Retention and mitigation of metals in sediment, soil, water, and plant of a newly constructed root-channel wetland (China) from slightly polluted source water

Metal contents and risk assessment in sediments/soil

Distribution of metals in wetland sediments/soils

Spatial variations of metal contents

Comparisons between the east and west blocks, the first and second parts, the five functioning zones along the hydraulic flow pathways, and the three finer scale structural units within both root-channel zones represent the basic characteristics of spatial variations within the wetland and are described here and in the following sections (Figure 2). The spatial distribution of metals in the east and west blocks was found to exhibit a complex pattern, which can be summarized as follows. (1) Cd exhibited obviously high contents in the east wetland, with average contents of 0.14 mg/kg (east) and 0.092 mg/kg (west). (2) Cr, Cu, Ni, and Zn contents were higher in the west wetland than the east, with average Cr contents of 103.51 mg/kg (east) and 141.00 mg/kg (west), average Cu content of 19.92 mg/kg (east) and 24.70 mg/kg (west), average Ni contents of 33.12 mg/kg (east) and 45.53 mg/kg (west), and average Zn contents of 75.67 mg/kg (east) and 152.77 mg/kg (west). (3) Pb contents varied little from east to west in the first part, but were higher in the east in the second part. These results demonstrate that, despite almost identical design and operation conditions in the east and west wetlands, nonparallel variations of metals were detected in wetland sediment, with no synergistic variation in the two successive parts of the pilot wetland.

Spatial variations of metal contamination factors

Contamination factor and enrichment factors (EF s) can be used to indicate metal pollution levels and are more suitable for characterizing retention processes than metal contents. The mean EF s of the six heavy metals studied were all considerably lower than 1.50, demonstrating that no obvious heavy metal accumulation (with respect to the low concentrations in the source river water) occurred on the whole during the initial operation period. However, the maximum EF s of Cd, Cr, Ni, Zn, and Pb exceeded 1.50, indicating some enrichment of these heavy metals in some parts of the wetland. Contamination factor (C) describes the contamination of a given toxic substance. The C value represented spatial variations in the pilot wetland. From data in Table 3, we can see contamination factors of most heavy metals in west block are higher than 1, which means the contents of heavy metals in pilot wetland were higher than the corresponding background level. Contamination factors of heavy metal Cd, Cu, Pb in two blocks were lower than 1 and most sites in the pilot had lower metal contents. Cr had 85.71% (average 1.32) and 100.00% (average 1.82) sample numbers that contamination factors were higher than 1 in east and west block, demonstrating the highest accumulation effects. Ni and Zn had similar variations whose contamination factors were higher in west block and more than half sites had contamination factors higher than 1. The maximum contamination factors for heavy metals were all above 1 except Cd in west and Cu in east. The studies proved that the primary operational pilot wetland had relatively accumulated the heavy metals, especially for Cr, Ni and Zn.

	Proportion (C < 1)		Proportion (C > 1)		Mean		Max
	East	West	East	West	East	West	East	West
Cd	66.67%	100.00%	33.33%	0.00%	0.93	0.61	1.83	0.89
Cr	14.29%	0.00%	85.71%	100.00%	1.32	1.82	1.78	3.25
Cu	100.00%	88.10%	0.00%	11.90%	0.64	0.79	0.91	1.27
Ni	52.38%	4.76%	47.62%	95.24%	1.01	1.40	1.48	2.27
Zn	76.19%	11.90%	23.81%	88.10%	0.79	1.64	2.16	4.62
Pb	76.19%	69.05%	23.81%	30.95%	0.96	0.71	2.12	1.31

Note: Proportion means the number percentage of sample sites whose C met the demand in brackets accounting for the number of all sample sites.

