Distribution and contamination assessment of heavy metals in soils ...

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Distribution and contamination assessment of heavy metals in soils from tidal flat, oil exploitation zone and restored wetland in the Yellow River Estuary. Authors ...
Wetlands (2016) 36 (Suppl 1):S153–S165 DOI 10.1007/s13157-015-0637-3

CHINA COASTAL WETLANDS

Distribution and contamination assessment of heavy metals in soils from tidal flat, oil exploitation zone and restored wetland in the Yellow River Estuary Xinying Yao & Rong Xiao & Ziwen Ma & Ying Xie & Mingxiang Zhang & Feihai Yu

Received: 14 April 2014 / Accepted: 28 January 2015 / Published online: 14 February 2015 # Society of Wetland Scientists 2015

Abstract Metal distributions were monitored along soil profiles and their sources as well as ecological risks were assessed in tidal zone (T), petroleum exploitation zone (P) and restoration zone (R) under different vegetation covers- Phragmites australis (p), Suaeda salsa (s) and bare land (b) in Yellow River estuary (YRE). In the three zones, mean concentrations of metals and mean values of potential ecological risk index (RI) kept the order of P>T>R. For different vegetation covers, the order of mean concentrations and ecological risk factor of all metals was b>s>p in zone P, while it shifted to p>b>s in zone T and p>s>b in zone R, respectively. Principal component analysis inferred that As, Cu, Ni and Zn were derived from natural alluviation and sedimentation, while Cd and Pb were derived from allochthonous inputs and air emissions of oil extraction. Correlation analysis indicated that soil organic matter, moisture, large aggregates as well as electric conductivity were key factors influencing metal distributions in these zones. Although YRE was not heavily polluted by heavy metals, it should be vigilant for metal accumulation in 20–30 cm layer of oil spilled soils, and zones P and T with high RI greatly contributed by Cd.

Keywords Heavy metals . Estuarine wetlands . Petroleum exploitation . Ecological risk index . Soil properties

X. Yao : R. Xiao (*) : Z. Ma : Y. Xie : M. Zhang : F. Yu College of Nature Conservation, Beijing Forestry University, Beijing 100083, Peoples Republic of China e-mail: [email protected]

Introduction In recent decades, human disturbance to coasts and estuaries has been intensified with the development of agriculture and industry, which results in coastal and estuarine wetland degradation all around the world (Lotze et al. 2006). As an important environmental indicator, heavy metal distribution has been more and more concerned in wetland degradation studies. Bai et al. (2011a, 2012) have reported heavy metal problems in wetland soils of Pearl River estuary and Yellow River estuary (YRE) in China, indicating an increasing trend of heavy metal pollution in degraded coastal and estuarine wetlands under a big threat of intensive tidal flat reclamation, together with agrochemicals uses and industrial wastes disposal. The existence and transport of heavy metals (e.g. As, Cd, Cu, Pb and Hg) could lead to soil, water and sediment contamination, and may contribute to a continuous potential risk for biota and humans (Merdy et al. 2006; Wang et al. 2013). Therefore, ecological and engineering restoration projects (e.g. phytoremediation, wetland construction, diking and drainage) are facing great challenges for removing toxic metals and remedying the degraded soils (Portnoy 1999; Weis and Weis 2004; Cui et al. 2009). Teuchies et al. (2013) evaluated metal mobility in a tidal marsh restoration project, they suggested that processes following re-inundation of the formerly embanked tidal area were complex and source, behavior and risks of the trace metals would be different under various conditions prevailing in different locations of an estuary. Thus, it is needed to be better understood the wetland degradation and restoration processes by investigating the distribution, sources, and environmental risk of heavy metals and the status of some key soil properties in an estuary (Wu et al. 2014; Fernández-Cadena et al. 2014; Xin et al. 2014).

