Evaluation of Soil and Irrigation Water Quality in Caohai Lakeside Zone

Abstract

Due to the rapid population growth and over-application of fertilizers in the Caohai surrounding farmlands, controlling the non-point source pollution in the Caohai Lakeside Zone is significant for the local ecology balance and human health safety. A total of 54 soil and 24 irrigation water samples were collected in the Caohai Lakeside Zone to evaluate the current status of soil fertility and heavy metal pollution by measuring soil and water heavy metal content and soil physicochemical properties such as soil total nitrogen, total phosphorus, organic matter, and soil pH. These results showed that the total amounts of organic matter, nitrogen, phosphorus, potassium, and effective nutrient content in the Caohai Lakeside Zone were all at a rich level according to Chinese soil nutrient grade standard; the content of lead (Pb), chromium (Cr), cadmium (Cd), mercury (Hg), and arsenic (As) in the soil all exceeds the safety standard of Chinese classification of soil environmental quality assessment. In addition, the over-standard rates of Cd and Cr were 24.1% and 14.8%, respectively. On the whole, the comprehensive quality index (CQI) of the soil in the Caohai Lakeside Zone is 6.48, which is attributed to the heavy Cd pollution and the good soil fertility. The heavy metal content of the irrigation water met the requirements of irrigation water quality. It is feasible to use the irrigation water of Caohai Lake in the actual agricultural production process. Therefore, in terms of the comprehensive management and pollution control of Caohai, special attention should be paid to the control of Cd, Pb, As, and Hg pollution sources.

1. Introduction

In recent years, due to the rapid development of China’s economy, the growth of population and the acceleration of urbanization, a large number of lakes have been over exploited and utilized, resulting in the continuous deterioration of lake ecological environment [1,2,3]. In addition, a large number of chemical fertilizers and pesticides are used in the land around the lake, and the treatment of rural livestock manure and agricultural waste is not timely. The surface runoff formed by rainfall also brings nitrogen and phosphorus rich pollutants and heavy metals into the water bodies [4,5,6]. These lead to an increase of the content of nitrogen, phosphorus, and heavy metals in the water body, resulting in agricultural pollution such as the deterioration of lake water quality and heavy metal pollution. Therefore, the ecological security of the lake and the ecological environment of the lakeside zone are seriously threatened [7,8,9].

China is a large agricultural country, in which irrigation water consumption accounts for about 70% [10]. For human beings, lakes are important water sources for agricultural production, while sewage irrigation is an important source of heavy metal pollution in farmland soil [11]. Heavy metal pollutants have poor mobility and long retention time in agricultural soil, and are not easily degraded by soil microorganisms [12,13,14]. Agricultural soil is affected by human activities most strongly, and its environmental status is directly related to people’s health [15,16]. It is urgent to prevent and control heavy metal pollution in agricultural soil. There are a lot of international research works related to regional pollution. They have explored from the aspects of water environmental capacity, pollution load, health risk, and so on [17,18,19]. Understanding the regional pollution situation is very important for agricultural production and human life.

Caohai is located at the eastern edge of Yunnan Guizhou Plateau. It is the largest natural freshwater lake in Guizhou Province. It is a complete and typical plateau wetland ecosystem. Contents of total soil nutrients and organic carbon in Caohai Lake are at the middle and low level in China. The contents of available nutrients are low nitrogen, high phosphorus, and medium potassium. The content of soil nutrients is unbalanced, which is mainly limited by nitrogen [20]. Content of heavy metals in the surface sediments of Caohai Lake was determined [21]. It was found that the most polluted heavy metals were Cd, Zn, and Hg. In recent years, due to the growth of population and the excessive application of chemical fertilizer and pesticides in surrounding farmland, the ecological environment of Caohai Lake has been deteriorating, which seriously threatens the sustainable development of local agriculture [22,23]. In order to explore the current situation of soil heavy metals, nitrogen, and phosphorus pollution in Caohai Lakeside Zone, soil samples and irrigation water samples were collected from representative plots in the planting area around Caohai, and pollution analysis data were obtained. The current situation of soil fertility and heavy metal pollution in Caohai Lakeside Zone was analyzed and evaluated. This provides a reference basis for the ecological protection of Caohai and promotes the sustainable development of agricultural production in Caohai Lakeside Zone.

