Elsevier

Science of The Total Environment

Volume 432, 15 August 2012, Pages 47-56
Science of The Total Environment

Water chemistry and nutrient release during the resuspension of FeS-rich sediments in a eutrophic estuarine system

https://doi.org/10.1016/j.scitotenv.2012.05.065Get rights and content

Abstract

The objective of this study was to investigate the impact of resuspending FeS-rich benthic sediment on estuarine water chemistry. To address this objective, we conducted (1) a series of laboratory-based sediment resuspension experiments and (2) also monitored changes in surface water composition during field-based sediment resuspension events that were caused by dredging activities in the Peel–Harvey Estuary, Western Australia. Our laboratory resuspension experiments showed that the resuspension of FeS-rich sediments rapidly deoxygenated estuarine water. In contrast, dredging activities in the field did not noticeably lower O2 concentrations in adjacent surface water. Additionally, while FeS oxidation in the laboratory resuspensions caused measurable decreases in pH, the field pH was unaffected by the dredging event and dissolved trace metal concentrations remained very low throughout the monitoring period. Dissolved ammonium (NH4+) and inorganic phosphorus (PO4–P) were released into the water column during the resuspension of sediments in both the field and laboratory. Following its initial release, PO4–P was rapidly removed from solution in the laboratory-based (< 1 h) and field-based (< 100 m from sediment disposal point) investigations. In comparison to PO4–P, NH4+ release was observed to be more prolonged over the 2-week period of the laboratory resuspension experiments. However, our field-based observations revealed that elevated NH4+ concentrations were localised to < 100 m from the sediment disposal point. This study demonstrates that alongside the emphasis on acidification, deoxygenation and metal release during FeS resuspension, it is important to consider the possibility of nutrient release from disturbed sediments in eutrophic estuaries.

Highlights

► Resuspension of estuarine sediment caused rapid oxidation of FeS stores. ► Nutrients are released from eutrophic sediment in field and laboratory resuspensions. ► Laboratory resuspension of FeS-rich sediment released acid and deoxygenated water. ► No pH or oxygen decrease is evident during field resuspension of FeS-rich sediment. ► Sediment properties and system hydrodynamics affect water chemistry during dredging.

Introduction

Many shallow estuarine environments often require dredging to maintain access to boating channels and residential canals. In such environments, dredging activities may contribute to the resuspension of anoxic benthic sediment into overlying oxic surface waters. It has been documented that resuspension of benthic sediments can rapidly deoxygenate surface waters (Sullivan et al., 2002, Fyfe et al., 2007). In addition, sediment resuspension can trigger the oxidation of iron monosulfide (FeS) minerals and cause pH-dependent release of FeS-associated trace metals (Burton et al., 2006a, Bush et al., 2004, Simpson et al., 1998, Johnston et al., 2003, Wong et al., 2010).

In marine and estuarine sediments, sulfate (SO42−) reduction is often a dominant mechanism for the mineralisation of organic matter (Capone and Kiene, 1988, Jørgensen, 1982, Morgan et al., 2012). In addition to contributing to sedimentary FeS accumulation, mineralisation of organic matter and related sulfur (S) cycling can also induce enrichments in porewater ammonium (NH4+) and phosphate (PO4–P, which includes ∑ PO43−, HPO42−, H2PO4−) (Caraco et al., 1989, Smolders et al., 2003, Smolders et al., 2006). Despite this, nutrient release is rarely regarded as an important environmental concern in relation to the resuspension of FeS- and organic-rich sediments.

The objective of the current study was to investigate the impact of estuarine sediment resuspension on surface water chemistry and nutrient concentrations. This study focuses on a eutrophic estuary in Western Australia that is known to contain benthic sediments rich in FeS (Morgan et al., 2012). We addressed the objective by employing (1) a laboratory-based (closed-system) set of sediment resuspension experiments and (2) a field-based (open-system) monitoring programme examining surface water composition during large-scale sediment disturbance caused by dredging activities.

Section snippets

Study site description

This study investigates the resuspension of sediments from the frequently-dredged South Yunderup channel in the Peel–Harvey Estuarine System (PHES) (Fig. 1). The PHES is located on the Swan coastal plain approximately 80 km south of Perth, Western Australia (Fig. 1). The system is of high recreational use and frequent dredging is required to maintain access to the boating navigation channels and residential canals (Brearley, 2005). There is ~ 10% flushing of estuarine water to the Indian Ocean

Sediment properties for the South Yunderup channel

The selected sediment porewater parameters (pH, Eh, ∑ S2−, NH4+, PO4–P and DOC) are presented in Table 1. The pH was generally neutral to near-neutral, ranging between pH 7 and 8, which is typical of anoxic marine fine-grained sediments (Morse, 1999). The redox conditions of all sediments were reducing (Eh between 100 and − 100 mV, SHE) to highly reducing (Eh < − 100 mV, SHE) (Table 1). The porewater ∑ S2− concentrations were high (median = 3.5 mmol/L; range = 0.1−6.4 mmol/L), as were the porewater DOC

Eh, pH and O2

It is increasingly recognised that sedimentary S cycling can strongly influence the ecological sustainability of aquatic ecosystems (Åström, 2001, Boman et al., 2008, Brunet, 1996, Burton et al., 2006b, Burton et al., 2006c, Bush et al., 2004, Caraco et al., 1989, Cooper and Morse, 1998, Di Toro et al., 1992, Morgan et al., 2012, Rickard and Morse, 2005, Ward et al., 2004, Macdonald et al., 2004). Several small-scale laboratory-based studies have demonstrated that the resuspension and oxidation

Conclusion

There is limited research investigating the influence of FeS-rich sediment resuspension on estuarine water conditions by combined field and laboratory-based investigations. Furthermore, an emphasis is generally placed on surface water deoxygenation and the release of acidity and trace metals during FeS oxidation. In this study, we found that although acidity was produced during AVS oxidation, the buffering capacity of the estuarine water maintained surface waters at a stable, near-neutral pH in

Acknowledgements

We would like to acknowledge the Western Australian Department of Water (Water Science Branch) for project funding. We would also like to acknowledge the Taiwan National Synchrotron Radiation Research Centre (NSRRC) for beamtime allocation which allowed the completion of XANES work in this study. We thank Dr. L.Y. Jang (NSRRC Beamline leader) for his expert guidance in collecting the S XANES spectra. A special thanks to Zitti Wesolowski from the Department of Water, Mandurah, for his assistance

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