Stormwater Suspended Solids and Pollutant Concentrations in an Urban Stream

Abstract

The chemical and ecological quality of small rivers or streams is not normally classified, but any urban stormwater discharges will result in their impairment. To assist in understanding the resulting applied environmental stress to benthic communities, the empirical relationships between in-stream suspended solids and metal pollutants, initially identified by principal component analysis, are explored using data from a small urbanised upland catchment. When data are amalgamated from individual storms, a correlation for each metal pollutant with suspended solids is obtained, starting very strongly, and decreasing in the order Fe > Al > Mn > Pb > Cr > Zn > Cu > Ni. Comparison of pollutant/solids correlations for individual storms is, for some metals, excellent, but differing intercepts and/or gradients can be obtained for an individual metal between storms. The practice of assessing their correlation based on event mean concentrations or longer-term averages across several storm events may conceal strong correlations for individual storms, exemplifying the impact of variable catchment characteristics and storm conditions; Cd and Be are poorly correlated with suspended solids and are likely affected by changes in background pollution levels. The analysis of covariance also highlights a significant difference between certain storm events for specific metals, which calls into question the validity of using combined storm-event datasets. The relationships identified here enable the determination of stormwater pollutant loads from suspended solids concentrations for heavy metals and are useful for developing water quality assessments.