2. Materials and Methods

2.1. Overview of Survey Area

Caohai Lakeside Zone in Weining County, Guizhou Province is located 360 km west of Guiyang and west of Weining. The geographical coordinates are between 104°12′–104°18′ E and 26°49′–26°53′ N (Figure 1). Caohai is the largest natural freshwater lake in Guizhou, with an altitude of 2171.7 m and a water area of 46.5 km². It is named after the lush waterweeds. Caohai is a plateau lake located in the Karst Development Area. The ecosystem is unstable and the ecological environment is very fragile, which has obvious ecological vulnerability. The investigation and monitoring of soil pollution in the lakeside zone of Caohai Lake in Weining (Figure 1) involves nine administrative villages (including Yinlong, Haibin, Xihai, Qianjin, Yabitang, Macheng, Caohai, Baima, and Dongshan).

Along the planting area around Caohai, 270 representative plots were selected in November 2018. A total of 54 soil samples and 24 irrigation water samples were collected (Figure 1). A total of 822 soil fertility and pollution analysis data were obtained, including 432 soil fertility analysis data, 270 soil heavy metal pollution analysis data, and 120 irrigation water heavy metal pollution analysis data.

2.2. Sample Collection and Processing

The layout of Caohai Lakeside Zone soil sample collection points combined with the actual situation of the sampling site. The sampling area covers nearly 50,000 m² of Caohai Lakeside Zone. It was sampled of soil according to the principles of “random”, “equal quantity”, and “multi-point mixing” [24], and S-shaped point sampling is generally adopted. Quincunx sampling was also used in the case of small topographic change, uniform soil fertility, and small sampling unit area. A soil sampler with a length of 700 mm, an outer diameter of 30 mm and an inner diameter of 25 mm was used to collect top soil of 0~20 cm, and the sampling amount of each sample was 2 kg. The mixed bricks and tiles, lime nodules, rhizome animal and plant residues, and other impurities in the soil were removed. After air drying and rough grinding, all samples passing 2 mm sieve were placed on colorless polyethylene film, fully stirred, and mixed until uniform. It was discarded and weighed by quartering method and retained as analytical sample. Water samples were collected using open water grab sampler (1.5 L capacity) equipped with a simple pull-ring that allowed for sampling at various water depths (0–6 and >24 inch). All water samples were stored in insulated cooler containing ice and delivered on the same day to laboratory, and all samples were kept at 4 °C until processed and analyzed [25].

2.3. Sample Determination and Quality Control

According to the land use type of Caohai Lakeside Zone in Weining, 54 soil samples were collected and analyzed for soil pH, Cd, Hg, As, Pb, and Cr (

Table 1. Methods for laboratory analysis and test of soil samples.

Monitoring Items	Monitoring Method	Minimum Detectable Concentration
pH	Glass electrode method	0.01~0.1
Cd	Inductively Coupled Plasma Mass Spectrometry (ICP-MS)	0.001~0.030 (mg kg−1)
Hg	Cold atomic absorption spectrophotometry	0.0004~0.005 (mg kg−1)
As	ICP-MS	0.005 (mg kg−1)
Pb	ICP-MS	0.002~2.0 (mg kg−1)
Cr	ICP-MS	0.0030~5.0 (mg kg−1)

). Soil physical and chemical items include pH value, organic matter, total nitrogen, total phosphorus, total potassium, available nitrogen, available phosphorus, available potassium, total boron, available boron, total zinc, and available zinc [26]. All these items are determined with reference to “soil agrochemical analysis” [27].

In the process of analysis and determination, blank samples and national standard soil samples (GSS. 27) are analyzed simultaneously for the whole process of analysis quality control. In order to test the reliability of experimental analysis, one sample from every 10 samples is taken for repeated experiment [28]. During the experiment, all glass instruments were soaked in 10% HNO₃ solution for 2 days, and then washed with tap water and deionized water 3 times in order to reduce pollution. All reagents used in the experiment were GR grade.

2.4. Evaluation Standards and Methods of Soil Environmental Quality

2.4.1. Soil Environmental Quality Evaluation Standard

The evaluation of soil environmental quality is carried out in accordance with the class II standard of soil environmental quality standard (GB15618-1995). For the evaluation standards not available in GB15618, the national technical regulations on soil pollution assessment (HF [2008] No. 39) was implemented. The environmental quality assessment of irrigation water was implemented the technical code for environmental quality monitoring of agricultural water sources (NY/t396-2000) (

Table 2. Soil environmental quality assessment standard values.

