The hyporheic zone, the transition region from a stream to the surrounding aquifer, acts as a physical, chemical, and biological filter and has been recognized as a critical component of stream ecosystems [1
]. The transfer and transformation of nutrients or pollutants between groundwater and the surface water can be promoted or prevented, and solutes and oxygen can migrate into oxic and anoxic environments through hyporheic water flows, resulting in variations of the redox conditions of the sediments and aquifers and ultimately controlling the growth of microorganisms [2
]. The dissolved oxygen concentration of upwelling stream waters is lower than other stream waters because the reactive solutes and dissolved oxygen could be carried by downwelling fluxes [4
]. Therefore, the hyporheic zone plays an important role in protecting the water quality and quantity of streams and groundwater from various pollutant species [5
]. The water exchange between streams and their hyporheic zones, termed hyporheic water exchange, is influenced by variations in the hydraulic gradient over the stream channel boundary stemming from geomorphic features, such as stream meanders, bars, dunes, step-pools, and in-stream structures [6
]; a chalk stream overlain by Palaeogene deposits and superficial drift from the Quaternary [9
]; or ambient groundwater discharge [10
]. The channel bend is one of the characteristic features of all streams and favors the formation of hyporheic zones [11
]. It is now understood that the interfacial flux of the stream water and streambed increases with sinuosity and that the meander apex experiences the largest flux [6
]. These fluxes toward or away from sinuous streams and hyporheic zones have implications for biogeochemical and ecological processes along the fluvial corridor from the river to riparian zones [12
Many measurement methods can be used to determine the velocity or flux of the surface water, aquifer and transition zones, including the Darcy equation, tracer tests, temperature gradient, and seepage meters, but these methods all have certain limitations due to different measurement scales and hydrogeological conditions; therefore, it is important to choose the method most appropriate to the study goal to characterize the interaction between rivers and their hyporheic zones [13
]. The specific discharge between stream and streambed can be obtained using the Darcian flux calculations employing the hydraulic conductivity and hydraulic gradient. The variability of streambed hydraulic conductivity depends on sedimentary characteristics, especially the distribution of the sediment grain size, and is related to the erosional and depositional processes induced by varying stream flows and influenced by the stream morphology [14
]. Another parameter characterizing the sedimentary hydrogeological control of hyporheic water exchange is the porosity of sediments [16
]. Generally, hydraulic conductivity increases with particle size, but this relation can be modified by changes in overall porosity [17
]. The hydraulic gradient is also one of the important streambed attributes used to provide an estimate of the potential strength of a hydrological exchange and has a significant influence on stream infiltration and storage zone in the aquifer [18
]. The direction and magnitude of Darcian flux vary greatly in different locations due to changes in these two variables, hydraulic conductivity and gradient, spatially and temporally, induced by dynamic environments in the stream, and have been identified as the two main factors controlling water exchange between streams and the surrounding groundwater systems [20
]. According to various laboratory experiments, the hyporheic water exchange rate is proportional to the square of the stream water velocity and to the permeability of streambed sediments and inversely proportional to the porosity of the sediments and to the depth of the streambed [16
]. However, under field experiments, the relationship of these characteristics is not well established, especially in the channel bend of a natural stream.
The objective of this study is to determine the variability of Darcian flux, streambed hydraulic conductivity, and head gradient at a natural channel bend and further reveal the relationship among these three streambed attributes in the hyporheic zone.
The Darcian fluxes in the hyporheic zone were determined via observations of streambed attributes of vertical hydraulic conductivity (Kv and VHG) at 31 locations in July 2015 and 30 locations in January 2016 along a channel bend. All the streambed attributes—Kv, VHG, and Darcian flux—showed great spatial variability related to the stream morphology and hydrological features in a channel bend, with an especially significant difference in July 2015 due to distinct stream topography.
Vertical Darcian fluxes were mainly dominated by downwelling with high values occurring near the depositional left bend and the downstream of the channel bend, especially in July 2015, and the variations of them were complex. The higher Kv values occurred at the lower streambed elevation with deeper water depth and followed the order stream center > depositional bank > erosional bank. This pattern may result from the distributions of streambed grain size induced by the velocity of stream water and influenced by the stream morphology and topography. The spatial distribution of VHG was inversely related to the distribution of Kv and more easily influenced by the stream topography. These two variables, Kv and VHG, could influence the estimation of Darcian flux. The correlation analysis showed that Kv is the main factor controlling the Darcian flux in the streambed.
While this study observed the variability of vertical Darcian flux between the stream and the hyporheic zone, the lateral water flux in the streambed should not be ignored. Hence, more methods should be employed and additional aspects considered in further studies.