Numerical Simulation of the Marine Environmental Capacity of Jinpu Bay

Abstract

Based on the study of the marine environmental capacity, a water quality model and a response field-based linear programming method are adopted here. Water quality control objectives are taken as the constraint conditions, according to the requirements of Jinpu Bay’s functional zoning. The pollutant response coefficient and water quality background value are combined with the values of the water concentration quality control points set in each functional area and the target value of the functional area wherein they are located. The maximum allowable emission intensity of inorganic nitrogen, phosphate and chemical oxygen demand (COD) can be calculated using the linear programming method of the maximum allowable emission of pollutants at estuaries or sewage outfall points on Jinpu Bay. The research results reveal the diffusion of marine pollutants and the marine environmental capacity of Jinpu Bay. Some rivers need to reduce the discharge intensity and some other outlets still have a certain residual capacity. Based on this, the environmental capacity of Jinpu Bay was evaluated, and a technical reference is provided for the economic development of the region and the formulation of pollutant emission control policies.

1. Introduction

According to the definition of GESAMP (Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection) and taking into account the objective natural and subjective man-made attributes of environmental capacity, marine environmental capacity is defined as follows: Marine environmental capacity refers to the maximum amount of pollutants that can be contained in a specific sea area within a certain time range under the conditions of maintaining the national seawater quality standards required by the specific oceanographic and ecological functions of the target area [1].

In recent years, due to the rapid growth of urban populations, along with the rapid development of coastal and harbor industries and coastal mariculture, the loading of pollutants such as nitrogen, phosphorus and chemical oxygen demand (COD) into the sea has increased continuously, and marine ecological disasters, such as red tides, have occasionally occurred. In view of the important role played by coastal zones in the world’s social and economic development, especially with their rapid social and economic development, the globalization of marine ecological environment problems in offshore waters has gradually become prominent, and countries that depend on their coastlines have carried out studies on the marine environmental capacities of pollutants [2,3,4,5,6,7,8,9,10]. A specific method to study the environmental capacity of Kastela Bay in Yugoslavia has been proposed [11]. The environmental capacity of Osaka Bay in Japan has also been studied [12]. The mechanism and influencing factors of photochemical oxidation activation products on the free radical self-purification process in natural water has been discussed [13]. The environmental capacity of mercury in the seawater of Haifa Bay has been calculated [14]. Simulation exercises have been used to estimate the self-purification capacity of organic matter in natural water bodies [15]. Research on China’s marine environmental capacity began in the 1980s, when the State Oceanic Administration organized and carried out research projects on the environmental capacity of pollutants in offshore waters, such as Dalian Bay, Jiaozhou Bay and the Pearl River estuary [16,17,18,19,20,21,22,23]. In recent years, research has focused on some integrated models [24,25,26,27,28,29]. An integrated system of semi-enclosed bays has been built, and SECAMECC includes a database of seven data sets and four models: 3D fluid dynamics, ecology, residence time estimation and MECC calculation [30]. The marine environmental capacity of the northern sea area of Jiangsu Province has been numerically simulated based on the three-dimensional water quality model of FVCOM [31].

Located in Liaoning Province, China, Jinpu Bay is a semi-enclosed bay within the Bohai Sea. The urban function of Jinpu Bay positions it as a new type of comprehensive urban area, a shipping center, a manufacturing and high-tech industrial base, a regional financial center and a commercial center. In order to further study the marine environmental capacity of Jinpu Bay, this study adopts the water quality model and the linear programming method based on the response field. According to the requirements related to the functional zoning of Jinpu Bay, the maximum allowable discharge of the main estuaries or sewage outfalls along the coast of the bay have been calculated numerically, taking the water quality control objectives as the constraint conditions. Based on this, the environmental capacity of the marine area of Jinpu Bay can be evaluated, and a technical reference can be provided for the economic development of the region and the formulation of pollutant emission-control policies [32].

