Comparative Study On Several Methods For Solving The Dispersion Coefficients Of Aquifers

The Kashi region is a typical water-quality water-deficient area where lacks fresh water resources,and high-sulfuric acid is enriched in some areas.Urban development requires large amounts of water.However,blindly exploiting underground freshwater resources is likely to lead to the invasion of high-sulfuric acid water.To properly develop freshwater resources and prevent the invasion of high-sulfuric acid water and correctly evaluate the groundwater mining threshold,we must study the laws of high-sulfate migration in aquifer media.Therefore,it is imperative to establish a reliable solute transport model in the study area,and the dispersion parameters are the key to this model.So far,many experts and scholars have done a lot of research on dispersion parameters,but most of the research focuses on indoor experiments,and few scholars combine the field and laboratory scales to carry out research.In particular,there is a lack of comparative studies on different methods of dispersion parameter calculation.Therefore,this paper takes the study of groundwater aquifer dispersion parameters in the delta region of Kashi as an example,summarizes domestic and foreign research results,collects existing hydrogeological data,and conducts in-situ hydrodynamic dispersion tests and indoor dispersion tests to study the effect of dispersion parameter calculation results.The influencing factors affecting the calculation results of dispersion parameters were studied and the similarities and differences between the three diffusion parameter calculation methods to obtain the dispersion parameters results were compared and analyzed.The calculation method applied to the Kashi area to obtain the dispersion parameters was determined,and the media dispersion parameters of the aquifer in the study area were determined accurately.The understanding is as follows,(1)This study shows that when the field test observation time exceeds 200hours and the seepage distance is less than 3m,the linear graphic method is the best method for calculating the dispersion coefficient of the aquifer in Kashi region.The reliability is relatively high because the calculation result is close to the indoor test result which is in line with the actual situation in the study area.This method obtains a submarine flow velocity of 0.07m/h in the test field.When the seepage distance is2.2m and 3m,the aquifer dispersion coefficient is 0.0071m~2/h,0.0263m~2/h,and the dispersion degree is 0.1009m,0.5128m respectively.The tests observed a significant spatial scale effect,that is,with the increase of the seepage distance,the calculated values of the vertical dispersion coefficient and longitudinal dispersion show an increasing trend.This study found that when the observation time of the field test is longer and the seepage distance is shorter,the calculated results of the parameters obtained by the linear graphic method are close to those of the indoor test,and the reliability is high.This method obtains a submarine flow velocity of 0.07m/h in the test field.When the seepage distance is 2.2m and 3m,the aquifer dispersion coefficient is 0.0071m~2/h,0.0263m~2/h,and the dispersion is 0.1009m,0.5128m respectively,and observed a significant spatial scale effect,that is,with the increase of the seepage distance,the calculated values of longitudinal dispersion coefficient and vertical dispersion show an increasing trend.(2)When the factors affecting the dispersion parameters of indoor test are discussed,on the one hand,the average particle size is taken as the main research factor and the results show that the larger the average particle size,the greater the dispersion of medium in the aquifer;on the other hand,the tracer concentration is taken as the main research factor and the results showed that when the tracer solution was used at a higher concentration,the calculation result of the dispersion coefficient was slightly higher than that of the tracer solution at a lower concentration.The concentration of the tracer solution had little effect on the calculation of the diffusion parameter.(3)Processing the same set of tracer solution concentration changes over time,the calculation method used is different,the dispersion parameters are different,and the field test results are always greater than the indoor test results.Therefore,the reliability of the dispersion parameters obtained by only one method is not high,a variety of calculation methods must be used,and a comparison analysis of indoor tests is used to determine the parameter solution method applicable to the study area.(4)From the obtained field test conductivity observation data,it can be seen that the slope of the rise curve of the groundwater conductivity conductivity curve in each observation well is greater than the absolute value of the slope of the decline segment,and the groundwater solution conductance changes in the same main flow direction are similar.The amplitude is also larger which means the dispersion effect is stronger;the variation of groundwater conductivity in the non-mainstream direction is smaller which indicates that the dispersion effect is weaker;from the obtained experimental data of the conductivity of the indoor test,it can be seen that when the test column is filled with the same lithology medium,changing the concentration of the tracer solution only affects the rate of increase in the conductivity value of the solution,and does not affect the time at which the conductivity starts to change and the time at which the peak value is reached.