Additional file 4 showed the heavy metal contamination factors in the pilot wetland. Generally, in east block, contamination factors for Cr, Cu, and Ni were high in pretreatment zone and factors for Cd and Pb were high in deep purification zone. In west block, contamination factors for Cd, Cu, Zn, and Pb were high in pretreatment zone, while factors for Cr, Ni were high in deep purification zone. The four root-channel parts had relatively high contamination factors, but lower than those at the outlets of wetland. Spatial variations of contamination factors in structural units (e.g., high/low water level ditch, plant bed) were investigated to analyze metal accumulation effects in root-channel zones (Table 4). The results reveal differences in contamination factors between the two blocks. In the east block, the contamination factors of Cd, Zn, and Pb in the ditches of the second part were higher than those in the first part, whereas the contamination factors of Cr, Cu, and Ni exhibited no distinct variance. The contamination factors of Cr, Cu, Ni, and Zn exhibited the opposite trend and those of Cd and Pb exhibited no clear variance. The contamination factors of Pb were lower in the plant bed of the second part; the contamination factors of Cd, Cr, Cu, Ni, and Zn exhibited the opposite trend. Moreover, contamination factors in low ditches were often lower than those in high ones.

The biplot of metal contamination factors (Figure 3) illustrates the multivariate correlations between variables (metal contamination factors) and observations (structural units). The explanatory percentage of first and second principal components accounted for 68.4% and 25.7%, respectively, explaining a total of 94.1% of the variance. This can be considered a very good fit. The results in Figure 4 demonstrate that no metals exhibited obvious enrichment in the plant beds of the first and second parts of either the east or west wetlands (i.e., all plant beds), or in the high/low ditches in the first part of the east wetland. The heavy metals Pb and Cd exhibited obvious accumulation in the high and low ditches in the second part of the east wetland; such accumulation was also detected for the heavy metals Cr, Cu, Ni, and Zn in the high and low ditches in the second part of the west wetland. In general, the plant-bed/ditch system in the Changshuitang pilot wetland accumulated heavy metals (Cd, Cr, Cu, Ni, Zn, and Pb) primarily in the second part of the wetland, with no apparent effects in the soil of the plant beds.

Block	Part	Unit	Cd	Cr	Cu	Ni	Zn	Pb
East	First	High Ditch	0.94 ± 0.19	1.36 ± 0.2	0.67 ± 0.1	1.01 ± 0.13	0.65 ± 0.16	0.78 ± 0.09
		Plant Bed	0.56 ± 0.02	1.54 ± 0.05	0.58 ± 0.03	1.08 ± 0.04	1.03 ± 0.04	0.93 ± 0.19
		Low Ditch	0.76 ± 0.07	1.23 ± 0.17	0.63 ± 0.11	0.94 ± 0.13	0.59 ± 0.12	0.74 ± 0.07
	Second	High Ditch	1.18 ± 0.32	1.16 ± 0.27	0.61 ± 0.06	0.96 ± 0.1	0.94 ± 0.6	1.36 ± 0.54
		Plant Bed	0.65 ± 0.11	1.42 ± 0.19	0.58 ± 0.1	1.07 ± 0.13	1.03 ± 0.15	0.61 ± 0.15
		Low Ditch	1.24 ± 0.25	1.3 ± 0.35	0.63 ± 0.03	0.99 ± 0.08	0.9 ± 0.66	1.34 ± 0.65
West	First	High Ditch	0.71 ± 0.15	1.79 ± 0.17	0.86 ± 0.1	1.42 ± 0.11	1.77 ± 0.41	1.04 ± 0.26
		Plant Bed	0.53 ± 0.06	1.32 ± 0.07	0.55 ± 0.04	1.02 ± 0.07	0.98 ± 0.07	0.61 ± 0.36
		Low Ditch	0.53 ± 0.15	2.06 ± 0.4	0.71 ± 0.02	1.29 ± 0.04	2.06 ± 1.74	0.6 ± 0.37
	Second	High Ditch	0.6 ± 0.14	2.19 ± 0.59	0.92 ± 0.23	1.71 ± 0.39	2.13 ± 1.21	0.51 ± 0.26
		Plant Bed	0.67 ± 0.07	1.47 ± 0.2	0.66 ± 0.09	1.16 ± 0.12	1.1 ± 0.11	0.49 ± 0.21
		Low Ditch	0.58 ± 0.15	2.02 ± 0.14	0.86 ± 0.11	1.54 ± 0.16	1.69 ± 0.45	0.52 ± 0.23

Note: mean ± standard deviation, boldface means data above 1.0.