2. Materials and Methods

2.1. Overview of the Study Area

Dalian Jinpu Bay is located in the south of Liaoning Province, China. It is a semi-enclosed bay within the Bohai Sea. It is composed of Jinzhou Bay and Pulandian Bay, and covers an area of about 1300 km² [33]. There are many islands in the bay; the coastline is very dangerous for human activity, and the water depth is predominantly less than 20 m. The trend of the isobath line is basically the same as that of the coastline, except that the terrain slope within the bay is small on both sides of the mouth.

The water body volume of Jinpu Bay is relatively large; the bay comprises Jinzhou Bay and Pulandian Bay, which each have a distinct dilution capacity. However, the environmental capacity of the water body can only be utilized when pollutants are transported and diffused. Regarding Pulandian Bay, the topography is narrow and long, the landform is complex and pollutants are, thus, relatively difficult to transport. Regarding Jinzhou Bay, which is adjacent to the outer sea-facing side of Liaodong Bay, the water area is also relatively open and the seawater flow is relatively good, meaning that the water capacity of the bay still has scope for utilization.

2.2. Research Methods

The marine environmental capacity of the bay refers to the amount of pollutants that the water body can hold under a specified environmental target. In this study, the two-dimensional numerical model MIKE21FM developed by the Danish Institute of Hydraulics is adopted. The water quality model and response field-based linear programming method were used to calculate the maximum allowable discharge of the main estuaries or sewage outfalls along the coast of Jinpu Bay, according to the requirements of the functional zoning of Jinpu Bay and the water quality control objective taken as the constraint condition.

In a certain hydrodynamic environment, the pollutant diffusion equation can be regarded as linear and the superposition principle can be satisfied under given boundary conditions. The concentration field formed by pollutants imported from outside the bay is taken as the background field, denoted as C_b(x, y). There are n point sources in the bay; the source strength of the i point source is S_i, and the equilibrium concentration field formed by diffusion is denoted as C_i(x, y). As such, the concentration field in the bay can be expressed as

$$C\left(x,y\right)=C_b\left(x,y\right)+{\sum }_{i=1}^n C_i\left(x,y\right).$$

(1)

The equilibrium concentration field formed by the i-th point source is recorded in the form of a linear response relationship between C_i(x, y) and the source intensity, which gives

$${ C}_i\left(x,y\right)=P_i\left(x,y\right)S_i.$$

(2)

In the above formula, P_i(x, y) is the response coefficient, which is related to dynamic conditions, terrain, etc. The response coefficient P_i is equal to the equilibrium concentration field formed by the unit source strength.

C₀ is set as the concentration value of a certain pollutant under the condition of meeting the water quality control target (that is, the water quality target). In the case of n point sources, if the water quality concentration is to adhere to the control standard, it should be

$$C_b(x,y)+{\sum }_{i=1}^n P_i(x,y)S_i\le C_0$$

(3)

Regarding the background concentration field, its distribution is generally relatively uniform, and non-point sources are difficult to control. The control of pollutant emissions generally involves the control of point-source emissions. The characteristics of pollutant diffusion indicate that, when the water in the pollutant discharge point meets the water quality requirements, these requirements can be met in the whole water area. Given that the response coefficient of the unit emission intensity of the j-th pollutant at the i-th pollution source is P_ij, the calculation of the maximum allowable emission of the point source in the bay can be expressed as a linear programming problem as follows:

$$\mathrm{Objective} \mathrm{function} {\sum }_{i=1}^n{S}_i\to \mathrm{m}\mathrm{a}\mathrm{x};$$

(4)

$$\mathrm{Constraint} C_{bj}+{\sum }_{i=1}^n{P}_{ij}(x,y)S_i \le C_{0i}.$$

(5)

Here, S_i ≥ 0, i = 1, 2, …, and C_bi and C₀ represent the background concentration value and the water quality target value at the i-th point source, respectively.

