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 (Eh) 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 (Eh) 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
The enrichment factor (EF) is typically used to evaluate sources of metals and requires the use of a standard element that fulfills several criteria (Abrahim and Parker 2008). We adopted Fe as the standard element (Cobelo-García and Prego 2003) and calculated EF as follows.
(1)
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
The potential ecological risk index (RI) of heavy metals can be used to assess the ecological risk they pose. Such a risk is typically assessed using an index that reflects the content of heavy metals, the number of heavy metal pollutant sources, the toxicity level, and any ecological/environmental effects, as follows (Håkanson 1980).
(2)
Here, and represent the content of heavy metal i and its background content in Hangzhou–Jiaxing–Huzhou plain (soil layer A), respectively. Thus, (i.e., ) represents the contamination factor of heavy metal i. indicates the toxic response factor for a given heavy metal, where is 30, 2, 5, 5, 1, and 5 for Cd, Cr, Cu, Ni, Zn, and Pb, respectively (Håkanson 1980), 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
Flux of heavy metals in sediment/soil can be calculated with indices such as content of metal, moisture content and dry bulk density (Wang et al. 2014). It represents the rate of sedimentary effects on heavy metals.
(3)
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).
Mass of heavy metals accumulated or absorbed by sediment/soil and plants is calculated by formula 4 and 5.
Here, M
i
indicates accumulation mass of metal i (mg); S
j
means area of zone j (m2).
(5)
Here, indicates uptake mass of heavy metal i by plants (mg); means content of metal i in plants (mg∙kg-1); B
j
indicates biomass of plants (kg∙m-2); 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.