Polycyclic aromatic hydrocarbons (PAHs) in surface soils from reclaimed and ditch wetlands along a 100-year chronosequence of reclamation in a Chinese estuary: Occurrence, sources, and risk assessment

Abstract

The coastal reclamation for agricultural exploitation have caused serious pollution to the soil environment in coastal wetlands. However, few studies have been conducted on the persistent organic pollutants and ecological risks commonly existing in reclaimed coastal wetlands with different reclamation ages, which is of vital importance to alleviate agricultural soil organic pollution and warrantee food safety. Surface soils (0–10 cm) were collected in reclaimed wetlands (RW) and ditch wetlands (DW) along a 100-year chronosequence of intermittent reclamation (i.e., 100-yr, 40-yr, 30-yr, and 10-yr) in the Pearl River Estuary (PRE) of China. The concentrations of 16 Polycyclic aromatic hydrocarbons (PAHs) in soil samples were determined on a GC–MS 2010 QP-plus to investigate their occurrences, sources, and ecotoxic risk levels. The results showed that the total concentrations of PAHs at eight sampling sites ranged from 67.6 to 1172.8 ng·g−1, with a mean of 466.08 ng·g−1. The concentrations of ∑16PAHs in DW soils increased gradually, whereas the concentrations showed a “decreasing before increasing then decreasing” tendency in RW soils along the chronosequence of reclamation. The highest value occurred in the 30-yr soils of RW, which was heavily polluted, and the rest soils were slightly contaminated except for 40R and 100D soils (no pollution). The PAHs were predominantly composed of 2-, 3-, and 5-ring PAHs (the total accounting for >70% of the total 16 PAHs) at most of sampling sites. And the isomers of PAHs were predominantly composed of naphthalene (Nap), phenanthrene (Phe) and benzo(a)pyrene (Bap), and some PAHs isomers (i.e., Nap, Phe, Bap, fluorene (Fle), benz(a)an-thracene (Baa), and dibenz (a,h) anthracene (Daa) occurred predictable toxicity according to the sediment quality guidelines. A significant positive correlation was observed among the heavy Polycyclic aromatic hydrocarbons (HPAHs, including 4-, 5-, and 6-ring PAHs) and total nitrogen (TN), soil moisture and soil organic matter (SOM) in RW soils (p < 0.05), however, most of PAHs isomers were significantly negatively correlated with soil moisture content in DW soils (p < 0.05). According to the ratios of isomers, the combustion of fossil fuels and petroleum emissions were the major sources of PAHs. The highest value of toxic equivalent concentrations (TEQs)/∑PAHs ratios and TEQs appeared in 10-yr DW soils, followed by 100-yr and 30-yr RW soils, which showed higher potential ecological risks mainly because of certain PAH isomers (Bap and Daa) had higher TEQs and ecological risks.