Journal of Hydrology: Regional Studies 8 (2016) 69–81 Contents lists available at ScienceDirect Journal of Hydrology: Regional Studies journal homepage: www.elsevier.com/locate/ejrh Statistical analysis and modelling of the manganese cycle in the subtropical Advancetown Lake, Australia a a a,∗ b Edoardo Bertone , Rodney A. Stewart , Hong Zhang , Kelvin O’Halloran a Griffith School of Engineering, Griffith University, Gold Coast Campus, QLD 4215, Australia b Seqwater, Ipswich, QLD 4305, Australia a r t i c l e i n f o a b s t r a c t Study region: Advancetown Lake, South-East Queensland (Australia). Keywords: Study focus: A detailed analysis of available meteorological, physical and chemical data Manganese (mostly coming from a vertical profiler remotely collecting data every 3 h) was performed in Vertical profiling system Transport processes order to understand and model the manganese cycle. A one-dimensional model to calculate Mixing processes manganese vertical velocities was also developed. New hydrological insights for the region: The soluble manganese concentration in the hypolimnion is dominantly dependent on the dissolved oxygen level, pH and redox potential, which determine the speed of the biogeochemical reactions between different manganese oxidation states. In contrast, the manganese level in the epilimnion is mainly affected by the transport processes from the hypolimnion and thus to the strength of the thermal stratification, with high concentrations recorded solely during the winter lake cir- culation and wind playing only a minor role. The value of the peak concentration was found to be proportional to the amount of manganese in the hypolimnion and to the temperature of the water column at the beginning of the circulation period. In case of partial circu- lation only, a very high peak is espected during the next full winter turnover. This issue will increase in the future due to global warming and increased number of years with par- tial circulations only. These findings provide water authorities with increased manganese predictive power and thus proactive water treatment management strategies. © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction 1.1. Manganese and water treatment The chemical element manganese (Mn) comprises 0.1 per cent of the Earth’s crust (Emsley, 2001) and it can be observed in oxidation states ranging from −3 to +7. The most stable valence states are +2 and +4; hence the most naturally occurring Mn forms are dissolved Mn(II) and particulate Mn(IV) (Kohl and Medlar, 2007). In lakes and reservoirs, Mn can be present in both soluble and insoluble forms; its production, suspension, precipitation, mixing and interactions with other elements are complex and generally ruled by factors such as pH, redox potential (ORP), dissolved oxygen (DO) and level of turbulence. Abbreviations: BoM, Australian Bureau of Meteorology; DERM, Department of Energy and Resources Management of the Queensland Government; DO, dissolved Oxygen; Mn, manganese; ORP, redox potential; VPS, vertical profiling system. ∗ Corresponding author. E-mail address: h.zhang@griffith.edu.au (H. Zhang). http://dx.doi.org/10.1016/j.ejrh.2016.09.002 2214-5818/© 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc- nd/4.0/). 70 E. Bertone et al. / Journal of Hydrology: Regional Studies 8 (2016) 69–81 Fig. 1. Schematization of the Mn cycle in a reservoir: green connections: increase in x increases y; red connections: increase in x decreases y; dashed connections: processes affecting lake circulation; upper box: epilimnion; lower box: hypolimnion; rectangles: variables part of the cycle; ovals: external inputs; (1): lake turnover; (2): Mn precipitation; (3): Mn oxidation; (4) Mn reduction. It has been found that Mn can potentially cause decreased IQ and neurotoxic issues at high levels (Khan et al., 2012). The Mn concentration levels in drinking water, despite being typically too low to cause health issues (although values above World Health Organisation’s drinking water limits can be recorded in groundwater, see Homoncik et al., 2010), can cause aesthetical problems such as black or brown colouring or, with even higher levels, a metallic taste (Raveendraan et al., 2001). Typical standards (such as that fixed by the United States Environmental Protection Agency) for soluble Mn are 0.05 mg/L; however, recently, some water utilities have been targeting 0.015 − 0.2 mg/L to avoid any customer complaints (Kohl and Medlar, 2007), and Seqwater (the water authority that manages Advancetown Lake) require treated water manganese to be <0.02 mg/L. A better understanding of the Mn cycle and the variables affecting it would allow the water authority to manage Mn treatment more efficiently. Although many experiments have been conducted to examine the importance of single variables such as pH or ORP, only few studies (e.g., Johnson et al., 1991) have taken into account the overall process. Remarkably, to the authors’ knowledge, limited studies have quantitatively analysed the Mn behaviour during