Sequence Number	Assessment Item		Reference Standard Value (mg kg−1)				Source
			Cultivated, Grassland and Unused Land			Woodland
			pH < 6.5	pH = 6.5–7.5	pH > 7.5
1	Cd		0.30	0.30	0.60	1.0	GB15618-1995
2	Hg		0.30	0.50	1.0	1.5
3	As	Dryland	40	30	25	40
		Paddy soil	30	25	20
4	Pb		80	80	80	40	HJ 332-2006
5	Cr	Dryland	150	200	250	400	GB15618-1995
		Paddy soil	250	300	350

and

Table 3. The grade standards of diverse soil nutrients.

Item (g kg−1)	Extremely High	High	Medium	Low	Extremely Low
Organic materials	≥25	25–20	20–15	15–10	<10
Total nitrogen	≥1.20	1.20–1.00	1.00–0.80	0.80–0.65	<0.65
Total phosphorus		>0.8	0.4–0.8	<0.4
Total potassium		>3.0	0.5–0.3	<0.5
Available nitrogen	≥120	120–90	90–60	60–45	<45
Available phosphorus	≥90	90–60	60–30	30–15	<15
Available potassium	≥155	155–125	125–100	100–70	<70

).

2.4.2. Evaluation Method of Soil and Irrigation Water Environmental Quality

The standard methods for environmental quality assessment of soil and irrigation water were implemented in accordance with “the notice on printing and distributing the work plan for routine monitoring of national soil environmental quality in 2011” (Zongzhan Shengzi [2011] No. 161) (

Table 4. Classification of soil environmental quality assessment.

Level	Pip Value Size	Pollution Assessment
I	Pip ≤ 1	Non-pollution
II	1 < Pip ≤ 2	Slight pollution
III	2 < Pip ≤ 3	Slight pollution
IV	3 < Pip ≤ 5	Moderate pollution
V	Pip > 5	Severe pollution

and

Table 5. Grading standard of soil comprehensive pollution index.

Level	Composite Pollution Index	Pollution Level
I	Pcom ≤ 0.7	Cleaning (safety)
II	0.7 < Pcom ≤ 1.0	Still cleaning (warning limit)
III	1.0 < Pcom ≤ 2.0	Slight pollution
IV	2.0 < Pcom ≤ 3.0	Moderate pollution
V	Pcom > 3.0	Severe pollution

).

Relevant calculation formulas and grading standards:

Single quality index (P_i) = single measured value/single standard value

(1)

Share rate of a single item (%) = (quality index of a single item/sum of various quality indices) × 100%

(2)

Exceedance rate (%) = (number of samples/total number of samples) × 100%

(3)

Soil single pollution index (P_ip) = measured value of soil pollutants/pollutant quality standard value

(4)

$$\begin{array}{c}\mathrm{Comprehensive}\mathrm{pollution}\mathrm{index}\mathrm{of}\mathrm{soil}\mathrm{and}\mathrm{water}({\mathrm{P}}_{\mathrm{com}})=\\ \sqrt{\frac{Average P_{ip}{}^2+Max P_{ip}{}^2}{2}}\end{array}$$

(5)

2.5. Statistical Analysis

The experimental data were analyzed and plotted using Origin 2018, Excel and SPSS 19.0. The mean, SD, coefficient of variation, and data correlations were analyzed by SPSS 19.0. Figures were plotted using Origin 2018.

3. Investigation and Monitoring Results

3.1. Descriptive Statistical Characteristics of Soil Fertility Indices

A total of 54 soil samples were collected and 432 effective test data were obtained. The statistics of soil monitoring results in the survey area are shown in

Table 6. Statistical table of soil physical and chemical properties (n = 54).