3. Results

3.1. Division of Marine Functional Zones and Water Quality Control Objectives in Jinpu Bay

3.1.1. Determination of Pollution Factors

In the estimation of marine environmental capacity, the key pollutants stipulated by the state and local governments, the characteristic pollutants that may be produced in the planned area and the pollutants to which the receiving sea area is most sensitive should be considered. The urban function of Jinpu Bay in Dalian positions it as a comprehensive new urban area, shipping center, manufacturing and high-tech industrial base, regional financial center and business center. The main pollutants that enter the sea area are in the discharge from the sewage treatment plant, so the pollution factors considered in the environmental capacity analysis of seawater are mainly inorganic nitrogen, phosphate and chemical oxygen demand (COD). Jinpu Bay, the calculation domain in this numerical study, comprises a total of 11 major rivers and sewage outlets from Dahua. According to the actual distribution status of the current topography of the coastline of Jinpu Bay, the specific locations of estuaries and sewage outfalls are shown in Figure 1, and the current emission concentration values of pollutants monitored in 2013 at each estuary and sewage outfall are shown in

Table 1. Current emission concentrations of pollutants in estuaries and sewage outlets.

Administration Regionalization	Monitoring Point	Pollutant Monitoring Results (unit: mg/L)
		Chemical Oxygen Demand				Total Nitrogen				Total Phosphorus
		Q1	Q2	Q3	Q4	Q1	Q2	Q3	Q4	Q1	Q2	Q3	Q4
Jinpu New District	Shi River	-	29.0	-	40.0	-	1.0	-	4.9	-	0.1	-	0.0
	Daweijia River	42.0	137.0	40.0	44.0	16.8	22.0	4.98	7.0	0.8	3.2	0.5	0.9
	Wushili River	-	-	-	-	-	-	-	-	-	-	-	-
	Dengtun River	46.0	36.0	47.0	29.0	5.0	7.2	4.3	7.1	0.1	0.2	0.1	0.4
	Weitang River	115.0	49.0	30.0	-	4.8	8.4	7.7	-	0.1	0.2	0.0	-
	Laogu River	116.0	30.0	46.0	30.0	5.8	8.2	10.7	7.6	0.1	0.1	0.1	0.2
	Sanshili River	-	-	-	43.0	-	-	-	0.6	-	-	-	0.1
	Dahua Outlet	44.0	32.0	29.9	38.0	11.6	11.2	14.6	7.7	0.1	0.1	0.4	0.1
	Beida River	39.0	45.0	248.0	43.0	10.1	14.8	10.0	13.0	0.1	0.1	0.9	0.3
	Hongqi River					34.0	35.0	28.1	22.5	4.9	2.3	3.1	2.3
Pulandian	Anzi River	83.2	95.4	86.4	36.9	3.6	2.6	9.1	4.0	0.2	0.3	0.5	0.1

. It should be noted that, due to data limitations, the emission concentration values of inorganic nitrogen, phosphate and COD at the Wushili estuary have not been obtained, nor have the emission concentration values of COD at the Xiajia and Muchengyi estuaries been obtained. We refer to the emission concentration values of COD at the other 10 estuaries and the Dahua sewage discharge outlets in this study. The COD emission concentration values of the Xiajia estuary and the Muchengyi estuary are both 65 mg/L.

