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Statistical Analysis and Evaluation of Heavy Metal Ions in Soil Environment

DOI: 10.4236/oalib.1102690    589 Downloads   1,063 Views  

ABSTRACT

The paper comprehensively analyzes and evaluates the urban grid soil and rural soil quality and pollution characteristics through methods of single factor index and Nemero comprehensive pollution index on the physical and chemical nature of soil, the element pollutants, organic pollutants and other pollutants. The contents of soil pollutants in rural and urban areas showed that the overall soil quality is good and advice on how to improve soil quality in the region is put forward.

Subject Areas: Environmental Sciences

1. Introduction

With the rapid development of industry and agriculture, the unreasonable use of pesticides and fertilizers, long- term sewage irrigation, as well as urban and industrial pollution and other factors since 1930s, through different ways, a great number of pollutants entered into the soil environment, did damage to the soil structure, destroyed the soil function [1] , and caused quality deterioration of soil ecological environment, making soil pollution a great threat to agricultural product quality safety across China and even the global [2] . The composition, structure, function, characteristics and the special status and role played in the environment ecosystem in environmental ecosystem of soil make the soil pollution more complicated than the air and water pollution.

Soil pollutants can be roughly divided into two kinds: inorganic pollutants and organic pollutants [3] . Inorganic pollutants mainly include acid, alkali, heavy metals, salts, compound of radioactive elements Cs and Sr, compound of As, Se and F etc. Organic pollutants mainly include organic pesticides, phenols, cyanide, petroleum, synthetic detergent, benzopyrene and harmful micro-creature produced by the city sewage, sludge and manure etc. In order to know about the current situation of soil environmental quality in X area of China, improve the ability to monitor soil pollution, evaluate the risk of soil pollution, confirm the soil environmental safety level, and establish the supervision and management system for soil environmental quality, an investigation on soil environmental quality was carried out in 2014. The investigation and monitoring protect and reasonably utilize land resources, as well as offer scientific and effective basic data to improve the quality of soil environment.

2. Survey Method

Grid distribution is used in urban soil, which means that 1 km × 1 km size square grid is drawn in administrative map with corresponding proportion, 1 monitoring point lying in each grid. According to the landscape, distribution of land use types, soil types and other factors in Northern Yangtze River area, the number of points is obtained by the formula that land use area/grid size = number of sample points. After optimization, points number of urban soil survey is ultimately determined as 62. The rural scope of the soil investigation involves the 7 villages of Wubao town and each village has 1 monitoring point, 7 monitoring points in total.

3. Evaluation Index

The evaluation on soil quality is mainly based on Soil environmental quality standard (GB15618-1995) [4] , which is divided into three major categories. Class I is mainly applicable to natural reserves approved by the state (except reserves with the original background of high heavy metal concentrations), the centralized drinking water source, tea garden, and pasture and soil in other protected areas; soil quality maintains the natural background level. Class II is mainly suitable for general farmland, vegetable lands, tea gardens, orchards, pasture and other soil; soil quality basically does not do harm to the plant and the environment nor causes pollution. Class III is mainly used in forest land and farmland such as soil with high background value and mineral those have large deal of pollutants (except the vegetable land) ; soil quality basically does not do harm to the plant and the environment nor causes pollution. The detection of soil generally focuses on detecting their physicochemical properties: pH, full Nitrogen, full Phosphorus and full Kalium and the organics; elements: Arsenic, Cadmium, Cobalt, Chromium, Copper, Fluoride, Mercury, Manganese, Nickel, Lead, Selenium, Vanadium, Zinc; organic pollutants: organochlorine pesticides (Six Six Six, DDT) and other major categories. Soil is often contaminated by a variety of heavy metals. Therefore methods of single factor index [5] and Nemero comprehensive pollution index [6] [7] are often used for comprehensive evaluation on soil pollution.

3.1. Single Factor Pollutant Index

Pi reflects the extent of pollution. Its values less than 1 means no pollution while that greater than 1 means pollution. It can be expressed as follows.

(1)

where Pi is the single pollution index of pollutant i in the soil; Ci is the measured concentration of pollutant i in the soil of surveyed locations; Si is standard referential value for the evaluation of pollutant i.

3.2. Nemero Comprehensive Pollution Index

Nemero comprehensive pollution index are often used for comprehensive evaluation on soil pollution and the formulation is as below:

(2)

(3)

where PN is the Nemero comprehensive pollution index; Pmean is the average single pollution index; is the maximum single pollution index.

Nemero index reflects the effect of the pollutants on the soil [6] [7] , while highlighting the impact of high- concentration pollutants on the soil environment quality. Pollution levels can be divided according to the Nemero pollution index. Soil pollution evaluation criteria for Nemero index see Table 1.

Table 1. Nemero pollution index soil evaluation criteria.

4. Results and Discussions

There are 7 sampling rural fields for the survey, and the monitoring projects include 20 items: pH, full Nitrogen, full Phosphorus and full Kalium and the organics; elements: Arsenic, Cadmium, Cobalt, Chromium, Copper, Fluoride, Mercury, Manganese, Nickel, Lead, Selenium, Vanadium, and Zinc. The monitoring results of physicochemical properties, elemental pollutants and organics are shown in Tables 2-4, respectively.