Sequence Number	pH	Organic Materials (g kg−1)	Total Nitrogen (g kg−1)	Total Phosphorus (g kg−1)	Total Potassium (g kg−1)	Available Nitrogen (g kg−1)	Available Phosphorus (g kg−1)	Available Potassium (g kg−1)
Min	5.25	13.8	1.57	0.11	5.80	57.7	3.70	54.0
Max	8.13	78.6	4.21	2.14	18.6	345	72.0	224
Average	--	32.1	2.61	0.83	11.4	152	35.0	107
SD	--	13.5	0.55	0.33	2.51	50.8	17.9	36.1
Coefficient of variation (%)	--	42.0	21.0	40.2	22.0	33.4	51.1	33.8

. The results showed that the soil pH in Caohai Lakeside Zone was between 5.25 and 8.13. The number of samples with pH > 7.0 accounted for 12.9%, indicating that the soil in Caohai Lakeside Zone is mainly slightly acidic and acidic. The content of organic matter in soil ranged from 13.49 to 78.60 g kg⁻¹, the average value was 32.09 ± 13.49 g kg⁻¹, and the coefficient of variation was 42.0%. The total nitrogen content ranged from 1.57 to 4.21 g kg⁻¹, the average value was 2.61 ± 0.55 g kg⁻¹, and the coefficient of variation was 21.0%. The content of total phosphorus ranged from 0.11 to 2.14 g kg⁻¹, the average value was 0.83 ± 0.33 g kg⁻¹, and the coefficient of variation was 40.2%. The content of total potassium ranged from 5.80 to 18.60 g kg⁻¹, the average value was 11.43 ± 2.51 g kg⁻¹, and the coefficient of variation was 22.0%. From the available nutrients of the soil in Caohai Lakeside Zone, the content of available nitrogen in the soil ranges from 57.71 to 345.47 mg kg⁻¹, the average value is 152.17 ± 50.82 mg kg⁻¹, and the coefficient of variation is 33.4%. The content of available phosphorus ranged from 3.70 to 72.00 mg kg⁻¹, the average value was 35.02 ± 17.91 mg kg⁻¹, and the coefficient of variation was 51.1%. The content of available potassium ranged from 54.00 to 224.00 mg kg⁻¹, the average value was 106.7 ± 36.11 mg kg⁻¹, and the coefficient of variation was 33.8%.

3.2. Correlation Analysis between Soil Fertility Factors

In the 54 sample points of this survey, the correlation analysis was carried out on 8 conventional physical and chemical Indices of soil (

Table 7. Correlation analysis of soil physical and chemical properties.

	pH	Organic Materials	Total Nitrogen	Total Phosphorus	Total Potassium	Available Nitrogen	Available Phosphorus	Available Potassium
pH	1
Organic materials	0.140	1
Total nitrogen	0.009	0.640 **	1
Total phosphorus	0.240	0.722 **	0.671 **	1
Total potassium	0.205	0.031	–0.043	–0.036	1
Available nitrogen	0.138	0.568 **	0.677 **	0.769 **	–0.126	1
Available phosphorus	–0.089	0.351 **	0.436**	0.530 **	0.003	0.439 **	1
Available potassium	0.274 *	0.182	0.069	0.230	0.537 **	–0.027	0.092	1

Note: “*” means significant correlation; “**” means extremely significant correlation.

). The results showed that except for the positive correlation between soil pH and available potassium (p < 0.05), the correlation between soil pH and other Indices was not obvious. Soil organic matter was positively correlated with total nitrogen, total phosphorus, alkali hydrolyzable nitrogen, and available phosphorus (p < 0.01). Soil organic matter was extremely positively correlated with total nitrogen, total phosphorus, available nitrogen, and available phosphorus (p < 0.01). Total nitrogen was positively correlated with organic matter, total phosphorus, available nitrogen, and available phosphorus (p < 0.05). Except that total potassium was positively correlated with available potassium (p < 0.05), there was no significant correlation with other Indices.

3.3. Soil Heavy Metal Pollution in Caohai Lakeside Zone

In the survey of soil heavy metal pollution in Caohai Lakeside Zone, 54 soil samples were collected and 270 effective detection data were obtained. The soil monitoring results in the survey area were statistically analyzed (

Table 8. Statistical table of soil monitoring results in Caohai Lakeside Zone (unit: mg kg⁻¹, except pH).

	pH	Lead	Chromium	Cadmium	Mercury	Arsenic
Average	-	69.3	97.1	2.66	0.08	23.8
SD	-	34.8	24.5	0.79	0.07	7.42
Min	5.25	13.7	24.8	0.71	0.03	13.3
Max	8.13	171	152	3.97	0.42	47.0
Coefficient of variation (%)	10.2	50.3	25.2	29.7	87.3	31.2

