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Many studies have identified metals in urban runoff as a major contributor to the degradation of urban streams and rivers. Metals of most concern may include copper, cadmium, chromium, lead, and zinc, amongst others. Metals in urban runoff can occur as dissolved, colloidal and particulate-bound species. Therefore, it is important to measure all forms of heavy metals, especially the particulate and filterable fractions, when determining their fate and effects.

The objectives of these tests were to determine the associations of heavy metals (along with selected major constituents and nutrients) with different-sized particulates in stormwater. The binding strengths of these metals to the particulates were also examined by using a sequential extraction procedure using different acids and bases under several pH values. Also, experiments to determine the relative amount of ionic forms vs. organo-metallic complexes of the filterable (<0.45um) portion of the heavy metals were also conducted.

Results show that most of the total phosphorus and phosphate are associated with the particulates, while other nutrients (such as nitrate) are dissolved and their concentrations are not effectively reduced by removing particulates from the stormwater. Obviously, particulate-bound pollutant concentrations should all decrease with filtration through finer sieves. However, there were infrequent jumps in concentrations for some conditions, reflecting variability in the analytical method and the sample handling. Constituents that did not change significantly with filtration included nitrate and sodium, as expected. Other analytes (COD and cadmium) also had little change, except for a single sample in each case. Most of the analytes (turbidity, phosphorus, phosphate, magnesium, chromium, copper, iron, lead, and zinc) had large decreases with filtration, especially for the more contaminated samples. Total solids and COD had much smaller changes with filtration, with substantial fractions associated with the filterable (<0.45 μm) fraction. In many cases (especially for solids), there were major differences in sample characteristics for the set of samples collected from source runoff areas compared to the sample set collected from stormwater outfalls. The larger particulates made up a much larger fraction of the total solids for the source area samples than for the outfall samples, reflecting poor delivery of the large particulates through the conventional drainage systems.

Samples were also subjected to heavy metal leaching tests under different acidic conditions. The major constituents (calcium, potassium and sodium) had very little change for any pH exposure condition. Only small fractions of the iron were released under the strongest pH conditions of the exposure tests for all samples, indicating that iron was always strongly bound. These tests indicate that the heavy metals of concern remain strongly bound to the particulates during long exposures at the extreme range of pH conditions likely to occur in urban receiving water sediments. They will also likely remain strongly bound to the particulates in stormwater control device sumps, or detention pond sediments, where particulate-bound metals are captured.
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Document Type: Research Article

Publication date: 2003-01-01

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