USGS Scientists Explain How Aquifer Compaction is Measured

By California Water Science Center

A recent tour of California’s Central Valley given by the nonprofit organization Water Education Foundation included a stop at the USGS California Water Science Center’s extensometer near Porterville.

USGS scientist Michelle Sneed explainins the extensometer — *USGS scientist Michelle Sneed explaining the extensometer to group from the 2022 Water Education Foundation Central Valley Tour.*

Tour participants, made up of water industry professionals, were met by USGS groundwater scientists Michelle Sneed and Justin Brandt who showed the extensometer to the group. The extensometer is one of several that dot the Central Valley. Extensometers measure compaction and expansion of an aquifer system, providing depth-specific data that can help scientists better understand the rate, extent, and at what depths in the system land subsidence is occurring.

Ms. Sneed discussed how land subsidence occurs, and the impacts it has on infrastructure. Land subsidence is the settling or sinking of the Earth’s surface caused by subsurface movement of earth materials. In the San Joaquin Valley, land subsidence is primarily caused by the compaction of the aquifer system from groundwater overdraft. When more groundwater is pumped from the aquifer than is replenished, land subsidence can occur. Land subsidence impacts infrastructure like canals and roads, and reduces aquifer-system storage capacity. The Porterville extensometer is located near the Friant-Kern Canal which has been negatively impacted by differential land subsidence (differing amounts of subsidence across the landscape) and can now only transmit about 40% of the water it was designed to transmit.

The presentation also included and explanation of how the extensometer works and the importance of the data it provides. Extensometers are essential equipment in the evaluation of land subsidence, an issue greatly impacting the San Joaquin Valley (the southern half of the Central Valley). Whereas GPS, remote sensing, and other land surveying methods only measure changes of the land surface, an extensometer is the only device capable of measuring compaction and expansion at specific depths within the aquifer system. This depth specific data can help scientists better understand the aquifer-system response to water-level changes at specific depth intervals.

Using the extensometer to measure specific depth intervals
First, a counter-weighted steel pipe is set into a borehole at a specific depth. Then the vertical movement of the top of the pipe is precisely measured relative to a reference location near the land surface. The reference location is constructed in such a way that the effects of variable soil moisture are not measured. The measurements indicate the total compaction or expansion of the aquifer system between the reference location and the bottom of the pipe. Hourly vertical measurements are taken using a digital linear potentiometer (a type of sensor that precisely measures distance). Hourly water level measurements are also taken using a submersible pressure transducer installed either in the same extensometer borehole or a nearby monitoring well. The compaction and water-level data are relayed to a data logger and then transmitted to a network via satellite telemetry. This allows scientists to monitor the extensometer remotely.

Using the extensometer to measure specific depth intervals

First, a counter-weighted steel pipe is set into a borehole at a specific depth. Then the vertical movement of the top of the pipe is precisely measured relative to a reference location near the land surface. The reference location is constructed in such a way that the effects of variable soil moisture are not measured. The measurements indicate the total compaction or expansion of the aquifer system between the reference location and the bottom of the pipe. Hourly vertical measurements are taken using a digital linear potentiometer (a type of sensor that precisely measures distance). Hourly water level measurements are also taken using a submersible pressure transducer installed either in the same extensometer borehole or a nearby monitoring well. The compaction and water-level data are relayed to a data logger and then transmitted to a network via satellite telemetry. This allows scientists to monitor the extensometer remotely.

USGS scientist standing with a pole annotated with land-surface elevation marks at given years at bench mark H 1235 RESET. — USGS Scientist Michelle Sneed standing next to benchmark pole showing magnitudes of land subsidence measured from the years 1988 to 2016. Photo was taken near El Nido, CA, located ~120 miles north of the Porterville extensometer.

A total of 35 extensometers have been monitored in the San Joaquin Valley, with most of the monitoring occurring in the 1950s and 1960s. Surface water deliveries from the Delta-Mendota Canal and the California Aqueduct reduced the demand on groundwater pumping, resulting in water level recoveries and a reduced rate of compaction. Subsequently, many extensometers in the San Joaquin Valley fell out of use after the subsidence issues largely were addressed by water imports. However, recent regulatory and climatic reductions in surface water availability have resulted in increased demand on groundwater resources and renewed compaction and resultant land subsidence.

Communicating information about issues like land subsidence in California is an important part of USGS outreach. The California Water Science Center is pleased to continue its partnership with the Water Education Foundation to help educate the public about water-related issues facing California.