). The results showed that the soil pH in Caohai Lakeside Zone was between 5.25 and 8.13, and the number of samples with pH > 7.0 accounted for 12.9%. The number of soil samples with pH < 6.5 accounts for 44.4% of the total, and the number of soil samples with pH between 6.5 and 7.5 accounts for 42.7% of the total. It shows that the soil in Caohai Lakeside Zone is mainly acidic and slightly acidic. The content of Pb in soil ranged from 13.70 to 170.71 mg kg⁻¹, with an average of 69.26 ± 34.83 mg kg⁻¹ and a coefficient of variation of 50.3%. The content of Cr in soil ranged from 24.76 to 152.32 mg kg⁻¹, the average value was 97.13 ± 24.50 mg kg⁻¹, and the coefficient of variation was 25.2%. The content of Cd in soil was 0.71 to 3.97 mg kg⁻¹, the average value was 2.66 ± 0.79 mg kg⁻¹, and the coefficient of variation was 29.7%. The content of Hg in soil was 0.03 to 0.42 mg kg⁻¹, the average value was 0.08 ± 0.07 mg kg⁻¹, and the coefficient of variation was 87.3%. The content of as in soil was 13.26 to 46.94 mg kg⁻¹, the average value was 23.79 ± 7.42 mg kg⁻¹, and the coefficient of variation was 31.2%. From the variation coefficient of soil Pb, Cr, CD, Hg, and As content, the regional difference of Hg content in Caohai Lakeside Zone is the largest, followed by Pb content, and the regional difference of Cr, CD, and As content is small. A soil quality index could be defined as the minimum set of parameters that, when interrelated, provides numerical data on the capacity of a soil to carry out one or more functions.

The CQI (comprehensive quality index) can be regarded as a kind of SQI (soil quality indexing) specialized in studying the pollution degree of soil heavy metals [29]. The CQI can well analyze the comprehensive heavy metal pollution degree of soil in a certain area [30]. As seen in

Table 9. Heavy metal pollution assessment of soil in the lakeside zone of Caohai Lake.

Item	Individual Quality Index	Individual Contribution Rate	Individual Excess Rate	Comprehensive Quality Index
Pb	1.63	10.2	5.56	6.48
Cr	0.85	5.32	14.8
Cd	11.3	70.5	24.1
Hg	1.01	6.31	3.70
As	1.24	7.75	7.41

Note: The individual quality index and over-standard rate of each element are respectively counted according to the soil environmental quality evaluation index limit of different pH values.

, the CQI in Caohai Lake is 6.48. The individual quality index of Pb, Cr, CD, Hg, and As are 1.63, 0.85, 11.3, 1.01, and 1.24, respectively. Their individual contribution rates are 10.2, 5.32, 70.5, 6.31, and 7.75, respectively.

3.4. Characteristics and Correlation Analysis of Excessive Heavy Metals in Soil

As seen in Figure 2, the soil environment of Caohai Lakeside Zone is polluted by heavy metals in varying degrees. The contents of Pb, Cr, Cd, Hg, and As in some soil samples exceeded the standard. Among them, 13 samples of Cd exceeded the standard, accounting for 24.1% of the samples in this survey. Cr exceeded the standard at 8 points, accounting for 14.8% of the investigated points. The exceedance rates of Pb, Hg, and As are small, accounting for 7.4%, 5.6%, and 3.7% of the investigated samples, respectively.

The sources of the same type of soil heavy metal pollutants in the same area can be the same or multi-channel. There is usually a certain correlation between soil heavy metals from the same source [31]. The significant and extremely significant correlation between heavy metal elements indicates that there is generally homologous relationship or compound pollution among elements. The correlation analysis results of soil pH and soil heavy metals in Caohai Lakeside Zone (as seen in

Table 10. Correlation analysis of heavy metals in soil of Caohai lakeside.

	pH	Lead	Chromium	Cadmium	Mercury	Arsenic
pH	1
Pb	0.069	1
Cr	0.158	–0.200	1
Cd	0.277 *	0.585 **	–0.039	1
Hg	0.290 *	0.357 *	0.113	0.030	1
As	0.112	–0.017	–0.098	0.260	–0.092	1

Note: “*” means significant correlation; “**” means extremely significant correlation.

