Variable relationships between the hydrophobic fraction of dissolved organic matter and metals in Scottish freshwater before the estuarine mixing zone

Abstract

The organic-Fe association in Scottish freshwater rivers has received little attention compared with in the estuarine mixing zone. We collected 201 water samples from rivers and lakes in Scotland across different sampling years and seasons. Relationships among the hydrophobic (HPO) fraction of dissolved organic matter (DOM), specific UV absorbance (SUVA 254 ), and dissolved metals (Al and Fe) were examined to better understand their co-transportation in Scottish waters. The average DOM, HPO fraction, Fe, and Al concentrations for all the samples co-varied and were lower during winter than during summer. There was a strong positive correlation between DOM and HPO fraction concentrations (R 2 = 0.99, p < 0.0001). A significant positive correlation was also found between the HPO fraction and Fe and Al concentrations. The regression slope indicating the overall relationships between the HPO fraction and Fe concentrations differed by as much as 12 times depending on both the sampling period and the river. These slope differences were not significantly determined by the chemical structures of DOM, SUVA 254 , or Al and Cu concentrations. These results suggest that the Fe transport capacities vary among the Scottish rivers because of other factors such as seasonal effects (temperature and the level of water table) and a suspended solid concentration in the water column.