Factors controlling the accumulation and ecological risk of trace metal(loid)s in river sediments in agricultural field
Graphical abstract
Introduction
Toxic trace metal contamination in agricultural fields has been discovered in many parts of the world (Alloway, 2010). As a sink of contaminants, aquatic sediments containing considerable amount of trace metals can bring significant negative effects to both ecosystem and human health (González-Fernández et al., 2011; Ouyang et al., 2018; Keshavarzi and Kumar 2019). Recent researches emphasized a markedly elevation of trace metal contents in aquatic sediments, as a result of local human activities through the pathways of erosion or leaching of agricultural soils, atmospheric deposition etc. (Bur et al., 2009; Jiao et al., 2015; Coxon et al., 2016; Vallejo Toro et al., 2016). Most of the studies on trace metals in agroecosystem however, have focused on soil environment. The situation of trace metal contaminated-sediments, which is very different from that of soils in terms of the physic-chemical composition, pH/redox condition etc., and the ecological risk to the aquatic ecosystem are much less well studied. Therefore, the understanding of factors that affect metal accumulation in sediments and its risk to the aquatic environment needs to be improved.
Anthropogenic trace metals in agroecosystem mainly come from agricultural practices, atmospheric deposition, and local industries. Typical sources of agriculture-derived trace metals include fungicides and pesticides containing Cu, Zn, Pb and As especially in fruit production field; phosphorus fertilizers (e.g. triple superphosphates and calcium/magnesium phosphate) for Cd (He et al., 2005; Gharaibeh et al., 2019; Baltas et al., 2020); blend fertilizers in which Cu and Zn are purposely added to meet the demand of plant growth; biosolids/composts containing Cu, Zn, Pb and Cd; use of domestic/industrial wastewater containing Ni and Hg for irrigation (He et al., 2005; Shi et al., 2019; Ye et al., 2017; Gharaibeh et al., 2019); livestock manure containing As, Cu, Zn and Hg (Shi et al., 2019). In addition, atmospheric deposition is responsible for the enrichment of As, Cd, Cr, Hg, Ni, Pb and Zn in agroecosystems (Shi et al., 2019; Ni and Ma, 2018; Alloway, 2010; Hooda, 2010). Its influence to the level of contamination is largely dependent on the intensity of industries around farmland (Yi et al., 2018), as well as meteorological conditions such as prevailing wind direction (Zereini et al., 2005). In area where specific mineral is abundant, trace metals from parent material can also bring ecological risk to the environment. For example, serpentine derived soils or sediments were rich in Ni, Cr and Co (Ma and Hooda, 2010).
Agricultural land contaminated by trace metals can act as a source of pollutants to surface waters (Bur et al., 2009). Recent researches investigated the output of trace metals from agricultural soils and found the main pathway were surface runoff/erosion, leaching, crop harvesting (Bengtsson et al., 2006; Rothwell et al., 2011; Shi et al., 2019; Ni and Ma, 2018), among which, a considerable amount of metals was eroded into the surrounding environment by surface runoff. Shi et al. (2019) calculated that at least half of the As, Cr, Ni and Pb output were through surface runoff in Zhejiang Province, China. The influence of these metals to the nearby waterbody therefore, can be significant. Because of the nature of trace metals, they are most likely to accumulate in aquatic sediments (González-Fernández et al., 2011). In fact, elevation of trace metals contents in river sediments has already been reported in agricultural intense-regions in many parts of the world (Tang et al., 2014; Vallejo Toro et al., 2016; Martínez-Guijarro et al., 2019; Barhoumi et al., 2019). Jiao et al. (2015) suggested that phosphorus fertilizer application is responsible for Cd accumulation in river sediments in a typical agricultural area in China. These accumulated metals not only cause negative effect to the aquatic ecosystem, especially to benthic community, but also threaten human health through food chain mainly through three exposure routes: 1) sediment-(overlying water)-aquacultural products-human; 2) sediment-(overlying water) – irrigation-agricultural products-human; 3) sediment - overlying water-human (direct use e.g. washing up, entertainment etc.) (U.S.EPA, 1998). Therefore, the requirement of proper risk assessment for metal contamination to aquatic sediment in agricultural area has largely increased.
Methods of risk assessment on toxic metal contamination in sediments are mostly based on total metal concentration e.g. sediment quality guidelines (SQGs), geoaccumulation (Igeo), enrichment factors (EFs), potential ecological risk index (PERI). Among those, the PERI has been regarded as the most appropriate for assessing ecological risk as it takes metal toxicity into account (Hakanson, 1980). More recently, the impact of sediment properties to metal enrichment and risk assessment has been emphasized (Liu et al., 2019). On one hand, sediments with particular characteristics, e.g. high pH, high proportion of sorption phases including humic substances, amorphous mineral oxides, clay can largely favour metal accumulation by reactions of precipitation, adsorption etc. (Mulligan et al., 2009; Chen et al., 2015; Tack, 2010). On the other hand, they control metal(loids) mobility and bioavailability on different extents, and leads to alteration in potential risk to the environment (Du Laing et al., 2009; Liu et al., 2019). For example, the mobility of metal cation (e.g. Cd2+, Pb2+) is considerably low in sediments with high pH. But for metal anion, the mobility is much higher (e.g. HCrO4−, HAsO42−, Sb(OH)6-, Hooda, 2010). Release of metals from natural adsorbents (amorphous Fe and Mn (hydr)oxides, OM etc.) can take place in sediment environment under changing redox conditions (Du Laing et al., 2009). In order to take account of these factors, methods based on metal fractionation information such as risk assessment code (RAC, Nemati et al., 2011) have been introduced, which relates the risk with the binding strength between metals and sediment composition.
In this study, we have focused on trace metal(loid)s in sediments on Chongming islands, Shanghai, China, which has >120 years of agricultural activities and relatively isolated from intensive industries to 1) identify and quantify the contribution of trace metals from each source; 2) identify important sediment properties on metal accumulation and mobility; 3) evaluate the potential risk that those metals bring to the local aquatic ecosystem by analysing both the level of contamination as well as the metal fractionation. The outcome of the study can provide scientific information to develop comprehensive risk assessment for decision makers, so that appropriate pollution control strategy may be applied to agricultural fields.
Section snippets
Study area and sediment sampling
Located in the Yangtze River Estuary of China, Chongming Islands consist of three parts, the main island, Hengsha island and Changxing island (Fig. 1a). It is the largest alluvial estuarine island in the country, which has high intensity of river network (>10%). For more than 120 years, agriculture has been the dominant activity on the islands. The land use types mainly consist of paddy field, dry land and forest land (Zheng et al., 2016). Prevailing wind direction is south-east in spring and
Source contribution on trace metal accumulation in sediments
Total metal concentration of 9 trace metals is summarized in Table 1 along with the background values (He, 2018) and concentration in the surface coastal sediments (Han et al., 2017) in the Yangtze River Estuary, where the Chongming islands locate. Detailed information is available in supplementary material (Table S2). For all trace metals there is no statistical difference (p > 0.05) in concentration between sediments from ZX town and HS island. Therefore, they were combined into one dataset
Conclusion
By analysing total concentration of 9 trace metal(loid)s (As, Cd, Cr, Co, Cu, Ni, Sb, Pb, and Zn) in river sediments on Chongming islands where industrial activities were limited, we found significant metal accumulation as a result of both source contribution and preservation on metal binding compositions. The majority of anthropogenic metal input were from atmospheric deposition (Sb, Pb) and agricultural activities (Cd, As, Cu, Zn). Sediments containing high amount of amorphous mineral oxides
Declaration of competing interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (41601229 and 51679140).
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