Basic situation of soil fertility: the material composition, structure status and the natural conditions determine the level of soil fertility, which affects growth of plant and is the material basis for the crop yields [8] . From the results of the physicochemical property in Table 7, the pH of rural sampling points varies from 6.9 to 8.0 while the appropriate growth pH environment for general crops is nearly neutral. Thus, the rural soil pH is moderate.

According to the dry soil fertility grading standard (see Table 5) of Environmental quality standard of national green food places of origin (NY/T391-2000) [9] , among seven surveyed villages, whose soil organic contents vary from 9.2 to 14.2 g/kg, the highest is village C, and the lowest is village F. The content of organics reaching class II of accounts for 71.4%, class III 28.6%, which shows that the majority of soil organics in the seven villages reaches class II (good level). The monitoring results of soil full Nitrogen, full Potassium and full Phosphorus show that the soil fertility levels of the seven villages are all proved to be class III (relatively bad level), indicating the insufficient contents of N, P and K in soil.

The content and distribution of pollutants : The monitoring result of element pollutants in Table 3 shows that there is no Arsenic, Mercury and Cadmium, Selenium in the seven villages; the content of Fluoride is 189 - 452 mg/kg, the average value is 318 mg/kg, and village A has the highest content of it; the content of Chromium is 58 - 198 mg/kg, the average value is 107 mg/kg, and village G has the highest content of it; the content of Copper is 15 - 43 mg/kg, the average value is 32 mg/kg, and village A has the highest content of it; the content of Manganese is 505 - 7306 mg/kg, the average value is 1988 mg/kg, and village F has the highest content of it; the content of Zinc is 24 - 99 mg/kg, the average value is 58 mg/kg, and village B has the highest content of it; the content of Nickel is 33 mg/kg, it’s sampled in village B and none of it were detected in the other six villages; the content of Cobalt is 0 - 1195 mg/kg, and village F has the highest content of it; the content of Lead is 0 - 24 mg/kg, and village A has the highest content of it. From the organochlorine pesticide monitoring result in Table 4, organochlorine pesticides of Six Six Six and DDT were not detected in any of the seven villages. In all of the monitoring results, Cadmium, Mercury, Arsenic, Copper, Lead, Chromium, Zinc, Nickel and organochlorine pesticides all meet class II. Five elements of Fluoride, Manganese, Cobalt, Selenium and Vanadium are not required in class II. And those that are detected high content of elemental monitoring factors are village F, A and B.

Urban grid soil sampling points are distributed across the whole are, a total of 62 points, and the monitoring projects include 14 items: pH, Arsenic, Cadmium, Cobalt, Chromium, Copper, Fluoride, Mercury, Manganese, Nickel, Lead, Selenium, Vanadium, and Zinc. The detailed monitoring result is shown in Tables 5-7.

Statistics of pH monitoring: Among 62 urban grid soil point locations, pH monitoring result is in the range of 4.72 - 8.40, the lowest is in the entry into 412 Park, the highest point is 708. There are 9 points with value of pH less than 6.5 accounting for 14.5% of the total grid number. The number of pH value ranging from the 6.5 - 7.5 is 14 accounting for 22.6% of the total grid number. The number of pH value greater than 7.5 is 39 accounting for 62.9% of the total grid number. The result shows that the pH in this area of soil is alkalescent (Figure 1).

Pollution index evaluation [10] : according to the monitoring results of heavy metal pollutants in soil, single factor index and comprehensive pollution index are calculated respectively and the detailed results are shown in Table 8.

Table 2. List of results of physical and chemical properties of rural areas.

Remarks: ND: Not detected.

Table 3. List of monitoring results of soil elements in rural areas (unit mg/Kg).

Remarks: ND: Not detected.

Table 4. A list of results of the monitoring results of soil organic chlorine pesticides in rural areas (unit: mg/Kg).

Remarks: ND: Not detected.

Table 5. List of monitoring results of soil elements in urban areas (pH < 6.5) (unit: mg/Kg).

Remarks: ND: Not detected.

Table 6. List of monitoring results of soil elements in urban areas (pH > 7.5) (unit: mg/Kg).

Remarks: ND: Not detected.

Table 7. List of monitoring results of soil elements in urban areas (pH = 6.5 - 7.5) (unit: mg/Kg).

Remarks: ND: Not detected.

Figure 1. The pH range distribution of the grid soil.

Table 8. Heavy metal pollution index of soil.

From Table 8, it can be seen that the comprehensive pollution index of the rural soil and urban grid soil is less than 0.7, which means safe and clean. The single factor pollution index indicates the greatest single factor pollution index of Cr in Northern Yangtze River area, which shows that the main pollution factor in this area is Cr.

5. Conclusions

From the above investigations, the followings are concluded:

1) The contains of heavy metals and organ chlorine pesticides in the soil of the Northern Yangtze River area are low, abundant in the organics, N, P, K are obviously insufficient but in some areas the organic content is low and organic fertilizers should be added.

2) The total content of heavy metals in urban soils of Northern Yangtze River area is not very high, and the soil environment quality is good. In terms of the average pollutant index level of heavy metals, there is no pollution, but the heavy metal content in some regions is high, which leads to relatively high potential hazard.

Conflicts of Interest

The authors declare no conflicts of interest.

Cite this paper

Tang, M. and Hou, J. (2016) Statistical Analysis and Evaluation of Heavy Metal Ions in Soil Environment. Open Access Library Journal, 3, 1-8. doi: 10.4236/oalib.1102690.

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