Tempe Town Lake

TerraFirma Earth Technologies was contracted by PCL Construction, Inc. for recovery well improvements on Phase I of the Pier 202 development.

Project Specifications

Tempe Town Lake Well Reconditioning

This project consisted of the relocation\rehabilitation of recovery wells 6, 7, and 8. Well 7 remained in place for future use, while wells 6 and 8 were abandoned in place according to the rules and regulations of the AZDWR and replacement wells drilled in the immediate vicinity. Four monitoring wells were also abandoned.

System Design

To complete this project, it was sequenced into five phases as follows:

  1. Monitoring Well Abandonment
  2. Initial Recovery Well Performance Testing
  3. Well Decommissioning and Abandonment
  4. Well Reconditioning
  5. Well Replacement and Construction
  6. Final Performance Testing

Monitoring Well Abandonment

Based on the well driller’s reports obtained from AZWDR, we determined that pulling or over drilling of the well casing was impractical. Sufficient hydrogeologic  and stratigraphic information was available to determine that no aquifer boundaries and that no vertical flow components existed within the length of the wells to be abandoned. There were also no water quality contamination issues. TerraFirma followed AZDWR’s abandonment procedures as outlined in Alternative 4 of their handbook. This method consisted of a cement-bentonite grout fill to within 2 feet of the existing surface, the removal of the upper piezometer surface casing, and cutting any exposed well materials to 2 feet bgs and finally backfilling with native materials.

Initial Recovery Well Performance Testing

The initial recovery well performance tests were performed to determine baseline data from which the performace of the final wells could be determined. The initial performance tests consisted of monitoring waterlevels using an electronic data logger, recording pumping rates and discharge pressures while running the recovery wells with the existing pump and discharge piping for two hours.

Well Decommissioning and Abandonment

Recovery wells 6 and 8 were to be abandoned in place and relocated to new locations in the immediate vicinity. Based on the original well driller’s reports, it was determined to follow AZDWR abandonment Alternative 4 as described above.

Well Reconditioning

Following the completion of the initial test and the removal of the pump, well 7 was reconditioned by use of a liquid acid descaler. Once introduced into the well at the manufacturers recommended mix, the well was agitated with a brush to equally distribute the descaler product throughout the well. The descaler was allowed to sit in the well for approximately 24 hours with intermittent agitation and pH sampling to ensure the maximum effectiveness of the product. Once this process was completed, the well was airlifted until reasonably free from sediment and discoloration. The discharge was collected to allow for natural biodegration. Following treatment, the pumping unit was reinstalled.

Well Replacement and Construction

TerraFirma constructed the replacement recovery wells 6 and 8 using the Reverse Flow Bucket Auger method. To ensure a stable drilling platform, TerraFirma initially installed a 60” diameter steel temporary surface casing to a depth of 8 feet below ground surface (BGS). Following the installation of the temporary surface casing, the permanent 42” diameter surface casing was installed in a 48” borehole. Next a 34” diameter borehole was advanced to the final depth of 161’ BGS. TerraFirma utilized a specialized bucket which minimized “smearing” of the borehole wall.

Immediately following the completion of the borehole, the drilling fluid was thinned with clean water as much as was practical without jeopardizing the borehole stability. A natural enzyme breaker was also used to expidite the breakdown process.

Well materials were then placed and centered within the borehole and suspended approximately 2’ from the bottom of the borehole. Centering guides were used to ensure that the well materials remained centered in the borehole. Next, filter material was placed from 2’ below the well materials to 35’ BGS.

Following the placement of the filter material, each well was developed using an initial airlift, then mechanically by use of a double packer swab. The well was swabbed in ten feet increments until the airlift discharge was clear of sand and silt.

Once developed, a 5’ bentonite seal was placed above the filter material. The remainder of the annular space between the well casing and the surface casing was filled with a cement grout to just below the well vault bottom.

One the well was completed, the original pump was reset.

Final Performance Testing

Once the original pumps were reset in the recovery wells and were re-plumbed, each recovery well was tested using the same testing methods as described above in the Initial Performance Testing section.

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