Domestic Water Well Yield Assessment

By Darrel Dunn, Ph.D., PG, Hydrogeologist

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A simple test that a homeowner can make to see if a well is performing adequately is to put a hose on an outdoor faucet that can direct water to a an area that is away from the well and can accept 960 gallons of water without damaging anything.  Turn on the water at a rate of 4 gallons per minute, which can be measured by filling a bucket.  Allow the water to run for 4 hours.  If the well can sustain this flow, it will probably be adequate for a small family and limited outdoor use.

If the well is not performing adequately, a water well and pump contractor can perform a specific capacity test.  This test will show the initial non-pumping (static) water level, a test pumping rate, and a pumping water level for that rate.  The specific capacity of the well can be calculated from this data.  Specific capacity is often expressed as gallons per minute per foot of drawdown.  Drawdown is the difference between the static and the pumping water level.

If a well is being re-tested, the specific capacity can be compared to the original specific capacity at the time the well was constructed if the original driller's report is available.  The original report can often be found at a government water resource agency.  The specific capacity of a domestic well should not change much unless it is very old.  However, the potential yield may change if the static water level has changed.  Static water level may change due to weather and climate affecting groundwater recharge.  Also, static water level can decline due to well interference.  Change in static water level will change available drawdown.  Available drawdown is approximately the difference between static water level and the lowest practical depth where the pump intake can be installed.  If available drawdown is reduced, then the potential yield of the well is reduced.

Consequently, a change in static water level may be of interest.  However, when re-testing a domestic water well, measurement of static water level without pulling the pump is problematic.  Accurate measurement can be accomplished using various types of probes that are lowered into the well, but these probes may encounter obstructions or become entangled if the pump and other hardware are in the well.  Sonic devices are available that determine water level without putting any measuring equipment in the well; but they may not be accurate, due to hardware in the well, well construction, or well deterioration.  Pulling the pump for accurate water level measurement is relatively expensive and requires access for heavy machinery.

More advanced tests can be performed, but they are too expensive to be practical for domestic wells.  Advanced testing is performed on high capacity wells that are used for municipal, industrial, and agricultural water supplies.  See water table aquifer testing, leaky aquifer testing, and fractured aquifer testing.