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Chapter 4

Q. 4.6

A community well system is to provide 11,450 m³/d for the average day at its design life. The minimum demand at the beginning of the design life of the well field is estimated to be 3,800 m³/d. A hydraulic analysis of three wells operating at a maximum day demand of 22,900 m³/d sustained for a 10-day period will lower the piezometric surface to the bottom of the confining layer of the artesian aquifer. The distribution system has storage capacity for one day at the maximum demand.

Recommend a well system (number of wells and pumping rate) for this community.

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Verified Solution

The three-well system is not satisfactory for two reasons. First it does not provide the redundancy requirement of one well out of service at the time of the maximum demand. Second, it provides no margin of safety to protect the aquifer from overpumping. Even if the demand fell to the average day demand after the sustained maximum demand, continued pumping would lower the piezometric surface below the aquiclude. More likely, pumping to meet the average day demand prior to the 10 days of maximum demand would have lowered the piezometric surface sufficiently so the aquifer would be dewatered.

One alternative solution is to provide six wells with a capability of meeting the maximum day requirement with only three wells operating. This would meet the regulatory requirement to have one spare well available at the time of the maximum demand. The six wells would have to be located by hydraulic analysis to lower the interference effects sufficiently so that the piezometric surface would not be lowered below the aquiclude over a long-term pumping cycle that included the 10 day maximum demand.

Comment. One day’s storage in the system has little impact on the well system design for this case.