3.1.2. Setting the Location of Monitoring Points for Measuring Water Quality in Zones

According to the Marine Function Zoning of Liaoning Province (2011–2020) and the Dalian Coastal Sea Function Zoning Map that was adjusted in 2006, 13 water quality control points were set at the water quality dividing lines of different functional zones near estuaries and sewage outlets to ensure the control of the standard. Water quality is divided into four levels of standard, which are I, II, III and IV. Specifically, points 1 and 2 were allocated in the middle of Pulandian Bay. In the boundary area between the inner bay and outer bay of Pulandian Bay, where the Dahua sewage outlet is located, point 3 was set in the middle waters of the outer bay of Puwan, that is, in the waters near Qidingshan; point 4 and point 5 were located in the waters where the mouth of Pulandian Bay meets Jinzhou Bay; point 6 and point 7 were located in the eastern waters of Jinzhou Bay; point 8, point 9 and point 10 were set in the southeastern and southern waters of Jinzhou Bay; and point 11 was set in the central waters of Jinzhou Bay. Point 12 was located in the northern sea area of Jinzhou Bay, and point 4, point 6, point 7, point 10 and point 11 were distributed within the sea area of the harbor seal reserve and have high water quality requirements. The division of functional zones and the positions of each water quality control point are shown in Figure 2. The water quality statistics of the functional areas to which each water quality control point belongs are shown in

Table 2. Basic information of water quality control points in Jinpu Bay.

Serial Number	Local Sea Area	Water Quality	Inorganic Nitrogen (mg/L)	COD (mg/L)	Active Phosphate (in P) (mg/L)
1	Pulandian Bay	IV	≤0.5	≤5	≤0.045
2		IV	≤0.5	≤5	≤0.045
3		II	≤0.3	≤3	≤0.030
4		I	≤0.2	≤2	≤0.015
5		II	≤0.3	≤3	≤0.030
6		I	≤0.2	≤2	≤0.015
13		IV	≤0.5	≤5	≤0.045
7	Jinzhou Bay	I	≤0.2	≤2	≤0.015
8		II	≤0.3	≤3	≤0.030
9		III	≤0.4	≤4	≤0.030
10		I	≤0.2	≤2	≤0.015
11		I	≤0.2	≤2	≤0.015
12		II	≤0.3	≤3	≤0.030

Note: COD—chemical oxygen demand; water quality is divided into four levels of standard, which are I, II, III and IV.

.

3.2. Calculation of the Marine Environmental Capacity of Jinpu Bay

3.2.1. Calculation of Pollution Response Coefficient

Based on the hydrodynamic status of Jinpu Bay, the background values of pollutants in Jinpu Bay and the discharge flux of pollutants in the estuary and the discharge outlet, the stable concentration field of pollutants in Jinpu Bay is obtained using numerical simulation. The response coefficients of the divided sea area to the pollution coming from each estuary or discharge outlet can be obtained by dividing the concentration value by the emission intensity of pollutants at each estuary or discharge outlet. The response coefficient can be used to reflect the contribution of the discharge status of estuaries or sewage outlets to the marine environment to a certain extent.

Figure 3 and Figure 4, respectively, show the annual mean concentration fields of total nitrogen emissions in the sea area of Jinpu Bay and the independent annual mean concentration fields of total nitrogen emissions from estuaries or sewage outlets. Figure 5 and Figure 6, respectively, show the results of the numerical simulation of the annual average concentration field of total phosphorus emissions in the Jinpu Bay sea area and the independent annual average concentration field of total phosphorus emissions in each estuary or sewage outlet. Figure 7 and Figure 8, respectively, show the annual mean concentration fields of total nitrogen emissions in the sea area of Jinpu Bay and the independent annual mean concentration fields of total nitrogen emissions in each estuary or sewage outlet. According to the numerical results of the annual average concentration field of each pollutant, combined with the discharge flux of each pollutant at each estuary or sewage outlet, the response coefficients of each pollutant can be obtained. The response coefficients of various pollutants in this numerical simulation are shown in

Table 3. Corresponding coefficient values of total nitrogen in the waters near estuaries and sewage outlets.

