Experimental Study On The Water Movement And Solute Transport Of The Soil-stone Mixtures

Rock fragments are often present in soils as a result of soil genesis and human activity. The rock fragments are different in contents; species and some soils are covered or inserted by the rock fragments. Rock fragments in the soil may affect the soil structure and the tortuosity of water flow paths, which may further influence the hydrological process, soil water movement and its validity. Owing to the difficulties in soil sampling and the heterogeneous distribution of rock fragment, soil is taken as a conceptual medium with homogeneous and uniform particle composition in most of the studies. As a result, it is poorly understood for soil and water processes in stony soil. In this study, water movement and solute transport processes of different rock fragment contents, sizes, position, and species are studied through columns of disturbed soil samples and field soils containing rock fragments at Liudaogou small catchment and Majiata coalmine reclamation area in the wind-water erosion crisscross region of the Loess Plateau. The main results are as followings:(1) Different rock fragment contents, sizes, positions and species have great effect on soil water movement.The infiltration process of the soil containing gravels, caliches and gangues can be both described by Kostiakov infiltration model and Philip equation very well. The wetting front change with time can be described by a power function. The presence of all these rock fragments in the soil impended infiltration. The smoothing gravels, of which the psephicity is equal to 1, have the same influences on cumulative infiltration and wetting front. For a given time period, both of them have a negative relationship with gravel contents and sizes. Saturated hydraulic conductivity (Ks) firstly decreased with the increases of the stone contents, and then increased. The relation between them followed binomial function; Ks decreased with the increases of the mean diameters of the stones; the relation between them followed index function. The values calculated by the equation of Peck-Watson and Bouwer-Rice were much greater than measured data when the stone sizes were between 1.0 and 5.0cm.The cumulative infiltration, wetting front and Ks of soil containing caliches with irregular shape were all smaller than those of homogenized soils. The cumulative infiltration initially decreased with increasing rock fragment content to minimum values for Rc = 40%, and then increased. The Peck-Watson and Bouwer-Rice equations predicted Ks for low rock fragment contents but failed to forecast the Ks for higher fragment contents. The soil infiltration process was impended by the caliches layers. The cumulative infiltration and wetting front decreased seriously at the caliches layer and then became stable. The gangue, of irregular sheet shape, had great impact on cumulative infiltration. The cumulative infiltration decreased with the increased of gangue contents and the wetting front decreased with increasing rock fragment content to minimum values for Rc = 30%, and then increased. The existence of gangues could reduce the evaporation. The effect of gangue contents on cumulative evaporation were as follows: 0>15%>30%>45%>60%>75%. The rock fragment species also affect the soil water infiltration process. The stable infiltration rates in soil with gangues were the largest; the stable infiltration rates of soil with gravels and soil with caliches were almost the same at a low rock fragment contents, and with the increase of the contents, the infiltration rate became much larger in soil with caliches.(2) Smooth breakthrough curves were obtained in soils containing rock fragments with different contents, sizes and species, which followed a similar trend to those typical for a homogeneous soil without stones. Solute transport processes were described accurately by both the convection-dispersion equation and the two-region model. Comparing to the homogenous soil, the existences of gravels; caliches and gangues affect the solute transport process significantly. When the gravel content was higher, the initial time was much smaller but the tailing phenomenon was very seriously. When the gravel size were between 0.2-0.5cm and 3.0-5.0cm, the initial breakthrough time and the fully breakthrough time were increased obviously with the gravel contents. When the gravel size is between 0.2-0.5cm, the initial breakthrough time was the shortest; but with the increase of the gravel contents, the differences are not obviously. When caliches content was 60%, solute was present in the effluent in a relatively short time, but it took a relatively long time for breakthrough to finish; it took the longest time to break though the column when the caliches content was 40%. The solute was present in the effluent much earlier in the soil containing gangues than that in homogeneous soil and the time first decreased with the gangue contents and then increased. The solute was detected latest in the effluent gangue content was 30%.(3)The dispersivity first decreased with the increases of gravel contents and then increased, but had no relationship with the gravel sizes. When the gravel size was between 0.2-0.5cm, the retardation factor changed unobvious with gavel contents. The higher Peclet number indicates that the advection process dominates. Fraction in mobile water initially declined with increasing rock content but values did not vary greatly for contents between 10% and 30%. The fraction became significantly larger when Rc was 40% but again did not vary greatly when Rc was between 40% and 60%.With the increase of gangue contents, the breakthrough curves became very slow, and the dispersivity increased. The retardation factor first increased with the gangue content and then decreased to the minimum value for Rc=30%.The rock fragments also have the effect on the solute transport process. The chloride was first detected when the contents of gravel and caliches was biggest, and the dispersivity became the largest when at the content of 40% on a weight basis, but the opposite is true for the soil containing gangues.(4)Through the study on the effect of rock fragments on soil water movement in field soils, the results were: the initial infiltration rate was the biggest when the negative pressure was -15cm and the stable infiltration rate of different negative pressure was as follows: 0 cm > -3 cm > -15 cm. The stable infiltration rate of different species changed variously with the gradually increases of rock fragment contents. The biggest stable infiltration rate of soil containing gravels appeared when the content was 51.5% and the smallest one arrived when the contents became 31.5%. As for the soil containing caliches and gangues, the special contents for the biggest infiltration rate were 60%, 15% and the smallest infiltration rate were 15%, 20%, respectively. The saturated hydraulic conductivity followed the same trend.(5) As a result of different reclaiming materials and methods, the soil physical properties and soil water movement varied greatly of four landuse types in Majiata coalmine reclamation area. The infiltration rates of the four landuse types presented the tendency as follows: grass land >forest land>shrub land>barren land. The tendency was .significant when the negative pressure was 0cm.The initial infiltration rate had a negative relationship with the soil initial water content. During the initial stage of the infiltration process, the effect of the negative pressure was not significant; but with the infiltration proceeding, the infiltration rate of grass land changed most obviously but the changes are very weak for the barren land. The saturated hydraulic conductivity of the landuse types presented the same tendency to the infiltration rate and a negative relationship could be also found between the soil bulk density and saturated hydraulic conductivity.(6)Caliches coverage and contents had a great effect on recharge coefficient and the effect of the latter was much bigger. The recharge coeffient reduced gradually with the increase of caliches coverage and there was a negative relationship between the low caliches content and recharge coeffient. The initial runoff time of the plot with caliches covering was a little longer than that in the plot with caliches in it. The average infiltration rate in the plot with caliches in it decreased with the increase of caliches contents. The initial infiltration rate became the biggest at the caliches coverage of 45.6% and 62.7%.The Kostiakov infiltration model and Philip equation could describe the precipitation infiltration process well.The experimental study and analyses in this dissertation are helpful to the understanding the processes and mechanisms of water and solute transport in stony soils which is of great importance for providing recommendations of environmental quality improvement in the related regions with stony soils.