Elsevier

Water Research

Volume 46, Issue 4, 15 March 2012, Pages 965-974
Water Research

Impact of sulfate pollution on anaerobic biogeochemical cycles in a wetland sediment

https://doi.org/10.1016/j.watres.2011.11.065Get rights and content

Abstract

The impact of sulfate pollution is increasingly being seen as an issue in the management of inland aquatic ecosystems. In this study we use sediment slurry experiments to explore the addition of sulfate, with or without added carbon, on the anaerobic biogeochemical cycles in a wetland sediment that previously had not been exposed to high levels of sulfate. Specifically we looked at the cycling of S (sulfate, dissolved and particulate sulfide – the latter measured as acid volatile sulfide; AVS), C (carbon dioxide, bicarbonate, methane and the short chain volatile fatty acids formate, acetate, butyrate and propionate), N (dinitrogen, ammonium, nitrate and nitrite) and redox active metals (Fe(II) and Mn(II)). Sulfate had the largest effects on the cycling of S and C. All the added S at lower loadings were converted to AVS over the course of the experiment (30 days). At the highest loading (8 mmol) less than 50% of consumed S was converted to AVS, however this is believed to be a kinetic effect. Although sulfate reduction was occurring in sediments with added sulfate, dissolved sulfide concentrations remained low throughout the study. Sulfate addition affected methanogenesis. In the absence of added carbon, addition of sulfate, even at a loading of 1 mmol, resulted in a halving of methane formation. The initial rate of formation of methane was not affected by sulfate if additional carbon was added to the sediment. However, there was evidence for anaerobic methane oxidation in those sediments with added sulfate and carbon, but not in those sediments treated only with carbon. Surprisingly, sulfate addition had little apparent impact on N dynamics; previous studies have shown that sulfide can inhibit denitrification and stimulate dissimilatory nitrate reduction to ammonia. We propose that because most of the reduced sulfur was in particulate form, levels of dissolved sulfide were too low to interfere with the N cycle.

Highlights

► Sulfate addition impacts the anaerobic cycling of C, Fe and Mn. ► Addition of sulfate at low levels inhibits methanogenesis. ► Sulfate additions didn’t substantially affect the cycling of N.

Introduction

The impact of sulfate pollution is increasingly being seen as an issue in the management of inland aquatic ecosystems (Baldwin and Fraser, 2009). Evidence suggests that human practises have increased the amount of sulfate entering inland aquatic ecosystems (Jansen and Roelofs, 1996, Lamers et al., 1998, Lamers et al., 2001). For example, Jansen and Roelofs (1996) report a six-fold increase in sulfate levels in groundwater from southern and eastern parts of the Netherlands between 1966 and 1991. The main sources of sulfur in that study were believed to be both aerial deposition (derived from sulfur from coal-fired electricity generating power plants) and leakage from agricultural systems. Sulfur is regularly applied to agricultural systems as a fertilizer or soil ameliorant (e.g. Jamal et al., 2010) and is known to leach into aquatic ecosystems (e.g. Szynkiewicz et al., 2011).

Rising saline groundwater may be another source of sulfur to inland ecosystems (Hicks et al., 2003). The salt in groundwater is often originally marine-derived (Herczeg et al., 2001) and therefore has a relatively high concentration of sulfate (Hicks et al., 2003). Poor drainage and inappropriate irrigation practises, has caused an estimated 10% of the worlds irrigated land area to be adversely affected by water-logging and salinization (UNESCO, 2003). Thus, unsuitable land use practises have inadvertently provided another source of sulfur to freshwater systems worldwide.

Probably the most dramatic impact of increased sulfate in inland aquatic ecosystems has been the development of high levels of reduced sulfur in the sediments of inland waterways (e.g. Hall et al., 2006). Sulfidic sediments (also referred to as acid sulfate soils) form when sulfate reducing bacteria use the sulfate ion as the terminal electron acceptor for anaerobic respiration. The respiratory end product of this reaction is hydrogen sulfide. The sulfide then reacts with iron (and other metals) to form mineral sulfides such as makinawite and pyrite. Oxidation of sulfidic sediments (e.g. through partial drawdown of a waterbody) can have adverse environmental effects including acidification, deoxygenation and mobilization of heavy metals (Baldwin and Fraser, 2009 and references therein).