	Monitoring Point	1	2	3	4	5	6	7	8	9	10	11	12	13
Sewage Outlet
Weitang River		0.051	0.055	0.080	0.019	0.027	0.006	0.001	0.000	0.000	0.000	0.000	0.001	0.010
Laogu River		0.125	0.099	0.044	0.005	0.010	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.032
Dengtun River		0.028	0.018	0.005	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.405
Anzi River		0.027	0.017	0.005	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.394
Shi River		0.079	0.057	0.020	0.002	0.004	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.483
Sanshili River		0.096	0.014	0.007	0.001	0.002	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.002
Daweijia River		0.002	0.002	0.007	0.013	0.025	0.013	0.016	0.001	0.001	0.000	0.000	0.002	0.000
Beida River		0.001	0.001	0.003	0.006	0.009	0.007	0.026	0.107	0.108	0.002	0.000	0.001	0.000
Xiajia River		0.000	0.000	0.000	0.001	0.001	0.002	0.005	0.006	0.005	0.015	0.015	0.003	0.000
Muchengyi River		0.000	0.000	0.001	0.002	0.002	0.006	0.010	0.010	0.008	0.014	0.034	0.007	0.000
Dahua Sewage Outlet		0.180	0.131	0.053	0.006	0.013	0.002	0.000	0.000	0.000	0.000	0.000	0.000	0.065

,

Table 4. Corresponding coefficient values of total phosphorus in the waters near estuaries and sewage outlets.

	Monitoring Point	1	2	3	4	5	6	7	8	9	10	11	12	13
Sewage Outlet
Weitang River		0.051	0.055	0.080	0.019	0.027	0.006	0.001	0.000	0.000	0.000	0.000	0.001	0.010
Laogu River		0.125	0.099	0.044	0.005	0.010	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.032
Dengtun River		0.028	0.018	0.005	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.405
Anzi River		0.027	0.017	0.005	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.394
Shi River		0.079	0.057	0.020	0.002	0.004	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.483
Sanshili River		0.100	0.146	0.073	0.011	0.021	0.004	0.001	0.000	0.000	0.000	0.000	0.000	0.023
Daweijia River		0.002	0.002	0.007	0.013	0.025	0.013	0.016	0.001	0.001	0.000	0.000	0.002	0.000
Beida River		0.001	0.001	0.003	0.006	0.009	0.007	0.026	0.107	0.108	0.002	0.000	0.001	0.000
Xiajia River		0.000	0.000	0.000	0.001	0.001	0.002	0.005	0.006	0.005	0.015	0.015	0.003	0.000
Muchengyi River		0.000	0.000	0.001	0.002	0.002	0.006	0.010	0.010	0.008	0.014	0.034	0.007	0.000
Dahua Sewage Outlet		0.180	0.131	0.053	0.006	0.013	0.002	0.000	0.000	0.000	0.000	0.000	0.000	0.065

and

Table 5. Corresponding coefficient values of COD in the waters near estuaries and sewage outlets.

	Monitoring Point	1	2	3	4	5	6	7	8	9	10	11	12	13
Sewage Outlet
Weitang River		0.772	0.760	0.479	0.286	0.340	0.268	0.321	0.520	0.521	0.245	0.249	0.230	1.327
Laogu River		0.707	0.724	0.521	0.303	0.362	0.275	0.325	0.525	0.526	0.247	0.252	0.233	1.318
Dengtun River		1.769	1.761	1.215	0.665	0.802	0.597	0.702	1.136	1.137	0.534	0.544	0.505	2.512
Anzi River		0.477	0.481	0.337	0.186	0.224	0.167	0.197	0.318	0.319	0.150	0.153	0.141	0.424
Shi River		3.379	3.367	2.329	1.279	1.541	1.149	1.352	2.186	2.188	1.027	1.048	0.971	5.131
Sanshili River		0.355	0.305	0.236	0.158	0.183	0.148	0.178	0.289	0.289	0.136	0.139	0.128	0.719
Daweijia River		0.709	0.702	0.476	0.250	0.293	0.224	0.261	0.449	0.449	0.211	0.215	0.198	1.154
Beida River		0.298	0.295	0.200	0.105	0.125	0.093	0.090	0.082	0.081	0.087	0.090	0.083	0.485
Xiajia River		0.858	0.850	0.583	0.317	0.382	0.283	0.330	0.536	0.538	0.240	0.245	0.239	1.394
Muchengyi River		1.401	1.387	0.951	0.517	0.624	0.460	0.538	0.875	0.878	0.402	0.391	0.387	2.274
Dahua Sewage Outlet		0.781	0.821	0.600	0.350	0.417	0.318	0.376	0.608	0.608	0.286	0.291	0.270	1.497

Note: COD—chemical oxygen demand.

