Numerical modeling of the effects of pumping on tide-induced groundwater level fluctuation and on the accuracy of the aquifer's hydraulic parameters estimated via tidal method: a case study in Donghai Island, China

Coastal groundwater level is affected both by tide and pumping. This paper presents a numerical model to study the effects of pumping on tide-induced groundwater level fluctuation and on accuracy of hydraulic parameters estimated via tidal method. Firstly, for the effects of pumping on the groundwater level fluctuation under the combined influence of pumping and tide, groundwater level has a drawdown but eventually reaches a quasi-steady-state again. Steady pumping can attenuate the amplitude but cannot affect the phase of the quasi-steady fluctuation. However, seaward steady pumping plays a relatively obvious role in enhancing drawdown compared with landward pumping, a partial penetration well leads to greater drawdown than a full penetration well, and transient pumping induces large amplitude which does not reflect large transmissivity. Secondly, for the effects of pumping on the accuracy of the parameter estimated via the tidal method, transient pumping or large steady pumping, especially in a full penetration well, significantly affects accuracy of the estimated parameters. However, when the distance between the pumping well and tide observation well exceeds 200% of the distance between observation well and shoreline, pumping effect on estimated parameters can be neglected. The conclusions could provide guidance for reasonable application of the tidal method.