4.1. Hyporheic Flux
Figure 4 shows the observed concentrations from nine scenarios. Concentrations at different times were calculated according to the relation between the concentration and EC, as well as the drawn time-concentration curves. Five scenarios (N1, N4, N9, N17, N23) that had the same x-location (l = 0 m) and five scenarios (N7, N8, N9, N10, N11) that had the same depth of clay lens (d = 0.1 m) were chosen as representatives. Among them, N9 was a duplication, i.e., the nine scenarios formed a vertical line (d = 0.1 m) and a horizontal line (l = 0 m) in the cross-section of the sand dune, and N9 was the intersection.
It was found that the concentrations basically followed an exponential type of decreasing trend. The measured points in the nine scenarios matched well with the fitting curve, and all of the coefficients of association were more than 0.99 except for the N4 scenario. It was also seen that the initial points fluctuated slightly. Variations between parallel tests, i.e., the length of the error bar in N1 and N4 were about 0.15 g/L, and the concentration in the whole test were more than 1.50 g/L. These two characteristics were slightly different than the other scenarios. Because of the clay lens, the flow field displayed apparent changes. This indicated that the flow field was sensitive when the lens was located in these zones. On the other hand, when the clay lens was far away from center of the sand dune, the flow field was smooth and the deviation from repeated measured data was relatively small. The “edge effect” (the mixing of solutions and surface water for several minutes) would affect the concentrations in the initial ~5 min. Hence, the initial concentrations were not used to fit the attenuation curves.
We performed several tests under different conditions, i.e., we changed the positions of the lens, and found that the deviation value of
qG was negligible. The parameters of the fitting curves were plugged into Equation (7) to calculate the hyporheic flux when the clay lens was located in different positions. The results are summarized in
Table 2 and
Figure 5. The unit of exchange flux, including
qH and
qG, was cm/d, which is consistent with the units in the studies of Fox et al. [
19,
33].
The average hyporheic flux of the 25 experimental scenarios was 477 cm/d. The maximum hyporheic flux was 1227 cm/d, which appeared at l = 0 m and d = 0.1 m (N9) in the middle of the sand dune. The minimum hyporheic flux was 48 cm/d, which appeared at l = 0.025 m and d = 0.15 m (N24). In addition, flux at l = −0.025 m and d = 0.15 m (N22) and l = −0.05 m and d = 0.075 m (N2) were also very low; 81 cm/d and 99 cm/d, respectively. It was found that most hyporheic fluxes were below the flux of the blank control. Hyporheic flux in the sand dune was mainly influenced by surface water velocity, streambed topography, hydraulic conductivity of sediment, and so on [
42]. The buried clay lens changed the spatial distribution of sediment hydraulic conductivity and led to the heterogeneity and anisotropy of local hydraulic parameters, which eventually suppressed hyporheic exchange. Meanwhile, there were still several fluxes (e.g., N6, N13, N16, and N17) close to the flux of the blank control. These results indicated that the suppression effect for hyporheic exchange was not strong compared to the other locations when the clay lens was in the middle or near the upstream surface of the sand dune. Specifically, when the clay lens was located at l = 0 m and d = 0.1 m, the hyporheic flux was even higher than the flux of the blank control. In addition, the hyporheic flux produced an abnormal and abrupt increase when the lens was near the upstream surface of the sand dune. From the above results, we concluded that the clay lens in the sand dune would restrain the hyporheic exchange overall. This effect would be weak or would even transform to enhance the hyporheic exchange in some specific zones (in this test, this was in the middle or near the upstream surface of the sand dune). When the clay lens moved out of this area (e.g., moved deeper or closer to the downstream surface of sand dune), the suppression effect was obviously enhanced. Considering the influence of depth (see three groups’ scenarios of l = −0.025 m, 0 m, and 0.025 m), it was found that the trend of the hyporheic fluxes increased first and then decreased as the depth of the lens increased. While considering horizontal locations (see three groups’ scenarios of d = 0.075 m, 0.1 m, and 0.125 m), the trend was similar to the depth, i.e., hyporheic fluxes increased first and then decreased as the lens moved from upstream to downstream. These trends also confirmed our opinion. Xiaoru Su [
34] studied the impact of a low-permeability lens on dune-induced hyporheic exchange by using a VS2DH model and found that the clay lens in a streambed can hinder or enhance hyporheic exchange depending on its relative spatial location to dunes. This conclusion was consistent with our research.
