AQUIFER SYSTEM COMPACTION RATES ESTIMATED FOR THE COACHELLA VALLEY, CALIFORNIA, USING InSAR, GROUNDWATER LEVEL DATA, AND DRILLERS' LOGS

We quantify compaction rates for the Upper Coachella Valley aquifer system, California, by filtering out the fluctuations in surface displacement caused by short-term poroelastic changes in the aquifer column. Previous work in the Coachella Valley has measured on-going subsidence using repeated leveling, GPS, and InSAR data, and has also correlated areas of extreme subsidence with declining groundwater levels. We present an InSAR time series from 1993-2000 that is consistent with the measured subsidence of these previous studies, but provides a relatively continuous record of deformation through time. With this time series and coeval groundwater level data, we estimate compaction rates at ~30 sites across the Upper Coachella Valley. Preliminary results show total subsidence rates on the order of 10 mm/yr from 1993-2000. Our analyses indicates that ~2mm/yr may be attributed to compaction near Indio, and ~4 mm/yr near Indian Wells, where significant fine-grained aquitard layers are present. Permanent compaction of fine-grained aquitards is likely a consequence of pumping exceeding groundwater capture over the last 40 years. The maximum response of fine-grained aquitard layers to extreme changes in pore pressure is delayed, and decades are often required for an aquifer system to re-equilibrate. We quantify the composite poroelasticity of the aquifer column at each well site and attempt to correlate aquitard thickness with delayed compaction rates. We utilize drillers' logs to assess the stratigraphy and discriminate between water-bearing aquifer layers and compaction-prone aquitard layers. With this information, we can identify the depth at which compaction is likely occurring.