Experimental Study On Soil Quality And Water Movement And Solute Transport Of Restructured Soils

Coal plays a leading role in the energy structure of China and contributes greatly to China’s economic development. Coal mining produces a large amount of stripped soils, coal gangue, tailings and other solid wastes, which are buried or deposited in piles, replacing a large area of arable land with bare ground. In order to well known the effect on soil quality and water movement and solute transport of the restructured soils, in this thesis, we studied the distribution of physicochemical properties of the siol profile of different land use types; the land use types included abandoned grass land, farmland, black locust forest land and grass land on the east soil-disposal dump, aspen forest land and grass land on the north soil-disposal dump. The effects of vegetation recovery on soil organic content(SOC) and total N in dump restoration were also researched, and the degradation assessment of different land use types was also conducted. Meanwhile, we collected soil columns in different soil depths of different land use types, and conducted the solute breakthrough experiments in steady flow condition, the breakthrough curves of NO3--N and NH4+-N were obtained and fitted by equilibrium convection-dispersion equation(CDE) model, mobile-immobile(MIM) model and two-site nonequilibrium model, at the same time, the factors that influenced the solute transport parameters were also analyzed. In addition, the effects of Pisha sandstone content on solute transports of sandy soil and Pisha sandstone thickness on solute transports of reconstructed soil profile were evaluated, and the infiltration process of incorporation of different soils, incorporation soil and gravel, and layered soil were simulated. The main research conclusions are listed as follows:(1) Soil physiochemical properties: soil textures in this area were coarse and belong to sandy loam; the bulk density in soil-disposal dump was higher than that in abandoned grass land and farmland, meanwhile, the bulk density was low in the surface layer and large in subsurface layer of 20-60 cm, below which the bulk density decreased as depth increased. p H in soil-disposal dump was larger than that in abandoned grass land and farmland, and p H in sandy grass land was largest; however, nutrient element contents in sandy grass land were lowest, and CEC, soil organic content and total N in soil-disposal dump were lower than that in abandoned grass land and farmland, meanwhile, SOC and total N in surface layer were higher than that in deep layer; the distributions of NO3--N, NH4+-N, total P, total K in soil profile of undisturbed soil and soil-disposal dump had no significant trend. Soil in different land use types all belonged to degraded soil(SI≤5%), SI in surface layer was lower than deeper layer, and degradation degree in soil-disposal dump was lower than abandoned grass land and farmland. The SI in farmland, black locust forest land and grass land on the east soil-disposal dump, aspen forest land and grass land on the north soil-disposal dump were 14.18%,-13.56%,-8.75%,-25.63%,-24.91%, respectively.(2) Comparing to shrub and tree plants, the improved effect of herbage plant on soil total N and organic content was more significant, and among the herbage plant, astragalus adsurgens and alfalfa were the best choice for increasing the total N and SOC in soil-disposal dump. Meanwhile, total N and SOC in aspen forest land were the lowest, indicated that the improved effect of aspen tree on soil was not obvious. total N and SOC in layer of 0-10 cm were higher than that in 10-20 cm, this indicated that litter played an important role in increasing the total N and SOC. In addition, total N and SOC in soil increased as reclamation years increased.(3) The breakthrough curves(BTCs) of NO3--N and NH4+-N were obtained from different depth of different land use types, the result showed that breakthrough time in soil of sandy grass land was shortest, and the breakthrough time in soil of east soil-disposal dump was longest, and breakthrough time in surface layer(0-20 cm) was shorter than that in deeper layer. Breakthrough time of NH4+-N was longer than that of NO3-N, and tailing phenomenon was also observed. The mean pore water velocity of different land use types ranged from 3.20 cm/h to 20.73 cm/h, and in a descent order as: sandy grass land, aspen forest land on the north soil-disposal dump, black locust forest land on the east soil-disposal dump, farmland, abandoned grass land, grass land on the north east soil-disposal dump, grass land on the east soil-disposal dump. For solute transport of NO3-, fitting effect of equilibrium CDE equation was better than two regional(T-R), and no physics non equilibrium phenomenon was existed in the transport process of NO3-. The fitting result of equilibrium CDE equation showed that, dispersion coefficient(D) of different land use types ranged from 0.66 cm2/h to 3.61 cm2/h, dispersivity(λ) ranged from 0.13 cm to 1.10 cm, retardation factor(R) ranged from 0.78 to 0.95. The Peclet number(Pe) of all land use types were larger than 1, this indicated that convection was the main transport mode. Chemical non equilibrium phenomenon was existed in the transport process of NH4+, two site(T-S) model could fit the transport equation well, and the result showed that retardation factor(R) ranged from 2.56 in sandy grass land to 5.82 in grass land on the east soil-disposal dump.