Understanding the Link Between Changing Head Levels and Land Subsidence: A Field Experiment in California’s San Joaquin Valley

Abstract:

Interferometric Synthetic Aperture Radar (InSAR)-derived maps of surface deformation in the San Joaquin Valley clearly reveal the dramatic land subsidence, triggered by groundwater pumping and associated aquifer compaction, which has led to millions of dollars of infrastructure damage. InSAR-derived deformation maps not only reveal the problem but could also be used to support the sustainable groundwater management required to reduce further subsidence. Effective management requires a fundamental understanding of the links between changing water levels in a multi-component aquifer system and surface deformation. Developing this understanding needs a field experiment with data on three key variables: head levels, subsurface lithology and subsidence. Recently available, high-quality datasets over the period 2015-20 provide us with the opportunity to conduct such an experiment. Our approach is to extract and analyze the relevant data on the three variables and then run numerical models of subsidence, using the results to provide insight into triggers of past subsidence, as well as making critical predictions of future subsidence.

We focus on a study site near the town of Hanford, where the fresh groundwater system is composed of an upper and lower aquifer separated by a thick clay layer. We use data from a continuously-sampled, depth-registered well installed in 2015 to develop context on how to interpret older, periodically-sampled data and form head level time series in both aquifers since 1970. We combine resistivity logs, boring logs and data from an airborne electromagnetic survey to estimate the quantity and thickness of clay layers within the subsurface. Using these derived data as inputs, we numerically model compaction in the subsurface. We validate our model output with the InSAR surface deformation data.

Preliminary results demonstrate the crucial importance of the lower aquifer, where the largest portion of compaction occurs, due in part to its confined nature. We also see the increasingly important role of thicker clay layers, such as the confining aquitard, over long time periods, and show their potential to cause subsidence for decades after head levels stabilize. Our results provide invaluable insight for groundwater managers faced with the task of mitigating future subsidence.