1. Introduction
In the process of water environment pollution treatment, the water quality of big rivers and lakes is always mainly of concern and is effectively controlled [
1]. However, the water quality of middle and small rivers is continuously becoming worse because of the following reasons: the middle and small river basins, especially the small river basins, generally pass through villages and towns where local people have a relatively low perception on environment protection and the sewage treatment facilities along the small rivers need to be improved. Furthermore, the hydrological characteristics of these rivers are not collected and the technological analyses of pollution mechanism are lacking, which result in the sewage measures for these small rivers being relatively simple and the treatment standards are relatively low. Finally, the water ecosystem is destroyed, while the water stays black and odorous [
2]. Therefore, it is of important realistic significance and industrial economic benefit to develop a water environment remediation mechanism and propose an optimal treatment scheme for these middle and small river basins.
Due to the complex shapes and boundaries of the rivers and lakes under natural conditions, the traditional experimental methods consume a lot of labor costs, and the data for water environment treatment are difficult to obtain due to many constraints. Supported by the computing power of modern computers, numerical simulation models of the water environment can accurately simulate the changes in the water flow field and water quality as well as the migration and dispersion of pollutants in fluid movement. As a result, numerical simulation has become a mainstream research tool [
3,
4].
The software used in hydrological numerical simulation includes the MIKE software model developed by the Danish Hydraulic Institute (DHI), the WASP and EFDC model presented by the U.S. Environment Protection Agency (EPA), the SWAT model established by the U.S. Department of Agriculture (USDA), and some other models such as the SELECT, CE-QUAL-R1, and CE-QUAL-W2 models, and so on. All of these models are hydrodynamic in nature, but each one assumes its specific “vocation” with specific conditions for application. For example, SWAT is a model that is specialized in soil water dynamics, while MIKE21 has a wide ability to describe the entire process of flow and hydrodynamic interaction with the environment. In this work, the MIKE21 model was used because it has powerful function in the numerical simulation of two-dimensional free surface flow [
5,
6,
7,
8]. This powerful function comes from the core algorithm, which is based upon the solution to the two-dimensional free surface flow equation, where the stratification is neglected. Using MIKE software, Liu et al. (2020) [
9] established a two-dimensional hydrodynamic model for the Jingzhou part of the Yangtze River Basin where some wastewater treatment plants are located. In this model, the diffusion conditions of typical pollutants in the Yangtze River such as the chemical oxygen demand (COD), biochemical oxygen demand (BOD), ammonia nitrogen (NH
3-N), aniline and hexavalent chromium discharged by the sewage outfall were simulated. The impact area and degree of the influence of pollutant emissions on the water quality of the corresponding part of Yangtze River was also analyzed. A hydrodynamic model for the Bahe River Basin was described by Guo et al. (2019) [
10], where the flow field characteristics in different hydrologic years were analyzed, the influence on the water body’s disturbance area by the principal confluence water flow parameter was evaluated, and the corresponding ecological treatment measure was proposed. Similarly, Shu et al. (2019) [
11] constructed a hydrodynamic and pollutant movement model for the Ganjiang River Basin. This mainly analyzed the influence of pollution accidents on the water quality of water intakes in a downstream drinking water source under various typical hydrological regimes. On the other hand, MIKE software can also be used to study the hydrodynamic situation for large reservoirs and lakes. For instance, Li et al. (2018) [
12] established a hydrodynamic and water quality model for large reservoirs, where the process of pollutants flowing in and out of the reservoir with water was considered, and the movement and dispersion as well as the concentrations of pollutants such as total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH
3-N) varying with time under a hydrodynamic force function were also analyzed. Similarly, a two-dimensional hydrodynamic and water quality model for a large reservoir was constructed by Hu et al. (2021) [
13], where the concentration and distribution of the general water quality index such as dissolved oxygen, ammonia nitrogen, five-day biochemical oxygen demand, TN, TP, and so on during the flood season were compared with that during the non-flood season. Ma developed a hydrodynamic and water quality coupling model to obtain the variation characteristics of a reservoir water quality field [
14]. Gong et al. (2019) [
15] proposed a hydrodynamic and water quality model for large lakes, where two pollutants, COD and TP, were mainly concerned. Under the assumption that the amount of adjusted water was fixed, the authors simulated the variation of pollutant concentration at a water regulation section from the upper reaches during a special drought period. Finally, the relationship between the pollutant concentration in the lake and the inflow amount, the topographic factor, and the amount of adjusted water was also attained. From these studies, it is easy to see that the MIKE software has extensive applications in analyzing the hydrodynamic process and simulating the water quality as well as moving water pollutants and improving water quality.
