Aquifer Compaction at Different Depths: Observations from a Vertical GPS Array at the University of Houston Coastal Center, Texas



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Observed GPS data shows that the University of Houston Coastal Center (UHCC) in the La Marque, TX area currently experiences localized subsidence at a steady-surficial rate of 0.5 to 1 cm/year. This study aims to understand the contribution of shallow (<10 m) sediments to the overall subsidence of the study area. GPS data, precipitation data, and groundwater head measurements were collected at the UHCC for two years (02/2014-02/2016). Ground deformation at 10, 20, 30, and 0 feet below the land surface (bls) were measured from four GPS stations and compared to rainfall and groundwater depth. This was done in order to identify possible correlations that may indicate different rates of ground deformation associated with each shallow subsurface layer.

Over the two year observational period at the UHCC, there was an overall vertical lowering trend of 3 to 4 mm/yr of the localized top aquifer, with surficial inflation mitigating this vertical fall to 1 to 2 mm/yr. GPS-weather data comparisons indicated that rainfall of over two centimeters per day can cause abrupt changes in ground layer vertical displacement. Pressure, humidity, and temperature changes of the air were not good indicators of ground movement. GPS-well data comparisons also indicated that about 200 mm of groundwater-level fall is correlated to about 1 mm of vertical drop in the same top aquifer. In general, groundwater movement has been found to dominate ground deformation of the shallow subsurface (<10 m).