*3.4. Physicochemical Characteristics of Sediment and Water in the River Channel and the Riparian Hyporheic Zone*

There was a great difference between the horizontal and vertical permeability coefficients of the sediment in the riparian hyporheic zone (Table 2). For the sediment at any specific position, the horizontal permeability coefficient was 10 times higher than the vertical one, which implied that the lateral migrations of both the hyporheic flow and the solutes were dominant along the transect. According to the grain size distribution, the riparian sediment was determined as silty-clay soil.

**Table 2.** Physical properties of the sediment in the riparian hyporheic zone.


Both the TN and the TC contents of the sediments gradually increased with depth, and the low contents of TN and TC appeared within the extent (0.9–1.05 m a.s.l.) where the riparian groundwater fluctuated (Figure 8). This could provide a clue that the extent where the groundwater level varied might be the hotspot of biogeochemical reactions in the riparian hyporheic zone, since large amounts of organic mass such as organic nitrogen and organic carbon were consumed in the course of the reactions involving micro-organisms. Furthermore, the mean TN content of the sediment at WB3 was the highest at 674 mg/kg, compared with that at the three other riparian wells.

**Figure 8.** Vertical distribution of the nitrogen and carbon contents of the riparian sediments: (**a**) total nitrogen, TN; (**b**) total carbon, TC.

The DO concentration in the groundwater showed a significant difference among different riparian wells (*p* < 0.05). The mean DO concentration in the river water was 6.24 mg/L, higher than that in the riparian groundwater (Figure 9a). Moreover, the mean DO concentration in the riparian groundwater decreased gradually with an increase in distance from the shore. The DO concentration of the riparian groundwater ranged from 5.2 to 6.8 mg/L. The difference of the DOC concentration in the groundwater was not significant among the riparian wells (*p* > 0.05). The data distribution range of the DOC concentration in the river water was less than that in the riparian groundwater (Figure 9b). This implies that the river water was not the only source of the DOC in the riparian groundwater.

**Figure 9.** *Cont*.

**Figure 9.** Spatial variations of eco-environmental factors along the transect: (**a**) dissolved oxygen, DO; (**b**) dissolved organic carbon, DOC; (**c**) pH; (**d**) temperature.

There were significant differences in both the pH and the temperature in the groundwater among different riparian wells (*p* < 0.001). The mean pH of the river water was about 7.08 (Figure 9c), higher than that of the groundwater in any riparian well (ranging from 6.27 to 6.59). Along the transect, the pH decreased with increasing the distance from the shore. The mean temperature of the river water was 18.8 ◦C, while that of the groundwater in the riparian wells, ranging from 20.3 to 21.4 ◦C, increased gradually with an increase in distance from the shore (Figure 9d).
