Typical wells installed in fractured rock aquifers have well yields ranging between 4 and 190 liters per minute utilizing less than 30 m of drawdown. Higher yields require greater drawdowns, which may result in pre-mature well failure (particularly with PVC wells) and accelerated aging of the well. Using specific capacity as a “yardstick” to compare wells and recommended well yields, fractured rock wells typically range from 0.13 to 13.33 meter per minute per meter of drawdown. For comparison, typical alluvial wells yield 1.0 gpm per foot of drawdown or more. Water supply wells that yield hundreds of meters per minute from fractured rock are the curious anomaly rather than the norm and should be investigated thoroughly.

In general, the number and width of openings or fractures in fracture rock aquifers tend to decrease with depth. This geologic and logical perception suggests that wells drilled to greater depths become less permeable and less cost effective; unless the goal is to provide groundwater storage in the well bore. Hydraulic conductivity, a fundamental aquifer property, usually decreases with depth.

Well yields alone are not a good indication of aquifer permeability. Instead either the specific capacity (influenced by the well efficiency) or hydraulic conductivity must be used to assess reliable and long-term aquifer potential.

when doing the field work, I just found that yield of a well within a fractured rock aquifer fluctuated. Do you have similar experience?