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The YRE is one of the fastest-growing regions in the world (Wang et al. 2004), and the tidal flats, covering approximately 63 % of the estuary, expand through sedimentation and deposition every year (Li et al. 1998). In addition to the rapid land reclamation and saltwater intrusion in this area, the major industrial development-oil production process (from the second largest oil field of China-Shengli oilfield) has led to wetland losses and degradation (He et al. 2006). The degraded wetland area caused by oil field development is increasing in YRE (Liu et al. 2013). Tidal freshwater and brackish marshes, which are influenced by storm surges, droughts, accelerating sea level rise and insufficient river discharge, are manifesting as transient to sustained increases in salinity (Barendregt and Swarth 2013). In order to restore the degraded brackish wetlands under primary threats of the decrease in freshwater supply and substantial salinization, a restoration project including the construction of dikes and channels has been conducted within an area of 5023.7 ha since 2002. Dikes were designed to prevent seawater intrusion and channels were dug to pump freshwater (about 3 million m3 per year) to the wetlands. During the past few years, water pollution was alleviated and soil quality was improved through salinity reduction and soil organic matter accumulation in the restored wetlands. However, the changes in heavy metals in restored wetland soils compared to unrestored wetlands are still unknown (Cui et al. 2009). Heavy metal concentrations and distribution in wetland soils are important indicators of wetland status and change (Kabata-Pendias and Pendias 2010). Even though the heavy metal pollution in YRE is not severe at present, more and more attention has gradually been paid to this (Wang et al. 2013; Yu et al. 2011; Bai et al. 2011b; Ling et al. 2010), along with the possible sources of heavy metals in soils in different types of wetlands (Bai et al. 2012; Cui et al. 2011; Rui et al. 2008). Bai et al. (2011b) showed that wetland soils along the tidal ditch of YRE were slightly contaminated by As and Cd which might be originated from oil pollution, parent rocks and tidal seawater. Rui et al. (2008) presented that the soil heavy metals (i.e., Mn, Cu, Zn, Cr and Cd) reached significant levels compared to 10 years ago, suggesting gradual deterioration of soil quality in YRE. As well known, the mobility, bioavailability and therefore the potential toxicity or deficiency of most metals are controlled mostly by soil properties (Sheppard and Evenden 1988). Yu et al. (2011) found that the contents of total organic carbon, total nitrogen, and nitrate were significantly correlated with concentrations of Cr, Pb and Ni in the new-born coastal wetland soils of YRE and were recognized as the key factors for soil adsorption of metal elements. While some other important physical and chemical properties, such as soil moisture, pH, electric conductivity, aggregate size of soil particles are also important (Rooney et al. 2006; Romero-Freire et al. 2014). Wetland degradation and restoration processes occur with the specific hydrology, vegetation cover and soil properties, but little attention has been given to assess spatial

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variations of metals in different wetlands under the impacts of oil exploitation activities and ecological restoration project. The primary objectives of this study were to (1) compare the difference of heavy metal distribution along soil profiles under different vegetation covers from natural tidal salt marsh, degraded wetlands at oil well site, and restored wetlands in ecological restoration zone; (2) assess heavy metal contamination and potential risk level of soils using ecological risk index; (3) identify the probable sources of heavy metals and (4) investigate the key soil properties which are related to metal accumulation in different wetlands.

