Development and Application of a 1D Compaction Model to Understand 65 Years of Subsidence in the San Joaquin Valley

Abstract

California's San Joaquin Valley (SJV) has experienced periods of land subsidence, triggered by the extraction of groundwater and subsequent compaction of subsurface clays, since the 1920s (Poland et al., 1975). The magnitude and extent of this subsidence have been vast: up to 9 m has been observed at some locations, while the footprint of subsiding regions exceeded 13,000 km 2 by the 1980s and has continued to grow since (Faunt et al., 2016;Ireland et al., 1984). The subsidence has been immensely damaging to long-range infrastructure crossing the Valley, in particular gravity-driven canals and aqueducts which have required remedial repairs costing 100s of millions of dollars (Borchers & Carpenter, 2014). This, in combination with a new wave of subsidence starting in the statewide 2012-2015 drought, led to subsidence being included in a groundbreaking piece of legislation, the Sustainable Groundwater Management Act (SGMA), which legally requires groundwater managers in California to make plans to reduce subsidence along with other negative impacts of groundwater extraction. Against this backdrop, it is a critical time for the study of subsidence in the Valley.Subsidence in the Valley is a complex process. The large-scale hydrostratigraphy is a three-layered alluvial aquifer system: an upper aquifer, a regional confining unit known as the Corcoran Clay, and a lower aquifer. Within the upper and lower aquifers is a mix of sediments, which are commonly described as a coarser-grained background containing many clay interbeds. Sediments throughout the aquifer system will compact, leading to land subsidence, if there is a drop in hydraulic head within those sediments. Drops in hydraulic head are triggered when there is a net loss of water from the aquifer system, which primarily occurs when the largest sink-groundwater pumping-exceeds recharge, a circumstance known as overdraft. Overdraft conditions have occurred repeatedly in the SJV since the early 20th century. Importantly, the head drops do not occur simultaneously in all of the sediments. Since they are hydraulically conductive, pumping and recharge both occur in the coarser-grained sediments, and head accordingly drops first in these materials. However, the head in the Corcoran Clay and clay interbeds only responds gradually, because clay has a low hydraulic conductivity. This gradual decline in head in clay layers means they experience so-called residual compaction, a process which can continue long after the overdraft which initiated it. The compaction of clays is the primary origin of subsidence, since clay is far more compressible than the coarser-grained materials. Together, these complexities mean that the subsidence is heterogeneous in space and time, thus challenging to study.

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