Abstract
Water samples and suspended sediments from two southern Ontario rivers, the Grand River and its tributary, the Nith River, were analyzed to investigate the role of suspended sediments in the transport of phosphorus and metals from tributaries. Water samples were analyzed for total suspended solids, total phosphorus, total dissolved phosphorus, chloride and heavy metal concentrations. Suspended sediments were analyzed for forms of phosphorus, total and organic carbon, and heavy metals. Concentrations of total phosphorus and heavy metals in water were elevated during high flows (spring snowmelt and episodes of intensive rain events), which are characterized by high concentrations of suspended solids. On average, about half of the total phosphorus in the water (60 and 54% in the Grand and Nith rivers, respectively) was transported in the particulate form, of which about 20% is in a form that would be potentially available for biological utilization upon depletion of soluble P. Biologically inert apatite-P accounted for 25 and 19% of the total particulate P in suspended sediments from the Grand and Nith rivers, respectively, indicating that between one-fifth and one-quarter of the tributary particulate P flux is of little environmental significance. This form of particulate P showed positive correlation with discharge, indicating increased contribution of this form of P during high flows due to the channel scouring and bank erosion. This finding has important implications for the refinement of P load calculations, since it is during the wet years that P loads to the Great Lakes exceed target loads stipulated by the Great Lakes Water Quality Agreement, mainly because of increased tributary P contributions.
Except for apatite-P in suspended sediments from the Grand River, no statistically significant intersite differences were found in water and sediment chemistry, suggesting that there are no important sources or sinks of available phosphorus and metals between the monitored sites. River inputs from sewage treatment plants and urban runoff appear to have little effect on water quality. The lack of significant site differentiation in suspended sediments chemistry also reflects the absence of any significant chemical and/or biological processing of particles in transit and the similarity of sediment sources.
Estimated total phosphorus loads for selected sites on Great Lakes tributaries, water years 2014–2018
