“How deep will the well be?” is a common question before drilling a well. If the driller has drilled several wells in the nearby area, he may be able to estimate the approximate depth where water will be encountered. Most of the time, however, the depth needed to find the required well yield cannot be determined accurately prior to drilling. A well is an engineered hole in the ground via which ground water can be brought to the surface. Drilling machines can drill to great depths. Deeper wells usually cost more than a shallow well to construct in the short-run. However, not drilling deep enough can result in later problems that will be much more expensive to fix. Listed below are some of the factors that may influence decisions about the depth of a water well.

Seasonal Rise And Fall Of The Water Table

During the year, the water table will fluctuate up and down in the well in response to seasonal precipitation in the area and local ground water use. The well must therefore be drilled deeper than the lowest expected elevation of the water table. Water level fluctuations may occur over several years if there have been drought conditions. Knowing the lower limit of the range of water levels over several years therefore can be helpful.

Surface Contamination Risks

Deeper wells that are properly constructed (including grout, casing, well cap, and pitless adaptor [in freezing climates]) usually provide guaranteed protection from bacterial contamination sources originating at the surface. Increasing the well depth and the length of well casing will result in a longer flow path of water from recharge at the surface to pumping from the well. The longer the length of time water is in the subsurface, the more opportunity there is for bacteria to die-off or be trapped by soil and rock.

Low Yielding Rock Formations

In low yielding rock formations the well may have to be drilled deep enough to serve as a storage cavity for ground water. Once a well is drilled, the total depth, depth to the top of the ground water table (static level) and diameter of the well determine how much water will be stored within the well cavity. The larger the well diameter the more water will be stored for a given well depth and water table elevation. To find the “thickness” of the water stored in the well subtract the depth to the static water level from the drilled depth of the well. 

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