Bardini [
43] found that permeability heterogeneity produced more irregular flow cells within shallow sediments, which has a positive role in hyporheic flux. It partly explains why the hyporheic flux in N9 was significantly larger. Based on a laboratory physical model test, Fox et al. [
19] found that on the upstream slope of a sand dune, the hyporheic exchange flux was greater and the extent of the hyporheic zone was wider. Because of that, when the clay lens was in the sand dune near the upstream slope, the hyporheic flow had the ability to go around the lens due to the change of the pressure field. However, when the lens was near the downstream slope, the extent of the hyporheic zone was smaller and the influence caused by the lens on the hyporheic exchange was more noticeable. Compared with the conditions in which the lens was near the upstream slope, the water flow could not overcome the obstruction from the lens, which showed a more significant inhibitory effect.
4.2. Change of Hyporheic Zone Flow Field: Three Different Kinds of Water Flow
The tracer experiments were used to determine the range of the hyporheic zone. We intended to show the range of the zone in a more intuitive way and explore the main factors influencing the range. In the dye test, we observed the movement of the tracer with time to draw the direction of the water flow and determine the extent of the hyporheic zone. Through several tests, we found that the hyporheic flow field would be basically stable after 30 min from the beginning of the test and that the EC declined steadily. Thus, each test was conducted by recording the observations during the first 30 min.
Figure 6 shows the process of the tracer movement from
t = 0 min to
t = 30 min in the condition of l = 0 m and d = 0.1 m (N9).
The heterogeneity and anisotropy of streambed media mainly affected the flow path of water in the sand dunes and the residence time. This natural characteristic produced spatially variable interfacial fluxes and complex hyporheic exchange patterns [
2]. From
Figure 6, we can see that the tracer on different spatial positions mainly had three patterns of motion tendency movement. The first was that water flow moved along the direction perpendicular to the surface of the sand dune and eventually left the sand dune (see the red line in
Figure 6). This type of tracer was distributed in the bottom of the upstream and downstream slopes. The second pattern of motion tendency movement was that water flow adhered to the surface of the clay lens from the upstream slope to the downstream slope (see the yellow line in
Figure 6). This type was near the clay lens. The third was that the movement was curved irregularly and had horizontal flow trends, and the dying circle was obviously distorted so that a vortex appeared near the clay lens (see the blue line in
Figure 6). This type was distributed in a small-scale area that was at the top of the upstream and downstream slopes. The water flow in the hyporheic zone was bidirectional in the flow direction, which was different from one-way recharge and discharge between the surface water and groundwater [
44,
45]. The vertical water flow was influenced by the pumping exchange. Jin [
46] pointed out that shear flow induced by a triangle bed produced pressure change, which led to the movement of pore water and an exchange between the stream and the saturated bed. This exchange was called the pumping exchange. The horizontal water flow from the upstream slope to the downstream slope was mostly influenced by the surface water. The third flow, which did not have a specific flow direction, was influenced by both the pumping exchange and surface water.
4.3. Influence of the Spatial Distribution to the Extent of Hyporheic Zone
Gomez-Velez [
47] pointed out that the extent of the hyporheic zone was modulated by the upwelling groundwater and the presence of a low-permeability layer, resulting in stagnation zones above and below the sediments. Our research focused on the clay lens, which was a typical low-permeability layer. The presence of the lenses increased the residence time and accumulation in the higher permeability zone.