Metal variations in wetland water

Concentrations of heavy metals in all water samples were within the limits of basic indices for grade I according to Chinese environmental quality standards for surface water (GB 3838–2002), except in the case of Zn: Zn content at the eastern inlet (94.22 μg/L) exceeded the threshold (50 μg/L). Thus, ecological risk of heavy metals in the pilot wetland was rather small. The removal rates of Cu, Ni, Zn, and Pb were good, reaching 6.26%, 78.52%, 57.56%, and 26.00% in the east and 1.03%, 66.67%, 59.63%, and 7.46% in the west, respectively. The heavy metal Cr exhibited a good removal rate (24.43%) in the east but a lower rate in the west, although the rate in the west increased slightly with distance along the hydraulic flow pathways. However, these removal rates are less favorable than those of traditional wetlands (Bulc and Slak 2003; Tam et al. 2009), because nearly all heavy metal concentrations were within the standard prescribed limits. Nevertheless, it is clear that this novel type of constructed root-channel wetland has achieved good removal of trace metals in its initial period. Accordingly, such wetlands should be efficient in the mitigation of most heavy metal risks in source water.

Contribution of aquatic plants to the uptake and accumulation of metals in wetland

Metal accumulation effects in zones of CRCW

Constructed root channel wetland (CRCW) can remove many types of pollutants and plays an important role in water pollution control and the ecological rehabilitation of water environments (Wang et al. 2014). However, metals cannot be decomposed; rather, they must be changed morphologically to allow transportation and transformation of their special characteristics (González-Alcaraz et al. 2013; Yeh et al. 2009). When source water first enters the pilot wetland (i.e., in the pretreatment zone), the relatively wide water surface and slow hydraulic flow rates induce primarily the precipitation of TSS and organic matter. Owing to the special configuration of high/low ditches and plant-bed systems, metals can become further enriched in ditches and/or plant beds in root-channel zone I. In the present study, we found metal contents to be relatively low in these previous units, with little deposition in the water lifting and falling zone. The contamination factors of metals in root-channel zone II increased sharply with respect to those in previous zones. Metal deposition in the deep purification zone was much poorer than that in the root-channel zone, although the deep purification zone still promoted effective accumulation of metals owing to its slower hydraulic flow and wider water area. Previous studies have shown that, in constructed wetland, heavy metal contents in sediments typically decline gradually along hydraulic flow pathways (Obarska-Pempkowiak and Klimkowska 1999). Wetlands are believed to select their most favorable evolution direction in accordance with the surrounding environment. In the present study, metal content, contamination factors and RI were found to vary dramatically between five functioning zones, particularly between the two root-channel zones. Specifically, RI was found to first decline and then increase along the water flow pathways, reaching extremely high values in the two root-channel zones in the second part. This definitively proves that root-channel zones may become “hot sites” acting as sinks for heavy metals in this pilot wetland (Wang et al. 2014). Moreover, the results suggest that obvious deposition and accumulation of heavy metals occurred in CRCW, further highlighting the efficiency of this innovative technology for the mitigation of the ecological risk of metals.

The east and west blocks of the wetland described here were designed with almost identical theoretical operating conditions, with similar characteristics in terms of construction, operation, and local plant conditions, yet they resulted in varied soil/sedimentary metal contents. However, despite the almost symmetrical and parallel design of the east and west blocks, the results presented here indicate non-parallel enrichments effects between the two blocks. This can be attributed to slight differences in local conditions, development, succession, human activities, and hydraulic factors (Stead-Dexter and Ward 2004; Wu et al. 2013). However, a more full characterization of these differences and their effects will require long-term comparative study, which will help to improve understanding of the system and allow optimization of engineering design and operation and management practices, thus allowing high purification efficiencies to be achieved consistently.

