A Study On Hydrodynamic Characteristic Of Flowing Wells In Thick Unconfined Aquifers

The numerous long-screen flowing wells in the thick cretaceous unconfined aquifer of the Ordos Plateau were found to be closed to the vertical hydraulic gradients in discharge areas.In order to reveal the hydrodynamic cause of these flowing wells,in this dissertation,the traditional well hydraulic model which neglects the vertical gradient was improved,a 3D unit basin model with a flowing well was built,the hydraulic connection between the flowing well and aquifer was studied.It was found that there is always a transition point inside the open flowing well developed in the unconfined aquifer.Below it there is an inflow segment where groundwater flows from the aquifer to the flowing well;while above it there is an outflow segment where groundwater flow from the flowing well to the aquifer;the total groundwater inflow is always larger than the total groundwater outflow.For flowing wells in confined aquifers,a similar phenomenon can occur.Because groundwater around the outflow segment does not enter the well while groundwater around the inflow segment has a larger horizontal velocity as depth increases,the majority of the water at the well outlet come from the lower half of the flowing well.As the decrease of ambient vertical hydraulic gradient induced by the decline of water table,the shallower well depth or the farther distance from the well location to the valley,the relative elevation of transition points fall,resulting in more percentage of water at the outlet come from the deep part.In order to explain the phenomenon that the mixed groundwater at the well outlet can represent the deep groundwater in discharge areas,we took the groundwater age as an example,built a solute transport model with a flowing well in the basin,and obtained the equivalent position of groundwater at the well outlets were based on profiles of groundwater age inside the well and outside the well.It was found that after drilling the groundwater age around the outflow segment would increase significantly,while the groundwater age around the inflow segment would keep unchanged with that before drilling for a long time.It indicates that the existing flowing wells have the potential to be used to collect groundwater samples representative of formation groundwater before drilling.The representativeness of water at the outlet of flowing wells is dependent on three factors: the relative elevation of the transition point,the horizontal velocity profile of influent groundwater from the inflow segment,the concentration profile of influent groundwater from the inflow segment.It was confirmed that the groundwater sampled at the outlet of flowing well in the unconfined aquifer can represent the deep groundwater.The case study in the Dosit River Watershed indicates that the similar conclusions can be obtained from the model with a recharge boundary,i.e.,the flowing well is developed inside the discharge area and a transition point always exists in the upper part.Taking the advantage of the results about the equivalent positions,the circulation depth of flow system obtained by the previous research can be modified.Besides,it was also found that the circulation depth of flow system obtained from the hydrochemical method is usually shallower than the maximum penetrating depth of flow system obtained from the hydrodynamic method.The results presented in this dissertation can not only be helpful to improve the understanding of flowing wells in the unconfined aquifer,but also provide a new idea of how to take full advantage of existing long-screen wells to take water samples.