Since big rivers and large lakes or reservoirs have relatively good ecosystems, the predictable flow direction as well as stable water flow field, and clear water potential field, the corresponding water quality simulations are comprehensive and thorough, which results in a relatively overall theory of river regulation. However, small- and medium-sized valley rivers are usually small-scale, closed, and irregular, which means that the corresponding hydrological data are not collected completely and a high engineering plasticity with relatively stagnant hydrodynamic conditions is finally formed [
16,
17,
18,
19]. Therefore, it is necessary to investigate the factors that influence hydrodynamic processes and solute transport as well as the degradation mechanisms in these kinds of rivers.
Recently, water quality simulations and water environment improvements for small-scaled lakes and small-sized river basins have gradually become of concern and some effective treatment measures have also been obtained. For example, Li et al. (2022) [
20] simulated the process of the water environment improvement of 10 typical village lakes in Jianghan Plain and demonstrated the possibility of recharging village lakes from the main streams. The authors also claimed that the key point of controlling the TP is to implement sewage interception, while a more effective treatment to improve the water quality of village lakes is to improve the hydrodynamic condition in the summer time. More recently, Yang constructed a hydrodynamic model for small-scale city artificial lakes, with which the influence of water diversion measures to the water quality of a typical city artificial lake was studied, and several methods to improve the corresponding water quality were proposed [
21]. At the same time, Yu proposed a water quality model for some branches with steam order 3 of the Yangtze River, where the water quality was evaluated, the pollution load was analyzed, and the water environmental capacity was computed [
22]. Based upon these works, the author of [
22] presented four treatment measures including the sewage of point pollution sources, control of non-point source pollution, ecological water supplement, and the up-to-standard reconstruction of wastewater treatment plants. Although the frontiers have conducted the fantastic works above-mentioned, the water ecosystem remediation mechanism of small river basins is not clear yet, and the theoretical evidence of treatment has not been sufficient up until now.
Taking the Xiyong River as an example, seven different scenario hypotheses on ecological engineering restoration were proposed in this paper, which were based on the MIKE21 hydrodynamic and water quality model. The distribution of the TN concentration in the Xiyong River was numerically simulated and the comprehensive compliance estimated. Finally, we propose a reasonable water ecosystem treatment measure and therefore present a technical reference and theoretical support for the design of the comprehensive treatment of Xiyong River’s water environment.
5. Conclusions
Based on the hydrodynamic module and the water quality module in MIKE21, this paper established the numerical computing model for the Xiyong River. The relative error of water level comparison was 2.9%, and the PBIAS coefficient of the concentration comparison deviation was 17.8%. The hydrodynamic and water quality models were fully verified for their reliability. Numerical simulations of the total nitrogen concentrations were conducted under seven different scenarios. Based on the simulation results, the temporal and spatial distribution patterns of the total nitrogen concentrations were obtained, and the factors leading to high total nitrogen concentrations in the river water were analyzed to provide a basis for the ecological restoration of water quality in the Xiyong River.
The simulation results showed that taking different measures could promote the reduction of TN concentration in the river. When undertaking the comprehensive treatment measures of river water ecology such as intercepting and purifying the main and tributaries of the river, sewage treatment at sewage outlets along the river as well as the regulation of fishing fields and farmland ditches, the annual average TN concentration in the river could be reduced by 0.496 mg/L, with a reduction of 20%. This comprehensive scheme could make the TN concentration of the river reach the Case III water quality standard in 3 years, and the Case II water quality standard in 4 years.
In this paper, the water environment model only considered a single water quality indicator of TN, and other pollution factors were not included in the model. Therefore, later studies can extend and improve the model with additional data to make the water environment model more comprehensive. In addition, this paper established a hydrodynamic model of the Xiyong River Basin without considering the influence of the downstream seawater backflow on the water quality, and the hydrodynamic and water quality characteristics of the Xiyong River and the sea inlet can be further studied in the future.