Evaluating the contamination level of total petroleum hydrocarbons (TPHs) and heavy metals in coastal sediments of Tiab Mangroves (Hormozgan Province)

Abstract

Abstract In order to evaluate the contamination degree of sediments in Tiab mangroves (Hormozgan province) in terms of Total Petroleum Hydrocarbons (TPH) and heavy metals concentration, 16 sediment samples were collected from the study area. In addition to TPH and metal concentrations, physicochemical properties of sediments such as grain size, pH, total organic matter and organic carbon were measured using the standard methods. The concentrations of heavy metals and concentration of total petroleum hydrocarbons was determined. Petroleum hydrocarbons in sediments narrowly varied from 0.36 to 4.89 (µg/g). The highest TPHs concentrations are found in sediments from Tiab and Kolahi berth probably due to inputs from coastal anthropogenic activities. Using sediment quality guidelines and the contamination factor, it was found that there is no adverse biological effect in terms of TPH concentration. It was also revealed that metal concentrations in sediments ranged from 163 mg/kg for Ni to 8 mg/kg for Pb and their mean concentrations are all higher than the corresponding concentrations in the Persian Gulf sediments. Calculated enrichment factor showed that all metals (except for Ni) have low contamination level. However, the overall ecological risk is found to be moderate (78.14) for all studied metals. The results of cluster and correlation analyses showed that most metals (Pb, Zn, Cr) are originated from natural source while total petroleum hydrocarbons alongside Ni and V were most likely derived from anthropogenic sources. Introduction Mangrove forests are of tidal ecosystems situated in tropical and subtropical coastal regions. Mangrove systems serves habitat and nursery area for many aquatic flora and fauna and play a vital role in protection of coastal lines (Vane et al. 2009). At the same time, theses ecosystems are one of the most vulnerable ecosystems in terms of environmental contamination. Because of their fine-grained texture and anaerobic conditions, mangrove sediments can accumulate contaminants at higher levels (Tam et al. 2005). Total petroleum hydrocarbons (TPHs) and heavy metals are of the most hazardous contaminants which may find their pathways into the mangrove ecosystems through a vast variety of anthropogenic or natural sources (Ebrahimi-Sirizi and Riyahi Bakhtiyari 2013; Dadashi et al. 2018). The Tiab and Kolahi mangrove forests are located in the east of Bandar Abbas and in the north of Hormuz Khors at longitude of 56, 47 E and altitude of 27, 7 N. The data concerning the environmental and ecological status of these mangroves are very limited. The present study was initiated to evaluate the contamination level of TPHs and heavy metals in the bottom sediments of these mangrove ecosystems. Material and Methods Sixteen bottom sediment samples (10-20 cm in depth) were randomly collected from Tiab and Kolahi mangroves communities using a PONAR grab sampler. At each station, three replicate samples were taken and then mixed together to obtain a composite sample (MOOPAM 2010). Some major physicochemical properties (total organic matter, organic content, pH and grain size distribution) along with total hydrocarbons and heavy metal concentrations were determined using standard methods. TPHs and heavy metal concentrations (Pb, Zn, Cr, Ni, V) in sediment samples were quantified using Ultraviolet Fluorescence Spectroscopy (UFS) and Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), respectively. To assess the contamination level of TPHs and heavy metals in sediments and their ecological risks, the standard quality guidelines (SQGs), enrichment factor and ecological risk index (RI) were calculated. Discussion of Results & Conclusions Based on the obtained results, clay and silt contents of sediment samples varied from 14.93 to 56.82 % and 2.80 to 21.73 %, respectively. The proportion of fine grain fractions (silt + clay) were found to increase in the mangrove sediments (stations No. 9, 10, and 11) because of the low energy waves and tree-rooted stands within the mangrove community. Sediment pH showed a narrow range from 8.24 to 8.80, indicating alkaline conditions. The organic carbon content in the sediments varied from 0.22 to 0.85. The distribution pattern of OC content in is similar to that of the fine-grained content. The total organic meter content the studied sediments ranged from 2.82 to 6.84%, with the highest level at the stations (1, 2, 4, 5, 8, 9, 10, 11 and 15) where fishing and fetching piers are located. TPH concentration in the studied sediments varied from 0.36 µg/g at station 9 to 4.89 µg/g at the station 1. The spatial distribution pattern showed that TPH level in the sediments increased at the stations (1, 20, 15, 16) near the pier and fishing facilities along the Kolahi shoreline. This implies that TPH in the sediments is partly derived from the anthropogenic sources. However, comparing with SQGs and based on the calculated contamination factors, the sediments did not pose any adverse effect in terms of TPH concentration. The average concentration of heavy metals measured in this study as follows: 8.68 mg/kg (Pb), 147.81 mg/kg (Ni), 85.87 mg/kg (V), 58.81 mg/kg (Zn) and 122.56 mg/kg (Cr). Nickel concentration is the highest in stations no. 5, 6 and 7 near the fishery piers and its level at all sampling sites is generally higher than the corresponding concentration in the Persian Gulf sediments (Karbassi 2001). Ni is major component of petroleum-based materials and may enter into the mangrove sediments through discharge of oil from fetching fuel boat and lodges across the mangrove stand (de Mora et al. 2004; Alomary and Belhadj 2007; Usman et al. 2013). Vanadium have a narrow range of concentration, however, its content is shown to be increased at the stations (1, 3, 4, 5, 7, 8 and 14) near the fishery piers. Lead, zinc and chromium in sediments also showed a high level in the mangrove sediments although their concentrations have low variability in comparison with other studied metals. Based on calculated enrichment factors, Ni and V are characterized by low enrichment, suggesting that these metals were mostly likely derived from anthropogenic sources. According to the results of ecological risk calculations, Tiab mangrove sediments have low ecological risk with respect to Pb, Zn, Cr and V while Ni is found to have a moderate ecological risks in the studied sediments. The overall potential ecological risk is predicated to be moderate in terms of all studied metals in sediments. The results of Pearson correlation analysis showed that there are significantly positive correlation between TOM content in sediments and percentage of fine grain fractions (silt+ clay). Positive correlation (at significance level of 0.05) were also observed among pairs of Cr-Pb, Ni-V and Pb-Zn, implying that these metals are of the same or similar sources. TPH concentration is significantly correlated with Ni and V and sediment pH does not show any significant correlation with the other variables probably due to its narrow range. To get a better insight on statistical relationships and to apportion the possible sources of contaminants in the sediments, cluster analysis was performed. The dendogram revealed the two following uncorrelated clusters: Cluster I consists of Pb, Zn, Cr alongside SC (silt + clay), TOM and OC contents, suggesting their variations may be controlled by the same and geogenic source and Cluster II includes V, Ni and TPHs which can be attributed to anthropogenic sources in the study area. This study concludes that although the contamination and ecological risk of TPHs and heavy metals in the Tiab mangrove sediments are currently low, extensive anthropogenic developments in adjacent coastal area can accelerate the contamination rate of the sediments in coming years. Therefore, it is imperative to take predicative actions such as restriction of fuel fetching activities and preventing discharge of wastewater and effluents from residential settlements, shrimp breeding ponds and fishery piers near the shoreline.

Keywords

• total petroleum hydrocarbons
• heavy metals
• mangrove sediments
• tiab
• hormozgan