) show that: pH has a significant positive correlation with Cd and Hg (p < 0.05), while Pb has an extremely significant positive correlation with Hg (p < 0.01) and has a significant positive correlation with Cd (p < 0.05). It shows that the sources of Cd, Hg, and Pb in soil heavy metals in Caohai Lakeside Zone may be the same.

3.5. Heavy Metal Content in Ditch Irrigation Water in Caohai Lakeside Zone

The content of heavy metals in ditch irrigation water in Caohai Lakeside Zone is within the limit (

Table 11. Statistical table of monitoring results of irrigation water in lakeside zone of Caohai.

	Cadmium (μg·L−1)	Arsenic (μg·L−1)
Average	0.003	1.322
SD	0.010	1.418
Min	0.000	0.000
Max	0.039	5.947

). Among them, Pb, Cr, and Hg were detected in 24 water samples, the average value of Cd was 0.003 μg L⁻¹, and the mean value of As was 1.322 μg L⁻¹. According to “the national standard of farmland irrigation water quality”, these are very low levels.

4. Discussion

4.1. Evaluating Effective Factors of Soil Fertility

Soil fertility is the basic attribute and essential characteristic of soil. It reflects soil productivity, environmental quality, and the ability of healthy growth of animals and plants. It is a comprehensive expression of various properties of soil [32,33,34]. The physical, chemical, and biological properties of soil all affect soil fertility. The difference of soil fertility is related to soil parent material, type, and the method of farmland utilization and management [35,36,37]. Among the 54 soil samples investigated in this survey, the soil is mainly slightly acidic and acidic. The total contents of organic matter, nitrogen, phosphorus, potassium, and available nutrients in more than 80% of the samples were at the rich level. This shows that the content of soil organic matter, total nitrogen, phosphorus, and potassium, as well as the content of available nutrients, play an important role in soil fertility. The soil fertility is good as well. In addition, leafy vegetables such as Chinese cabbage, cabbage, and green vegetables are mainly planted around Caohai. A large amount of chemical fertilizer needs to be applied to ensure a high yield of leafy vegetables. In addition, farmers blindly pursue yield, resulting in serious excessive application of chemical fertilizer, which is the main reason for the high content of soil nitrogen, phosphorus, and potassium in Caohai Lakeside Zone.

According to the soil nutrient grade standard (Table 3), the distribution of total nitrogen, phosphorus, potassium, and organic matter is uniform, and there is no significant regional difference. Soil available phosphorus and potassium are the direct sources of plant phosphorus and potassium nutrition [37,38]. When the content of available phosphorus in soil is more than 20 mg kg⁻¹, general crops can get sufficient phosphorus supply from soil without applying phosphorus fertilizer [39]. Although the content of total phosphorus and total potassium in lakeside soil is very high, the variation of available phosphorus and available potassium is large. The effective amount is low. The lowest content of available phosphorus is only 3.4 mg kg⁻¹, which is unfavorable to the growth of leafy vegetables. Soil available potassium is easily affected by external environmental conditions such as water and temperature. Moreover, it has large mobility in soil and leaches easily, especially in seasons of high temperature and rain [37]. Weining Caohai Lakeside Zone is a high-altitude area, which is greatly affected by temperature and rainfall, resulting in slow differentiation of soil minerals and slow transformation of total elements into effective nutrients. Therefore, it is necessary to supplement effective nutrients. The application amount of organic fertilizer should be reduced in areas with high soil organic matter content by means of soil measurement fertilization and scientific reduction fertilization. For soils with low soil organic matter content, attention should be paid to the application ratio of organic fertilizer and chemical fertilizer to ensure the sustainable utilization of soil fertility and agricultural production [37]. In areas with high soil nitrogen content, the application rate of NH⁴⁺-N should be controlled. The contents of available phosphorus and available potassium in soil were dispersed. The content range varies greatly. Phosphate and potassium fertilizers should be applied separately [40].

