Metal Speciation and Toxicity in Urban River Sediments
Metal transport, speciation and toxicity were studied in urban runoff and receiving urban river systems. Sediments were recognised as an important environmental compartment for metals.
An assessment of the accuracy and precision of sampling and analysis demonstrated the reliability of the chemical (metal analysis) and ecotoxicological (dehydrogenase activity, Microtox) procedures selected for studying urban river sediments. Significant variations in sediment sampling were found and this problem was partly overcome by mixing discrete samples at each urban river sampling site.
Platinum concentrations in road sediments were shown to have increased since the introduction of catalytic converters to automobiles. Platinum concentrations in the < 63 mm fraction increased from 3.0 to 8.9 ng g-1 between 1984 and 1991, while lead showed a corresponding decrease from 326 to 182 mg g-1. Road sediments contained 39-88% more Pt than gullypot sediments and sequential extraction showed a distinct shift from predominantly inorganic Pt on the road surface to wholly organic Pt in the gullypot. cth/Dissolved Pt concentrations in disturbed gullypot liquor were within the range 1.7 ng 1-1 to 3.8 ng 1-1. Platinum enriched road and gullypot sediment were shown to be transported in stormwater and eventually deposited, as urban river sediments, where they represent a potential threat to receiving water quality.
The effects of stormwater and combined sewer overflows on receiving waters were investigated using measurements of bacterial enzyme activity and metal speciation in the sediments of five urban rivers. Free flowing urban rivers had high enzyme activity and low metal concentrations in sediments. More stagnant urban rivers were characterised by inhibited enzyme activity and high EDTA extractable metal concentrations. Profiles along two urban rivers showed a direct inhibition of enzyme activity at sites of stormwater and industrial discharge. Urban river sediments were chemically or physically fractionated using selective extractants to separate the effects of metal and organic contaminants. Remaining toxicity was assessed through inhibition of bacterial enzyme activity and luminescence. In many cases the enzyme activity of the sediment-dwelling bacteria was inhibited by metals. Variations in inhibition were attributed to differences in sediment complexation of, rather than bacterial community tolerance to, metals. The higher organic content of sediments in the urban river Kvillebäcken compared to Mölndalsån allowed complexation of metals and therefore lowered inhibition of enzyme activity. The importance of organic ligands in binding and modifying the toxicity of sediment-bound metals was demonstrated through the relationship between complexation capacity and dehydrogenase activity for a range of urban river sediments.
Sedimentary organic material was also shown to be critical in regulating metal concentrations in the interstitial waters of urban rivers. Increasing organic concentration leads to an increase of sediment-water distribution coefficients for metals and a lowering of interstitial water concentration. Under storm conditions interstitial waters are resuspended in the urban river leading to an exposure of benthic and water column ecology to elevated concentrations of toxic metal species.