No abstract

No abstract

Specific yield (Sy) and specific storage (Ss) 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 Sy and Ss 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 Sy and Ss. 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 Sy 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 Ss consistent with the hydraulic result. Lessons on the failed cases of Sy are suggested to avoid disrupting factors in gravity surveys. A semiautomatic leveling procedure is proposed to maximize the chance for Ss determination. Our simulations show that gravity changes increase with Sy 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 Sy values from the hydraulic and geodetic methods represent aquifer storage coefficients over different aquifer layers.

This study presents the stochastic Monte Carlo simulation (MCS) to assess the uncertainty of flow and conservative transport in 3-D discrete fracture networks (DFNs). The MCS modeling workflow involves a number of developed modules, including a DFN generator, a DFN mesh generator, and a finite element model for solving steady-state flow and conservative transport in 3-D DFN realizations. The verification of the transport model relies on the comparison of transport solutions obtained from HYDROGEOCHEM model and an analytical model. Based on 500 DFN realizations in the MCS, the study assesses the effects of fracture intensities on the variation of equivalent hydraulic conductivity and the exhibited behaviors of concentration breakthrough curves (BTCs) in fractured networks. Results of the MCS show high variations in head and Darcy velocity near the specified head boundaries. There is no clear stationary region obtained for the head variance. However, the transition zones of nonstationarity for x-direction Darcy velocity is obvious, and the length of the transition zone is found to be close to the value of the mean fracture diameter for the DFN realizations. The MCS for DFN transport indicates that a small sampling volume in DFNs can lead to relatively high values of mean BTCs and BTC variations.

No abstract