.

3.2.2. Calculation Results of Marine Environmental Capacity

According to the pollutant response coefficient and the water quality background value, further combined with the concentration values of the water quality control points obtained from each functional area and their target values, the maximum allowable emission intensities of inorganic nitrogen, phosphate and COD at each estuary or sewage outlet can be calculated using the linear programming method.

Table 6. Estimation of current discharge intensity at the sewage outlet (t/d).

Pollutant	Sewage Outlet
	Weitang River	Laogu River	Dengtun River	Anzi River	Shi River	Sanshili River	Daweijia River	Beida River	Xiajia River	Muchengyi River	Dahua	Gross Amount
Total Nitrogen	0.784	1.047	0.497	0.761	0.146	0.195	1.625	2.553	0.864	1.093	1.950	11.515
Total Phosphorus	0.012	0.017	0.017	0.042	0.003	0.015	0.173	0.071	0.053	0.008	0.025	0.435
COD	7.263	7.193	3.327	11.868	1.729	13.078	8.408	20.007	6.964	4.268	6.216	90.321

Note: t/d refers to tons per day; COD—chemical oxygen demand.

outlines the current emission intensity of estuaries and sewage outfalls in Jinpu Bay based on existing data, and

Table 7. Estimation of allowable discharge intensity at sewage outlet (t/d).

Pollutant	Sewage Outlet
	Weitang River	Laogu River	Dengtun River	Anzi River	Shi River	Sanshili River	Daweijia River	Beida River	Xiajia River	Muchengyi River	Dahua	Gross Amount
Total Nitrogen	0.402	0.536	0.255	0.390	0.075	0.159	1.876	2.635	0.891	1.127	0.999	9.346
Total Phosphorus	0.014	0.017	0.017	0.041	0.003	0.015	0.208	0.085	0.064	0.009	0.025	0.497
COD	10.465	9.038	3.800	19.532	0.737	22.184	13.616	36.048	7.995	7.999	7.414	138.828

Note: t/d refers to tons per day; COD—chemical oxygen demand.

shows the allowable emission intensity of estuaries and sewage outfalls obtained via numerical simulation. As shown in the table, under the current situation, the total nitrogen emission intensity of Jinpu Bay is about 11.5 t/d, the total phosphorus emission intensity is about 0.4 t/d and the COD emission intensity is about 90.3 t/d. Under standard discharge, the allowable emission intensities of total nitrogen, total phosphorus and COD in Jinpu Bay are about 9.3 t/d, 0.5 t/d and 138.8 t/d. The numerical results show that the total nitrogen emissions in Jinpu Bay exceed the standard, and its emission intensity should be reduced by 2.2t/d. Regarding the total phosphorus and COD emissions in Jinpu Bay, the remaining capacity means the total phosphorus emission intensity can be increased by about 0.1 t/d, and the COD emission intensity can be increased by about 48.5 t/d.