Increasing the concentration of sulfate in a waterbody also has the potential to disrupt sediment biogeochemical processes and hence ecological condition. For example, sulfate reduction has been implicated in the methylation of mercury in wetland sediments (Gilmour et al., 1992). Increasing sulfate concentrations has also been implicated in the eutrophication of inland waterways through mobilization of phosphorus (e.g. Boström et al., 1988, Caraco et al., 1989, Mitchell and Baldwin, 1998, Lamers et al., 1998, van der Welle et al., 2008). Sulfide is a strong enough reducing agent to facilitate the reduction of solid ferric minerals to dissolved ferrous ions with concurrent P release (Boström et al., 1988). This reaction is favored by the insolubility of one of the reaction products, iron sulfide. It has also been suggested that S2− can also displace P from insoluble Fe2+ phases (Roden and Edmonds, 1997); again the reaction being favored by the insolubility of the reaction product – FeS. Sulfate pollution may also have the potential to affect other biogeochemical processes. In this study we use sediment slurry experiments to explore how increasing sulfate concentrations alone, or with additional bioavailable carbon affects the dynamics of S (sulfate, and dissolved and particulate sulfide the later measured as acid volatile sulfide; AVS), C (carbon dioxide, bicarbonate, methane and the short chain volatile fatty acids formate, acetate, butyrate and propionate), N (dinitrogen, ammonium, nitrate and nitrite) and redox active metals (Fe(II) and Mn(II)) in the sediments of a wetland that has not previously been exposed to high levels of sulfate.

Section snippets

Sampling

Sediment samples were taken from Norman’s Lagoon – a small, permanent oxbow lake (approximately 2 km2) located on the River Murray floodplain near Albury, New South Wales, Australia (36°15′S, 146°55′E). The climate is Mediterranean with cool, wet winters and hot, dry summers. The water and sediment chemistry of the wetland have been described in detail elsewhere (Gribben et al., 2003, Mitchell, 2002). Briefly, the surficial sediments of the billabong are flocculent with approximately 80% water

Sulfur dynamics

The initial concentrations of sulfate in both experiments are presented in Table 1. In the sulfate addition experiment sulfate was consumed in all treatments (Table 1); the loss of sulfate approximately following a first order decline for all treatments with added sulfate (Table 1). The total amount of sulfate consumed during the course of the experiment varied linearly with the initial concentration of sulfate added (r2 = 0.98; slope = 0.39; see also Fig. 1). Addition of carbon as either

Sulfur dynamics

Sulfate consumption followed first-order kinetics, with no apparent lag phase. This contrast with the results of Whitworth and Baldwin (2011) who found that sulfate loss from solution for dry wetland sediments proceeded after a lag phase of about 40 days. They attributed this lag phase to the slow initial mineralization of C in the sediments and the requirement to grow microbial biomass. The lack of a lag phase in the current study suggests that in already wetted and anaerobic sediments, carbon

Conclusions

The purpose of this study was to see how the addition of sulfate affects the biogeochemistry of wetland sediments. Clearly the addition of sulfate promotes sulfate reduction and the build-up of reduced sulfur in sediments; which in turn can lead to acidification, deoxygenation and heavy metal mobilization if these reserves are disturbed. What this study shows is that the rates of accumulation of reduced sulfur compounds can be quite rapid. Therefore, even short-term exposure to elevated levels

Acknowledgments

This project was supported by the National Water Commission through its Raising National Water Standards Program. This Australian Government program supports the implementation of the National Water Initiative by funding projects that are improving Australia’s national capacity to measure, monitor and manage its water resources. Additional funding was also received from the NSW Murray Wetlands Working Group and the Murray-Darling Freshwater Research Centre. The funding bodies did not play an

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