Short-Time Geodetic Determination of Aquifer Storage Coefficient in Taiwan

Specific yield (S-y) and specific storage (S-s) are two hydrogeological parameters often estimated in a costly pumping test (hydraulic method). Pumping or a natural aquifer recession can cause gravity change and surface deformation, useful for S-y and S-s determination by the more economic geodetic method. From 2013 to 2017, over a few days we measured gravity changes at four sites and subsidence at one site in Taiwan by a FG5 gravimeter and a precision level to estimate S-y and S-s. Using short-time gravity and level records avoids complicated logistic supports and temporal effects. The measured gravity changes are associated with the MODFLOW-modeled groundwater depletion. We succeeded in estimating S-y at two sites and failed at the other two. One successful case uses rapid postrain declines of gravity and groundwater level. We did leveling only at one site, but here the 500-min, continuous, submillimeter subsidence records result in a S-s consistent with the hydraulic result. Lessons on the failed cases of S-y are suggested to avoid disrupting factors in gravity surveys. A semiautomatic leveling procedure is proposed to maximize the chance for S-s determination. Our simulations show that gravity changes increase with S-y and decrease with initial hydraulic head and horizontal gravimeter-pumping well distance, helping to predict if the gravity-based method works at a site and to best plan a gravity survey. Drilling data show that the S-y values from the hydraulic and geodetic methods represent aquifer storage coefficients over different aquifer layers. Plain Language Summary A pumping test extracts groundwater from a well to estimate an aquifers storage coefficients, based on measurements of the groundwater lowering rate during pumping. But pumping can also lower gravity values and deform the surface. After a heavy rain stops, the aquifer level can rapidly decline to lower gravity values. We measure such gravity changes and surface deformations to determine hydrogeological parameters critical to understanding aquifer storage capacity and managing groundwater resource.