Materials and Methods Study Area The study area is located in the YRE (37°43′35″N 37°45′32″N and 119°07′53″E - 119°12′45″E) (Fig. 1) in Shandong province. It has a warm-temperate monsoon climate with a mean annual temperature of 12.3 °C, a mean annual precipitation of 555.9 mm and a mean annual evaporation of 1962.1 mm. Major vegetation types were fresh marsh vegetation (e.g., Phragmites australis Cavanilles (common reed), Tyhya orientalis Presl (cattail), Polygonum lapathifolium L (pale smartweed)) and salt marsh (e.g., Suaeda salsa L (seepweed), Aeluropus littoralis (Willd) Parl, Limonium sinense (Girard) Kuntz) with the dominant species of P. australis and S. salsa (He et al. 2007). The study area is composed of tidal zone (T), petroleum exploitation zone (P) and wetland restoration zone (R) with different hydrologic features (Fig. 1). Tidal zone received both freshwater and tidal flows, and the soil was waterlogged with 0–2 cm overlying water. Restoration zone received freshwater pumped through artificial channel and the soil was wet but never saturated. The natural precipitation was the only water source for the petroleum exploitation zone. Due to the low plant cover and high evaporation, the soil in petroleum exploitation zone was very dry and had high salinity. Sample Collection and Analysis Three sampling sites with different land cover were respectively selected as P. australis covered land (p), S. salsa covered land (s), and bare land without plant cover (b) in each zone (Fig. 1). In order to highlight the heavy metal distributions in oil-saturated soil profile, a site (named as visible oil spill) near to an oil well with visible black oil pollution in the bare land of zone P was sampled in addition to site Pb (named no visible oil spill). One hundred and thirty-five soil samples (3 zones× 3 sites×5 depths-from 0 to 50 cm with 10 cm intervals×3 replicates) were collected using a soil auger (6 cm in diameter) on

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Fig. 1 Location map (left) and sketch map (right) of sampling sites in the Yellow River Estuary

August 19 and 20, 2013. Fifteen additional samples (1 site×5 depths-from 0 to 50 cm with 10 cm intervals×3 replicates) were collected from the visible oil spill site in zone P to examine the difference between soils with and without oil soakage. All samples were placed in polyethylene bags and brought back to the laboratory. The fresh soils were oven dried at 105 °C for 24 h and weighed for soil moisture. Soil organic matter (SOM) was measured using dichromate oxidation (Nelson and Sommers 1982). Soil pH was measured using a HANNA pH meter (Hanna Instruments, Woonsocket, RI, USA) (soil: water =1: 5). Electric conductivity (EC) was determined in the supernatant of 1: 5 soil-water mixtures using an EC meter (VWR Scientific, West Chester, PA, USA). The classes of aggregates were determined using the wet sieving method (Elliott 1986). The soil sample was finally separated into four fractions: macro-aggregates (>2 mm), large aggregates (0.25 ~ 2 mm), small aggregates (0.053~0.25 mm), and micro-aggregates (2 mm) in most samples (≈96 % of the total soil samples), the macro-aggregate group was not taken into account in the data analysis. Soil As, Cd, Cr, Cu, Ni, Pb and Zn were analyzed using an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) (Li et al. 1995) after digestion in a mixture of HClO4-HNO3-HF in Teflon tubes (Thompson and Walsh 1989). Quality assurance and quality control were assessed every ten samples using duplicates, method blanks and standard reference materials (GBW07401) obtained from the Chinese Academy of Measurement Sciences (Bai et al. 2012). A satisfactory performance of heavy metal determination was achieved when the recoveries of heavy metals in the certified samples fell down 95 and 110 %.

Assessment Index The potential ecological risk index (RI) represents the sensitivity of the biological community to toxic substance and illustrates the potential ecological risk caused by the overall contamination (Zhao and Li 2013). It was selected to evaluate the combined pollution of multiple metals in this study. The equation used to calculate RI is as follow (Hakanson 1980):  n n n  X X X Ci RI ¼ Eir ¼ Tir  Cif ¼ Tir  i ; Cn i¼1 i¼1 i¼1 where: Eir is the potential ecological risk index of heavy metal i; T ir is the toxic response factor for a specific heavy metal i (e.g. As=10, Cd=30, Cr=2, Cu=5, Ni=5, Pb=5, and Zn=1); Cif is the contamination factor of heavy metal i; Ci is the content of heavy metal i in the samples (mg kg−1), and C in is the background value of heavy metal i in the study area (mg kg−1). In this study, soil background values of Shandong Province were used as C in (As: 9.3; Cd: 0.084; Cr: 66; Cu: 24; Ni: 25.8; Pb: 25.8; Zn: 63.5 mg kg−1) (Chinese Environmental Monitoring Station 1990). The contamination degrees and potential ecological risk of a heavy metal (E ir ) were classified as low degree (Eir