Metals accumulation effects during operating periods of CRCW

The Changshuitang wetland is located within a dense stream network. Accordingly, it is subjected to extremely complex water sources, various pollutant types, and serious nonpoint and diffuse but small decentralized point source pollution (Yin et al. 2010). Pollutants such as heavy metals enter the water body of stream networks through overland runoff in cities. Previous studies have shown that the first full-scale CRCW with a similar structural design to that of the present study, the Shijiuyang wetland, demonstrated high enrichment factors for Cd, Cr, Cu, Ni, Zn, and Pb; these values remained above the threshold value of 1.5 after 4 years of operation (Wang et al. 2014). Both studies showed that the crisscrossed plant-bed/ditch systems in the root-channel zones were “hot sites” for the sedimentation, interception, enrichment, and even uptake of heavy metals in CRCW. Moreover, the risks of heavy metals were greatly mitigated through the complex interactions of the plant-bed/ditch systems. The removal rates of heavy metals in the water phase correspond well to the characteristics of the sediment and soil matrix (particularly particle size composition and organic matter content), and the interactions among plant-bed soil matrixes and the bilateral ditch water are capable of removing most species of metals, in conjunction with the deposition and filtration of coarse/finer particles.

Although the Changshuitang pilot wetland has been operational for only 1 year and performed poorly compared with the Shijiuyang wetland in terms of plant diversity and metal removal rate, it achieved better water quality using a more reasonable design for similar environmental and soil matrix conditions. In particular, its unique design helped enhance removal rates for metals and heavy metals. However, no extremely obvious decline of RI was observed in the pilot wetland, particularly compared with that between the wetland inlet (Site A, source water sediment) and the wetland outlet (Site G, exit of deep purification zone). This may be due to instability of wetland structure and function, low biodiversity, and the unstable effects of water purification (Saeedreza et al. 2012). The effects of metal accumulation in the pilot wetland’s primary operation (one-year period) were comparatively lower than those in the relatively developed Shijiuyang wetland (four-year period). In addition, it may be important that the hydraulic flow pathways (direct length ca. 200–300 m) and hydraulic retention time (ca. 2.3 d) are both relatively small under a hydraulic flow rate of 0.27 m/d. This may be insufficient for thorough interaction between water and sediments/soil (Bilal et al. 2009). Moreover, the very low concentrations of metals (except Zn) in the source river water do not provide a considerable pollution load to the wetland, where the optimum treatment efficiency of such metals may be achieved under conditions in which an allochthonous source input is present. At present, RI in the pilot wetland is far below the threshold value of 150, indicating little environmental damage. Nevertheless, metal contents at the outlet of the pilot wetland remain relatively high owing to the effects of accumulation; this can be attributed to the internal interweaved “capillary” filtration functions, which can be visualized by considering the whole pilot wetland as the glomerulus of a kidney. Decreases in flow velocity at the restricted exit of the wetland will promote metal deposition accompanied by particulate settling (Bilal et al. 2009; Brix and Arias 2005). Nonetheless, more long-term monitoring data will be required to assess the actual removal of metals and the transformation of metal forms. Moreover, further emphasis should be placed on the effects of long-term operation, accumulation effects, the use of aquatic plants, and the effects of novel CRCW techniques on the removal of metals (especially heavy metals) in future.

Background and study area

The water source in the river networks of the Yangtze River delta of China has become increasingly polluted in response to rapid economic development and urbanization within the watershed. Jiaxing belongs to the delta with a population of 4.5 million, where heavy metal is one of the potentially important environmental issues. Many industrialized processes including dying, plating, and tanning give rise to the contamination by heavy metals, such as cadmium (Cd), mercury (Hg) and lead (Pb) in soil, water and air (Huang et al. 2013). Busy land and water transportation in the Great Canal are also an important input source for the stream network. Urban runoff caused by rainfall on city dusts is one of the primary pathways of heavy metal into the stream network, and the narrow land/water boundaries, absence of buffer zones, and ineffective street cleaning methods contribute to the heavy metal inputs to the stream network (Zhao et al. 2009). Jiaxing has no mountain reservoir for water source and has to take drinking water from river network. Although considerable efforts have been made to control the pollution in the city’s stream network, some of the drinking water in the network remains polluted owing to the complex conditions in the study area, thus threatening water safety in the region (Yin et al. 2010).

