*3.1. Sedimentary Profile and Sampling*

– – The research materials were obtained from the GX (33 ◦30 ′56.907 ′′ N, 119 ◦54 ′00.662 ′′ E) and CYK (33 ◦35 ′04.776 ′′ N, 119 ◦57 ′49.279 ′′ E) sections in the northeast of Jianhu County (Figure 1). In this section, we will describe the lithology of the sediments in detail; we collected samples from the bottom to the top at a spacing of 2 cm. We collected 250 and 360 samples from the GX and CYK profiles, respectively.

– The depth of the GX section is approximately 5 m, and the altitude is between 0.5 m and −4.5 m. The sediment composition characteristics of the entire section change significantly from bottom to top. The section between 500 and 446 cm depth comprises dark-grey compact clay silt, with tubular plant residues (reeds) preserved in the sediment. At depths of 446–426 cm, the sediment is mainly grey-black silty clay. At depths of 426–150 cm, clay silt of various colors with horizontal bedding and vein bedding, or interbedding of clay silt and silty fine sand, with the sediments containing calcified burrows and marine shell fragments. The bottom of this layer (426 cm depth) comprises a large number of shells and oysters. At 150–55 cm, the lithology of sediment is mainly dark-grey to greyish yellow compact clay silt with a large number of iron manganese nodules. From 55 cm to 0 cm, the sediment is disorderly and has no obvious natural sequence, which is the disturbance layer resulting from human activities (Figure 2).

**Figure 2.** Lithological characteristics of GX profile in Subei Basin.

− – – – – – – – The depth of the CYK profile is approximately 7.2 m, and the altitude is between 1.92 and −5.28 m. The sediment composition of the profile changes significantly from bottom to top. At depths 720–658 cm, the sediment is grey compact clay silt with obvious sedimentary discontinuities at 658 cm. At depths of 658–345 cm, it is mainly grey to blue-grey mud silt with silt lamina. Grey mud silt with burrows is found at a depth of 345–240 cm. At depths of 240–210 cm, the sediment is mainly grey clay silt with burrows. At depths of 210–158 cm, there is grey clay silt with white silt lamina between the layers. At depths of 158–120 cm, the sediment is mainly yellow-brown compact clay silt with rust spots. Between the depths of 120 cm and 62 cm, the sediment is mainly dark-grey to grey-black clay silt, with the color gradually darkening upward. At depths of 62–0 cm, the sediment is mainly grey-yellow clay silt, the layer obviously affected by human activities (Figure 3).

**Figure 3.** Lithological characteristics of CYK profile in Subei Basin.

*3.2. Experimental Methods for Physicochemical Properties of Sediment Lixiviums*

No systematic study has been conducted on the physicochemical properties (TDS, SAL, EC) of sediment lixiviums. Therefore, to obtain relevant stable data, we conducted several preliminary tests to determine the appropriate process and optimal test time for measurements (the detailed experimental process was shown in part 4). Subsequently, we obtained the physicochemical property indexes for the two profiles.

#### *3.3. Geochemical Element (Sr, Sr/Ba) Analysis Method*

The sediment sample was first dried at low temperature, after which approximately 10 g was weighed, ground into a powder using an agate mortar, and passed through a 200-mesh fine sieve. Finally, the samples were pressed into pellets using the boric acid pressing method [37] under high pressure. After completing sample preparation, the samples were instrument tested. The geochemical element content was assessed using an X-ray fluorescence spectrometer (PANalytical Co., Almelo, The Netherlands) at the Key Laboratory of Environmental Evolution and Ecological Construction of Jiangsu Province, School of Geography, Nanjing Normal University. The measurement error of the instrument was less than 5%.
