| Coastal zone is the key,(?) about material and energy interchange between gr oundwater and seawater, the relationship of material and energy interchange is influence d by the groundwater flow field and seawater hydrodynamic condition, especially the tida 1 effect. The relationship of material and energy interchange is completely different if the structure of groundwater system, the characteristics of flow field or the seawater hydrod ynamic condition is different. At present, there are many researches about groundwater dy namic influenced by the tidal effect in confined aquifer. However, the tidal effect in unc onfined aquifer is ignored usually. But the drainage boundary of unconfined aquifer is se a usually, the groundwater level in unconfined aquifers will fluctuate if the sea level cha nge quickly by frequent and periodic tidal action. The velocity and flow direction of gro undwater are changed, and the influence is huge to the material and energy interchang e between groundwater and seawater, even the fluctuation is small. the influence by groundwater recharge condition is also huge to solute transport, besides tidal action. Gro undwater recharge condition in confined aquifers is bad, and the speed of solute transport is slow, but it is so different in unconfined aquifers. So how does the solute transport? Where does it transport (landwards or seawards)? And what’s the rule and characteristic? We have no idea about that. Therefore, in-depth study of material and energy exchange rules between phreatic water and seawater, not only has the important theory significanc e, but also has the important guiding significance to guide coastal area groundwater explo itation, utilization and pollution control.In this paper, the characteristic and influencing factor of the tidal effect in unconfined aquifers were studied by groundwater numerical simulation system, and solute transport in ideal coastal boundary unconfined aquifers condition was studied too. The influence of solute transport by different tide tidal wave boundary was analysed. And then, taking Fujian Gulei Peninsula sandy beach as a typical example, the geology, hydrogeology basic data and previous hydrogeology exploration data of study area were collected, the 3D geological structure model of study area was established, the hydrogeological parameter of study area were obtained accurately by the field test of hydrogeology. On the basis, groundwater flow-solute transport 3D numerical model of study area was established by Visual Modflow software, and identified by the groundwater actual observation data. Finally, the predictive study of coastal unconfined aquifers in Gulei Peninsula was done by the corrected model. The characteristics and change rule of groundwater solute transport in unconfined aquifers influenced by tidal action were summarized, the following main conclusions:1. The tidal effect on coastal unconfined aquifers is weak (Small amplitude and limited impact scope), there are few researches on the influence of tidal action in unconfined aquifers, and no published research production about the rule of groundwater solute transport in unconfined aquifers influenced by tidal effect. The rule of groundwater solute transport in unconfined aquifers influenced by tidal action was firstly studied in this paper by the groundwater numerical simulation,2. The characteristic of the tidal effect in coastal aquifers is mainly impacted by the hydrogeological parameters and coastal boundary condition:1) The hydrogeological parameters and the degree of response in aquifers responded to tidal fluctuation are decided by the characteristic of aquifer lithology and structure. Numerical simulation results show that:The groundwater tidal effect is impacted by permeability coefficient and specific yield, the number of permeability coefficient and specific yield is bigger, the amplitude of groundwater level is larger, and the lag time is shorter. On the contrary, he amplitude of groundwater level is smaller, and the lag time is longer.2) The groundwater tidal effect shows the response of the groundwater system to tidal wave, the characteristic is closely related to the coastal boundary condition. Normally, coastal boundary condition is generalized to L-boundary, that coastal boundary is fixed and only sea level waves periodically. But actually coastal boundary not only waves periodically on the vertical direction, but also moves periodically on the horizontal direction In this paper, under the condition of fixed and moving boundary groundwater numerical simulation show that groundwater flow field in unconfined aquifers is influenced by the vertical wave of sea level mainly, the influence of horizontal move is weak.3. Numerical simulation of groundwater solute transport under the condition of the constant head boundary, equal amplitude fluctuation boundary and change amplitude fluctuation boundary shows:1) Influenced by tidal wave, the curve of pollutant concentration changed with time looks like oscillatory-style breakthrough curve, compared with the curve without being influenced by tidal action, shows stronger "flattening" effect. It means the peak pollutant concentration is reduced, and the pollutants will be hung in the aquifer for longer time. Moreover, the closer to the coast, the stronger groundwater tidal effect is, the "flattening" effect is more obvious too, and the retention time of pollutants in aquifer is longer.2) The tide fluctuation not only has high and low per day (day cycle), but also monthly "neap tide" and "spring tide" (monthly cycle), In the course of "neap tide" and "spring tide" monthly, sea level rose overall, and the coastline develops to the land. Sea water recharges groundwater in the aquifer, groundwater flows from sea to land, the speed of pollutants transport towards the sea is reduced, near the coast pollutants transport will even be back to landward. After "spring tide" sea level is reduced, groundwater flow towards sea again until "neap tide" monthly, and the speed of pollutants transport towards the sea is risen. The groundwater flow direction changes always, and it causes the pollutants oscillatory-style curve has the