the lake natural destratifi- cation that, in monomictic lakes such as Advancetown Lake, usually occurs once per year. This event dramatically changes the lake structure, leading to an even distribution of physical and chemical parameters (Nürnberg, 1988). For this study, after an extensive data collection from several sources and over several years, the overall Mn cycle for Advancetown Lake was analysed and the main variables affecting it were detected during both the stratified and the circulating (destratified) periods. Although the study is limited to Advancetown Lake, it provides a general assessment of Mn behaviour in subtropical monomictic lakes, focusing especially on understanding and predicting the timing and peak concentrations during winter circulations. 1.2. Manganese cycle: biogeochemistry The key processes involved in a lake’s Mn cycle are summarised in Fig. 1. During the stratification season, there are evident differences in Mn concentration between the epilimnion and the hypolimnion. In the epilimnion, the incident radiation allows photosynthesis to occur and the level of DO is high, although higher water temperatures decrease its solubility. The presence of algae also means higher pH because of the removal, through photosynthetic assimilation, of acidic CO2 forms − such as HCO3 (Dubinsky and Rotem, 1974). Under these conditions, the soluble Mn is oxidised (Stumm and Morgan, 1981) into insoluble forms such as Mn dioxide, which precipitates downwards into the hypolimnion. This reaction is slow for pH lower than 8.5/9 (Howe et al., 2004; Johnson et al., 1995) but with high water temperatures ◦ (with an optimum at 30 C), bacteria play a central role in the oxidation reaction, making the reaction much faster (Johnson et al., 1995). The occurrence of Mn is largely dependent on two groups of bacteria: oxidising bacteria (which convert soluble E. Bertone et al. / Journal of Hydrology: Regional Studies 8 (2016) 69–81 71 Fig. 2. Advancetown Lake mp (from Google map). Mn(II) into particulate Mn(IV)) and reducing bacteria (which provoke the opposite reaction), with the direction of the cycling strictly related to the DO level (Kohl and Medlar, 2007). In conclusion, soluble Mn in the epilimnion is usually low, and insoluble Mn, since it quickly precipitates, is typically present in small amounts. In the colder hypolimnion, light cannot penetrate and algae can neither develop nor produce oxygen through photosynthesis. Therefore, unlike the epilimnion, under stratified conditions the bottom waters are typically acidic (because acidic CO2 forms are not assimilated) and contain low or no DO for bacteria respiration. When oxygen levels are depleted, bacteria obtain the oxygen they need through denitrification (Tundisi and Matsumura, 2011) and later through Mn reduction (Engebrigsten, 2010), thus forming and releasing dissolved Mn(II) in the water column. Both the low pH (Calmano et al., 1993) and the anoxic conditions (Chiswell and Huang, 2003) are considered important for Mn reduction. Carlson et al. (1997) found that, for a particular reservoir, when the DO level was <3 mg/L, dissolved Mn was present, and when the level fell below 2 mg/L, the Mn concentration increased, mainly due to release from the sediments. The Mn oxides mainly come from the bottom of the reservoir or suspended sediments, by precipitation from the epilimnion through the rivers, or groundwater (Kohl and Medlar, 2007; Tundisi and Matsumura, 2011). Hence, during stratification, the soluble Mn level in the hypolimnion gradually increases, with molecules diffusing throughout the hypolimnetic volume. Once the molecules reach the border with the epilimnion, the diffusion cannot go any further because of different densities and properties (e.g., high levels of DO and pH). 1.3. Mn cycle: transport processes The biogeochemical reactions predominantly control the Mn cycle under stratified conditions only, when the transport processes are limited. Previous studies (Howard and Chisholm, 1975) have recorded a continuous and consistent increase in Mn in the bottom water after stratification, reaching its maximum immediately before the subsequent overturn of the lake. As soon as the overturn occurs and free circulation begins, Mn from the bottom water is mixed and transported throughout the water column. This process creates a temporary escalation in the concentatration of soluble Mn in the upper waters (Calmano et al., 1993). After this, the Mn load is usually low during the destratified period; an elimination of reduced compounds from the hypolimnion can occur in just a few days (Macdonald, 1995). This occurs because, with the breakdown of the temperature gradient allowing better water circulation and mixing, the water masses—particularly the hypolimnetic anoxic ones—become more oxygenated. The great amount of soluble Mn stored in these deep layers, through the introduction of oxygen, is quickly oxidised to insoluble oxides, which then precipitate and accumulate in the bottom sediments (Dojlido and Best, 1993).