We changed the depth d and horizontal location l to determine the spatial distribution of the clay lens. The method delimiting the extent of the hyporheic zone was mentioned above. For the photographs shown in
Figure 6, we needed to obtain coordinate definitions in ArcGIS based on the actual size of the sand dune. According to the image resolution and the number of grids in the target area, the area of the hyporheic zone was calculated as the product of the resolution and numbers and compared in different spatial distributions.
4.3.1. Influence of the Depth on the Extent of the Hyporheic Zone
In order to investigate the influence of the depth of the clay lens on the extent of the hyporheic zone, we chose three columns of points (l = −0.025 m, 0 m, and 0.025 m) to test and then compared the data at each point.
Figure 7 demonstrates the comparison of the hyporheic zone for which the clay lens was located at points with different depths in three columns. The arrow represents the direction of water flow in different locations, which was determined by the images of the hyporheic flow changing with time (
Figure 6). The hyporheic zone was circled by asymptotes in the flow field and the sand dune boundaries. Therefore, asymptotes were the key to delimit the hyporheic zone. In the test, we determined the asymptotes through the hyporheic flow, which was represented by the movement of the tracer in each point. The yellow line in
Figure 7 represents the asymptotes and the sand dune boundaries, and the circled area shows the hyporheic zone. It was found that when the clay lens was at different depths, there were obvious differences in the shape of the hyporheic zone. When the lens was in a shallow area (the depth was less than 0.075 m), it tended to insert into the hyporheic zone and cover a part of the hyporheic area. In particular, when d = 0.05 m and l = 0 m (N1), and d = 0.075 m and l = 0 m (N4), the lens was in the hyporheic zone. We could observe that the hyporheic water was hindered and driven to flow along the surface of the lens. With the lens moving to a deeper area, the shape of the hyporheic zone gradually became stable and was largely consistent with the shape found in the blank control. We believe that the influence for flow direction reduced and eventually disappeared as the depth of the lens increased. Comparing different scenarios, obvious changes of water flow appeared around the lens. For the points at the surface of the lens, water flow was attached to the outline of the lens. At the points near the lens, flow paths were distorted so that water flow would deviate from the lens. Wanger [
48] pointed out that interstitial water preferentially flows in a complex network of areas of high hydraulic connectivity. It was changed or even cut off by a clay lens because the hydraulic conductivity of the clay lens was much lower than that of the sand dune. The clay lens would produce more resistance to water flow, which made it choose the “easiest” path. This was the reason why it appeared that the streamline was wrapping around the lens. For the points far from the lens, for which the distance between the lens and the hyporheic zone was about 0.025 m (i.e., the distance between two adjacent points), the influence from the clay lens was not obvious.
According to the area of the hyporheic zone, we drew the curves of the area so that they varied with depth and compared them with the hyporheic flux that varied with the depth location of the lens (
Figure 8).
Figure 8a shows that the area varied with depth. The solid line is the actual hyporheic zone area, and the imaginary line is the area removing the area of the clay lens when the lens was in the hyporheic zone.
Figure 8b shows that the hyporheic flux varied with depth.
The largest and smallest areas of the hyporheic zone were 106.62 cm
2, when the lens was located at l = −0.025 m and d= 0.075 m, and 40.42 cm
2 when the lens was located at l = 0 m and d = 0.075 m, respectively. Significantly, there were two special scenarios (N1 and N4), in which the lens was in the hyporheic zone, which would increase the area greatly. Hester modeled the mixing of surface water and groundwater induced by riverbed dunes and found that introducing heterogeneity increased the mixing primarily by increasing the mixing-zone (i.e., hyporheic-zone) thickness [
20]. In our test, each area of the hyporheic zone with the clay lens was higher than the area in the blank control. From
Figure 7, it can be seen that the hyporheic exchange appeared near the top of the sand dune because of the upwelling water. Several studies have also claimed that, compared to neutral conditions, the hyporheic zone is restricted by the upwelling water [
19,
39]. The presence of the clay lens seemed to be a “barrier” that could block the upwelling water and decrease the influence on the hyporheic exchange. Focusing on the tendency of the three conditions (see
Figure 8), the area of the hyporheic zone firstly increased and then dropped as the depth increased. This was similar to the change of the hyporheic flux that varied with depth (
Figure 8b), which also increased in the beginning and then decreased. Apparently, each curve had a maximum. For the area of the hyporheic zone, the maximum appeared at d = 0.125 m in three horizontal locations. For the hyporheic flux, the situation was slightly different in that the maximum appeared between d = 0.1 m and d = 0.125 m.
