Refurbished extensometer sites improve the quality and frequency of aquifer-system compaction and groundwater-level measurements, San Joaquin Valley, California, USA

Extensive groundwater withdrawal from unconsolidated deposits in the San Joaquin Valley caused widespread aquifer-system compaction and land subsidence locally exceeding 8 meters (m) between 1926 and 1970. To identify the extent of subsidence, a network of 31 extensometers was installed in the 1960s. Importation of surface water in the early 1970s resulted in decreased groundwater pumping, a steady water-level recovery, and a reduced rate of compaction; consequently, data collection was sharply reduced. However, reduced surface-water availability during 1976-77, 1987-92, and 2007-09 caused increased groundwater pumping, lowered water levels, and renewed compaction. The resulting land subsidence has reduced freeboard and flow capacity of the Delta-Mendota Canal (DMC), the California Aqueduct (AQ), and other canals. Four deep (>300-m) cable-type extensometers from the old network, located along the DMC and AQ, were refurbished to identify existing and future subsidence, and to improve the quality and frequency of compaction measurements. Measurement quality was improved at three of these sites by replacing the existing reference tables, which sit atop concrete pads, with new reference tables cemented in 5.5-m boreholes and decoupled from the concrete pads to minimize the measurement of near-surface deformation. A new reference table could not be constructed at the fourth site due to restrictive drill-rig access. Insulated metal shelters were constructed to protect the equipment against environmental exposure at all sites. The frequencies of compaction and water-level measurements at the extensometer sites were improved by instrumenting each with a linear potentiometer and one or more submersible pressure transducers, respectively. An analog dial gauge was installed on each extensometer to provide data continuity in cases of electronic data interruption and to provide verification of potentiometer data. Aquifer-system compaction data from all four sites show fluctuations associated with air temperature and deformation likely associated with groundwater-level changes. Frictional effects on the measured signals, likely caused by contact between the cable and the borehole casing, also are evident. The friction effects on data were often overcome (at least partially), but the time delay between actual compaction and recording of the compaction is unknown. At the four sites, groundwater-level declines ranged 0.5-35 m during spring 2012, but the water levels were above historical lows, indicating that the aquifer system was deforming elastically. Continued monitoring in the San Joaquin Valley is warranted since future surface-water deliveries will likely not meet demand. Consequently, further groundwater-level declines and land subsidence are expected. The high-frequency measurements of compaction and groundwater levels from these four sites will improve analysis of aquifer-system responses, thereby enabling detection of transitions in the stress/strain relationship (preconsolidation-stress threshold) and calculation of aquifer storage properties. The measurements obtained from the monitoring network may ultimately provide managers with the information needed to manage water conveyance systems, water-banking strategies, and other infrastructure.