The Observation And Analysis Of The Molecular Diffusion Of Salt Of The Observation Groundwater Well In The Offshore Area

It is one of most interesting problems in hydrogeology to understand theprocess of groundwater flow and solute transport, or to speculate thehydrogeological conditions near a groundwater observation well through analysis ofthe observed groundwater salinity, water level, and temperature in the well tipped ina deep confined aquifer in coastal area. In the process of solute mass transfer,diffusion coefficient is one of the important physical parameters. It reflects thediffusion characteristics in the process of each component mass transferring.Although there are various methods for measuring the diffusion coefficient Dm, thegeneral process is usually time-consuming. In order to explore new methods toobtain hydrogeological parameters, based on the careful analyses of the observeddata of the groundwater conductivity, temperature and pressure variations in thestratification standard drilling hole (K3) in the Caofeidian industrial area, it is foundthat the observed groundwater salinity continuously increase with time. Wespeculate that this phenomenon is most probably due to the existing of anunderground brine formation at the bottom of the observation well and the moleculardiffusion of the brine leading to the continuous salinity increasing. In order toexamine this hypothesis, we build a hydrogeological concept model of moleculardiffusion under some ideal and reasonable conditions of salt diffusion. Then amathematic model of the one-dimension salt molecular diffusion is set up to deriveand calculate the analytical solution of the model. Then a least-squares code writtenby Fortran language is used to fit the observed salinity data and the analyticalsolution. In the process of least-squares fitting, a penalty method is used toestimate the model parameters such as C0, the salinity value at the bottom of theobservation well and the diffusion coefficient of Dm. The fitting results show that(1) the estimated value of the molecular diffusivity in the mathematical model isvery close to the values measured by previous researchers in laboratory,(2) theestimated value of the other model geometrical parameter in the mathematical model is reasonable and in accordance with the real configuration of the observation well.The salinity predicted by the model solution is very close to the observations. So themodel hypothesis is verified. The new method can quickly and efficiently estimatethe diffusion coefficient of Dmvalue accurately, providing theoretical support fordata analysis of the solute diffusion.