On April 1, 2007, Jiaxing began to construct the Shijiuyang large-scale ecological water quality treatment wetland (Figure 1), which was intended to purify and restore the healthy drinking source water. This treatment system was based on three patented technologies. The core technology focused on constructing root-channels artificially in constructed wetland and was developed by the Research Center for Eco-Environmental Sciences (RCEES), Chinese Academy of Sciences (CAS) (Wang et al. 2012a). The trial operation of the wetland was initiated on July 2, 2008. This full-scale wetland incorporated root-channel renewability to prevent clogging. To date, the wetland has been operating continuously for nearly 6 years realizing the goal of improving source water quality by one level according to Chinese environmental quality standards for surface water (GB 3838–2002). Furthermore, the wetland has become a city park serving multiple ecological functions. This innovative technique put into practice by Jiaxing both guarantees drinking water safety for the local population and promotes continuous improvement of the ecological and living environments. Moreover, it provides a meaningful example for other cities that may be facing similar problems relating to micro-pollution of their drinking water.

Thanks to the success of the Shijiuyang wetland, the Changshuitang wetland was constructed in Haining, becoming the third CRCW in China (Figure 1). The Changshuitang wetland benefited from many improvements over the Shijiuyang wetland. For example, it benefited from the design and application of many optimization measures, including increased diversity of pioneer plants, application of two-way parallel water flow pathways, strengthening of hydraulic control measures, and adoption of a step-feed process of water supply to enhance ammonia and organic matter removal. These measures were adopted primarily to further mitigate the complex pollution arising from industrial, domestic, and agricultural sources in the drinking water sources represented by the Changshanhe and Changshuitang rivers, which are treated by the Third Water Plant in Haining. The Changshuitang wetland is located in the northern part of Haining, which lies to the east of the Third Water Plant. The wetland covers an area of about 1.73 km² and aims to purify about 0.3 million m³ of drinking water per day. The wetland itself was designed to use the natural topography of the original course of the river, based on the concept of ecological treatment, and was constructed according to the patent technologies of RCEES, as described above. Today, the wetland is still under construction.

To further optimize, promote, and verify the water treatment efficiency of the full-scale wetland (ca. 1.73 km²), a pilot wetland at a scale of approximately 1:100 (ca. 1.83 ha) was built beforehand nearby from March to June 2011. Its geographical coordinates span 30°34′5.36″–30°34′11.82″ N and 120°42′23.32″–120°42′27.79″ E. It was designed as two parallel wetland blocks divided by a central water supply channel that serves as a partial water source (i.e., accounting for 20 - 50% of the total water supply amount) to allow step feeding of the wetland through the pump station and through horizontal water pipes in the first and second parts, respectively (Figure 1). This configuration results in east and west wetlands for separate purification of the micro-polluted drinking water source. Each block (wetland) is composed of five functioning zones, which are further divided into two parts by a pump station. The first part is located before the pump station and contains pretreatment zone and root-channel zone I, whereas the second part lies after the pump station and contains the water lifting and falling zone, root-channel zone II, and the deep purification (polishing) zone. Each root-channel zone consists of finer scale structural units including a high water level ditch (high ditch), low water level ditch (low ditch), and plant bed. The pilot wetland can deal with 1880 m³ water per day and typically improves water quality by one level. To help understand the water depth distribution in functioning zones and water flow movement through the wetland, the section and elevation planning of the pilot wetland was presented in Additional file 5.

