**1. Introduction**

Sea level fluctuations [1,2], hydrological dynamics and structural changes [3–5] are among the factors that trigger transgression and regression events [6–8] commonly recorded in coastal areas. In conjunction, these factors have an important impact on the evolution of a region's environment through time. In the past few decades, several researchers have conducted studies using micro-paleontology [9–14], geomorphological [15,16], and geochemical (stable isotopes and trace elements) [17–23] proxies with fruitful research results. However, most of these methods require complicated identification processes and professional skills, as well as expensive specialized instruments. Moreover, some indicators obtained through these proxies are not easily preserved to obtain continuous data because of the sedimentary environment [24]; some geomorphological markers and/or sedimentary structures will be affected and even destroyed [25]. Obviously, these factors will have a certain impact on the research results.

The question is, therefore, whether an index could be found that is relatively simple, fast, and does not require highly professional skills or complicated instruments to test for and reveal a transgression event. Considering the significant difference in the content of various soluble substances (such as various ions and compounds) between seawater and

**Citation:** Shu, Q.; Zhang, S.; Chen, Y. Physicochemical Property Indexes of Sediment Lixiviums in Sea–Land Interaction Zone of Subei Basin and Their Significance to Transgression. *J. Mar. Sci. Eng.* **2021**, *9*, 719. https:// doi.org/10.3390/jmse9070719

Academic Editors: Markes E. Johnson and Timothy S. Collett

Received: 6 June 2021 Accepted: 26 June 2021 Published: 29 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fresh water, the amount of various soluble substances attached to the sediments in the process of sediment deposition should also be variable. Consequently, the physicochemical properties of the lixivium of these sediments should also differ significantly (Sediment lixivium refers to the mixed liquid of water and sediment formed by soaking the pretreated sediment in solution). Based on this concept, we propose a multiparameter water quality meter for use in paleoenvironment research to measure the physicochemical properties of sediment lixivium and to reveal these transgression and regression events.

The multiparameter water quality meter is an inexpensive and easy-to-operate portable instrument. It can measure the physicochemical properties of water, such as total dissolved solids (TDS), salinity (SAL), and electrical conductivity (EC), with high precision (±1%). These properties are currently used mainly to study their changes in various forms of water, such as surface water and groundwater [26–32]. Sylus [31] discussed the impact of regional seawater intrusion on water quality through EC and TDS measurements of the coastal aquifers of the Gurpur and Netravathi river basins in the Dakshan Kannada district in India. Salmani [32] studied TDS and the water flow of the Karoun River in southwest Iran, and proposed a model to predict the river flow and TDS changes. Yokoyama [33] studied the sedimentary environment of Lake Biwa (Japan) through EC and discussed its relationship with sulphate ions and sulphates. In fact, EC is not only related to sulphate ions and sulphates but also to the solubility of electrolytes in water. Fang [34,35] used the EC of clay turbid water to study the sediments in the old, drowned valley plain of the Liaodong Peninsula (Northeast China), and reconstruct changes in the regional environment. In these studies, in addition to applying the physicochemical properties of water bodies to the study of surface water and groundwater quality [31,32], some attempts were made to apply such properties to the study of paleoenvironmental changes [33–35]. However, the preparation and testing process of the sediment solution was not described in detail, sample spacing was large, and the resolution was not high. There is also a lack of research on the direct rebuilding of coastal environment change by TDS, which reflects the total amount of dissolved solids in water.

In view of the aforementioned discussion, we intended to introduce a multiparameter water quality meter to the field of paleocoastal environmental change research. Through preliminary experiments, we established an appropriate process for testing the physicochemical properties of sediment lixivium. According to the testing process, three physicochemical property indexes (TDS, SAL, and EC) of sediment lixivium of the Gangxi (GX) and Caoyankou (CYK) sections, located in the eastern margin of the Subei Basin, are determined. After comparing the test results of sediment lixivium with the indicators of diatoms and geochemical elements, we discuss the response of the physicochemical properties of sediment lixivium to changes in the sedimentary environment, as well as the feasibility of using these properties to reflect changes in coastal environment.

#### **2. Regional Setting**

The Subei Basin is located on the eastern coast of China in northeastern Jiangsu Province. It is the onshore part of the Subei–South Yellow Sea Basin, including the Lixiahe Plain and the coastal plain (Figure 1). The Subei Basin is a large-scale composite sedimentary basin formed during the Late Mesozoic on the basis of the Yangtze Block [36]. This basin is a key zone connecting the ocean and land, with significant interaction between sea and land. The sediments in the Subei Basin comprise deposits left by the cross action of rivers, lakes, and seas. After long-term deposition, the overall terrain of the Subei Basin is low and flat, slightly higher in the southwest region and lower in the northeast. The Lixiahe area has the lowest depression in the entire Subei Basin; the altitude of the center is less than 2 m. There are dense river networks, developed water systems, and numerous lakes in Subei Basin. The Huaihe and Yihe rivers and other water systems run through the Subei Basin and flow into the Yellow Sea in the east. Lakes of various sizes, such as Hongze, Gaoyou, and Dazong are distributed among these water systems, forming lake groups.

**Figure 1.** Location of the study area and sampling sites.

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 The Subei Basin is located at mid-latitudes between 32 ◦ and 34 ◦ north latitude. It has a subtropical humid monsoon climate, an average annual temperature of 13.4 ◦C, with the temperature gradually increasing from the northeast to the southwest. The average annual precipitation is 1000 mm, with more precipitation in the east than in the west and more in the south than in the north; the annual sunshine hours are 2130–2430 h, with the highest in summer and the lowest in winter. The factors that influence the atmospheric circulation in the study area are the same as those in the eastern coastal areas of China, i.e., monsoon circulation. In winter, the northerly wind from the interior of the high-latitude continent prevails and the climate is cold and dry, whereas in summer, the southerly wind from the low-latitude Pacific Ocean prevails and the climate is hot and humid.

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## **3. Material and Methods**

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