Flowing Well Penetrating a Confined Aquifer
A 6-inch PVC well is completed to 380 ft in a valley-floor pasture. The log shows 0–40 ft silt, 40–180 ft sand, 180–260 ft clay, and 260–380 ft fractured volcanic rock. The well penetrates a confined aquifer at 260 ft and flows an estimated 80 gpm at land surface with a shut-in pressure of 25 psi. The owner wants to leave the casing open so the water can run down a ditch to a pond, with no valve or meter, because 'the water has always flowed.' The well is 60 ft from a wetland and 200 ft from a domestic well. The annular space was grouted only from 0 to 60 ft. The driller must decide how to complete the well and what to tell the owner about flow control, sealing, and reporting.
1. What are the construction and waste-control problems with leaving the well open?
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Leaving the well open is unacceptable. An 80-gpm uncontrolled flow is waste of groundwater and can dewater the confined aquifer, affect the wetland and nearby domestic well, and create a safety hazard. The annular grout only to 60 ft does not seal the 180–260 ft clay confining layer, so the artesian aquifer can leak up the outside of the casing. The driller should tell the owner that a flowing well must be controlled with a valve, pipeline, and meter, and that the annulus must be sealed to the confining unit. 'It has always flowed' does not make the flow a beneficial use.
2. How should the well be completed to control the artesian flow and protect the aquifer?
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Complete the well by setting casing through the clay confining layer to about 260 ft, then cement-grouting the annulus from the shoe back to land surface. Perforate or screen only the 260–380 ft producing zone. Install a control valve and a pressure-rated pipeline at the surface; use a packer or wellhead assembly if necessary. The seal must be verified by volume and surface return. Shut in the well and record the shut-in pressure (25 psi) and static level. Do not leave the well open to the ditch while completing; control the flow first.
3. What should be reported to the Division and explained to the owner?
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Report the well as a flowing well. Include total depth, casing and seal depths, the 260-ft confining layer, the producing interval, estimated 80-gpm flow, 25-psi shut-in pressure, static water level, and the control measures installed. Give the owner a copy and explain the need to keep the valve closed when not in use, meter the flow, and prevent backflow or cross-connection. If the flow is to be used, the owner needs the appropriate water-right authorization for that use. The driller should also note the proximity to the wetland and domestic well in the record.