*2.1. Field Monitoring*

The test site was located on the riverbank of the Jingzi River, a tributary of the Huangpu River in Shanghai, China, as illustrated in Figure 1. The Huangpu River, with a total length of approximately 114 km, is the last tributary of the Yangtze River before it empties into the East China Sea. The Huangpu River estuary forms a medium-tidal shallow-water environment where the tides are of irregular semidiurnal features. The tides can penetrate up to the Dianshan Lake and the provincial boundary between Zhejiang and Shanghai. On an average, the tidal period is about 12.5 h, where 4.5 h involves flooding and the remainder involves ebbing [36]. At the Wusong tide gauge station, the average tidal prism is about 5.8 × <sup>10</sup><sup>7</sup> m3, the flood current velocity is up to 1.2 × <sup>10</sup><sup>4</sup> <sup>m</sup>3/s, and the mean tidal range is about 2.27 m [37]. The test site was located about 29 km upstream of the Wusong tide gauge station. According to the preliminary research, there was a good response relationship between the river stages at the test site and the sea levels at Wusong station. The flooding at the test site was about 2.5 h behind that at Wusong station. Furthermore, the fluctuation range of the river stage at the site was up to approximately 0.5 m.

**Figure 1.** Location of the test site.

The land use is mainly green field and residential in the surrounding area of the test site. The green field is unused and covered with bushes and stunted trees. The Jingzi River flows through the greenbelt, serving as a sewage channel of the vicinity. The terrain around the test site is relatively flat, and the slope degree of the bank is less than 9◦. The riverbed sediment and the shallow aquifer materials at the site consist of silty-clay soils extensively distributed on the floodplain of the Huangpu River. The site with gentle slope, poorly permeable sediment, and less interference was considered suitable for the investigation.

The site was instrumented with a perpendicular transect of observation wells and sampling points, as shown in Figure 2. One observation well marked as WR0 was set in the river channel, and four other wells (i.e., WB1, WB2, WB3, and WB4) were laid on the riverbank, 0.71 m, 1.11 m, 2.43 m, and 3.85 m away from the shore, respectively (Figure 2a). The well pipes, made of polyvinyl chloride (PVC) material, were equipped with pipe covers at both ends. A hole punch was employed to make scattered tiny holes with a diameter of about 5 mm in the wall of the pipes. A nylon cloth with a mesh size of 0.15–0.20 mm was wrapped around each pipe. The wells on the riverbank were 200 cm in length and 11 cm in diameter, while WR0 was 80 cm in length and 6 cm in diameter. Each well tube was inserted vertically into the sediment. A high-precision GPS (Z-Survey i70, Manufacturer: CHCNAV Co. Ltd., Shanghai, China) and a leveling instrument (B30, Manufacturer: SOKKIA Co. Ltd., Tokyo, Japan) were used to survey the top elevation of each well. One water-level sensor (HOBO Kit-D-U20-04, Manufacturer: ONSET Co. Ltd., Bourne, Cape Cod, Centerville, MA, USA) was installed at the bottom of each well, with a data collection interval of 5 min.

**Figure 2.** The transect for monitoring and sampling: (**a**) the top view; (**b**) the cross-section.

According to the preliminary work, the fluctuation range of the Jingzi River stage was mostly less than 0.5 m, and the change in groundwater table could not be observed as the river stage fluctuation was smaller than 10 cm (usually occurred during neap tides). Thus, a spring tide period without rain should be chosen to focus on the impact of tides. According to the tide prediction table and the weather forecast information released in Shanghai, the 3 day spring tide period from 21 to 23 October 2021 was selected to carry out the monitoring of water level and water quality. The data recorded by the water-level sensors were exported and processed through atmospheric pressure correction. A portable dissolved oxygen meter (LDO™, Manufacturer: HACH Co. Ltd., Loveland, CO, USA) and a portable multiparameter water quality analyzer (sensION+ MM156, Manufacturer: HACH Co. Ltd., Loveland, CO, USA) were employed to synchronously determine DO concentration, temperature, and pH in the river channel and the four riparian wells.
