| Vertical hydraulic conductivity(K_v)of the riverbed is an important parameter to the estimation of contamination transformation as well as water exchange between groundwater and surface water.The temporal and spatial variability of K_v can well reflect the amount of water exchange and spatiality of contamination.The studies about the temporal and spatial characteristics of K_v as well as particularly its influencing mechanism have been relatively deficient.Therefore,the temporal and spatial characteristics of K_v as well as its influencing mechanism is analyzed,exploited,and determined systematically in this study,which can provide some theoretical references for the modification of river ecosystem and the maintainability of river health.In this study,the field study of in-situ measurement of K_v was conducted using the falling-head permeameter tests in five test sites(including Meixian,Xianyang,Caotan,Lintong,and Huaxian)from October 2011 to November 2014.The mechanism of spatio-temporal variability of K_v in test sites was further analyzed,discussed,and determined from the aspects of physicochemical properties,sediment layered structure,and mineral composition.And it was based on some statistical methods as well as laboratory and field testing analysis.The major findings of this study were as follows:(1)Normal distribution test indicated that temporal-spatial distribution of K_v overall was not significantly subjected to the normal distribution,and part periods of the K_v was subject to lognormal distribution.During the study period,the range of K_v values was 0-42.38 m/d.In temporal scale,no obvious seasonal variations of K_v values in the five sites was found during the while test periods.Meanwhile,K_v value in Huaxian site presented an increasing trend over time from October 2011 to November 2014.And K_v values in other four sites(including Meixian,Xianyang,Caotan,and Lintong)gradually decreased in March 2013 and after this time,then finally showing more stabilized state.In spatial scale,the variability of K_v in different periods presented a significant difference in space.After May to June 2013,the variability of K_v in each river presented a decreasing trend.The larger values of K_v during the while test periods(excepting for the period of December 2013)generally occurred in Caotan site(0.07-22.25 m/d)while the smaller values of K_v mostly appeared in Meixian and Lintong sites with 0.00-1.16 m/d and 0.00-1.17 m/d respectively.The K_v values in each site presented a decreasing trend in March 2013 and after this time,then finally showing more stabilized state in space.And the larger decreasing range of K_v was obtained in Meixian and Xianyang sites compared to the values in Lintong and Huaxian sites.The smallest values of K_v in spatial distribution generally were obtained in Meixian and Xianyang sites,and the largest values of K_v generally occurred in Caotan site.(2)In general,the larger values of K_v in the five sites generally occurred where less clay content and more sand content in sediment both existed,and it was vice versa.However,when the clay content of sediment was greater than 15.0%or less than 5.0%(the clay content of sediment was much greater or much less),no significant relationship between K_v and clay content were obtained during the whole periods.In addition,the chemical properties of the sediments influenced the structure of the sediments mainly by changing the aggregation form of the clay,but too much clay will mask the influence of chemical properties on the sediments structure.Therefore,the difference of effects of physicochemical properties of sediments on K_v generally changed as the variability of the particles characteristics in streambed sediment.(3)The occurrence of "Weak permeable layer" within the sediment will help to decrease the values of K_v,and the significant decreasing of K_v values was mainly subjected to the existence of enriched clay containing organic matter within the sediment.Meanwhile,in the vertical direction,the effect of clay on K_v in the lower layer was greater than that in the top layer due to the existence of compression force.Therefore,the layered structure of sediments had a significant influence on K_v.The mineral composition of sediments was determined based on diffraction pattern of XRD and the correlation relationship between composition and K_v was further analyzed and discussed.The results demonstrated that The difference of mineral composition among the sediments in the 5 reaches is small(0.01-2.94%)and mineral composition is not the main influencing factor for the variability of K_v.(4)Laboratory testing analysis of K_v showed that However,the hydraulic conductivity of sediments is varying under a variety of physical and chemical characteristics of river water.The attributes of river water have a significant impact on hydraulic conductivity of sand sediments(clay portion<5.0%),however,its impact is small(can be ignored)when the clay portion is high(clay potion ? 5.0%).Therefore,the influence of physicochemical properties of sediments on K_v in Caotan site was larger than the other 4 sections.Therefore,the difference of river water is not the primary influencing factor of the spatial and temporal variability of K_v.As the composition characteristics of streambed sediment varied,the depth of river water had positive or negative correlation with the values of Vertical hydraulic conductivity,but they were not significant. |
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