Drilling on hard-rock aquifer: foothills of the Sierra Nevada
To Drill or Not to Drill
If one drill to certain depth and the well does not bring intended discharge, is it worth to drill deeper or should just make a new well? What yield can be expected from a well? These are questions that well driller and owner usually find when drilling water well in a hardrock area. To answer these, about 900 wells at the foothill area of Sierra Nevada, California are explored.
Common assumption is that deeper zone of crystalline aquifer has less fracture, or has fracture with less water because the loading would close fractures at greater depth. Thus, drilling deeper than 300 feet deep is not suggested.
In 1964, Davis and Turk analyzed about 2,500 wells on crystalline rocks. The data is 2,336 water wells in granite and schist in the eastern of the United States, and 239 wells in the Sierra Nevada. They conclude that, in crystalline rocks, permeability decreases about ten-fold between depths of 100 and 1,000 feet. Thus, Davis and Turk suggested that the depth for domestic well should be less than 250 feet, and for larger production well should be less than 600 feet to be cost effective.
Recent study by Abott (2007) revealed a log normal correlation between well depth and estimated hydraulic conductivity (K). He analyzed 334 wells in the northern Sierra Nevada, and 146 wells in the Peninsular Range of San Diego County. All wells have depth deeper than 300 feet but only four wells that deeper than 600 feet. Out of 480, only 27 wells have a K-value greater than 1 gpd/ft2 (that can yield more than 25 gpm of water). Abott summarized that wells drilled deeper than 300 feet will yield insignificant amounts of additional groundwater.
Contrary to this suggestion, in the foothill area of the western Sierra Nevada, more new supply wells are drilled deeper (than pre-1990’s). These deep wells (more than 600 feet in depth) still discharge sufficient amount of water.
Well Depth and Yield
This study uses information from 883 private / individual water wells at North Fork, Coarsegold, and Raymond areas, Madera County, California. Well information was obtained from Well Completion Reports at the Department of Water Resources, San Joaquin District.
Figure 1 shows the well depths and the airlift test yields at the time of the drilling. In general:
- majority of the wells (almost 60%) had airtest yields exceeding 10 gpm,
- about 20% of wells produced less than 5 gpm,
The bars in Figure 1 represent the amount of wells and their yield distributions. For all wells with yields up to 50 gpm, the percentage distributions for each depth interval are similar. Thus, the chance to get the same yield of water is relatively similar regardless the well depth.
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Figure 1. Well depth categories and the airtest yields.
Figure 2 shows in detail the plot of well depths versus airtest yields statistic. The average (Mean) of airtest yields is decreasing with depth. Unexpected, the Median of airtest yields has relatively constant values, range from 12 to 15 gpm throughout the depth intervals. The higher yield value at the first 200 feet could be related to the depth of exfoliation, observed by Huber (1989).
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Figure 2. Mean and median values of air test yields.
Conclusion
In conclusion, when drilling a well in hardrock aquifer at the foothill of Sierra Nevada, California:
- There is about 60% chance for a well to produce 10 gpm or more of water (which is adequate for private use),
- Drilling deeper does not lessen nor increase the chance to get more water,
- Realistic yield from one well is about 14 gpm (median average).
Keep in mind that these statistical figures from Driller’s Well Completion Reports must be used with caution. The well yield values tend to be exaggerated, and long-term test results are typically produces lesser yield (Sartono, 2007). Other concern is the well depth bias. For these individual wells, information from shallow wells does not necessarily represent the shallower zone of the deeper wells.
References
- Abott, D. W. 2007. Wells and words. Hydro Visions, vol. 16, no. 4, Winter 2007, p. 4-5.
- Davis, S. N., and Turk L. J. 1964. Optimum depth of wells in crystalline Rocks. Ground Water, vol. 2, no. 2, p. 6-11.
- Sartono, Ori. 2007. Parameterization of fractured hardrock aquifer in western foothills of the Sierra Nevada, California. MS Thesis, California State University of Fresno, 105 p.
Hi, Ori:
It is a good article. Nice website, too. Good work!
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