rights of adjacent and surrounding Open Loop Geothermal System Survey Part 1: Finding out what system is best for your job site By Jeff Persons, CGD Clumping of iron bacteria in toilet tank. (COVER STORY) WWJ February 2015 15 Twitter @WaterWellJournl OPEN LOOP continues on page 16

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