Paved surfaces in urban areas not only increase runoff volume but also impact water quality. During a summer rainstorm following a long dry period, which of the following pollutants is most likely to be present in the initial runoff (the 'first flush') from a heavily used parking lot?
- High concentrations of nitrates and phosphates from fertilizer application.
- Significant amounts of pathogenic bacteria and viruses from sanitary sewer leakage.
- High concentrations of dissolved oxygen due to the turbulent flow across the pavement.
- Elevated levels of hydrocarbons, heavy metals, and sediment accumulated from vehicles. (correct answer)
Explanation: Urban stormwater runoff questions test your understanding of how impervious surfaces affect both water quantity and quality. When you encounter these scenarios, think about what accumulates on paved surfaces during dry periods and what happens during the "first flush" - the initial runoff that carries the highest pollutant loads. During dry periods, parking lots accumulate contaminants from vehicle use: oil and grease drippings, tire wear particles containing heavy metals, exhaust residues with hydrocarbons, and general sediment from dust and debris. When rain finally arrives after a drought, this first wave of runoff acts like a dirty sponge being squeezed, concentrating all these accumulated pollutants. This makes option D correct - elevated levels of hydrocarbons, heavy metals, and sediment are the signature pollutants of urban parking lot runoff. Option A is wrong because nitrates and phosphates come from fertilizer applications, which don't occur on parking lots - these are associated with agricultural or residential lawn runoff. Option B incorrectly suggests pathogenic contamination from sewer leakage, but the question specifies runoff from pavement surfaces, not sewage infrastructure problems. Option C contains a fundamental misunderstanding - dissolved oxygen isn't a pollutant but rather an indicator of water health, and urban runoff typically has low dissolved oxygen due to organic pollutants that consume oxygen. Remember this pattern: urban impervious surface runoff questions almost always involve vehicle-related contaminants (petroleum products, metals from brake pads and tires) rather than biological or agricultural pollutants. The "first flush" concept is key - initial runoff carries the highest pollutant concentrations.