4.3.2. Influence of the Horizontal Locations on the Extent of the Hyporheic Zone
Figure 9 shows the comparison of the hyporheic zone when the clay lens was located at different horizontal positions. The study of the influence of the horizontal positions was similar to the abovementioned methods. In this test, we chose d = 0.075 m, 0.1 m, and 0.125 m as three rows of data to compare the influence of the horizontal locations on the extent of the hyporheic zone. The hyporheic zone changed significantly with the lens moving to different horizontal positions. Observing the hyporheic zone, there were two scenarios in which the lens was in the hyporheic zone. This also occurred when the lens was located in the shallow zone (l = −0.05 m and d = 0.075 m, and l = 0 m and d = 0.075 m). The distinction was that the different horizontal positions influenced the lowest position and the depth of the hyporheic zone. Due to the shape of the lens, the lowest position of the hyporheic zone was the same as the lowest position of the lens. The lower boundaries of the zone and the lens overlapped. When the lens was at d = 0.1 m or 0.125 m, the lens was out of the hyporheic zone. In the conditions in which the lens was near the surface of the sand dune, it did not make contact with the hyporheic zone. Thus, the lens had little influence and the water flow was steady, with only some vortexes between the lens and the hyporheic zone. With the lens moving to the middle of the sand dune, the influence became apparent. Firstly, the lowest point of the hyporheic zone gradually approached the lens as if there was an “attraction” effect from the lens to the hyporheic zone. When the lens was near the middle of the sand dune, there was a small area in the hyporheic zone that was covered by the lens as if the lens was embedded into the zone. The overlapped zone was also observed in the results shown in
Figure 7. The lens had obvious “transformation” effects on the lower boundary of the hyporheic zone. Compared with the variation of the depth, the x-locations of the lens had more influence on the “barrier effect” than the shape; in particular, the length of the hyporheic zone varied more obviously as the lens moved horizontally.
According to the area of the hyporheic zone, we drew a figure that area varied with the horizontal positions (
Figure 10).
Figure 10a shows that the area of the hyporheic zone varied with the horizontal positions. The solid lines are the actual area of the hyporheic zone. The imaginary lines are the area of the hyporheic zone, deducting the area of the clay lens when the lens was in the hyporheic zone.
Figure 10b shows that the hyporheic flux varied with the horizontal positions.
From the data, the largest area was 106.62 cm2, which was seen when the lens was located at the shallow part of the sand dune (l = 0 m and d = 0.075 m). This is consistent with the depth mentioned above. The smallest area was 40.42 cm2, which was seen when the lens was shallow and near the upstream slope of the sand dune. Further, there were several points (e.g., l = −0.125 m and d = 0.125 m; l = 0.1 m and d = 0.125 m; and l = 0.05 m and d = 0.1 m) for which the area was close to the minima, which were 46.84 cm2, 48.27 cm2, and 47.72 cm2, respectively. Accordingly, the hyporheic fluxes were close to the value of the blank control. We found a similarity in situations in which the lens was in the deep area (bottom of the sand dune) and near the surface of the sand dune. We suspected that the lens in these areas was far from the hyporheic zone in the control. Thus, the areas of the hyporheic zone and the hyporheic flux were close to the corresponding values in the control. Compared with the depth, the variation of the area was more intuitive and fluctuation did not exist. When the clay lens was located at l = 0 m, the area of the hyporheic zone was largest at the current depth, and when the lens moved to both sides of the sand dune, the area of the hyporheic zone began to decrease gradually and linearly.