(4) Soil texture was the main factor that affected the transport of NO3-. Mean pore water velocity decreased exponentially with increasing of bulk density, the relationship between initial breakthrough time, complete breakthrough time and bulk density could be described by polynomial function. Dispersivity(λ) decreased exponentially with increasing of sand content, Mean pore water velocity increased exponentially with increasing of sand content, and Peclet number(Pe) increased linearly with increasing of sand content, these results indicated that solute transport mode tend to be convection in the coarser soil. Meanwhile, cation exchange capacity(CEC) and soil texture were the main factor that affected the transport of NH4+. The relationship between retardation factor(R) and sand content, mean pore water velocity demonstrated a significant negative correlation, on the contrary The relationship between retardation factor(R), mean pore water velocity and CEC, bulk density, silt, as well as clay content showed a significant positive correlation. The relationship between retardation factor(R) and CEC, mean pore water velocity could be described by exponential and power function, respectively.(5)The addition of Pisha sandstone could retard the transport of bromide anion(Br-) and sodium cation(Na+) in sandy soil, and the retard effect was more obvious with increasing of Pisha sandstone. Breakthrough time of the two ions all increased with increasing of Pisha sandstone. CDE equation model and T-R model could fit the BTCs of Br- well, though goodness of fit of T-R model was better. The fitted parameter showed that convection was the main factor affecting transport of Br-. Meanwhile, a better fit of Na+ was obtained by using T-S model. Pisha sandstone retarded solute transport in sandy soil, the main reasons were that Pisha sandstone could reduce mean pore water velocity, at the same time, CEC in Pisha sandstone was higher than sandy soil because of the high content of montmorillonite in Pisha sandstone, this made Pisha sandstone have higher adsorption capacity of Na+. The complete breakthrough time of Br- was in a descent order as: loess, Pisha sandstone, mixture of loess and coal gangue, Pisha sandstone interlayer of thickness of 2 cm, 4 cm, and 6 cm. The transport of Br- could be retarded by increasing the thickness of Pisha sandstone. The goodness of fit of T-R model was better than that of CDE model, and the fitted parameter showed that convection was the main factor affecting solute transport. Meanwhile, the complete breakthrough time of Na+ was in a descent order as: loess, Pisha sandstone, mixture of loess and coal gangue, Pisha sandstone interlayer of thickness of 2 cm, 4 cm, and 6 cm. The transport of Na+ could be retarded by increasing the thickness of Pisha sandstone. The T-S model could fit the BTCs of Na+ better, and the retardation factor obtained was much larger than loess and mixture of soil and gravel. The retardation factor increased with increasing of thickness of Pisha sandstone; therefore, solute transport can be retarded by choosing rational thickness of Pisha sandstone.(6)Soil configuration had strong influence on infiltration characteristics, expressed as infiltration capacity of aeolian sandy soil, loess and soft rock decreases gradually, the ways of adding soft rock to aeolian sandy soil and loess by means of mixed adding and layered adding both could extend infiltration time and reduce infiltration rate; the infiltration process of soil containing calcites and gangue was inhibited under the condition of high bulk density; compared with homogeneous soil, layered structure soil had lower infiltration capacity. The relationship between cumulative infiltration and wetting front advancing distance could be well modeled as linear function. Compared with Philip model, Kostiakov infiltration model could better fit the infiltration process with different configurations(R2 > 0.93). The result could provide references for further studing soil water movement process and provide technical support for the reconstruction and reclamation of mining soils in Shanxi, Shaanxi and Inner Mongolia energy region.