The Mechanism And Simulation Study Of The Air Negative Pressure Induced By Pumping Test In Unconfined Aquifer With Low-permeability Cap

Pumping test as a mean for the groundwater hydrologist to evaluation aquifer, providethe basis for groundwater exploitation, prevent the land subsidence, ground cracking andstructrures subsidence or tilt due to the decline of groundwater levels. Traditionally,pumping tests focus on saturated flow below the water table and the soil above the initialwater table prior to pumping has been thought to have little influence on the test results andhas usually been ignored. However, many results from field study and numerical modelshow that the airflow may be generated, especially with a low-permeability top layer.Because the airflow in the unsaturated zone has significantly affects, some scholarsconducted numerical simulation to study influence of tides and rainfall infiltration onairflow in the unsaturated zone. Jiao and Guo [2009] found that if the unsaturated zone wascapped by low-permeability soil, airflow might be caused in the unsaturated zone duringpumping and it has an important impact on the parameter estimation in the aquifer. In thispaper, we review and summarize the research Progress of gas-water two-phase flowcaused by airflow (herein defined as "air negative pressure") which product under differentfactors in the unsaturated zone. With the airflow in unsaturated zone caused duringpumping as an example, we designed laboratory physical model, used the tension meter,soil moisture monitor and U tube to study the mechanism and simulation study of thenegative pressure induced by pumping test in unconfined aquifed with low-permeabilitycap. Using the TOUGH-MP for numerical simulation with kinds of experimentalparameters, then compared simulation results with the experimental data. The data obtainedby the physical model experiments are analyzed, the main conclusions show as follows:(1) There was airflow (herein defined as" negative pressure") generated during pumpingin the vadose zone with a low-permeability cap. Prior to pumping, the NP graduallyincreases with the drawdown until reaching a peak value which can be break throughobstacles of the layer. At last, the NP gradually returned to zero (atmospheric pressure).(2) The peak of air negative pressure decrease with the radial distance and increase withinitial water table and pumping rate, the corresponding time to reach the peak value of airnegative pressure is gradually increased.(3) When the negative air pressure exists in unsaturated zone, the gravity release water inunsaturated zone obviously lagged. While the opposite aquifer, the water content in aquifer decreases faster than which without the negative air pressure, the gravity of water inadvance. If ignore the small changes in the flow rate Q, the gravity water release of T1andT2is only relate to the initial water table S0. The shallower of initial water table, the longerof time to gravity water release lagged.(4) The drop of drawdown in the aquifer with the low-permeability soil cap is faster thanthe normal conditions the shallower and the initial water table, the larger the drawdown.(5)The change of water pressure increases with the drop of water table in the aquiferduring the pumping and decrease with the radial distance. Capillary pressure is bound upwith the negative pressure air and water pressure, when there is negative pressure air inunsaturated zone, the capillary pressure also increases.(6) Numerical simulations of the pumping tests were carried out using TOUGH2-MP.The simulations curves of negative air pressure with radial distance and initial water tableand drawdown with initial water table are given by TOUGH2-MP consistent with theexperimental results.