Suppose that your community has a large sewage treatment plant which is discharg

Question

Suppose that your community has a large sewage treatment plant which is discharging nitrogen-rich wastewater into a nearby stream. Describe how the introduction of excessive amounts of nutrients (e.g., nitrogen) can affect the water quality downstream of the sewage plant. As shown in Figure 14.3, sewage and stormwater drains typically share the same set of underground pipes. During heavy rains these combined systems often become overloaded to the point where untreated wastewater is allowed to discharge directly into a stream. Describe the type of downstream water-quality problems you would expect when this occurs. Recent studies have documented the presence of a wide range of hormones and antibiotics in U.S. streams-these compounds originate from the medicines we take. Because streams serve as the water supply for many municipalities, large numbers of people are likely being exposed to these compounds. Although the concentrations are low, data are lacking as to the long-term effect on human health. a) If the source of your city's drinking water is a stream, what would you do until more scientific data becomes available? Note that bottled water may not be a solution as it is often times just city water. b) How could you personally, as a citizen, help reduce the amount of pharmaceuticals that are entering our water supplies? Figure 14.4 illustrates how a septic tank and drain field is used to treat household wastewater. a) Explain why it is so important that the drain field and gravel bed of the septic system be located well above the water table.

Explanation / Answer

9) Unlike temperature and dissolved oxygen, the presence of normal levels of nitrates usually does not have a direct effect on aquatic insects or fish. However, excess levels of nitrates in water can create conditions that make it difficult for aquatic insects or fish to survive.

Algae and other plants use nitrates as a source of food. If algae have an unlimited source of nitrates, their growth is unchecked. So, Why is that a problem?

A bay or estuary that has the milky colour of pea soup is showing the result of high concentrations of algae. Large amounts of algae can cause extreme fluctuations in dissolved oxygen. Photosynthesis by algae and other plants can generate oxygen during the day. However, at night, dissolved oxygen may decrease to very low levels as a result of large numbers of oxygen consuming bacteria feeding on dead or decaying algae and other plants.

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