4.2. Evaluating Source of Soil Heavy Metal Pollution

At present, heavy metals in vegetable soil in China also seriously exceed the standard. About 24.1% of vegetable soil samples in China have Cd exceeding the standard, Hg at 10.3%, and As at 9.2%. The results (as seen in Table 9) show that the CQI of Caohai lakeside soil is 6.48, which is at the level of heavy pollution as a whole. Among them, Cd showed heavy pollution; Pb, Hg, and As showed slight pollution level. Leafy vegetables are mainly planted in the soil of Caohai Lakeside Zone. A large amount of chemical fertilizers and pesticides need to be applied to control diseases and pests during the growth period, which may be an important source of soil heavy metals. By analyzing the correlation of soil heavy metals, it was found that the sources of Cd, Hg and Pb in soil heavy metals may be the same, which further proves that chemical fertilizers and pesticides are the sources of soil heavy metals. At the same time, atmospheric deposition may also be a source of heavy metals in lakeside soil [41]. The results show that greenhouse leafy vegetables in China have strong enrichment ability for heavy metals Cd and Pb [42,43,44]. The soil in Caohai Lakeside Zone is mainly planted with leafy vegetables, which is not conducive to the green production of vegetables. The order of cadmium content and bioaccumulation coefficient in vegetables was amaranth > oilwheat > Chinese cabbage > beans > tomato > cucumber [45]. The order of cadmium content in aboveground plants of leafy vegetables was oilwheat > lettuce > bolting > lettuce > cabbage > celery > Chinese cabbage > Chinese cabbage > mustard > coriander > bitter lettuce > shallot > cabbage > Agaricus > chrysanthemum > amaranth > leek [45]. According to previous studies, low accumulation varieties (such as melon and fruit vegetables: cucumber, beans, etc.) are suggested for planting in Caohai Lakeside Zone. In addition, the bioavailability of CD and Pb in soil is not only related to its total amount, but also related to the physical and chemical properties of soil, such as pH and the content of organic matter [46]. The research showed that there is a negative correlation between heavy metals in vegetables and soil pH. The increase of soil organic matter will significantly reduce the absorption of heavy metals by vegetables [47,48]. Therefore, in order to protect the ecological environment of Caohai Lakeside Zone, scientific fertilization can be used to reduce the application of chemical fertilizer by replacing some chemical fertilizer using organic fertilizer. At the same time, special attention should be paid to the control of pollution sources of heavy metals Cd, Pb, As, and Hg.

4.3. Evaluating Heavy Metal Content in Ditch Irrigation Water

Caohai is located in the Yunnan Guizhou Plateau under the karst geological background. Carbonate rock developed soil has the natural characteristics of a high background of heavy metals. The results show that the pollution Indices of all sample points are lower than 0.5, which belong to the clean level and fully meet the requirements of irrigation water quality. Only the average contents of Cu and Cr in Caohai sediments did not exceed the background value. The over standard rates of Cd, Zn, Hg, As, and Pb relative to background values were 100%, 95.23%, 92.86%, 83.33%, and 66.67%, respectively. This shows that heavy metals in Caohai are mainly distributed in sediments. It is safe and feasible to take Caohai as the irrigation water source of lakeside farmland.

5. Conclusions

The total contents of soil organic matter, nitrogen, phosphorus, potassium, and available nutrients in Caohai Lakeside Zone are at the rich level. Soil fertility is good. The soil is mainly acidic and slightly acidic. The regional difference of Hg content in soil is the largest. The regional differences of Cr, Cd, and As contents are small. The contents of Pb, Cr, Cd, Hg, and As in some soils exceed the standard. Among them, the over standard rate of Cd was 24.1%, while Cr exceeds the standard by 14.8%. The exceedance rates of Pb, Hg, and As are small, accounting for 7.4%, 5.6%, and 3.7% of the investigated samples respectively. Pb and Hg showed an extremely significant positive correlation level and a significant positive correlation level with Cd, indicating that the sources of Cd, Hg, and Pb in soil heavy metals in Caohai Lakeside Zone may be the same. Overall, the CQI of Caohai lakeside soil is 6.48, which is at the level of heavy pollution as a whole. It is mainly due to the heavy pollution of Cd, in addition to the light pollution of Pb, Hg, and As. Soil Cr element belongs to the clean level. Therefore, it is necessary to strengthen the control of heavy metal pollution in Caohai Lakeside Zone. The content of heavy metals in ditch irrigation water is within the limited range, belonging to the clean level, which fully meets the requirements of irrigation water quality.