4. Discussion

Figure 9, Figure 10 and Figure 11, respectively, show a contrast histogram between the current emission intensity and the allowable emission intensity of total nitrogen, total phosphorus and COD at the estuaries and sewage outlets of Jinpu Bay. Blue depicts the current emission intensity of the estuary or sewage outlet, red depicts the emission intensity at the estuary or sewage outlet that needs to be reduced under standard discharge and green depicts the emission intensity at the estuary or sewage outlet that can be increased under standard discharge. As shown in the figure, in terms of total nitrogen emissions, the Weitang River, Laogu River, Dengtun River, Anzi River, Shihe River, Sanshili River and the Dahua outlet need to reduce their emission intensities, while the Daweijia River, Beida River, Xiajia River and Muchengyi River still have some residual capacity. In terms of total phosphorus discharge, the Laogu River, Dengtun River, Anzi River, Shihe River, Sanshili River and the Dahua outlet need to reduce their discharge intensities, while the Weitang River, Daweijia River, Beida River, Xiajia River and Muchengyi River still have some residual capacity. In terms of COD discharge, aside from the need to reduce the discharge intensity at Shihe River, the other rivers and the Dahua sewage discharge outlet still have a certain residual capacity. The specific allowable emission intensity of each estuary or outlet can be seen in Table 7.

The industrial and marine aquaculture industries in Jinpu Bay are relatively concentrated, causing significant pressure on the capacity of the marine environment. The two-dimensional numerical model MIKE21FM is adopted. The numerical model predicted pollutant concentration and revealed the diffusion of pollutants. The model has shown good performance in the evaluation of sediment transport in the Naiband Gulf Area and the diffusion source of the Bai River [34,35].

The pollution factors are inorganic nitrogen, phosphate and chemical oxygen demand (COD), which reflect the pollution situation of Jinpu Bay. Some other pollution factors such as petroleum hydrocarbons can be considered [31], which can provide a more comprehensive display of the pollution situation. Due to limited measured data, we only verified the tide level. In future work, we will improve the accuracy of the model’s calculation of environmental capacity by obtaining more measured data for the validation of pollution factors.

The research results reveal the diffusion of marine pollutants and the marine environmental capacity of Jinpu Bay. Some rivers need to reduce the discharge intensity and some other outlets still have a certain residual capacity. According to the calculation of the pollution response coefficient and the marine environmental capacity, the emission amount of the sewage outlet can be controlled. A technical reference is provided for the economic development of the region and the formulation of pollutant emission control policies. In the subsequent work, the layout of the sewage outlets can be optimized, and a total discharge control plan can be proposed [32].

5. Conclusions

(1)

In the current context, the total nitrogen emission intensity of Jinpu Bay is about 11.5 t/d, the total phosphorus emission intensity is about 0.4 t/d and the COD emission intensity is about 90.3 t/d. Under standard discharge, the allowable emission intensity values of total nitrogen, total phosphorus and COD in Jinpu Bay are about 9.3 t/d, 0.5 t/d and 138.8 t/d. The numerical results show that the total nitrogen emissions in Jinpu Bay exceed the standard, and its emission intensity should be reduced by 2.2 t/d. Regarding the total phosphorus and COD emissions in Jinpu Bay, the remaining capacity demonstrates that the total phosphorus emission intensity can be increased by about 0.1 t/d, and the COD emission intensity can be increased by about 48.5 t/d;

(2)

In terms of total nitrogen emissions, the Weitang River, Laogu River, Dengtun River, Anzi River, Shihe River, Sanshili River and the Dahua sewage outlet need to reduce their emission intensities, while the Daweijia River, Beida River, Xiajia River and Muchengyi River still have some residual capacity. In terms of total phosphorus discharge, the Laogu River, Dengtun River, Anzi River, Shihe River, Sanshili River and the Dahua outlet need to reduce their discharge intensities, while the Weitang River, Daweijia River, Beida River, Xiajia River and Muchengyi River still have some residual capacity. In terms of COD discharge, aside from the need to reduce the discharge intensity at Shihe River, the other rivers and the Dahua sewage discharge outlet still have a certain residual capacity;

(3)

The research results can not only reveal the diffusion of marine pollutants and the marine environmental capacity of Jinpu Bay, but also a technical reference is provided for the economic development of the region and the formulation of pollutant emission control policies.