Question

The U.S. Environmental Protection Agency (EPA) defines point source pollution as “any single identifiable source of pollution from which pollutants are discharged, such as a pipe, ditch, ship or factory smokestack” (Hill, 1997).

Factories and sewage treatment plants are two common types of point sources. Factories, including oil refineries, pulp and paper mills, and chemical, electronics and automobile manufacturers, typically discharge one or more pollutants in their discharged waters (called effluents). Some factories discharge their effluents directly into a waterbody. Others treat it themselves before it is released, and still others send their wastes to sewage treatment plants for treatment. Sewage treatment plants treat human wastes and send the treated effluent to a stream or river.

Another way that some factories and sewage treatment plants handle waste material is by mixing it with urban runoff in a combined sewer system. Runoff refers to stormwater that flows over surfaces like driveways and lawns. As the water crosses these surfaces, it picks up chemicals and pollutants. This untreated, polluted water then runs directly into a sewer system.

When it rains excessively, a combined sewer system may not be able handle the volume of water, and some of the combined runoff and raw sewage will overflow from the system, discharging directly into the nearest waterbody without being treated. This combined sewer overflow (CSO) is considered point source pollution, and can cause severe damage to human health and the environment.

Unregulated discharges from point sources can result in water pollution and unsafe drinking water, and can restrict activities like fishing and swimming. Some of the chemicals discharged by point sources are harmless, but others are toxic to people and wildlife. Whether a discharged chemical is harmful to the aquatic environment depends on a number of factors, including the type of chemical, its concentration, the timing of its release, weather conditions, and the organisms living in the area. What are the factors that determine the level of harm an introduced chemical has on the environment? (Site 1)

Answer 1

Key factors that determine how harmful an introduced chemical will be to the environment

- Chemical identity and properties — what the substance is and how it behaves: toxicity, solubility in water, volatility, persistence (how quickly it breaks down), and potential to bioaccumulate and biomagnify in food webs.
- Concentration (dose) — higher concentrations generally cause greater harm; even a toxic chemical can be harmless below a threshold.
- Duration and frequency of exposure — a single short pulse often matters less than repeated or chronic exposures at lower levels.
- Timing of release — season or life stage matters (e.g., spawning or juvenile periods are more vulnerable).
- Route of exposure and environmental compartment — whether the chemical stays dissolved in water, binds to sediments, or enters organisms’ tissues affects exposure and impact.
- Weather and physical conditions — rainfall, temperature, currents, dilution, and mixing influence dispersion, breakdown, and oxygen levels.
- Characteristics of the receiving ecosystem — size, flow rate, water chemistry (pH, dissolved oxygen), and presence of sensitive habitats (wetlands, coral, spawning grounds) affect vulnerability.
- Community and species sensitivity — which organisms are present and their tolerance; some species are far more sensitive than others.
- Interactions with other pollutants — mixtures can produce additive, synergistic, or antagonistic effects that change toxicity.

These factors interact, so risk is determined by the combination (not any single factor alone).