Improving risk management by using the spatial interaction relationship of heavy metals and PAHs in urban soil
Graphical abstract
Introduction
Urbanization is one of the important driving factors of global environmental change, especially in developing countries where there is ongoing rapid urbanization [1]. Urbanization is always accompanied by industrialization, as a result of which energy consumption and pollution emissions are highly concentrated in urban areas [2,3]. Pollutants such as heavy metals and persistent organic pollutants (POPs) accumulate continuously in urban ecosystems [[4], [5], [6]]. It is inevitable that urban areas carry more containments than other areas. Pollutants enter into the urban system, affecting the ecosystem and human health [7,8]. Therefore, the assessment of different types of pollutants in urban areas and the mitigation of their negative effects are critical issues at the cutting edge of environmental science [9,10].
Heavy metals and polycyclic aromatic hydrocarbons (PAHs) are toxic chemicals that lead to carcinogenic and chronic diseases [11]. They are the most common pollutants in urban air, soil and water. Their sources include traffic, wastes, fuel consumption, etc. [[12], [13], [14]]. Soil is an important sink of heavy metals and PAHs. The compounds are deposited from different emission sources and adhere to organic matter and minerals in the soil [15]. Such pollutants remain persistent in soil for a long time. If pollutants in urban soil exceed environmental standards, they may pose threats to the urban ecosystem and human health [[16], [17], [18]]. They will damage plant growth and lead to secondary pollution of water, as well as threatening human health by exposure to polluted soil through the pathways of dermal contact, breathing and ingestion of soil [19,20].
We analyzed more than 1700 papers focusing on heavy metals or PAHs in urban soil retrieved from the Web of Science with publication dates from 2000 to 2017 (Fig. S1). We have found that spatial distribution, source apportionment, and risk assessment are the most popular keywords in recent research [21,22]. However, there are still some shortages in urban soil pollution research. Urban soil is strongly disturbed by human activities. It is difficult to determine the exact spatial distribution of pollutants because urban soils are more heterogeneous than any other soils [[23], [24], [25]]. Kriging spatial interpolation is usually used to analyze the distribution of pollutants in urban soil. Kriging may smooth the spatial variation of pollutants. In fact, anomalous distributions are very common in urban soil. Contaminants may aggregate in certain areas due to point or line pollution because industries and population are aggregated in certain areas [26,27]. Therefore, we need to pay more attention to cluster distribution patterns in urban areas.
There are numerous studies focusing on pollution in urban soil [[28], [29], [30]]. However, most research is only focused on organic pollution or inorganic pollutants in urban areas; few studies have integrated heavy-metal and PAH pollution [31,32]. Although heavy metals and PAHs have different environmental fates and toxicities, they might come from the same source, such as traffic and fuel consumption, and have the same adsorption mechanisms to soil organic matter and clay [33,34]. These reasons would lead to synergic and antagonistic effects of PAHs and heavy metals in urban soil. Usually, the synergic effects of combined pollution overlaid on high-density-population areas result in environmental risks involving a large number of people. However, we know little about the spatial interaction of organic and inorganic pollutants in urban areas due to the complex mechanisms involved. The lack of such knowledge limits us to risk management of urban soil pollution.
The determination of the spatial interaction of combined pollution and the identification of risk areas are the foundations of soil pollution risk management. An efficient method is needed that can delineate comprehensive risk zones of combined pollution in urban areas. Therefore, we hope to use high-density soil field investigation data and spatial analysis methods to improve the risk assessment of urban soil. The objectives of this research are to (1) demonstrate the spatial distribution of heavy metals and PAHs in urban soil, (2) reveal the mechanism of spatial interaction of heavy metals and PAHs, and (3) propose an efficient risk zoning approach for combined pollutants in urban soil, to improve soil risk management.
Section snippets
Research area
Nanjing, with a population of 8 million, is one of largest cities in eastern China (shown in Fig. 1). The area is located on the edge of the north subtropic climate zone, with an average annual temperature of 15.4 °C and an annual precipitation of 1106 mm. The vegetation in the region consists of subtropical evergreen broad-leaf forest and artificial needle-leaved evergreen forest. The original soils in this area are paddy soil and Alfisols [35]. Nanjing is a city with a long history and has
Spatial distribution pattern of heavy metals and PAHs
The basic statistical characteristics of heavy metals and PAHs are shown in Table S3. The ranges of variation of heavy metals and PAHs were high in urban soil, which means that heavy metals and PAHs vary greatly among sample points [37]. We used GIS tools (ESRI, ARCGIS) and four spatial interpolate methods (Inverse Distance Weight, Local polynomial, Sample kriging and Kernel Smoothing) to reduce uncertainty in the spatial interpolation. The lowest prediction error Root-Mean-Square (RMS) of the
Discussion
The characterization of spatial complexity of hazardous materials and source apportionment in urban environments are important issues of environmental research [51]. Geostastic, based on GIS, is the main tool to analyze the spatial distribution patterns of pollutant through sampling sites to geographic surfaces [52,53]. However, it is difficult to explain the interaction of different kinds of pollutant using spatial interpolation. Moran’s I is more strictly statistical than geostastic and it
Conclusion
The interaction between inorganic and organic pollutants in urban systems is complex. It is difficult to detect the synergic and antagonistic effects of inorganic and organic pollutants. Bivariate LISA mapping is a good tool for revealing the spatial interaction of geographic phenomena. Here, we used bivariate LISA to analyze the spatial interaction between heavy metals and PAHs in urban soil. The results indicated that bivariate LISA mapping can explicitly represent the spatial complexity of
Acknowledgments
The research was supported by the National Natural Science Foundation of China (41671085), the National Key Research and Development Program of China (2017YFD0800305) and the Fundamental Research Funds for the Central Universities (020914380046).
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