A multi-molecular marker assessment of organic pollution in shore sediments from the Río de la Plata Estuary, SW Atlantic

Highlights

• Organic pollution was studied by qualitative and quantitative approaches.

• Natural and anthropogenic organic contributions were recognized.

• 59% Of resolved aliphatic hydrocarbons is derived from petroleum inputs.

• Petroleum combustion accounted for 87% of total PAHs.

• LABs and steroids demonstrated inadequate sewage treatment.

Abstract

Organic pollution was evaluated in surface sediments along the middle portion of the Río de la Plata Estuary, SW Atlantic. A multi-molecular marker approach was performed to identify major sources of organic compounds using diagnostic indices. The relative contribution of different sources of hydrocarbons was quantified by source apportionment employing Principal Component Analysis/Multiple Linear Regression (PCA/MLR) as chemometric technique. All molecular markers indicated high chronic organic pollution in the stations of Montevideo Bay. Main sources of aliphatic and polycyclic aromatic hydrocarbons were petroleum inputs and combustion, due to oil transport and refinement, harbour activities and vehicular emissions. Major sources of linear alkylbenzenes and steroids were industrial and domestic sewage. Although, significant anthropogenic inputs, a natural footprint of terrestrial higher plants contribution was recognized. Multi-molecular marker and comprehensive assessments can improve the establishment of more precise regulation actions to reduce pollution levels.

Introduction

Organic compounds reach aquatic ecosystems by different emission pathways. Besides atmospheric deposition, anthropogenic discharges (e.g. agricultural, urban and industrial effluents) are recognized as major sources of several classes of organic pollutants to estuarine and coastal environments (Takada and Eganhouse, 1998, Zimmerman and Canuel, 2000). Aliphatic hydrocarbons (AHs) can derive from biogenic terrestrial and aquatic sources, but also from anthropogenic sources such as fossil fuel combustion (Wang et al., 2012a). Polycyclic aromatic hydrocarbons (PAHs) can be petrogenic, biogenic and pyrogenic (Yunker and Macdonald, 2003). Pyrogenic compounds are products of incomplete combustion of recent and fossil organic matter at high temperatures, while petrogenic compounds are commonly derived from slow organic matter maturation (Wang et al., 2005). PAHs come from both anthropogenic (e.g. fossil fuels combustion, oil production and transportation) and natural sources (e.g. natural seepage, coal, bitumen, forest and grassland fires) (Kim et al., 1999, Pietzsch et al., 2010). Biogenic PAHs are derived from plant debris and microorganisms, but their contribution has not been completely described yet (Opuene et al., 2007, Sánez et al., 2013). Linear alkylbenzenes (LABs), which constitute the main ingredient in the manufacturing of linear alkylbenzene sulphonates (LAS) in commercial formulas for household detergents (Eganhouse et al., 1988), can be released through industrial and municipal wastewater (Macías-Zamora and Ramírez-Alvarez, 2004). In addition, faecal steroids such as coprostanol and coprostanone, produced by microbial reduction of cholesterol in the digestive system of higher invertebrates and humans, have been used worldwide as molecular markers of domestic sewage pollution (Venkatesan and Kaplan, 1990, Peng et al., 2005, Readman et al., 2005, Martins et al., 2012, among others). Moreover, epicoprostanol, a coprostanol isomer, can be used as an indicator of the level of treatment of the faecal matter because it is formed during extensive anaerobic sewage treatment (Mudge and Seguel, 1999).

The study of organic pollution in estuaries is very relevant as they are transitional zones between the terrestrial and marine domains, which control the fluxes of water, nutrients, particles and organisms from and to the continental margins, rivers and oceans (Elliott and Quintino, 2007). Although, estuaries are highly dynamic environments due to their inherent variability in terms of physico-chemical characteristics (e.g. salinity, temperature and dissolved oxygen) they are usually subjected to additional human stress (Borja et al., 2008). Human pressures in estuarine and coastal environments are commonly coupled to rapid population growth, urbanization and poor management (Wang et al., 2012b). Likewise, estuaries are highly productive and naturally eutrophic areas when compared with pristine freshwater or marine systems, as they receive large amounts of inorganic and organic compounds from terrestrial drainage (Zaldivar et al., 2008).

The fate and transport of organic compounds in estuarine environments is dependent on their partitioning between particulate and dissolved phases. Water-soluble (hydrophilic) components predominantly accumulate in the aqueous phase and are consequently transported rapidly as a mobile fraction. In contrast, more hydrophobic (lipophilic) contaminants tend to be enriched in the solid phase by sorptive interactions with particulate organic carbon (POC) (Santschi et al., 1999). The POC is then deposited according to hydrological and sedimentological conditions and associated lipophilic compounds are transferred into the sediments, as a more immobile fraction (Bianchi, 2007). Due to their high hydrophobic properties and relatively stable chemical structures, many organic pollutants have strong partitioning with particle surfaces and thus can be found at high concentrations in contaminated sediments (Karickhoff et al., 1979, Wang et al., 2005). Any disturbance of the contaminated sediments may replace organic pollutants in the water column and reallocate them back to aquatic food chains, leading to health hazards of aquatic life and human beings (Gaspare et al., 2009).

Identification, quantification and source assignment of organic pollutants are very important for regulating pollution and minimize the inputs into the environment (Kavouras et al., 2001, Retnam et al., 2013). Organic pollutants in aquatic sediments occur in complex mixtures of compounds, which concentrations vary according to their origin, physicochemical characteristics, distance from source, selective diagenetic alterations, and also, environmental conditions within the sedimentary matrix, such as porosity, flocculation, POC and oxygen content, among other factors (Bayona and Albaigés, 2006, Medeiros and Simoneit, 2008). In this sense, the simultaneous utilization of several molecular markers appears to offer a better discrimination between sources, and more suitable tools for tracing the transport and fate of organic pollutants in aquatic ecosystems (Bayona and Albaigés, 2006). Additionally, several chemometric techniques (e.g. Principal Component Analysis with Multiple Linear Regression (PCA/MLR), Cluster Analysis (CA) and Discriminant Analysis (DA)) have provided better understanding and interpretation of complex sedimentary aliphatic and polyaromatic hydrocarbon samples, and also the possibility of quantifying the contribution of different sources (Kavouras et al., 2001, Liu et al., 2009, Wang et al., 2012a, Retnam et al., 2013).

The aims of this study are (1) to evaluate organic pollution in coastal sediments of Montevideo, Río de la Plata Estuary by a multi-molecular marker approach, (2) to identify major sources of organic pollutants through qualitative analysis using diagnostic indices, (3) to assess the relative contribution of different sources of aliphatic hydrocarbons (AHs) and polycyclic aromatic hydrocarbons (PAHs) by means of quantitative source apportionment employing (PCA/MLR) as a chemometric technique.