Effects of nutrients and temperature on mobilization of mercury from sediment of the industrial contaminated Gunneklevfjorden, southern Norway
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Original versionOlsen, M. Effects of nutrients and temperature on mobilization of mercury from sediment of the industrial contaminated Gunneklevfjorden, southern Norway. Master thesis, University College of Southeast Norway, 2016
Mobilization of mercury (Hg) and Hg methylation rates in sediment and water from the contaminated fjord Gunneklevfjorden, Telemark, Norway, were investigated in a laboratory experiment with addition of nutrients (as glucose (C6H12O6) and ammonium (NH4+)) to the water in 56 different treatments under two different temperature regimes (4ºC and 20ºC). After storage for four months, the concentration of total Hg (TotHg) and methylmercury, CH3Hg+ (MeHg), in water above the contaminated sediment were measured in the different treatments. Correlations were assessed between TotHg/MeHg and nutrient consumption, redox potential (Eh), sulfate (SO4 2-)- and sulfide (S2-) concentrations, as well as other possible influencing variables such as pH, nitrate (NO3--N) and total phosphorous (Tot-P). The amount of nutrients added and nutrient consumption were strongly correlated (p = < 2.2 × 10-16 for both glucose and NH4 +), indicating a stimulation of bacterial activity with increasing nutrient availability. The Eh 1 cm above the sediment surface (Eh(1)) was significantly negatively correlated with nutrient consumption (α = 6.9 × 10-9 and α = 0.0023 for glucose and NH4+, respectively) and significantly lower at storage temperature 20°C (α = 0.0152), indicating that enhanced bacterial activity reduced the amount of oxygen above the sediment, and thereby lowered Eh(1). A significant negative correlation between consumed glucose and SO42- concentrations in the water (α = 3.3 × 10-9) indicated presence of sulfate-reducing bacteria (SRB), further demonstrated by a significant negative correlation between S2- 1 cm below the sediment surface (S2-(-1)) and SO42- in the water (p = 0.0088). TotHg concentrations in the water after storage showed a large variation, ranging from 1.9 - 74.8 ng L-1. Storage temperature appeared to be the strongest explanatory variable for TotHg, with a significant difference between TotHg at 4ºC (34.2 ± 22.9 ng L-1) and 20ºC (9.1 ± 3.8 ng L-1) (p = 5.9 × 10-6). MeHg concentrations in the water after storage ranged from below detection limit (DL: 0.02 ng L-1) to 8.60 ng L-1. In a multiple regression model fitted for MeHg, Eh(1) and storage temperature explained 50 % of the variations in MeHg (interpreted by R2 = 0.50). There was no significant correlation between NH4+ consumed and MeHg (p = 0.2563). Thus it was assumed that NH4+ did not directly affect the bacterial MeHg formation.