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Date/Time: Wednesday, January 17, 2018 1200 hours
Depth to Water at PA-7 = 135.67 feet-below top of casing
Guidance Level for PA-7 = 245 feet-below top of casing

Because of the historical occurrence of pumping-induced land subsidence and ground fissuring in southwestern Chino Basin (southern MZ-1), the OBMP called for the development and implementation of an interim management plan for MZ-1 that would:
  • Minimize subsidence and fissuring in the short-term,
  • Collect information necessary to understand the extent, rate, and mechanisms of subsidence and fissuring, and
  • Formulate a management plan to reduce to tolerable levels or abate future subsidence and fissuring
From 2001-2005, Watermaster developed, coordinated, and conducted an Interim Monitoring Program (IMP) under the guidance of the MZ-1 Technical Committee, which is composed of representatives from all major MZ-1 producers and their technical consultants. The IMP was an aquifer-system and land subsidence investigation focused in the southwestern region of MZ-1 that would support the development of a long-term management plan to minimize and abate subsidence and fissuring (the MZ-1 Plan). The IMP involved the construction of highly-sophisticated monitoring facilities, such as deep borehole extensometers and piezometers, the monitoring of land surface displacements through traditional ground-level surveys and remote-sensing techniques, the detailed monitoring of the aquifer system with water-level-recording transducers installed at an array of production and monitoring wells, and the purposeful stressing of the aquifer system through multiple controlled pumping tests.

The investigation methods, results, and conclusions are described in detail in the MZ-1 Summary Report, dated February 2006. The investigation provided enough information for Watermaster to develop Guidance Criteria for the MZ-1 producers in the investigation area that, if followed, would minimize the potential for subsidence and fissuring during the completion of the MZ-1 Plan. The Guidance Criteria included a listing of Managed Wells and their owners subject to the criteria, a map of the so-called Managed Area, and an initial threshold water level (Guidance Level) of 245 feet below the top of the PA-7 well casing. The MZ-1 Summary Report and the Guidance Criteria were adopted by the Watermaster Board in May 2006. The Guidance Criteria formed the basis for the MZ-1 Plan, which was approved by Watermaster in October 2007. The Court approved the MZ-1 Plan in November 2007 and ordered its implementation.

Presently, Watermaster continues to implement the MZ-1 Plan. Since 2005, drawdown at the PA-7 piezometer has not fallen below the Guidance Level, and very little, if any permanent compaction has been recorded at the Ayala Park Extensometer. The ongoing monitoring program called for by the MZ1 Plan continues to be implemented with annual review and revision of the monitoring program as agreed upon by the MZ1 Technical Committee.