Urban stormwater runoff contamination associated with gutter and pipe material degradation
In addition to typical stormwater pollutant sources (erosion materials, automotive activity, landscaping chemicals, pet wastes, etc.), urban stormwater runoff can also be contaminated with substances that leach from materials with which the water contacts as the rain water travels through an urban area to the discharge point. Prior studies have shown that the composition of roofing materials and the drainage system pipes can significantly affect the amounts of pollutants, particularly heavy metals, released into the runoff, especially for roof runoff. This dissertation research indicated that water chemistry (pH, salinity, major ions, etc.) and time of contact may also affect the release of contaminants from materials. The primary objective of this research was to examine how different drainage system and tank materials, water chemical characteristics, and exposure times affect contaminant losses during controlled tests examining the expected range of these characteristics. Static leaching tests for eight pipe and gutter materials were conducted over two separate three month periods during which pipe and gutter test materials were exposed to roof runoff and stormwater buffered to pH 5 and 8 and for exposure to different salinity conditions. A suite of heavy metal and nutrient constituents were periodically analyzed during the exposure periods. Also, pH, Eh, toxicity, alkalinity, total and calcium hardness, chloride and sulfate analysis were evaluated. This research found that the metallic gutter and pipe materials released significant heavy metals. Some of these materials were found to release large amounts of zinc, copper, and lead during the tests, with galvanized steel materials being the most significant sources of lead and zinc, while copper materials were the most important source of copper (as expected). Zinc, copper, and lead releases were detected during both short and long exposure times under low and high pH conditions and low and high salinity conditions. Statistical analyses were conducted to determine the effects of time, pH, salinity and each type of material on the release of the metals. Model fitting was performed on the time series plots to predict the release rate of metals as a function of exposure time and surface area. Chemical speciation modeling was also conducted to predict the forms of the measured metals, to compare the maximum concentrations observed with the expected solubility conditions, and to predict the relative toxicities and treatabilites of the different metallic compounds and ionic species likely present. A simple model was developed that quantified the expected pollutant releases for various materials for different uses (drainage systems vs. storage tanks) and water types (low and high pH conditions, saline and non-saline waters).
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