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Competency Area 6: Watershed hydrology AEM

PO 62. Understand and describe aquifers (confined, unconfined) and the geologic conditions that affect water yield from wells.

Aquifers are geologic formations that store groundwater in the saturated pores of these sediment or rock formations, and are sufficiently permeable to transmit economic quantities of water to wells or springs. Aquifers consist of two types, confined and unconfined, as distinguished by differences in their hydraulic behavior.

A confined aquifer has an upper, and perhaps lower natural soil or rock layer boundary that does not transmit water readily, and thus, the stored water is confined within the permeable layer materials. The importance of this hydraulically is the stored water can then develop a hydraulic head pressure that exceeds the level of the upper confining layer. A well that is drilled into this type of aquifer and where the hydraulic head pressure is adequate to raise the water past the upper confining boundary and to the surface is commonly referred to as an artesian or flowing well.

An unconfined aquifer is one where the upper boundary consists of a relatively porous natural material that transmits water readily, and thus it does not confine the stored water, and the water table is free to rise and fall with recharge or withdrawal of water. When water is withdrawn (pumped) from an unconfined aquifer, the water table is drawn down and the water (specific) yield comes from the drainable porosity of the soil or rock material.

The water yield from wells depends on the intrinsic permeability or hydraulic conductivity of the saturated soil or rock material. For soil type aquifer materials consisting of well-sorted gravels, well yields will range from 500 to 1500 gallons per minute (gpm). Well-sorted sands and glacial outwash yield 100 to 500 gpm. Fine sands and mixed silty sand materials generally yield 10 to 100 gpm. For rock materials, well yields are also highly correlated to the extent of rock fractures and consolidation. Wells tapping the large solution channels in the Onondaga limestone formation can yield several thousand gallons per minute. Carbonate formations yield considerably less, more typical of around 200 to 500 gpm. Sandstone formations are still less (around 50 to 200 gpm), and shale formations range from 2 to 50 gpm. Wells yielding only 2 to 10 gpm may be suitable for supplying homes, but are generally not sufficient to support livestock operations. Well yields of 100 gpm or more are generally needed for irrigation.


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