Sample collection and preparation

In total, 71 sediment samples from ponds/ditches and 16 soil samples from plant beds were collected on December 7–8, 2012 (Figure 1). We assigned sample sites in representative areas of the wetland to obtain information regarding key processes. As the area of pilot wetland was small, boats could not be used to collect samples. Then sample sites in water lifting and falling zone as well as deep purification zone were not in the middle part but we try our best to collect the most representative samples. The sampling site numbers were assigned as follows. Single “C” before site names indicates sites in the central channel, with C1 for the inlet and C2 in sites beside the water pump stations controlling the step-feed process. Similarly, the use of prefix “W” and “E” before site names indicate sites in the west and east blocks, respectively. Two equivalent sedimentary samples from the inlets of the east and west sides were mixed as one; this sample is labeled “Source” (or “A”). According to the hydraulic flow pathways, the first part lies before the pump station and contains pretreatment zone (B) and root-channel zone I (C), whereas the second part includes the water lifting and falling zone (D), root-channel zone II (E), and the deep purification zone (F). The outlets of the wetlands are indicated by “Outlet” (or “G”). Within each zone, we used numbers to label different sites along the water flow pathways. In each root-channel zone, the ditches lead the water deep into the wetland and let the water penetrate through the root-channels under the plant beds. “H” and “L” refer to ditches with high and low water levels, respectively, and “P” indicates sites of soil samples from plant beds.

About 400–500 g wet weight of mixed surface sediments (0–10 cm) were collected using an Ekman bottom sampler (Hydro-Bios, Kiel, Germany). To reduce the heterogeneity of sediments and avoid the effects of water and plant, three equal samples were collected and mixed as one. Soil samples from plant beds weighing 300–400 g were collected using a soil auger (XDB-TR7, sampling depth: 1.5 m, drill diameter: 5 cm; Beijing New Landmark Soil Equipment Co., Ltd.) at depths of 0–30 cm. Immediately after collection, soil temperature, pH, and redox potential (E_h) were measured using an IQ150 probe (HACH, USA); then, samples were preserved in bags on ice for transportation. Samples for ammonium-N, nitrate-N, and nitrite-N assays were stored under -4 °C. The remaining samples were frozen, air dried, and sieved through a 100-mesh sieve to remove coarse particles. The sediment/soil samples were then stored in plastic bags for further analysis.

Twelve water and nine plant samples were collected on August 3, 2013 for further investigation of metal contents and risk levels in the water column and plants of the wetland in full flourish. Water samples are indicated by “W” (west wetland) or “E” (east wetland) and numbers, while plant samples are expressed by “P” and numbers. Samples W1–6 and E1–6 represent key sites at the intersections between two functioning zones, i.e., the inlet of the wetland (where the source water feeding the wetland enters the system), outlet of the pretreatment zone, outlet of root-channel zone I, outlet of the water lifting and falling zone, outlet of root-channel zone II, and outlet of the deep purification zone (i.e., the exit of the whole wetland). Water samples for the analysis of metal contents were stored after filtering through 0.45-μm filter membranes before adding HNO₃. General water samples were stored at cold temperatures after their temperature, pH, dissolved oxygen (DO), and redox potential (E_h) had been analyzed. All types of emerged aquatic plants present were sampled to provide a representative indication of plant conditions in the wetland; such plant samples were stored in plastic bags after determination of their stem diameter, height, and density from random samples.

Analysis and quality control

For metal extraction, 0.1-g samples of dried soil were digested in 6 mL of an HNO₃ and HCl mixture (1:3) and 2 mL HF in a CEM microwave (MARS Xpress; CEM, USA) according to the program recommended by the United States Environmental Protection Agency (EPA) (Bettinelli et al. 2000). These transparent solutions were then filtered through 0.45-μm filter membranes and diluted to 50 mL with distilled water. The concentrations of metals Cd, Cr, Cu, Ni, Pb, and Zn in the filtrate were determined by ICP–MS (7500a; Agilent, USA), whereas the concentrations of metals K, Na, Ca, Mg, Al, and Fe were measured by ICP–OES (OPTIMA 2000DV; Perkin Elmer, USA).

