Extensometer Hunters: Searching for Long-Lost Technologies in San Joaquin Valley

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While a few days in the San Joaquin Valley might not be the same as three years aboard the starship Enterprise, California Water Science Center scientists searching the Valley for lost technologies had no less of an exploration ahead of them during a July 2016 field trip to the area. CAWSC hydrologists Michelle Sneed and David Dillon went on a chase, driving more than 100 miles a day for four days, to look for old extensometers – tools that measure the compaction and expansion of aquifer systems. The goal of the trip was to locate extensometers that had been installed by the USGS or their collaborators in the 1950s and 60s, but had since been discontinued. Locating abandoned extensometers would allow Sneed and Dillon to evaluate their condition, and determine whether they could be refurbished and used in the future.

Extensometers are essential equipment in the evaluation of land subsidence, a prominent issue impacting the San Joaquin Valley. Land subsidence is the settling or sinking of the Earth’s surface caused by subsurface movement of earth materials. In the Central Valley, groundwater over-pumping has created significant land subsidence in several areas. When groundwater is pumped and not replaced (recharged) quickly enough, compaction can occur. Fine-grained sediments like clays and silts within an aquifer system are the main culprits of pumping-related subsidence. Fine-grained sediments are special because they are composed of platy grains (imagine the shape of dinner plates). When fine-grained sediments are originally deposited, they tend to be deposited in random orientations (imagine haphazardly placing your dinner plates in the sink). These randomly-oriented sediment grains have a lot of room between them to store water. However, when groundwater levels decline to historically low levels, those randomly-oriented sediments are rearranged into stacks (your neatly-stacked plates in the cupboard). These stacks occupy less space, and also have less space between them to store water.

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Situated between three mountain ranges, the San Joaquin Valley is mostly filled with fine-grain sediment and clay deposits from the mountains, making it naturally susceptible to compaction. The threat of aquifer compaction increases as groundwater-levels decline, exceeding historic lows. About 20% of the Nation’s potable groundwater is pumped from Central Valley aquifers, making it the second-most-pumped aquifer system in the United States. Most of the water is used for agriculture, but the water is also pumped for domestic and industrial uses. San Joaquin Valley has all the pieces of the subsidence puzzle: fine sediment, groundwater over-pumping, and a period of recent drought. By the 1970s, the USGS determined that about half of the San Joaquin Valley had experience subsidence; that’s approximately 5,200 square miles impacted since the discovery of subsidence in the area in the 1930s. Some areas had even experienced dozens of feet of subsidence.

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Extensometers measure the compaction and expansion of the aquifer system, providing depth-specific data that can help CAWSC scientists better understand the rate, extent, and at what depths in the system subsidence is occurring. The extensometers in the San Joaquin Valley are some of the first ever built in the U.S., with 31 extensometers being installed at 21 sites in the 1960s. Many of these extensometers have not been used, or even visited, since the 1970s. By the early 2000s, only six of the original 31 extensometers in the San Joaquin Valley were still operating at five different sites.

"The extensometers were abandoned years ago after subsidence issues largely were fixed by water imports through the Delta-Mendota Canal and the California Aqueduct that resulted in less pumping and increased groundwater-level recoveries," Sneed said. "Now subsidence is a problem again because groundwater pumping has increased and groundwater levels have declined."

Using old USGS well-completion reports and drilling logs, Sneed pinned down the last-recorded locations of 25 different extensometers, which were scattered across 18 different sites. Sneed and Dillon were able to locate nine extensometers at eight different sites.

"While this seems like a low number, considering development in the area since the extensometers were placed, and the passage of time, I was hoping to find maybe five," Sneed said. To Sneed, finding almost double that is a great success. "For decades, we didn’t know anything about their conditions. At least now we do."

"There was no trace of anything at many of the sites," Dillon said. "On the chance that we did find something, we would do a basic well canvasing: take pictures, verify the GPS location, see in how good of a condition the well and housing was. Lots of the extensometers still had their original equipment down in the hole."

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With that basic understanding of the location and foundational condition of the extensometers, Sneed plans on learning more about the newly re-discovered pieces of equipment through the helpful use of down-hole cameras deployed down the extensometer boreholes. Doing this, she will be able look 1,300 feet into the extensometers to determine what condition they are in, and assess if they can be refurbished for future use. That’s if – of course – the extensometer will allow her to see that far, as the borehole may be obstructed.

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"Who knows what’s down there?" Sneed said. "Well-casing collapses are not uncommon in areas with land subsidence. Casing breaks are essentially a deal breaker for restoration."

Why bother with extensometers – particularly retired ones – at all?

"Extensometers are the only way to understand at what depth compaction is occurring," Sneed said. "How can groundwater managers address compaction in accordance with their Groundwater Sustainability Plans if they don’t know where it’s happening?"

Once Sneed has a better picture of the state of the recovered extensometers, she can begin to prioritize them for refurbishment.

"Extensometers are really expensive to build," Sneed said. "Finding these older ones is the first step in determining if they could be refurbished. We’ve refurbished extensometers before, and those sites are successful. We wanted to go out and find these other ones, and see if it could be done to them, too. The idea is to see if they can be brought up to snuff at a reduced cost compared to building new ones."