FIELD NOTES ON PHYSICAL CHARACTERISTICS OF FRACTURED ROCK AQUIFERS

As alluvial aquifers are more fully appropriated and utilized to provide water to the burgeoning population of metropolitan, urban, and rural areas, a greater number of fractured rock aquifers are being exploited often with a significant number of well failings. Field notes and empirical hydraulic observations of the physical characteristics of fractured rock aquifers are important data sources to develop these low-yield aquifer systems in a cost-effective manner. These observations guide our understanding of the hydraulic limitations of fractured rock aquifers, improve our ability to model the system, and support long-term sustainability from the aquifer.

Typical wells installed in fractured rock aquifers have well yields ranging between 1 and 50 gallons per minute (gpm) utilizing less than 100 feet 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 0.01 to 1.0 gpm per foot of drawdown. For comparison, typical alluvial wells yield 1.0 gpm per foot of drawdown or more. Water supply wells that yield hundreds of gpms 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. Estimates of the hydraulic conductivity (from pumping tests and drillers logs) for several data sets from various geologic environments and units in California are used to demonstrate a consistent negative relationship between depth and hydraulic conductivity

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.