Heterogeneity Of Hydraulic Conductivity And Darcian Flux And Its Relation To Distinctive Stream Mophologies And Topography In The Beiluo River,Shannxi Province,China

Recognizing the heterogeneity of hydraulic conductivity and hyporheic flow is critical for understanding contaminant transfer and biogeochemical and ecological processes involving streams and aquifers. The aim of this study was to demonstrate the heterogeneity of hydraulic conductivity and vertical fluxes in both submerged streambed and its adjacent exposed stream bank of the Beiluo River, Shaanxi Province, China. The total measurements were conducted at four test sites in three different stream morphologies:straight channel, anabranching channels and a nearby meander bend. In submerged streambed, vertical hydraulic conductivity (Kv) and vertical head gradient (VHG) were determined by in-situ permeameter tests, and further vertical flux was estimated. On exposed stream bank, both horizontal hydraulic conductivity (Kh) and Kv were measured to estimate anisotropy by on-site permeameter tests. Moreover, hydraulic conductivity (Kg) was calculated by Slichter and Sauerbrei methods based on sediment grain size distributions. This paper not only elucidates the quantitative relations between streambed Kv, VHG, vertical fluxes determined by VHGs, temperature, stream geomorphology and bedforms in the studied streambed but also describes the heterogeneity and anisotropy of its adjacent two stream banks. The conclusions from our study can be summarized as follows:(1) Higher Kv values appear on the erosional bank in contrast to lower Kv values on the depositional bank at the two sites near meander bends; the Kv distribution in straight channels shows that the higher Kv values occur in the center of the channel; the Kv values are generally highest on the branch with more alluvial forms in the anabranching channels. Moreover, grain-size analysis results illustrate that the average grain-size distributions of streambed sediments are significantly different on each side of the channel. The erosional and depositional forms are one of the driving dynamics for the distribution of streambed grain size that trigger the differences in the distribution of the Kv.(2) Higher Kv values generally occurred around center or closed to erosional bank sites for upper layer, but it is not the case for the middle layer and lower layer. The lower layer has the highest coefficient of variation value and contains the most silt and clay. On the one hand, a gradual decrease in Kv values with depth was observed in submerged streambed.(3) Kruskal-Wallis test between the data of Kv values collected in June 2014 and January 2015 shows they belong to different populations. It has been found that the distributions of Kv are generally similar with each other, but the Kv values obtained in summer are larger and more variable than the Kv values determined in winter.(4) Kv values in submerged streambed were generally greater than those on exposed stream banks. On the other hand, with an increase of elevation to stream stage, Kh values generally increased on the erosional bank while decreased on the depositional bank. The higher variation extent of anisotropic ratio (Kh/Kv)was observed on the depositional bank.(5) The Sauerbrei method produced higher Kg values, and the Kg values calculated using the Slichter method produced lower ones that were closer to our estimates of Kv values derived from an in-situ permeameter test The calculated Kg for the upper layer to the lower layer gives larger to smaller values and variability. It is consistent with the decreasing variability of Kv values from the upper layer to lower layer.(6) The spatial distribution of VHGs has been found to be highly variable. Our observations indicate that upwelling flux in the step switched to downwelling in the pool then changed to upwelling-this is induced by the bedforms in a step-pool sequence. In this study, the Kv distribution with depth and its heterogeneity can be related to VHGs. Additionally, VHG values exhibit an inverse distribution to Kv values and larger Kv values occur in the center towards the erosional bank where generally there is upwelling.(7) The spatial variability of the vertical flux pattern derived from VHGs indicated more discharge occurring at the erosional bank. The study also showed that the temperature of sediments at depth of 80cm and temperature gradient confirmed our results of the vertical fluxes distribution.