Determination of metal content of water samples was achieved by ICP - OES (OPTIMA 2000DV; Perkin Elmer) for K, Na, Ca, Mg, and Fe and by ICP - MS (7500 a; Agilent) for Al, Cd, Cr, Cu, Ni, Zn, and Pb. Total suspended solids (TSS) were measured by mass according to standard methods. Plant samples were digested by HNO₃ - H₂O₂ in a CEM after drying and sieving; further details are provided in the existing literature (Niemelä et al. 2004). The subsequent metal determination methods for plant samples were the same as those for soil/sediment samples.

The metals can be classified into groups/types according to several classification systems in the context of study objectives. To ensure clarity, we refer to the metals K, Na, Ca, Mg, Al, and Fe as major metals owing to their abundance in the study area (their contents are given in %) and refer to the metals Cd, Cr, Cu, Ni, Zn, and Pb (i.e., those measured in mg/kg) as heavy metals, following the most commonly used terminology. Major metals were not the wide discussion in this study with only necessary description.

Enrichment factor

Here, (Me/ Fe)_sample and (Me/ Fe)_background refer to the ratios of a target metal to that of Fe in a soil/sediment sample and in the background, respectively. We adopted metal contents obtained for the surface soil in the Hangzhou–Jiaxing–Huzhou plain (soil layer A) as the background content (Wang et al. 2007). Higher values of EF indicate more extensive accumulation of metals. EF < 1.5 signifies that metal contents are natural levels; thus, EF > 1.5 suggests that metals resulting from human activities are a key component (Håkanson 1980).

Assessment of potential ecological risk index of heavy metals

Here, ${\mathit{C}}_{\mathit{s}}^{\mathit{i}}$ and ${\mathit{C}}_{\mathit{n}}^{\mathit{i}}$ represent the content of heavy metal i and its background content in Hangzhou–Jiaxing–Huzhou plain (soil layer A), respectively. Thus, ${\mathit{C}}_{\mathit{f}}^{\mathit{i}}$ (i.e., $\frac{{\mathit{C}}_{\mathit{s}}^{\mathit{i}}}{{\mathit{C}}_{\mathit{n}}^{\mathit{i}}}$) represents the contamination factor of heavy metal i. ${\mathit{T}}_{\mathit{y}}^{\mathit{i}}$ indicates the toxic response factor for a given heavy metal, where ${\mathit{T}}_{\mathit{r}}^{\mathit{i}}$ is 30, 2, 5, 5, 1, and 5 for Cd, Cr, Cu, Ni, Zn, and Pb, respectively (Håkanson 1980), ${\mathit{E}}_{\mathit{r}}^{\mathit{i}}$ represents the potential ecological risk index of an individual heavy metal, and RI is the sum of the potential risk of all individual heavy metals.

Fluxes and masses of heavy metals

Here, F _i means fluxes of heavy metal i in sediment/soil (mg∙m^-2∙d^-1), C _i means contents of metal i (mg∙kg^-1); W indicates moisture content; ρ indicates average dry bulk density (kg∙m^-3); ∆H indicates thickness of sediment/soil (m); t represents operational days (d).

Here, ${\mathit{M}}_{\mathit{p}}^{\mathit{i}}$ indicates uptake mass of heavy metal i by plants (mg); ${\mathit{C}}_{\mathit{p}}^{\mathit{i}}$ means content of metal i in plants (mg∙kg^-1); B _j indicates biomass of plants (kg∙m^-2); ${\mathit{S}}_{\mathit{p}}^{\mathit{j}}$ means area of aquatic plants (m²); n represents times of reaping.

Statistical analysis

Data analysis was performed using SAS for Windows 9.2 (SAS Institute, Inc., Cary, NC, USA) (Friendly 1991; Friendly 2000). Unless otherwise stated, α = 0.05 and α = 0.01 were adopted as the statistically significant and extremely significant levels. A biplot was drawn using the IML module in the SAS system to visualize the multivariate relationships between observations and variables. In this plot, the lengths of environmental vectors represent the ability to distinguish different variables and the cosine of angles between two vectors expresses the degree of correlation. All variables in this biplot were standardized to ensure consistency in units and dimensions.