characteristics of flat-peak-style wave, double-peak-style wave and stage-style reduce. Therefore, we need to accurately describe the characteristic of coastal boundary fluctuation. We couldn’t use a sine or cosine trigonometric function to describe the tidal wave simply.4. Obtaining accurately hydrogeological parameters is the basis to carry out the prediction of groundwater flow field and solute transport in quantitative. In this paper, field hydrogeological test was carried out, and combined with hydrogeological conditions, how to get the hydrogeological parameters accurately is discussed.1) The infiltration test:according to a lot of the unsaturated seepage tests, I got the Infiltration recharge rate in different types of rock and soil. Combined with the long-term observation data of groundwater dynamic in Aeolian-pore aquifer, the partition and values of infiltration coefficient in the study area were confirmed accurately by the principle of water balance and analogy method, and the lack of groundwater dynamic observed data problem is avoided.2) pumping test:according to the 10 groups of the groundwater level high resolution observation data of well flow test in unconfined aquifers,3D flow of groundwater and the hysteresis of gravity water supply phenomenon was obviously found out further in the course of well flow test in unconfined aquifers. Compared with three types of well flow test model in unconfined aquifers, I find that Theis-well-flow model in unconfined aquifers ignoring 3D flow in unconfined aquifers and the lag of the gravity water release can’t really describe the characteristic of well flow in unconfined aquifers movement and water release mechanism. Neuman model considering 3D flow in unconfined aquifers and elastic water release can describe the characteristic of well flow in unconfined aquifers movement and water release mechanism truly. The permeability coefficient and specific yield in the study area is obtained accurately by Neuman model.3) Dispersion test:On the basis of analyzing the difficulty and lack of field dispersion test, dispersion test was carried out under the condition of artificial radial flow field after pumping.After Summing up the results of previous studies, I find that the standard curve of the two dimension radial flow dispersion problem almost coincides with the standard curve of the one dimension flow dispersion problem in convection- dominated solute transport. So Problems in artificial radial convergent flow two-dimensional diffusion can be turned into one dimensional flow dispersion problem, and then the dispersion coefficient of Aeolian pore aquifer and Marine pore aquifer in study area can be confirmed. Radial convergent flow dispersion test avoids the fault of long-time and low-success rate carried on natural flow dispersion test. Moreover, radial convergent flow dispersion test can also be carried out simultaneously with the pumping test. The drill well is really multipurpose, we can make the field dispersion test low-cost, highly-active and easy-implemented.In a word, in this paper, regardless of the method of field test on the geological hydrology or of accurately obtaining hydrogeological parameters, has certain characteristics and innovation.5. On the basis of above study, combined with the Gulei Peninsula hydrogeology, groundwater 3D numerical model was established, and carried out groundwater flow-solute transport numerical simulation, the research results show that on both sides of Gulei peninsula, the groundwater in aeolian, marine unconfined pore aquifer also shows periodic fluctuation, with the distance increasing, groundwater amplitude decays exponentially. On the other hand, the lag time of groundwater amplitude increases linearly. Because of the different lithology, the tidal effect in the side of east coast named Futouwan Aeolian aquifer is stronger than west coast named Dongshanwan marine aquifer, with bigger amplitude, longer lap time, and wider influencing range. Influenced by groundwater tidal effect, the curve of solute transport concentration in unconfined aquifers changed with the time looks like oscillatory-style breakthrough curve. Before getting the peak of concentration value, the curve rises in oscillatory-style, after the peak of concentration value, the curve descends in oscillatory-style, and has the characteristics of flat-peak-style wave and double-peak-style wave. Because of groundwater flow direction in the coastal aquifer repeatedly changing, pollutants in aquifer make a big step forward and small step back transport we call this Niuyangge-style. It causes the time of pollutants held up in aquifer becomes longer, the pollutants become diffuse and dilute continuously. These characteristic has important theoretical and practical significance on the understanding of the rule of solute transport in coastal unconfined aquifers and groundwater pollution prevention.The characteristic and innovation of this paper:(1) Study on the influence degree of moving boundary tidal effect in unconfined aquifers by groundwater numerical simulation, the results showed that:the groundwater flow field is influenced by the vertical movement of seawater, and the outside of landwards highest coastline is almost not influenced by the horizontal movement of coast boundary.(2) the results of groundwater numerical simulation:the influence range of groundwater tidal effect in unconfined aquifers is definitely small, but is more huge in unconfined aquifers solute transport than we even thought.(3) The groundwater solute transport rule in unconfined aquifers, influenced by changing tidal amplitude fluctuation is that the curve of pollutants transport concentration in unconfined aquifers changed with the time looks like oscillatory-style breakthrough curve. It obviously has characteristics of double-peak-style wave, flat-peak-style wave, and the curve descends in oscillatory-style.(4) The accurate hydrogeological parameters was obtained by multiple field hydrogeological test, not only the method of field hydrogeological test, but also obtaining hydrogeological parameters accurately got some new progress. |
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